Joseph Makau BSc Dessertation on Acacia Drepanolobium survival

DAMAGE STRATIFICATION ON

Acacia drepanolobium at Ol Pejeta Conservancy

Black Rhino habitat co-existence and survival: Interaction and effects of Mega-herbivores,

fire and drought on Acacia drepanolobium in Ol Pejeta Conservancy, Kenya

By

Joseph K. Makau

Reg. No.: WM/16/06

Supervisor: Prof. G. M. Wahungu

Moi University, Chepkoilel Campus

School of Natural Resource Management

Department of wildlife management

P.O. Box 1125, ELDORET

30100, KENYA.

Personal Contacts

P.O. BOX 117, MACHAKOS.

Email: [email protected]

Phone No: +254-726-051-176.

@2010

mailto:[email protected]

Damage stratification on Acacia drepanolobium at Ol Pejeta conservancy

ii

Joseph K. Makau @ 2010

DISSERTATION

A senior research project submitted to Moi University, the Department of Wildlife

Management in partial fulfillment of the requirements for the award of a Bachelor of Science

Degree in Wildlife Management.


iii


DECLARATION

I JOSEPH KYALO MAKAU solemnly declare that this research project is my original

work, with assistance of my supervisor and without any form of plagiarism and that it has

never been submitted to any university, or any other institution for the award of any

certificate, diploma or degree.

Signature: … … Date: …29th

May, 2010…


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ACKNOWLEDGMENT

I wish to express my heartfelt gratitude to the Ol Pejeta conservancy Earthwatch principal

investigator, main financier of this project and my supervisor Prof. G. M. Wahungu for his

continued support throughout the project period. I acknowledge the efforts of my lecturers:

Dr. Karanja and Mr. Kimuyu for empowering me with the knowledge and skills on research

methods.

I thank the Ol Pejeta conservancy management for providing me with permission to do this

research and the chance to be attached in the conservancy, especially the Ecological

Monitoring Department staff; Mr. M. Mulama, Mr. Nathan, Mr. Mutisya, Miss. Ngw'eno and

Mr. Kamaru. It is during the attachment period when I developed this research topic and the

EMD staff assisted me in getting the background information as well as the literature review.

I would also like to acknowledge, Lucy Mureu, Odera George, James Wambugu (the ranger),

Mwangi Benson (the driver) and Mwaniki (research assistant) who enthusiastically supported

and worked with me throughout the study period.

Finally, this list will not be complete without acknowledging the Almighty God for His

power and strength that has sustained me up to this far without which it would not have been

possible.


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DEDICATION

I dedicate this proposal to my beloved family members: Mum; Philomena, Sisters; Frida,

Noreen and Miriam, and my Brother; Stephen.


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ABSTRACT

This study was carried out in Ol Pejeta Conservancy (OPC) which lies on the Laikipia

plateau between Mt Kenya and the Aberdare mountains at an altitude of 1800 m. Damage

stratification was examined in Acacia drepanolobium. The study was done in a period of

eight months: June 2009 to February 2010. In total, 5722 Acacia drepanolobium trees were

counted and measured in fourteen sampling plots and in two seasons (dry and wet). At the

study site, Acacia drepanolobium form approximately 75% of the endangered black rhino

diet. Frontiers

The Acacia drepanolobium was subjected to three main treatments: burnt, unburnt and

control plot (damage free zone). The 14 sampling plots were distributed on these treatments

based on the relative size for a good replicate and representation of each treatment. Five

radiating transects were laid originating from a marked center tree each measuring 100 x 2 m.

All the Acacia drepanolobium trees were inspected for their damage status and various

parameters measured and recorded.

Rhinos, elephants, giraffes, small browsers, fire and natural death were recorded as the main

damagers of Acacia drepanolobium. Both parametric and Non-parametric tests were used to

test the hypotheses at a 0.05 significance level. Damage on the acacia trees was significant

(p<0.00), with a damage level of 61% over the undamaged level of 39%. Natural damage was

more in bunt areas (71%) than in unburnt areas (29%) hence Acacia drepanolobium trees are

more susceptible to drought when burnt than when unburnt. Ol Pejeta conservancy Acacia

drepanolobium population is composed of 73% seedlings, 17% saplings and 10% mature

trees. Various levels of damage alters the Acacia drepanolobium population structure

significantly (p<0.00). High damage level: burnt and other damage types combined

maintained the Acacia drepanolobium trees short (48.58±73.51 cm), Medium damage level:

Unburnt but with other damage types maintained the Acacia drepanolobium trees at a

medium height (73.11±99.959 cm) while absence of damage made the Acacia drepanolobium

trees grow tall (121.43±136.161 cm). Browsing by the other mega herbivores facilitates for

the availability of rhino browse material. The conditions at the time of the study indicated

significantly more Acacia drepanolobium trees in the conservancy had suffered some

damage, and this damage was higher in the burnt areas and over the dry season.

Key words: Acacia drepanolobium, mega herbivores, burnt and season.


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TABLE OF CONTENTS

DISSERTATION ....................................................................................................................... ii

DECLARATION ...................................................................................................................... iii ACKNOWLEDGMENT........................................................................................................... iv DEDICATION ........................................................................................................................... v ABSTRACT .............................................................................................................................. vi TABLE OF CONTENTS ......................................................................................................... vii

LIST OF TABLES AND FIGURES....................................................................................... viii CHAPTER ONE ........................................................................................................................ 9 1.0 INTRODUCTION ............................................................................................................... 9

1.1 Background information ................................................................................................. 9

1.2 Problem statement ......................................................................................................... 10 1.3 Conceptual Framework ................................................................................................. 11 1.4 Justification of the Study ............................................................................................... 12

1.5 Research Objectives and Hypotheses ............................................................................ 13 CHAPTER TWO ..................................................................................................................... 15 2.0 LITERATURE REVIEW .................................................................................................. 15

2.1 Giraffe impacts on Acacia drepanolobium ................................................................... 15

2.2 Elephant impacts on acacia ........................................................................................... 16 CHAPTER THREE ................................................................................................................. 18

3.0 RESEARCH DESIGN ....................................................................................................... 18 3.1 Study area ...................................................................................................................... 18

3.1.1 Location ...................................................................................................................... 18 3.1.2 Climate and hydrology ............................................................................................... 18 3.1.3 Wildlife ...................................................................................................................... 18

3.1.4 Vegetation and soil type ............................................................................................. 19 3.2 Research tools and equipment ....................................................................................... 19

3.3 Methodology ................................................................................................................. 21 CHAPTER FOUR .................................................................................................................... 23 4.0 DATA COLLECTION AND RESULTS .......................................................................... 23

4.1 Data collection .............................................................................................................. 23

4.2 Results ........................................................................................................................... 23 CHAPTER FIVE ..................................................................................................................... 36

5.0 DISCUSSION .................................................................................................................... 36 CHAPTER SIX ........................................................................................................................ 41 6.0 CONCLUSIONS AND RECOMMENDATIONS ............................................................ 41

6.1 Conclusions ................................................................................................................... 41 6.2 RECOMMENDATIONS .............................................................................................. 43

APPENDICES ......................................................................................................................... 44 Program of activities ........................................................................................................... 44 Data sheet ............................................................................................................................ 45 Key ...................................................................................................................................... 47

REFERENCE ........................................................................................................................... 49


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LIST OF TABLES AND FIGURES

Figure 1; Map of the Ol Pejeta Conservancy Showing the Vegetation Types ........................ 19

Figure 2: Radiating transects ................................................................................................... 21

