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5 - Long‐term effects of herbivory on plant diversity and functional types in arid ecosystems

Published online by Cambridge University Press:  16 November 2009

By
David Ward
Edited by
Kjell Danell ,
Roger Bergström ,
Patrick Duncan  and
John Pastor
David Ward
Affiliation:
University of KwaZulu‐Natal
Kjell Danell
Affiliation:
Swedish University of Agricultural Sciences
Roger Bergström
Affiliation:
The Forestry Research Institute of Sweden
Patrick Duncan
Affiliation:
Centre National de la Recherche Scientifique (CNRS), Paris
John Pastor
Affiliation:
University of Minnesota, Duluth
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Summary

INTRODUCTION

There is a widely held belief that abiotic factors outweigh biotic factors in arid ecosystems, and that herbivory by mammals is relatively unimportant in ecosystem functioning and biodiversity maintenance (reviewed by Noy‐Meir 1973). The over‐riding importance of abiotic factors in arid regions is ascribed to the high temporal and spatial variation in rainfall. Indeed, changes in plant species composition in arid and semi‐arid regions of Africa and Asia as a consequence of grazing are positively correlated with mean annual rainfall (Milchunas & Lauenroth 1993, Ward 2004). There is a strong negative correlation between the coefficient of variation in mean annual rainfall among years and median annual rainfall of arid regions (Ward 2001, 2004). Similarly, spatial variation in rainfall is high and is not correlated with distance among stations (Ward 2001, Ward et al. 2000, 2004). This high variability in rainfall results in high spatio‐temporal variability in plant abundance and availability to herbivores. For example, Ward et al. (2000) showed that, in the Negev desert of Israel, only 1% of plant species were present in their permanent plots in all years and approximately half the plant species were found once only in ten years. Spatial variation in forage availability is also enormous (see e.g. Fig. 5.1) and most plants may be restricted to ephemeral water courses (so‐called contracted vegetation) (Whittaker 1975) in some arid regions. Furthermore, geological substrates vary considerably among arid regions, particularly in their nutrient status and distribution, as well as in their water retention capacities.

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Large Herbivore Ecology, Ecosystem Dynamics and Conservation , pp. 142 - 169
Publisher: Cambridge University Press
Print publication year: 2006

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