1. Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, UK
2. Department of Biology, McGill University , Montreal H3A 1B1, Quebec, Canada
3. Redpath Museum, McGill University , Montreal H3A 2K6, Quebec, Canada

Correspondence to: Angus Buckling¹ Correspondence and requests for materials should be addressed to A.B. (e-mail: Email: angus.buckling@plant-sciences.ox.ac.uk).

Abstract

External agents of mortality (disturbances) occur over a wide range of scales of space and time, and are believed to have large effects on species diversity. The "intermediate disturbance hypothesis"^{1, 2, 3}, which proposes maximum diversity at intermediate frequencies of disturbance, has received support from both field^{4, 5} and laboratory^{6, 7} studies. Coexistence of species at intermediate frequencies of disturbance is thought to require trade-offs between competitive ability and disturbance tolerance⁸, and a metapopulation structure, with disturbance affecting only a few patches at any given time^{9, 10, 11}. However, a unimodal relationship can also be generated by global disturbances that affect all patches simultaneously, provided that the environment contains spatial niches to which different species are adapted¹². Here we report the results of tests of this model using both isogenic and diverse populations of the bacterium Pseudomonas fluorescens. In both cases, a unimodal relationship between diversity and disturbance frequency was generated in heterogeneous, but not in homogeneous, environments. The cause of this relationship is competition among niche-specialist genotypes, which maintains diversity at intermediate disturbance, but not at high or low disturbance. Our results show that disturbance can modulate the effect of spatial heterogeneity on biological diversity in natural environments.