Figure 3: Acacia drepanolobium damage status in the fourteen sampled plots. ..................... 24

Table 1: Interaction between the habitat status and the damager ............................................ 24

Figure 4: Interaction of burning and the level of damage for each damager ........................... 25

Figure 5: Acacia drepanolobium population structure ............................................................ 26

Table 2 A general linear model; Univariate analysis of variance table of results ................... 26

Figure 6: Effects of fire on the Acacia drepanolobium population structure .......................... 27

Table 3: ANOVA table of results ............................................................................................ 28

Figure 7: Effects of fire on the Acacia drepanolobium density. .............................................. 28

Table 4: Damage stratification ANOVA test results ............................................................... 28

Figure 8: Damage stratification across the Acacia drepanolobium height structure ............... 29

Table 5: ANOVA test on the damagers’ damaging diameter .................................................. 29

Figure 9: Acacia drepanolobium tree diameters for the various damagers. ............................ 30

Figure 10: Herbivore preference for sprouting Acacia drepanolobium trees. ......................... 31

Figure 11: The relationship between the symbiotic ants and the damager .............................. 31

Table 6: Effects of season on damage status ANOVA test result’s. ........................................ 32

Figure12: Effects of season on the damage status ................................................................... 32

Table 7: The interaction between the season and the damager ................................................ 32

Figure 13; The interaction between season and damager ........................................................ 33

Table 8: Effects of damage on Acacia drepanolobium tree height structure ........................... 33

Figure 14: Effects of fire on Acacia drepanolobium tree height structure .............................. 34

Table 9: Effects of fire on Acacia drepanolobium diameter,................................................... 34

Figure 15: Effects of fire on the diameter of Acacia drepanolobium trees. ............................ 35


9


CHAPTER ONE

1.0 INTRODUCTION

1.1 Background information

The Ol Pejeta conservancy was established in September 2004 after the merge of the

SweetWaters Game Reserve and the Ol Pejeta ranch. The SweetWaters Game Reserve (SWGR)

was established in 1989 mainly as a Black rhino sanctuary under a collaborative effort between

Kenya Wildlife Service and Lornho East Africa. The establishment of the Game Reserve was

necessitated by the rapid decline in black rhino population in Kenya in line with the

government’s policy to intensively manage the remaining populations. Prior to 2004, much of

the land was under cattle ranching. The ranch supported a variety of wildlife population ranging

freely within and outside its boundary. In 2007 merging of the SWGR and the Ol Pejeta ranch

was completed, together with the construction of an electric perimeter fence. This merging was

after a recommendation from various stakeholders and high wildlife population pressure (mainly

the elephants, giraffes, rhinos and other plain game like the buffaloes) within the former SWGR.

Before the 2007 it was noted that the Acacia drepanolobium habitat in the game reserve had

been severely damaged by herbivores while that in the cattle ranch side was free from browsing

pressure and healthy. The expansion availed more food resources for herbivores and thereby

reducing browsing pressure in the former SweetWaters Game Reserve and introduced herbivore

damage to the Acacia drepanolobium habitat in the Ol Pejeta cattle ranch. The reserve was

expanded almost three fold, thus bringing the current 75,000 acres (303.51Km2) into an

integrated wildlife conservation area with livestock production. Although the conservancy is

fenced all round, three wildlife movement corridors have been constructed to allow selective

dispersal of all mammals except the rhinos.

Acacia drepanolobium, which is a keystone species in the area, covers over 20% of the total area

of the conservancy and it constitutes 40% of the trees population. The Acacia drepanolobium

constitute approximately 75% of the black rhino diet. Giraffes in Ol Pejeta have been found to

spend 90% of their feeding time browsing Acacia drepanolobium. The presence of the wildlife

movement corridors makes the populations of elephants and giraffes populations to vary.

However, it is estimated that the resident population is approximately 300 elephants, 300 giraffes


10


and the current black rhino population is 82 in total (Mutisya 2009). Currently with the

assistance of the Earthwatch Institute, the Ol Pejeta conservancy ecological monitoring

department monitors the Acacia drepanolobium growth rate, damage and mortality occasioned

by elephants, rhinos and giraffes. Earlier studies by Birkett (2002) indicated that Acacia

drepanolobium trees in the Ol Pejeta conservancy are generally maintained at less than 3m tall

through browsing of the crown by giraffe or by having the stem broken off by elephants;

however they can grow up to 20m once out of high herbivore browsing pressure.

1.2 Problem statement

Herbivore damage on the Acacia drepanolobium trees across different height classes affects the

habitat architectural structure. Browsing damage at various heights is known to affect the Acacia

drepanolobium differently like decrease in browse availability. These effects if not well

understood and active management interventions taken can compromise the objectives of

conserving an endangered species. Studies conducted by Birkett (2002) in OPC indicated

overstocking of elephants in the former SweetWaters Game Reserve (SWGR) lead to severe

damage of the Acacia drepanolobium trees. As a result the rhino carrying capacity declined from

estimated 90 to about 50 rhinos. Based on the recommendations of this study, a total of 56

elephants were trans-located from OPC to Meru national park in 2001. Consequently, Acacia

drepanolobium mortality caused by elephants reduced significantly. The study also

recommended that the game reserve be expanded and this was completed in April 2007. The

former SWGR was expanded from 24,000 acres to the current Ol Pejeta conservancy 75,000

acres. The expansion availed more food resources for herbivores and thereby reducing browsing

pressure. In addition to the expansion, three corridors were opened to the north of OPC, thereby

enabling dispersal of elephants to the greater Laikipia ecosystem.

Following the above major transformations, monitoring results showed that elephant induced

mortality of Acacia drepanolobium declined (Wahungu & Mureu, 2008); this significant

reduction was realized as a result of elephant redistribution due to the expansion and opening of

the movement corridors. Habitat monitoring of Acacia drepanolobium woodland after the 2001

translocation showed gradual recovery of these habitat; 14% annual regeneration of Acacia

drepanolobium recorded thereafter. A recent study on the interactions of herbivores and Acacia

drepanolobium in OPC revealed that elephant related mortality in Acacia drepanolobium trees


11


was 35% and increased predictably with increase in tree height. Giraffe browse significantly

reduced flowering and fruiting in Acacia drepanolobium but did not directly influence height

increment. However, giraffe browsing increased susceptibility to drought. Rhinos browse on

seedlings and trees below 2m thereby affecting the rate of recruitment of seedlings into trees

(Wahungu & Mureu, 2008).This study is therefore, going to act as a post expansion examination

of the damage status on the Acacia drepanolobium trees in the Ol Pejeta conservancy since 2007.

1.3 Conceptual Framework

Mega herbivores, small browsers, fire and natural damage all interact interdependently in

changing the architecture of the Acacia drepanolobium population.

Acacia drepanolo

bium

FIRE

NATURAL DAMAGE

SMALL BROWS

ERS

MEGAHERBIVORES


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1.4 Justification of the Study

Ol Pejeta conservancy strives to conserve the highly endangered black rhino (Diceros bicornis

michaeli). Acacia drepanolobium makes up a high percentage (~75%) of the black rhinos’ diet.

As stated in the problem statement, the presence of the elephants and giraffes in Ol Pejeta

possess a threat of compromising the standards of conserving the endangered black rhinos. The

three mega-herbivores: rhino, elephants and giraffes, greatly affects the status of the Acacia

drepanolobium (Wahungu & Mureu, 2008). The elephants kill trees entirely or reverse growth

by breaking the main stem, giraffe affect flowering and fruiting and the rhinos reduce

recruitment of seedlings to mature trees. There are other browsers like the elands and the impalas

which browse on the seedlings hence having the same impacts as the rhinos.

