Habitat Fragmentation and Edge Effects

Topics:

•Definition

•Root in Island Biogeography

•Ecological Consequences

•Case studies:

•East African Forest Fragments

•EDF Birds

•Ecosystem Sensitivity to Fragmentation

Habitat Fragmentation:

Breaking up of habitat into

smaller pieces

More Specifically:

•Reduction in habitat area

•Decrease in patch size

(increase in edge effects)

•Increase in distance among

patches (change in

connectivity)

Cadiz township, WI

Curtis 1956

Habitat Fragmentation:

Variations in base model:

•Rate of recovery of

disturbance patches;

•Disturbance history/natural

vegetation dynamic;

•Type of matrix

Cadiz township, WI

Curtis 1956

MacArthur and Wilson. 1967. A theory of island

biogeography. Princeton Press.

Species Area Relationship

Species Area Relationship

S=cAz

number of species=intercept*area slope

Smaller islands have fewer species

than large islands.

Why?

MacArthur and Wilson. 1967. Theory of Island

Biogeography

MacArthur and Wilson. 1967. Theory of Island

Biogeography

Small island

Large island

S2

MacArthur and Wilson. 1967. Theory of Island

Biogeography

Near island

Far island

S2

MacArthur and Wilson. 1967. Theory of Island

Biogeography

Implications for Habitat Islands?

Habitat as islands?

If habitats in an inhospitible matrix

act as islands, then we expect

smaller, more isolated habitat

patches hold fewer species.

Ecological Consequences of Fragmentation

•Reduction in habitat area

- reduction in population sizes;

- decreased habitat heterogeneity.

Ecological Consequences of Fragmentation

•Reduction in habitat area

- reduction in population sizes;

Figure 2. Relation critical reserve size and female home range size calculated for

10 species of large carnivore. r2 = 0.84, F1,8 = 42.1, P < 0.005. ) Critical reserve size

estimated by using the logistic regression models to predict the area at which populations

persisted with a probability of 50%. (Woodroffe and Ginsberg 1998)

Ecological Consequences of Fragmentation

•Reduction in habitat area

- reduction in population sizes;

- decreased habitat heterogeneity.

Pre-isolation Post-isolation

Atotal Afragment

Stotal

Soriginal

Sfragment

Number of

Species

0

Brooks et al. 1999

Bird Extinctions in Forest Fragments: Kenya

(Brooks et al. 1999)

Fragment Afragment Soriginal Snow Sfragment Half-life

Malava 100 32 19 18 23

Kisere 400 39 32 26 55

Ikuywa 1450 48 44 36 38

Yala 1500 48 44 36 42

Kakamega 8600 62 59 56 80

Ecological Consequences of Fragmentation

•Reduction in habitat area - reduction in

population sizes; decreased habitat

heterogeneity.

•Reduction in Patch Size - Increasing edge

effects

Hypotheses about edge effects

Hypotheses about edge effects

Bird distributions across clearcut/forest edges

Ecological Consequences of Fragmentation

•Reduction in habitat area - reduction in

population sizes; decreased habitat

heterogeneity.

•Reduction in Patch Size - Increasing edge

effects

•Patch Isolation - Less exchange of organisms

How prevalent is fragmentation among

ecosystem types?

•Ecosystem Properties – Biomass

Accumulation

Biomass Accumulation Hypothesis

Early Seral Patch Later Seral Patch

2. Microclimate (e.g.,

wind speed)

1. Vegetation biomass

3. Species Guild

Abundances

High Biomass Ecosystem

2. Microclimate (e.g.,

wind speed)1. Vegetation biomass

3. Species Guild

Abundances

Low Biomass Ecosystem

Ma

gn

itu

de

Hig

hL

ow

Ma

gn

itu

de

Hig

hL

ow

Edge

Early Seral Patch Later Seral PatchEdge

Early Seral Patch Later Seral Patch

2. Microclimate (e.g.,

wind speed)

1. Vegetation biomass

3. Species Guild

Abundances

High Biomass Ecosystem

2. Microclimate (e.g.,

wind speed)1. Vegetation biomass

3. Species Guild

Abundances

Low Biomass Ecosystem

Ma

gn

itu

de

Hig

hL

ow

Ma

gn

itu

de

Hig

hL

ow

Edge

Early Seral Patch Later Seral PatchEdgeHansen et al. in review

Aboveground Biomass of Major Forest

Ecosystem Types.

Hansen et al. in review

Microclimate Results

Hansen et al. in review

0 100 200 300 400 500

0.0

0

.4

0.8

Light Intensity

AGB (t/ha)

ME

I y= 0.0006 * AGB + 0.7068

R 2 =0.44

0 100 200 300 400 500

-0.6

-0

.4

-0.2

0

.0

Humidity

AGB (t/ha)

ME

I

y= 0.0006 * AGB + 0.0221

R 2 = 0.86

Species Results

Hansen et al. in review

Variation in Interior Species with Biomass

Birds

y = 0.07x - 0.6979

R2 = 0.6782

Beetles

y = 0.0771x + 2.3499

R2 = 0.6457

Mammals

y = 0.0664x - 5.7805

R2 = 0.8958

-10

0

10

20

30

40

50

60

0 100 200 300 400 500 600

Biomass

Pe

rce

nt

of

Sp

ec

ies

Sp

ec

ializin

g o

n F

ore

st

Inte

rio

rs

Beetles Birds Mammals Linear (Birds) Linear (Beetles) Linear (Mammals)

Discussion

How do the three drivers of landscape pattern

(biophysical, disturbance, land use) interact to influence

the effects of fragmentation?

What are implications for managing various ecosystem

types?