Modeling the Impact of the African Elephant, Loxodonta africana, on Woody Vegetation in Semi ...

178 Pages·2004·1.33 MB·English

Modeling the Impact of the African Elephant, Loxodonta africana,
on Woody Vegetation in Semi-Arid Savannas

by

Peter William Joseph Baxter

B.A. (University of Dublin, Trinity College) 1996

A dissertation submitted in partial satisfaction of the requirements for the degree of

Doctor of Philosophy
in

Environmental Science, Policy and Management

in the

GRADUATE DIVISION

of the

UNIVERSITY OF CALIFORNIA, BERKELEY

Committee in charge:

Professor Wayne M. Getz, Chair
Professor Dale R. McCullough
Professor Cheryl J. Briggs

Summer 2003
The dissertation of Peter William Joseph Baxter is approved:

Chair Date

Date

Date

University of California, Berkeley

Summer 2003
Modeling the Impact of the African Elephant, Loxodonta africana,
on Woody Vegetation in Semi-Arid Savannas

© 2003
by
Peter William Joseph Baxter

Abstract
Modeling the Impact of the African Elephant, Loxodonta africana,
on Woody Vegetation in Semi-Arid Savannas
by
Peter William Joseph Baxter
Doctor of Philosophy in
Environmental Science, Policy and Management
University of California, Berkeley
Professor Wayne M. Getz, Chair

Concerns over elephant impacts to woody plants in African savannas have highlighted
shifts in vegetation community composition with implications for possible reductions in
biodiversity.
I developed a grid-based savanna model that differs from previous elephant-
vegetation models by accounting for tree demographics, tree-grass interactions,
stochastic environmental variables (fire and rainfall) and spatial contagion of fire and tree
recruitment. The vegetation component of the model produces long-term tree-grass
coexistence and realistic fire frequencies. The tree-grass balance of the model is more
sensitive to changes in rainfall conditions and tree growth rates while less sensitive to
fire regime. Introducing elephants into this model savanna has the expected effect of
reducing tree cover, although at an elephant density of 1.0 per square kilometer, woody
plants still persist for over a century. I tested the effect of plant responses to elephant
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impact: faster growth was a more successful strategy than elephant-enhanced
germination or adult resilience to impact.
I elaborated the model by including a second, more “r-selected” tree species to
investigate the effects of elephant impacts on species composition within the tree
community. The model produces similar dynamics when run with either tree species
alone; when both species are included it replicates ecological succession, with
competitive exclusion of the early-successional species by the later-successional species
on a timescale of centuries. Increases in growth, fecundity or survival of the early-
successional species increase the likelihood of its persistence over 500 years. Inclusion
of the faster-growing tree species in the model enables both species to survive greater
elephant densities. Spatial heterogeneity of the woody plant component increases with
elephant density. I examined the interaction of the two tree strategies – adult resilience
and elephant-enhanced germination – with elephant preference for either species. Adult
tree resilience was the more successful strategy and may act synergistically between tree
species. Fire suppression also moderates the effects of elephant damage.
I conclude that while elephants may cause woodland to decline, they may also
enhance biodiversity at lower densities, and increase spatial heterogeneity. Conservation
workers should be conscious of the array of species types and their interactions when
planning to manage savannas and/or elephant populations for biodiversity.
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TABLE OF CONTENTS

List of Figures ii

List of Boxes iii

List of Tables iii

Acknowledgements iv

Chapter One Elephant-vegetation interactions in African savannas. 1
Peter W. J. Baxter

Chapter Two An African savanna model: effects of tree demography, 17
rainfall, fire and elephants.
Peter W. J. Baxter and Wayne M. Getz

Chapter Three Effects of elephant impacts on two competing tree species 97
in an African savanna model: insights into coexistence
and management.
Peter W. J. Baxter and Wayne M. Getz

References 135

Appendix 146

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LIST OF FIGURES
Chapter Two
Figure 1. Model results using the default parameter set. 65

Figure 2. Mean trajectories for different rainfall scenarios 67

Figure 3. Sensitivity of vegetation to rainfall, fire probability, 68
resprouting ability and growth rates.

Figure 4. Effects of elephant introduction. 71

Figure 5. Sensitivity of vegetation to elephant population densities 72
and effect of various plant strategies.

Figure 6. Spatial extent of woody dominance for different rainfall scenarios. 76

Figure 7. Spatial extent of woody dominance for different plant 78
strategies, with 1.0 elephants per square kilometer.

Figure 8. Likelihood of quasi-removal for various elephant 80
densities and plant strategies.

Figure 9. Mean fire return periods for selected runs. 83

Chapter Three
Figure 1. Trajectories for savanna model with only one woody species 110

Figure 2. Trajectories for savanna model with both woody species 111

Figure 3. Sensitivity of persistence and cover of both woody species, 113
to species v’s vital rates.

Figure 4. Effects of elephant introduction for selected 115
parameter combinations.

Figure 5. Species shifts following elephant introduction. 117

Figure 6. Effect of elephant preferences and plant responses 118
on vegetation state.

Figure 7. Typical spatial output from the model. 120

Figure 8. Spatial heterogeneity of cover for various elephant densities. 122

Figure 9. Effect of fire suppression on vegetation state. 124
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LIST OF EXPLANATORY BOXES

Box 1 Schematic flowchart of the savanna model. 30
Box 2 Woody plant growth algorithm. 34

LIST OF TABLES

Table 1 Parameters and variables used in savanna model. 49
Table A1 Parameters and variables used in two-tree-species savanna model. 163

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ACKNOWLEDGEMENTS
This dissertation is the product of (exactly!) six years’ work on three continents, and
there are many people who deserve thanks for helping this man from Dublin study
African savannas in California. I’ve stood on the shoulders of many giants.

Enormous thanks must first go to my advising professor, Wayne Getz, who always had
faith in me, and was a shining beacon of inspiration throughout the process. His
astonishing mind is matched only by his genuine concern for his students’ welfare and
success.
Also at Berkeley, I’d like to thank the members of my committees for their
guidance and nurturing throughout my graduate career – Tracy Benning, John Harte, Ye
Qi, but primarily Cherie Briggs and Dale McCullough, who’ve been available and
helpful since I first got here. My lab-mates and fellow students also provided many
hours of discourse and assistance. Nelleke van Deusen-Scholl found the time to teach
me Afrikaans (and Dutch) for a year and made it thoroughly enjoyable: Baie dankie!

In South Africa, many scientists were willing to share their time and valuable experience
with me. Norman Owen-Smith was the main driving force behind the study, and
provided hours of stimulating discussion and guidance. Johan du Toit generously hosted
me at the University of Pretoria’s Mammal Research Institute, and I benefited from
fruitful discussions with him and the MRI students. Tim O’Connor provided great
insights into savanna vegetation; Kevin Duffy and Bruce Page did the same for
elephants. Angela Gaylard, Michele Hofmeyr, Steve Higgins and Nick Zambatis
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