The need for a comprehensive understanding of elephants, rhinos and giraffes interactions with

Acacia drepanolobium woodland in modeling the ecosystem at Ol Pejeta conservancy is

necessary. Ol Pejeta conservancy is the south most and wettest of the ranches in the Samburu-

Laikipia ecosystem. Because of this, there has been an influx of wildlife particularly elephants

escaping drought in the north (Samburu) into the conservancy. This has resulted in heightened

elephant damage on the Acacia drepanolobium (Wahungu & Mureu 2008). The corridor

movement of animals has made it difficult to accurately monitor the population demographics of

the elephants, giraffes and other animals with the exception of the rhinos because the corridors

are rhino proof.

This may imply that the herbivore damage is high at certain seasons due to the opening of

migratory corridors. This study will give recommendations to the Ol Pejeta conservancy

management based on its findings in making decisions on the protection of the Acacia

drepanolobium from extensive damage.


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1.5 Research Objectives and Hypotheses

1.3.1 Main objective

The main objective of the study was to assess the extent and the effects of damage on Acacia

drepanolobium in the Ol Pejeta conservancy.

1.3.2 Specific objectives

The Specific objectives of this research were:

1) To assess the extent of damage on the Acacia drepanolobium in Ol Pejeta conservancy.

2) To examine the Acacia drepanolobium population structure at the Ol Pejeta conservancy.

3) To evaluate the effects of season on the Acacia drepanolobium in Ol Pejeta conservancy.

4) To examine the effects of fire on the Acacia drepanolobium population structure in Ol Pejeta

conservancy.

1.3.3 Hypotheses

1. i) Ho: The occurrence of Acacia drepanolobium damage is independent of the sampling plots

in Ol Pejeta conservancy.

Ha: The occurrence of Acacia drepanolobium damage is dependent on the sampling plots

in Ol Pejeta conservancy.

ii) Ho: There is no interaction between the habitat status and the damager.

Ha: There is interaction between the habitat status and the damager.

2. i) Ho: Acacia drepanolobium population structure is independent of burning.

Ha: Acacia drepanolobium population structure is dependent on burning.


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ii) Ho: There is no difference in the Acacia drepanolobium density in the burnt and unburnt

areas.

Ha: There is difference in the Acacia drepanolobium density in the burnt and unburnt

areas.

iii) Ho: The damagers damage the Acacia drepanolobium trees at the same height.

Ha: The damagers do not damage the Acacia drepanolobium trees at the same height.

iv) Ho: All damagers damage Acacia drepanolobium trees of the same diameter.

Ha: All damagers do not damage Acacia drepanolobium trees of the same diameter.

v) Ho: The damager is independent of the sprouting status of the Acacia drepanolobium trees.

Ha: The damager is dependent on the sprouting status of the Acacia drepanolobium trees.

vi) Ho: The damager is independent of the symbiotic ants on the Acacia drepanolobium trees.

Ha: The damager is dependent of the symbiotic ants on the Acacia drepanolobium trees.

3. i) Ho: Acacia drepanolobium damage status does not vary with season.

Ha: Acacia drepanolobium damage status varies with season.

ii) Ho: There is no interaction between the damager and the season.

Ha: There is interaction between the damager and the season.

4. i) Ho: Fire does not affect the Acacia drepanolobium height structure.

Ha: Fire affects the Acacia drepanolobium height structure.

ii) Ho: Fire does not affect the diameter of Acacia drepanolobium.

Ha: Fire affects the diameter of Acacia drepanolobium.


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CHAPTER TWO

2.0 LITERATURE REVIEW

Acacia drepanolobium is a swollen-thorn Acacia species native to East Africa. The Acacia

drepanolobium trees have an average growth height of 2 meters and are covered with long

thorns, some of which have large bulbous bases. These swollen thorns are naturally hollow and

occupied by any one of the several symbiotic ant species. The common name of the plant is

derived from this: when wind blows over the bulbous thorns in which ants have made entry/exit

holes, they create a whistling sound thus the common name of the whistling thorn Acacia.

Whistling thorn Acacia is the dominant tree in some areas of upland East Africa, sometimes

forming a nearly monoculture woodland, especially on "black cotton" soils on impeded drainage

with high clay content. It is browsed upon by the black rhinos, elephants and giraffes.

2.1 Giraffe impacts on Acacia drepanolobium

Giraffa camelopardalis reticulata spend most of their feeding time within different areas of

different diverse habitats however; they have a seasonal preference for certain plant species

(Foster 1966). How these plant species are utilized depends on their tannin content and browsing

pressure placed upon them. An increased disturbance on any given habitat takes long time and

energy for equilibrium to be restored; this explains why there is a need to examine the herbivore

damage extent and advice on management actions to be taken. A case study of the Nairobi

national park shows that the Acacia drepanolobium has been severely affected by large herbivore

browsing damage. As a result, they only grow to an average height of only 67cm, whereas

outside the park they grow to 120cm because there is no browsing by specialist (Foster 1966).

Species abundance is related to browsing pressure although some species can adapt to heavy

browsing. Due to giraffe heavy browsing pressure in the Nairobi national park the Acacia

drepanolobium have become smaller and short, but more numerous due to the absence of fires

(Foster 1966). An indication of giraffes preferred food species is being over browsed is when

they start feeding at a low level when they are adapted for reaching high levels (Foster 1966). It

has been shown that there is clear stratification in the African browsing ruminants in which the

male giraffe feed at a significantly higher level than females (Toit, 1990). This is accounted for

by two theories, one being that this might imply the existence of competition between species


16


(Toit et al.,1990) and the other is that this competition is reduced by having different significant

feeding levels (Sinclair and Norton Griffith 1979, Pellew 1983;).

Booth (1997) reported that, giraffes in Ol Pejeta spend over 85% of their time budget feeding on

Acacia drepanolobium. For a greater percentage of these feeding observations (86%), giraffes

fed at the top soft crown of Acacia drepanolobium (Booth 1997). Severe browsing may have a

considerable effect on Acacia drepanolobium and may alter the structure of the vegetation

associations (Coe et al,. 1987). This is evident at OPC where distinct architectural difference

exists in the Acacia drepanolobium habitats. Although moderate browsing by giraffe may

stimulate sprouting in Acacia drepanolobium, severe browsing may reduce re-growth feed-back

loop (Pellew, 1983). Although there is evidence to suggest that giraffe browsing is having an

effect on the structure, the giraffes are still feeding at high levels and not utilizing all available

food species in the conservancy. This indicates that the fitness of the Acacia drepanolobium is

not being compromised.

'The diameter growth rate does depend on the general vigor of the tree' (New 1984), Booth

(1997) found that the Acacia drepanolobium of SweetWaters were in good condition. However,

they are maintained at a height of 2.8 m below their capacity of 4.8 m, with smaller canopy

width and percentage leaf coverage as a result of giraffe browsing. She suggested that if over

population of giraffe in the Game Reserve will occur, it would be expected that the preferred

food source; Acacia drepanolobium, would be severely exploited and the vegetation to contain

abnormal amounts of tannin due to an increased browsing pressure. This would result to

mortality of those individuals with the weakest physiological tolerance. This study will therefore

focus on assessing the structure and condition of Acacia drepanolobium in relation to the

browsing pressure.

