INDIRECT EFFECTS OF LARGE HERBIVORES ON SNAKES IN AN AFRICAN SAVANNA

Aug 23, 2026

Many large mammal species are declining in African savannas, yet the ecological consequences for other taxa remain poorly understood. This study measured the locally common olive hissing snake, Psammophis mossambicus, over 19 months in experimental plots with and without large herbivores. Snake abundance increased significantly where large herbivores were removed and rodent numbers were high, while observed patterns were not explained by predation-rate differences. Instead, snakes responded numerically to increased small-mammal prey availability, illustrating indirect trophic-web effects of dominant species.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages7

Reading Time18 min

AccessFree to download

Introduction

Dominant and keystone species and indirect effects

Large herbivore declines in African savannas

Hypotheses for snake abundance

Methods

Study site

Ecology, 87(10), 2006, pp. 2657–2663 􀀁 2006 by the Ecological Society of America INDIRECT EFFECTS OF LARGE HERBIVORES ON SNAKES IN AN AFRICAN SAVANNA DOUGLAS J. MCCAULEY, 1,5,6 FELICIA KEESING,2 TRUMAN P. YOUNG,3,5 BRIAN F. ALLAN,4 AND ROBERT M. PRINGLE 1,5 1Department of Biological Sciences, Stanford University, Stanford, California 94305 USA 2Department of Biology, Bard College, Annandale-on-Hudson, New York 12504 USA 3Department of Plant Sciences, University of California-Davis, California 95616 USA 4Department of Biology, Washington University, St. Louis, Missouri 63130 USA 5Mpala Research Centre, P. O. Box 555, Nanyuki, Kenya Abstract. Many large mammal species are declining in African savannas, yet we understand relatively little about how these declines inﬂuence other species. Previous studies have shown that the removal of large herbivorous mammals from large-scale, replicated experimental plots results in a dramatic increase in the density of small mammals, an increase that has been attributed to the relaxation of competition between rodents and large herbivores for food resources. To assess whether the removal of large herbivores also inﬂuenced a predator of small mammals, we measured the abundance of the locally common olive hissing snake, Psammophis mossambicus, over a 19-mo period in plots with and without large herbivores. Psammophis mossambicus was signiﬁcantly more abundant in plots where large herbivores were removed and rodent numbers were high. Based on results from raptor surveysand measurements of vegetative cover, differences in snake density do not appear to be driven by differences in rates of predation on snakes. Instead, snakes appear to be responding numerically to greater abundances of small-mammal prey in areas from which large herbivores have been excluded. This is the ﬁrst empirical demonstration of the indirect effects of large herbivores on snake abundance and presents an interesting example of how the inﬂuence of dominant and keystone species can move through a food web. Key words: Africa; indirect effects; large herbivore; Saccostomus mearnsi; small mammal; snake; Psammophis mossambicus. INTRODUCTION Dominant and keystone species are recognized by their profound impacts on the composition, dynamics, and functioning of ecological communities (Power et al. 1996) . Direct effects of these species on their prey, predator, or competitor species are relatively apparent, but in most cases constitute only a portion of their total effect on community dynamics. Indirect effects are also essential components of food web ecology, but are often more difﬁcult to detect. Indirect effects or interactions follow when one species produces a change in a variable associated with a second species by changing one or more other variables, at least one of which is associated with a third species (Wootton 1994, Abrams et al. 1996) . The general role and importance of indirect effects in food web ecology have been extensively considered (Schoener 1993, Menge 1995, Abrams et al. 1996, Schmitz et al. 2004) . Indirect effects of dominant or keystone species typically are examined in the context of trophic cascades, deﬁned as the impact of predators on abundance or biomass of prey that penetrates at least two trophic levels below the predator (Pace et al. 1999) . Manuscript received 20 July 2005; revised 28 March 2006; accepted 3 April 2006 . Corresponding Editor: A. Bronikowski. 6 E-mail: [dougm@stanford.edu](dougm@stanford.edu) Other indirect pathways for the inﬂuence of these species, for example via competition for shared resources or bottom-up penetrance through multiple trophic levels, are less often considered. In African savannas the dominant species are often native large herbivorous mammals (hereafter ‘‘large herbivores’’) . Native African large herbivores are declining as a result of habitat destruction, poaching, disease, and competition with domestic stock (du Toit and Cummin

FAQs

What ecological question does the study address?

How the removal of large herbivores in African savannas indirectly affects snakes, a predator of small mammals.

How was snake abundance measured in relation to herbivores?

Psammophis mossambicus abundance was surveyed over 19 months in plots with and without large herbivores.

What mechanism best explained the differences in snake density?

The results indicate numerical responses to higher small-mammal prey abundance, rather than changes in predation rates on snakes.

Aug 23, 2026

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