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How many elephants should there be?
There are too few!
There are too many!
They breed too fast, or too slow!
They threaten our well-being!
We Love Numbers
Title: Savannah elephant populations are massively depleted even
in protected areas
Authors: Ashley S. Robson1, Morgan J. Trimble1, Andrew Purdon1, Kim D. Young-
Overton1, Stuart L. Pimm2, Rudi J. van Aarde1*
Affiliations:
1Conservation Ecology Research Unit (CERU), Department of Zoology and
Entomology, University of Pretoria, South Africa 0002.
2Nicholas School of the Environment, Duke University, Box 90328, Durham, NC 27708,
USA.
*Correspondence to: [email protected]
Based on information provided by SANParks
1995 2000 2005 2010 20155000
10000
15000
20000
Boltzmann sigmoidal, r2=0.965
Year
Num
ber
of
elep
han
ts
Population trend for Kruger’s elephants - 1995 to 2015 -
Population growth rate as a function of detrended
elephant numbers (1998-2012)
A b u n d a n ce (d e tre n d ed )
Gro
wth
ra
te
-0 .1 5 -0 .1 0 -0 .0 5 0 .0 0 0 .0 5 0 .1 0
-0 .0 5
0 .0 0
0 .0 5
0 .1 0
0 .1 5
r2 = 0 .4 4 ; F 1 , 1 2 = 9 .5 7 ; P < 0 .0 1
(Robson & van Aarde 2016 In preparation)
What do elephants do to Kruger?
C o C o h e n 's d 9 5 % C I
D iv e rs ity
P o s it iv eN e g a t iv e 0
A b u n d a n c e
A b io t ic c o m p o n e n ts (3 ; 2 7 )
H e rb s (3 ; 3 4 )
In v e r te b ra te s (8 ; 2 7 )
V e rte b ra te s (7 ; 2 5 )
T re e s (8 ; 2 5 )
T re e s (5 4 ; 5 2 8 )
H e rb s (8 ; 4 3 )
In v e r te b ra te s (1 3 ; 6 4 )
V e rte b ra te s (1 0 ; 8 6 )
379 Publications, 72 with relevant information (sample
size and measures in presence and absence of elephants)
Meta analysis of elephant impact
C o h e n 's d 9 5 % C I
T re e s (7 ; 1 1 2 )
H e rb s (2 ; 3 1 )
In v e r te b ra te s (2 ; 2 3 )
D iv e rs ity
P o s it iv eN e g a t iv e 0
A b u n d a n c e
T re e s (1 ; 1 )
H e rb s (2 ; 3 0 )
In v e r te b ra te s (2 ; 1 6 )
A b io t ic c o m p o n e n ts (3 ; 2 7 )
Meta analysis of elephant impact - Kruger -
53 Publications, 13 with relevant information (sample
size and measures in presence and absence of elephants)
Apparent impact as a function of density - all studies -
0 1 2 3 4
-5 .0
-2 .5
0 .0
2 .5
5 .0
D ensity (e lephants/km2)
Co
he
n's
d72 of 379 studies at 29 sites
Impact as a function of density - Kruger -
0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8
-5 .0
-2 .5
0 .0
2 .5
5 .0
D ensity (e lephants/km2)
Co
he
n's
d
13 of 54 papers published between 1969 and 2015
E lep h an t n u m b ers
Pe
rce
nta
ge
of
1k
m2 c
ell
s
8 0 0 0 1 0 0 0 0 1 2 0 0 0 1 4 0 0 0 1 6 0 0 0
5
6
7
8
9
r2 = 0 .8 7 ; F 1 ,1 3 = 8 3 .1 7 ; P < 0 .0 1
Occupancy as a function of elephant numbers
Occupancy as a function of numbers close to rivers (<5km)
and away from rivers (>5km)
N u m b er o f e lep h an ts
Pe
rce
nta
ge
of
1k
m2 c
ell
s
8 0 0 0 1 0 0 0 0 1 2 0 0 0 1 4 0 0 0 1 6 0 0 0
4
6
8
1 0
Ecological density as a function of numbers
- Kruger -
8 0 0 0 1 0 0 0 0 1 2 0 0 0 1 4 0 0 0 1 6 0 0 0
6
8
1 0
1 2
1 4
< 5 k m f r o m r iv e r s
> 5 k m f r o m r iv e r s
N u m b er o f e le p h an ts
Ec
olo
gic
al
de
ns
ity
(e
lep
ha
nts
/km
2)
Habitat use
0
1 0
2 0
3 0
0
1 0
2 0
3 0
4 0
B in ce n te rs fo r p e rce n tag e w o o d y co v e r
5 1 5 2 5 3 5 4 5 5 5 6 5
0
1 0
2 0
3 0
B in cen te rs fo r d is tan ce to r iv e rs (m )
1 0 0 0 7 0 0 0 1 3 0 0 0 1 9 0 0 0 2 5 0 0 0 3 1 0 0 0
0
1 0
2 0
3 0
4 0
Pe
rc
en
tag
e u
se
A erial su rvey d a ta
T elem etry d a ta
Wilcoxon p = 0.97
rs = 0.98
Wilcoxon p = 0.23
rs = 0.98
Aerial survey and telemetry data provided similar patterns of habitat use.