2.2 Elephant impacts on acacia

Elephant damage on trees includes bark-stripping, breaking of branches and the main stem and

uprooting of the complete tree. Van de Vijver et al., (1999); Ruess et al., (1990) and Croze

(1974b) show that elephants push over tall mature trees (over 5m tall) but break branches and the

main stem of smaller trees. Birkett (2002) showed that 72% of the trees sampled were below 3m

in height and consequently the breaking of branches will be a significant mode of damage at


17


SweetWaters. Bond and Loffel (2001) assessed browse damage by recording the status of branch

ends. Between 1998 and 2001, studies conducted by Birkett in OPC indicated overstocking of

elephants in the former SweetWaters Game Reserve leading to severe damage of the Acacia

drepanolobium trees. As a result the rhino carrying capacity declined from estimated 90 to about

50 rhinos. Based on the recommendations of this study, a total of 56 elephants were translocated

from OPC to Meru national park in 2001 in conjunction with KWS. Consequently, Acacia

drepanolobium mortality caused by elephants reduced significantly. The study also

recommended that the game reserve be expanded and this was done in April 2007. The former

SWGR was expanded from 24,000acres to 75,000 acres. The expansion availed more food

resources for herbivores and thereby reducing browsing pressure. In addition to the expansion,

three corridors were opened to the north of OPC, thereby enabling dispersal of elephants to the

greater Laikipia ecosystem.

Following the above major transformations, monitoring results showed that elephant induced

mortality of Acacia drepanolobium declined from 4.4% to 2.5% (2005-2007); this significant

reduction was realized as a result of elephant redistribution since the expansion and

establishment of the movement corridors. Habitat monitoring of Acacia drepanolobium

woodland after the 2001 translocation showed gradual recovery of these habitat; 14% annual

regeneration of Acacia drepanolobium recorded there after. This study is therefore, going to act

as an examination on the variations of the elephant damage to the Acacia drepanolobium since

the establishment of the movement corridors in 2007. To accomplish this, secondary data from

the EarthWatch institute on Acacia drepanolobium damage since 2007 will be used to compare

and contrast with the data collected from this study.


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CHAPTER THREE

3.0 RESEARCH DESIGN

3.1 Study area

3.1.1 Location

Ol Pejeta Conservancy (OPC) is located in central Kenya, 230km north of Nairobi, near

Nanyuki, on the equator at longitude 36°56'E. It lies on the Laikipia plateau between Mt Kenya

and the Aberdare mountains at an average altitude of 1800m.

3.1.2 Climate and hydrology

Ol Pejeta conservancy is in a semi-arid area with an average annual rainfall of 739 mm with

peaks from March to May, and from October to December. There is a smaller peak in August

with an average of 78mm. The annual average maximum and minimum temperatures are 28ºC

and 12ºC respectively. Two permanent rivers; Ewaso Nyiro and Ng’obit which traverse OPC and

several seasonal rivers and streams (lagga), man-made dams, troughs and pipelines provide water

to wildlife, livestock and staff.

3.1.3 Wildlife

Ol Pejeta conservancy is home for approximately 68 species of mammals. The conservancy

is currently the largest Black rhino Sanctuary in East Africa with a population of 82 black

rhinos (Diceros bicornis). Other large mammal herbivores at the study site include

elephants (Loxodonta africana), giraffes (Giraffa camelopardalis), cape buffalos (Syncerus

caffer), elands (Taurotragus oryx), Grevy’s zebras (Equus grevyi), Burchell’s zebras (Equus

burchelli), Beisa oryx (Oryx beisa), Jackson’s hartebeests (Alcelaphus buselaphus jacksoni),

Waterbuck (Kobus defassa), Grant’s gazelles (Gazella granti), steinbucks (Raphicerus

campestris) and domestic cattle. Predators include Lion (Panthera leo), leopard (Panthera

pardus, cheetah (Acinonyx jubatus), wild dog (Lycaon pictus), Silver-backed jackal (Canis

mesomelas) and Spotted hyena (Crocuta crocuta). Primates include Olive baboon (Papio

anubis), Patas monkey (Erythocebus patas), Vervet monkey (Cercopithecus aethiops) and

Lesser bushbaby (Galago senegalensis).


19


3.1.4 Vegetation and soil type

Ol Pejeta conservancy has got four distinct habitats; a mosaic of grassland, Acacia

drepanolobium mixed woodland, Euclea bushes (Euclea divinorum), and Riverine woodland

(Acacia xanthophloea). The Grasslands are dominated by Themenda triandra and Penisetum

mezianum grass species. The Acacia drepanolobium is mixed with shrubs of Euclea divinorum,

Carissa edulis, Psidia punctulata and Scutia myrtina. The dominant soil type is black cotton soil.

Figure 1 below shows the habitat distribution map of the Ol Pejeta conservancy courtesy of the

OPC Ecological Monitoring Department.

Figure 1; Map of the Ol Pejeta Conservancy Showing the Vegetation Types (Map adopted from

Olpejeta Earthwatch’s Black rhino habitat Project)

3.2 Research tools and equipment

A computer with Geographical information system software


20


A global positioning system unit (GPS)

A 100 m measuring tape

A 30 m steel measuring tape

Measuring rod

Prismatic compass

Stationeries


21


3.3 Methodology

A Geographical information system (GIS) map of OPC showing the vegetation types (shown

above) was used to identify Acacia drepanolobium habitat. Within these habitats, fourteen

sampling plots were selected to cover the main sectors/blocks of the conservancy. Each plot was

circular in shape with a radius of 100 m and covering 31428.57 m2. Four plots were set in areas

that had a record of burning in past two or three years. One control plot was also established in

an enclosure free from herbivore and fire damage: Morani and the remaining nine sampling plots

were distributed in the rest of the Acacia drepanolobium habitat with mixed types of damages.

For each sampling plot an Acacia drepanolobium tree was identified, marked and its position

marked as a way plot in a GPS unit for subsequent identification and measurements. For each

plot the marked tree was used as the starting plot to lay down five radiating transects each

measuring 100m as shown in figure 2 below.

Figure 2: Radiating transects

The first transect was oriented straight in to any direction with help of a leader at the end of

transect and a follower at the starting point to position the leader. Then its bearing was measured

using a prismatic compass held by the follower in order to get the bearing of the next transect by

adding 72°. The same procedure was repeated respectively to lay down all transects. Once

transect was laid down all the Acacia drepanolobium trees within a belt of 2m (i.e. 1m on either

side of transect) were given a number and several variables were measured and recorded against

its number.

First, the tree height was measured, its breast height diameter and its damage status; whether

present or absent were recorded against the tree’s number. Once an Acacia drepanolobium tree

100m

720

2m


22


was identified as damaged, the damage height was measured and recorded. The type of damage

was described as whether the main stem was broken, snapped off/ secateurs’ type of cut, side

branch broken or snapped off, the tips snapped off, tree leaning or uprooted, tree burnt, tree

drying from the top or tree dry/dead stamp.

The age of the damage was estimated by direct observation of the damage site, Croze (1974 b)

stated that the age of damage could be categorized by inspection of the color and condition of the

wood at the damage site. Birkett (2002) stated that elephant damage older than 18 Months was

frequently missed due to the subsequent growth of the branch. Damage over 18 months old was

therefore not sampled due to the relatively low probability of encountering representative

samples.

