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Interactions among keystone species: effects of Odontotermes and ungulates on savanna biodiversity
Alison K. Brody, University of Vermont; Todd M. Palmer, Univ. of Florida; Dan Doak, University of California, Santa Cruz, USA
Introduction
Preliminary Results
Fig. 1.
An Odontotermes mound in fore-ground.
Distinguishable from matrix by green vegetation with a
distinct margin and absence of trees and shrubs.
MEAN % CARBON
Perhaps because their presence across landscapes is so widespread and conspicuous (Fig. 1), the importance
of termites to the structure and function of African savanna communities is almost cliché. Despite this, few hard
data exist on the magnitude of and mechanisms for community-wide effects of termites in these unique and
threatened ecosystems. Here, we propose manipulative experiments to assess the influence of an abundant
termite on a widespread model ecosystem, the “black cotton savanna” of East Africa and how these effects are
mediated by interactions with another commonly identified “keystone” guild, grazing ungulates (Sinclair & Arcese
1995, Jackson 1997, Mduma et al. 1999, Larter et al. 2000, Gordon et al. 2004).
% ORGANIC CARBON
3.0
2.5
2.0
1.5
-5 m
edg
e
+ 5m
+10m
+ 1
5 m
+ 2
0 m
-5 edge 5 10 15 20
Fig. 2. Mean % organic C at 5-m intervals
radiating from mound centers (N=4
mounds). Vertical line = mound edge.
DIST FROM MOUND (m)
MEAN No.
INSECTS
# INSECTS / TRAP
Understanding the links between species and the structure and function of ecological communities has long
been a primary goal of ecologists. As human modification of the biosphere intensifies, understanding these
linkages has become a fundamental conservation concern as well (Chapin et al. 1997, Walker & Noy-Meir 1982).
Species differ in the extent to which they impact the communities they inhabit. In particular, species that create,
modify, or maintain physical habitats (“physical ecosystem engineers” sensu Jones et al. 1994), may strongly
influence local and regional biodiversity and species interactions (Gutterman et al. 1990, Folgarait 1998, Williams
et al. 1986, Doak and Loso 2003, Castilla et al. 2004).
Important soil characteristics are strongly positively correlated with proximity to termite mounds (Fig. 2), and invertebrates are 3x m
30
20
Fig. 3. a) Mean number of insects caught in
pitfall traps on mounds vs. > 10 m from
mound edges.
10
0
MOUND NON-MOUND
Ongoing Experiments
• We will quantify patterns by sampling plants and invertebrates on and off termite mounds. We will remove
termites from some mounds to measure how termite activity per se produces the patterns observed.
2) We will conduct an experiment in which termites, native ungulates and cattle are allowed or excluded to
partition each of their effects on plant and invertebrate communities.
3) We will examine the patterns of plant and invertebrate diversity on and off mounds across a range of cattle
grazing intensity to examine how cattle and termites interact in producing patterns observed.
Hypotheses
Hypothesis I: Termite mound building activity leads to predictable patterns of density and diversity of plant,
invertebrate and vertebrate community by creating a mosaic of soil fertility and soil structure. The landscapescale effect of these islands of fertility is to enhance and structure biodiversity.
Hypothesis II: High nutrient levels of termite mound soils are maintained and/or enhanced by preferential use
of mounds by large herbivores that deposit nutrients in dung and urine. This positive feedback loop contributes
to nutrient heterogeneity both directly through deposition, and indirectly through enhancement of nitrogen
mineralization rates.
Hypothesis III: High levels of cattle grazing will reduce termite populations and at high numbers cattle function
as exporters of nutrients from mounds. These negative effects, in turn, cascade throughout the plant,
invertebrate, and vertebrate communities to ultimately impact the diversity and stability of savanna ecosystems.
ACKNOWLEDGEMENTS:
We thank Dr. Nick Georgiadis, Director, and the staff at Mpala Research Center for supporting this work and making it all possible.
We give special thanks to John Lemboi and Wilson Ndurito for help in the field. The project is sponsored by NSF DEB-0519223.
Scientific Significance
Termite mounds appear to be a major organizing feature of the black-cotton ecosystem (Young et al. 1998,
Palmer 2003) and play important roles in other semi-arid savannas as well (Scholes & Walker 1993). One of
the most visible and striking characteristics of termites is the degree to which their subterranean mounds are
associated with above-ground heterogeneity in plant communities. In East Africa, habitat heterogeneity has
consequences for plants and animals at all levels of ecological organization (Sinclair & Arcese 1995).
Understanding the forces that generate and maintain this patchiness has direct bearing on our ability to
manage these systems (Walker & Menaut 1988, Sinclair 1996).
Broader Significance
A primary goal of the project is to train both U.S. and Kenyan researchers and students and to interact with
the broader, Kenyan public and surrounding land-owners and ranch managers. The project will support a
Post-doc, one African Master’s student, provide partial support for one U.S. graduate student, employ three
full-time technicians (two in Kenya and one in the US) and provide part-time support to at least 2 additional
Kenyan field assistants.