Habitat selection
P e rce n tag e w o o d y c o v e r
Lo
git
(P
rob
ab
ilit
y o
f se
lec
tio
n)
0 1 0 2 0 3 0 4 0 5 0 6 0
-1
0
1
ß = 0 .2 3 ; S E = 0 .0 9 ; p = 0 .0 1
D is tan ce to r iv ers (m )
0 5 0 0 0 1 0 0 0 0 1 5 0 0 0 2 0 0 0 0 2 5 0 0 0 3 0 0 0 0
-3
-2
-1
0
1ß = -0 .7 9 ; S E = 0 .1 9 ; p < 0 .0 0 1
In Kruger, elephants selected for areas with high woody cover close to rivers.
(Example: Northern district, 2006)
D ensity (elep han ts/km2)
De
pe
nd
en
ce
(G
AM
)
0 .4 0 .6 0 .8 1 .0
-0 .2
-0 .1
0 .0
0 .1
0 .2
Contribution of woody cover to selection decreases
with increased density
Generalization of habitat selection
1998 2003 2009 2012
Low
High
Elephant densities do not explain impact
0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8
-5 .0
-2 .5
0 .0
2 .5
5 .0
D ensity (e lephants/km2)
Co
he
n's
d
Patterns of spatial use in response to local environmental
conditions may explain impact
What do elephants do to Kruger?
Increase in occupancy away from rivers
N u m b er o f e lep h an ts
Pe
rce
nta
ge
of
1k
m2 c
ell
s
8 0 0 0 1 0 0 0 0 1 2 0 0 0 1 4 0 0 0 1 6 0 0 0
4
6
8
1 0
8 0 0 0 1 0 0 0 0 1 2 0 0 0 1 4 0 0 0 1 6 0 0 0
6
8
1 0
1 2
1 4
< 5 k m f r o m r iv e r s
> 5 k m f r o m r iv e r s
N u m b er o f e le p h an ts
Ec
olo
gic
al
de
nsi
ty (
ele
ph
an
ts/k
m2)
Trends in occupancy reduces ecological density
Generalization of habitat
selection
C o h e n 's d 9 5 % C I
T re e s (7 ; 1 1 2 )
H e rb s (2 ; 3 1 )
In v e r te b ra te s (2 ; 2 3 )
D iv e rs ity
P o s it iv eN e g a t iv e 0
A b u n d a n c e
T re e s (1 ; 1 )
H e rb s (2 ; 3 0 )
In v e r te b ra te s (2 ; 1 6 )
A b io t ic c o m p o n e n ts (3 ; 2 7 )
Will the population be limited without management interferences?
???
1970 1980 1990 2000 20106000
8000
10000
12000
14000
Year
Nu
mb
er
of e
lep
ha
nts Density independent growth driven by
fluctuations in reproduction and primary
productivity modified by ENSO
Density dependent growth
driven by survival and/or dispersal
Summary
- what do elephants do to Kruger? -
We do not know - science on impact not all that good.
Trends in occupancy reduces density and may mask
impact.
Changes in habitat selection driven by changes in
woody cover, EVI and water distribution might
ameliorate impact.
Most published studies do not fulfil the minimum
standards for inclusion in a meta-analysis.
Elephant densities do not explain impact.
Patterns of spatial use in response to local
environmental conditions may explain impact.
After 68 years of studying elephant impact