The cause of the damage was identified and recorded. Elephant damage was distinguished from

rhino damage through inspection of the mode of breakage (Laws 1970; local information) where

the branch or the main stem appeared to have been ripped of, the damage was attributed to

elephants. Where a secateurs type of cut was identified, that was attributed to rhino. Selective

removal of the soft twigs at the crown of tall Acacia drepanolobium trees was attributed to

giraffe browsing and that on short Acacia drepanolobium trees was attributed to small browsers

like Impalas. Where the Acacia drepanolobium trees were seen to be drying from top this was

attributed to natural damage either because of prolonged droughts, age or nutrients deficiency.

The identification of damage signs was backed up by information from the accompanying

security guards who have good knowledge on the local vegetation and animal behavior. Also the

presence of the symbiotic crematogaster ants’ species was observed and recorded. Any other

relevant information such as whether the tree was sprouting, regenerating, flowering or bearing

seeds (pods) was recorded as an additional comment.


23


CHAPTER FOUR

4.0 DATA COLLECTION AND RESULTS

4.1 Data collection

The data was collected in two seasons starting from June 2009 to December 2009: dry season

and January 2010 to February 2010: wet season. The collected data was first entered in

Microsoft office excels spreadsheet, and coded for analysis with the statistical package for social

sciences (SPSS). The data was then analyzed using parametric tests in case of a normal

distribution and where the sample number was equal to or greater than the total number of trees

sampled. Non-parametric tests were used where the data did not follow a normal distribution.

The hypotheses were tested at a 0.05 significance level.

4.2 Results

In total 5,722 Acacia drepanolobium trees were counted and measured in fourteen sampling

plots. Each sampling plot was measured twice in the entire study period: in dry and wet seasons.

2,770 Acacia drepanolobium trees were sampled in the dry season and 2,952 Acacia

drepanolobium trees in wet season. Out of the fourteen sampling plots four of them were in areas

which had been burnt two years before and the total number of Acacia drepanolobium trees

sampled in these plots was 2,104. Nine sampling plots were located in unburnt areas where a

total of 3,259 Acacia drepanolobium trees were sampled. One control sampling plot was set up

in an enclosure free from herbivore damage and also unburnt; in this plot 359 Acacia

drepanolobium trees were sampled.

A Pearson chi-square test indicated that the occurrence of Acacia drepanolobium damage was

dependent on the sampling plots in Ol Pejeta (χ2 =5.793; DF = 13, P <0.00).

This study showed that 61% (N= 3467) of the Acacia drepanolobium trees had some of damage

from either herbivore browsing, fire or drying naturally and 39% (N= 2255) were intact and

without any form of damage. Out of the 14 plots 12 of them the level of damage was higher than

the undamaged. The control (plot 10) and plot 13 had very low damage level: the number of

damaged trees was less than the undamaged ones therefore; Acacia drepanolobium damage is


24


distributed all over the Ol Pejeta conservation area with some areas highly damaged than others

(Figure 3 below).

Figure 3: Acacia drepanolobium damage status in the fourteen sampled plots.

A general linear model; Univariate analysis of variance indicated that there was a significant

interaction between the habitat status and the damager (Table 1 below).

Table 1: Interaction between the habitat status and the damager

Tests of Between-Subjects Effects

Dependent Variable: Plot number

Source of variance

Type III Sum

of Squares DF Mean Square F Sig.

Habitat Status 1218.365 1 1218.365 105.238 .000

Damager 7841.844 6 1306.974 112.891 .000

Status * Damager 689.762 5 137.952 11.916 .000

Error 66094.738 5709 11.577

0

100

200

300

400

500

600

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Sampling plots

Damaged

Undamaged


25


Relatively herbivores preferred browsing on burnt Acacia drepanolobium habitat i.e. 55% in

burnt areas over 45% in unburnt areas. However, of all damages recorded, others; small

herbivores like the impalas, elands, zebras and hartebeest had the highest preference for burnt

habitat with 60% of their total damage being on burnt habitats and 40% on the unburnt areas.

Black Rhino had the second high preference for burnt habitats with 55% on the burnt Acacia

drepanolobium habitat and 45% on unburnt habitats. 70% of the elephant damage was found on

unburnt areas while only 30% was on burnt areas. Natural damage was more in bunt areas (71%)

than in unburnt areas (29%) hence Acacia drepanolobium trees were more susceptible to drought

when burnt than when unburnt (Figure 4 below).

Figure 4: Interaction of burning and the level of damage for each damager

Ol Pejeta conservancy Acacia drepanolobium population is composed of 73% seedlings, 17%

saplings and 10% mature trees (Figure 5 below).

-200

0

200

400

600

800

1000

1200

Me

an d

amag

e

burnt

unburnt


26


Figure 5: Acacia drepanolobium population structure


interaction between the Acacia drepanolobium population structure and burning (table 2 below).

Table 2 A general linear model; Univariate analysis of variance table of results


Dependent Variable: Plot No.

Source of

variation

Type III Sum

of Squares Df Mean Square F Sig.

Habitat Status *

Age class 12669.651 5 2533.930 439.735 .000

Error 21770.336 3778 5.762

Burning increased the seedlings of Acacia drepanolobium by 8% in a period of two years. Fire

was more detrimental on the Acacia drepanolobium saplings where there was a 6% decrease;

burning also decreased the mature Acacia drepanolobium trees (Figure 6 below).

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Seedling (1-50 cm) Sapling (51-200 cm) Mature Trees (>200cm)

No

. of

sam

ple

d t

ree

s


27


Figure 6: Effects of fire on the Acacia drepanolobium population structure

An ANOVA test indicated that there were significantly more trees in burnt areas (mean 58± 39)

than in unburnt areas (mean 48 ± 33) (table 3 below).

-100

-50

0

50

100

150

200

250

300

seedling (1-50cm)

sapling (51-200)

mature trees(>200)

Me

an N

o. o

f A

. d

rep

burnt

unburnt


28


Table 3: ANOVA table of results

ANOVA

Number of trees per transect

Source of

variation

Sum of

Squares DF

Mean

Square F Sig.

Habitat status 92920.395 1 92920.395 68.719 .000

Error 5113962.476 3782 1352.185

Fire increased the density of the Acacia drepanolobium by an average of 24% after a recovery

period of two years (figure 7 below)

Figure 7: Effects of fire on the Acacia drepanolobium density.

An ANOVA test illustrated that the damagers significantly damaged the Acacia drepanolobium

trees at different heights (Table: 4 below).

Table 4: Damage stratification ANOVA test results

ANOVA

Tree damage height

Source of

variation

Sum of

Squares DF Mean Square F Sig.

Damager 15522790

6 2587131.698 1.1613 .000

0

20

40

60

80

100

120

burnt unburnt

Me

an N

o. o

f Tr

ee

s P

er

tran

sect


29


Error 1.273E7 5715 2227.799

The rhino browsing intensity was concentrated below 50cm (Mean 22.13±26.83 cm), other

browsers like the Impalas and the hartebeests also fed at the same height as the rhinos (Mean

22.14±31.54 cm), the elephants were found to browse the Acacia drepanolobium between the

heights 30- 200 cm (Mean 118±85.94 cm), fire mostly affected the Acacia drepanolobium

measuring up to 160 cm (mean 76.6381.84 cm), natural damage affected the Acacia

drepanolobium trees of 180 cm and above (Mean 179±149 cm) , giraffes browsed Acacia

drepanolobium trees measuring >200 cm(mean 223.21±110.1 cm) in height (Figure 8 below).

Figure 8: Damage stratification across the Acacia drepanolobium height structure

An ANOVA test indicated that all damagers damaged Acacia drepanolobium trees of

significantly different diameters (table 5 below)

Table 5: ANOVA test on the damagers’ damaging diameter

ANOVA

Tree Diameter mm

Source of

variation

Sum of

Squares DF

Mean

Square F Sig.

Damager 1767934.836 6 294655.806 631.904 .000

Error 2664895.810 5715 466.298

-50

0

50

100

150

200

250

300

350

400

Rhino Others Fire Elephant Natural Giraffe

Me

an D

amag

e h

eig

ht

(cm

)

Damager


30


Small browsers like Impalas and hartebeests were found to damage Acacia drepanolobium trees

with an average of 9.8±13.76 mm diameter, rhinos; 11.88±10.72 mm, natural damage;

46.12±33.63 mm, giraffe; 57.65±24.68 mm, fire; 58.56±17.44 mm and elephant; 59.92±37.83

mm (figure 9 below).

Figure 9: Acacia drepanolobium tree diameters for the various damagers.

A Pearson Chi-Square indicated that the damagers were dependent on the sprouting status of the

Acacia drepanolobium trees (χ2 =18.737; DF = 6, P =0.005).

Most of the sampled sprouting Acacia drepanolobium trees were intact without any form of

damage on them. Black Rhino showed the highest preference for browsing on the sprouting

Acacia drepanolobium trees followed by the elephants, other browsers, natural and giraffe,

respectively, as shown in Figure 10 below.

-20

0

20

40

60

80

100

120

Others Rhino Natural Giraffe Fire ElephantA.

dre

p. A

vera

ge d

iam

ete

r (m

m)

Damager


31


Figure 10: Herbivore preference for sprouting Acacia drepanolobium trees.

Pearson Chi-Square indicated that the damager was dependent on the Symbiotic ants on the

Acacia drepanolobium trees (χ2 =27.124, DF =6; P <0.000).

Most Acacia drepanolobium trees hosting the symbiotic ants had not been damaged. Rhino

damaged the highest number of Acacia drepanolobium trees with symbiotic ants, elephants

damaged relatively lower number of Acacia drepanolobium trees hosting the symbiotic ants

compared to rhinos followed by other browsers, natural damage and giraffe in a descending

order. No burnt Acacia drepanolobium tree that had hosted the symbiotic ants (Figure 11 below).

Figure 11: The relationship between the symbiotic ants and the damager

-20

0

20

40

60

80

100

120

140

160

Nodamage

Rhino Elephant Others Natural Giraffe

Me

an N

o o

f sa

mp

led

sp

rou

ts

Damage Cause

0

50

100

150

200

250

300

350

Nodamage

Rhino Elephant Others Natural Giraffe Fire

No

. of

Tre

es

wit

h S

ymb

ioti

c A

nts

Damage cause


32


An ANOVA test indicated that damage status on Acacia drepanolobium trees varied

significantly with season (Table 6 below).

Table 6: Effects of season on damage status ANOVA test results.

ANOVA

Tree damage status

Source of

variation

Sum of

Squares DF

Mean

Square F Sig.

Season 84.568 1 84.568 377.396 .000

Error 1281.753 5720 0.224

73% of the Acacia drepanolobium trees had been damaged during the dry season while 49% of

the Acacia drepanolobium habitat had been damaged in wet season (figure 12 below).

Figure12: Effects of season on the damage status


interaction between the season and damager (Table 7 below)

Table 7: The interaction between the season and the damager


Dependent Variable: Plot number

0

500

1000

1500

2000

2500

Dry Wet

Ave

rage

No

. o

f A

. dre

p

Present

Absent


33


Source

Type III Sum

of Squares DF Mean Square F Sig.

season * Damager 12450.418 12 1037.535 85.757 .000

Error 69058.558 5708 12.099

Elephant damage was high over the dry season; 58% compared to 42% in wet season, Rhino

damage was relatively constant over the two seasons (i.e. 51% in dry and 49% in wet). Giraffe

damage was very high in wet season; 70%, and low in dry season; 30%. Most small browsers

preferred the Acacia drepanolobium habitat in dry season; 75%, than in wet season; 25%.

Natural damage on the Acacia drepanolobium habitat was very high in dry season; 74%, and low

in wet season; 26%. Damage by fire was constant over the two seasons since there was no

burning during the study period (Figure 13 below).

Figure 13; The interaction between season and damager

An ANOVA test indicated that fire significantly affected the Acacia drepanolobium height

structure (Table 8 below).

Table 8: Effects of damage on Acacia drepanolobium tree height structure

ANOVA

Tree height (cm)

0

2

4

6

8

10

12

14

Me

an d

amag

e

Dry

Wet


34


Source of

variation

Sum of


Treatment 1898440.592 2 949220.296 107.383 .000

Error 5.054E7 5718 8839.587

High damage level (burnt areas with other damage types) maintained the Acacia drepanolobium

trees short (Mean 48.58±73.51 cm; seedling height), medium damage level (Unburnt areas but

with other damage types) maintained the Acacia drepanolobium trees at a medium height (Mean

73.11±99.959 cm; sapling height) while absence of damage (control plot) made the Acacia

drepanolobium trees grow tall (Mean 121.43±136.161 cm; mature trees’ height) (Figure 14

below).

Figure 14: Effects of fire on Acacia drepanolobium tree height structure

An ANOVA test showed that the diameter of Acacia drepanolobium varied significantly with the

level of damage (Table 9 below).

Table 9: Effects of fire on Acacia drepanolobium diameter,

ANOVA

Tree Diameter mm

-50

0

50

100

150

200

250

300

Burnt & other damages Unburnt but with otherdamage types

Control: Un damaged

Me

an t

ree

he

igh

t


35


Source of

variation

Sum of


Treatment 97480.844 2 48740.422 64.296 .000

Error 4335349.801 5719 758.061

High damage level (burnt areas) maintained Acacia drepanolobium trees of small diameter

(15.732±21.581 mm), medium level of damage (unburnt) maintained Acacia drepanolobium

trees of relatively larger diameter (22.538±30.3606 mm) while the absence of damage

maintained large diameters (30.532±31.3912 mm) (Figure 15 below).

Figure 15: Effects of fire on the diameter of Acacia drepanolobium trees.

-20

-10

0

10

20

30

40

50

60

70

Burnt & otherdamages

Unburnt but withother damage types

Control: Un damaged

Me

an t

ree

Dia

me

ter

(mm

)

treatment


36


CHAPTER FIVE

5.0 DISCUSSION

Woodland–grassland ecosystems are inherently dynamic (Dublin, 1995) with factors such as

browsing, fire and rainfall being critical in determining whether the habitat will be stable or

subject to change (Cumming, D.H.M. (1982)). Norton-Griffiths (1979) and Dublin (1995)

reported that fire and browsing pressure impact the vegetation structure of Serengeti- Mara

ecosystem and limit the natural regeneration of East African woodlands. Elephants have been

reported to cause spectacular changes in vegetation structure and in composition of savannahs in

Africa (Cumming, 1982; Ruess & Halter, 1990). Furthermore, elephants have been reported to

kill large Acacia trees (Western & Praet, 1973; Croze, 1974), resulting in disappearance of

woodland. However, Western & Praet (1973) recognized that some trees in Amboseli were dying

without appreciable elephant damage, while apparently healthy trees were able to tolerate

significant amounts of debarking and branch removal. Their study suggested that long-term

climatic change was also a more fundamental cause of tree mortality.

In this study, the Acacia drepanolobium woodland was exposed to the conditions of wet and dry

seasons, different levels of browsing by large herbivores (mainly elephants, giraffes and black

rhinoceros) and subject to fire. These impacts appeared to be sufficient to cause rapid reductions

in tree density, height and canopy. Dry season, burning, low rainfall and intense browsing

pressure reduced tree growth. Growth retardation was height and canopy specific, whereas

growth increment was diameter specific. The present study has found that in the burnt and

browsed area, there was a low average growth rate in plant height and canopy compared with the

trees in the fenced plot (control area). The results show that heavy browsing pressure on Acacia

trees resulted in lower average incremental growth in the height and canopy cover of the trees in

the damaged areas as compared to the fenced and protected plot. Hence, the impact of heavy

browsing pressure on Acacia trees in the damaged area resulted in a lower average growth in

diameter as compared to the fenced plot where average growth in stem diameter was higher.

Therefore, high damage pressure (burning combined with herbivore browsing) decreased growth

in stem diameters of the trees and growth of tree height and canopy. Although the impact of


37


browsing on the height and canopy of Acacia trees was considered to be high, it did not kill trees,

and seedlings and saplings were found growing in all plots, with the exception of the few

instances where the elephants uprooted the all tree. Birkett (2002) studying the impacts of

giraffes, rhinos and elephants within the black rhino sanctuary habitat in the Sweet Waters Game

Reserve in Kenya found that Acacia drepanolobium trees subject to high levels of giraffe

browsing and low rainfall grew by only 7.5 ±0.5 cm/year in an unprotected area as compared to

19.1± 2.1 cm/year in a protected area. This study reported extensive damage of trees due to

elephant destruction. Ruess & Halter (1990) observed vegetation changes in Serengeti National

Park due to the combined effects of fire, elephants and giraffes, reporting a high degree of stem

and branch damage that resulted in high mortalities among trees.

A recent study on the interactions of herbivores and Acacia drepanolobium in OPC revealed that

Elephant related mortality in Acacia drepanolobium trees was 35% and increased predictably

with increase in tree height (Wahungu & Mureu, 2008). Giraffe browse significantly reduced

flowering and fruiting in Acacia drepanolobium by selectively browsing on the soft flowering

and seed bearing shoot tips. However, giraffe browsing increased susceptibility to drought hence

did not directly influence height increment (Wahungu & Mureu, 2008). Rhinos browse on

seedlings and trees below 2m thereby affecting the rate of recruitment of seedlings into trees

(Wahungu & Mureu, 2008).

In this study, significant differences were found in the average damage level in Acacia trees in

both rainy and dry seasons. This shows that Acacia drepanolobium is highly preferred by all

browsers in dry season due to the unavailability of other alternative browse materials in OPC.

Low rainfall not only lowered the rate at which trees were replaced but also slowed grass and

herb growth. Elephants eat a mixture of grass and browse in the wet season but increase the

browse proportion during the dry season (Field & Ross, 1976; Dublin, 1995). Similarly, rhinos

will eat more woody species (Oloo, Brett & Young, 1994) as the availability of herb species

decreases during dry periods. Green grass biomass peaks within a month of the rains starting and

then grass quantity and quality decline (Dublin, 1995) during the dry period. The crude protein

content of grasses was reported to fall from 11 to 3% during a 3-month dry period (Pellew,

1984). Woody species retain their high protein levels (Pellew, 1984). This accounted for the high


38


preference of Acacia drepanolobium in OPC which is an enclosed reserve. Elephant damage was

recorded the highest in dry season and also higher than that of the other damagers. This can be

attributed to the fact that elephants are in competition for the available grass with other grazers

such as zebras and buffaloes. Zebras in particular can browse grass to such a low height that

elephants have difficulty feeding. The higher the density of competing grazers, the earlier in the

dry season elephants will be forced to switch to trees and seedlings, hence the more they will

damage trees. During this study several buffaloes died during the six months drought and there

were clear signs that the rest of the other big mammals had lost weight.

Fire increased the density of the Acacia drepanolobium by an average of 24% after a recovery

period of two years. The increment was on the Acacia drepanolobium seedlings by 8%.

However, fire was more detrimental to the Acacia drepanolobium saplings and mature trees.

Most mature trees, especially those above 3 m tend to have desiccated bark and colonies of

lichen that make them very susceptible to fire than shorter trees. Also the high woody content of

the mature trees increases their vulnerability to fire. These trees were seen to dry from top and

drop off chunks of their canopy after the first year of burn (Wahungu et. al., 2009). The burnt

trees were also more susceptible to breakage by animals rubbing and scratching against them. All

these factors lead to rapid reductions in heights and mortality (Wahungu et. al., 2009).

Of great importance to note in this study was the great significant difference in the damage

caused by drought between the burnt and the unburnt areas. Acacia drepanolobium trees were

more susceptible to drought when burnt than when unburnt. Although, fires caused mortalities to

adult Acacia drepanolobium, the most significant effect was tree reversals into seedling height

class as trees resprouted. Although fire may increase browse biomass of A. drepanolobium

available for black rhino, it is not an appropriate black rhino habitat management tool because

burnt areas attract many seedling predators that lower seedling recruitment into adult trees

(Wahungu et. al., 2009). This study also found similar observations whereby, burnt plots

attracted small browsers such as the impalas, Jackson’s hartebeest, elands and zebras. The small

browsers predate on Acacia drepanolobium seedlings hence lowering the rate of seedling

recruitment into saplings and mature trees. This signifies that burning of Acacia drepanolobium

habitat should be discouraged. Burning makes the acacia trees more vulnerable to drought


39


especially in this era of drastic climate change characterized by prolonged droughts. Mega

herbivores: the elephants and the giraffe do not prefer feeding on burnt Acacia drepanolobium

plots. This probably is due to the absence of their browse material, since fire maintains short

Acacia drepanolobium trees which are below their browsing level. There was no relationship

between damage by rhinos and rainfall. Tree deaths that appeared to be caused by drought

increased steadily during the study and continued to increase even after the rains began.

It is noted that the current browser population of ~300 elephants, ~300 giraffes and 82

rhinoceros in the vast 303.51Km2 area of the Ol Pejeta conservancy has not reached a critical

level to cause much negative impacts on the Acacia drepanolobium trees. Findings of this study

showed that although heavy browsing reduced the height and canopy of Acacia trees, seedling

regeneration took place simultaneously with higher rate of regeneration in burnt area and over

the wet season but with very little seedling regeneration in undamaged areas (control plot).

The results clearly indicated that most of the damage took place in the height above 50 cm.

Pellew (1983) in Serengeti National Park, reported more giraffe browsing impact on Acacia

tortilis trees of 200–300 cm height class, but Birkett (2002) observed that giraffes browsed

extensively in the 250–450 cm height class, whereas black rhino concentrated on lower <200 cm

height class. This clear height stratification of the level at which each browser browses leads to

forage facilitation. Facilitation allows species to co-exist in a community, especially where one

species increases resource access by another through its feeding behavior. Vessey Fitgerald

(1960) established that trembling and feeding by elephant exposed medium height grass to

buffaloes which in turn through their feeding activities generated shorter grass for topi. This

study found out that rhino, elephant and giraffe damage were well stratified across the height and

diameter structures. Natural and giraffe damages maintained the acacia trees short (~200cm)

such that the elephants could access. Elephants damaged the Acacia drepanolobium trees by

breaking the main stem or the side branches at an average height of ~150cm. The feeding

behavior of the elephants then reverses the Acacia drepanolobium trees to a height which is

accessible to the rhino. Elephants also facilitated the accessibility of rhino browse material by

breaking the main stems or side branches of mature Acacia drepanolobium which later

resprouted producing fresh browse for the rhino.


40


Elephants, giraffes and natural death damaged the Acacia drepanolobium trees with diameters

larger than that of the browsing level of the rhino. This reversed the trees to the rhino browsing

level hence maintaining maximum browse availability for the black rhino. Absence of damage

allowed the acacia trees to grow large and tall far above the browsing level of the rhinos hence

making the browse material for rhino inaccessible.

Despite the presence and impacts of large herbivore damage, fire, and natural damages, results of

this study indicated that the Acacia drepanolobium habitat was still at a balance. The addition

and recruitment of seedlings into saplings was at a relatively high rate except in burnt areas

where saplings had been reduced by 6%. The conditions at the time of study indicated that the

damage in Acacia drepanolobium habitat was significant, and was serious in burnt areas and

over the dry season. This was a warrant for management intervention to look for alternative

methods of pasture management other than burning.

It will be of high importance for the management to monitor the fluctuating populations of the

elephants and giraffes within the conservancy. This is necessitated by the fact that Ol Pejeta

conservancy is the south most and wettest of the ranches in the Samburu-Laikipia ecosystem.

This caused an influx of wildlife particularly elephants escaping drought in the North into the

conservancy through the wildlife movement corridors. This has resulted in heightened elephant

damage on the Acacia drepanolobium during dry seasons.


41


CHAPTER SIX

6.0 CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

The Acacia drepanolobium habitat in the 303.51Km2 area of the Ol Pejeta conservancy in Kenya

is being altered as populations of elephant, giraffe and black rhino increase. Damage height-

specific impact data was recorded for a period of eight months in a total number of 5,722 trees of

the dominant species, the whistling thorn; Acacia drepanolobium. Rhinos, elephants, giraffes,

small browsers, fire and natural death were recorded as the main causes of damage on the Acacia

drepanolobium habitat. Damage on the Acacia trees was significant (p<0.00), with a damage

level of 61% over the undamaged level of 39%. Natural damage was more in burnt areas than in

unburnt areas indicating that Acacia drepanolobium trees are more susceptible to drought when

burnt than when unburnt. Burnt plots attracted more Acacia drepanolobium seedlings predators

(rhino and small browsers such as the impalas, Jackson’s hartebeest, elands and zebras). This has

the implications that the rate of seedling recruitment into saplings and mature trees is lowered in

the burnt areas.

In Ol Pejeta conservancy the population structure of Acacia drepanolobium is pyramidal where

seedlings dominate the population followed by the saplings with mature trees being the least.

This indicated that the habitat type is still in balance with the addition of recruitment of seedlings

and saplings except in burnt areas where the saplings were reduced by 6%. Fires caused

mortalities to mature Acacia drepanolobium trees. The most significant effect of fire was tree

reversals into seedling height class as trees resprouted hence more seedlings in burnt areas than

in unburnt.

The damagers were well stratified across the Acacia drepanolobium height and diameter

structures. This clear height stratification of the damage level of each damager leads to forage

facilitation. Facilitation allows species to co-exist in a community, especially where one species

increases resource access by another through its feeding behavior. Natural and giraffe damages

maintained the acacia trees short such that the elephants could access. The feeding behavior of

the elephants then reverses the Acacia drepanolobium trees to a height which is accessible to the


42


rhino and other small browsers. Elephants also facilitated the accessibility of rhino browse

material by breaking the main stems or side branches of mature Acacia drepanolobium which

later resprouted producing fresh browse for the rhino.

There was a significant interaction between the season and the damager (p<0.00) such that

damage level increased over the dry season and decreased in wet season. Acacia drepanolobium

therefore, is a highly preferred browse by browsers in dry season at OPC. The elephants and the

other browsers such as the impalas, elands, zebras and the Jacksons hartebeests switched to

Acacia drepanolobium diet during the dry season. This indicated that Acacia drepanolobium in

OPC is under great browse pressure in dry season. In contrast, the giraffes fed more on Acacia

drepanolobium in wet season than in dry season.

Various levels of damage altered the Acacia drepanolobium population structure significantly

(p<0.00). High damage level: burnt and other damage types combined together maintained the

Acacia drepanolobium trees short at seedling height. Medium damage level: Unburnt but with

other damage types maintained the Acacia drepanolobium trees at a medium height a health

structure with more saplings than old mature trees. While absence of damage made the Acacia

drepanolobium trees grow tall with very few seedlings and saplings growing.


43


6.2 RECOMMENDATIONS

Fire is very detrimental to the Acacia drepanolobium saplings. Also, fire increases the trees’

vulnerability to drought especially in this era of climate change characterized by prolonged

droughts and unpredictable seasons. These conditions are unsustainable and will result in habitat

change and may affect rhino breeding and population growth in OPC. Therefore, based on the

findings of this study I recommend that:

1. Burning as a tool of pasture management should be practiced away from Acacia

drepanolobium dominated or mixed woodlands. Alternative method of pasture management

should be adopted and use of controlled fire should be practiced in open grasslands only.

2. It is important for the OPC management to monitor and control the fluctuating populations of

the elephants, giraffes and zebras within the conservancy especially during the dry season.

3. An extensive case study on the population dynamics of the Acacia drepanolobium habitat in

Ol Pejeta conservancy should be carried out to determine the population density, growth and

mortality factors. This will be very useful in projecting future Acacia drepanolobium population

trends.


44


APPENDICES

Program of activities

Activity Time

Research topic , literature review, proposal writing and

approval by supervisor

May- June 2009

Data collection fieldwork June 2009-feb 2010

Data analysis March 2010

Report writing March and April 2010

Defending before a university panel April 2010


45


Data sheet

STRATIFICATION OF HERBIVORES’ DAMAGE ON ACACIA DREPANOLOBIUM

Investigator _____________ __________Transect number ________ Date &

Time____________________________

Plot No.___ GPS Coordinates X_________ Y_________

Local Name ______________________________________

Tr

No

Transec

t No.

height

(cm)

Diamete

r (mm)

Dam

age

status

Descri

ption

(type)

Dama

ger

D.

height

(cm)

Damag

e age

Ants &

commen

ts

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19


46


20


47


Key

Growth stage

1) Seedlings: 0-0.5m height

2) Saplings: 0.51-2.0m height

3) Mature trees: Above 2m height

4) Regeneration: Sprout with tree stump present

Damage status

P- Present

A- Absent

Damage description

MSB = Main stem broken off

BB = side branch broken or ripped off.

TSB = Terminal shoot bitten/ browsed.

TU = Tree up-rooted

TL = Tree leaning sideways

Others = any other form of damage apart from the above mentioned which should be explained

in the comments column.

The comments column should include the trees condition such as;

Seedling,

Sprout,

Flowering,

Bearing seeds and any other observable condition that will be useful in understanding and

explaining the observed data.

Damage identification

I) Rhino damage: secateurs type of cut.

II) Elephant damage: Branch ripped off, main stem broken,

or the all tree uprooted.

III) Giraffe damage: selective removal of the upper soft twigs

at the tree crown.

IV) Other: small browse marks at the supple tips at low levels

by small browser mammals.


48


Damage aging

i) Fresh; within 48 hours after browsing, white and wet/green

ii) Recent: Two weeks to 12 months old, dry and yellow to brown in color

iii) Old: 18 months old; brown to dark color and some recovery of the bark.

Crematogaster ant’s species

The head thorax and abdomen colors: R: red, B: black.

1. RRB

2. RBB

3. BRB

4. BBR


49


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Documents

Joseph Makau BSc Dessertation on Acacia Drepanolobium survival