Ecological determinism increases with organism size

Background/Question/Methods

Understanding the patterns in species occurrence is one of the major goals in ecology. Two very different paradigms have emerged to explain such patterns. Niche theory assumes that species occurrences are limited both by the different habitat requirements of each species as well as antagonistic interactions between species within suitable habitats. Neutral theory assumes that dispersal is limiting and species are ecologically equivalent, such that species occurrence is strongly influenced by the stochastic processes of colonization and demographic drift. Theoretical models and natural communities have shown varying degrees of support for these two paradigms. The critical question for ecology is therefore which attributes of species predict the dominance of niche or neutral processes. We outline two hypotheses by which organism size could determine which processes structure ecological communities, and test these hypotheses by comparing the community structure in bromeliad phytotelmata of three groups of organisms –bacteria, zooplankton and macroinvertebrates- that encompass a 10,000-fold gradient in body size but live in the same habitat.

Results/Conclusions

Organisms varying by four orders of magnitude in size all showed some deterministic patterns predicted by niche processes as well as some stochastic patterns consistent with neutral processes, but differed in the importance of specific niche processes. Bacteria had no habitat associations, as would be expected from neutral processes, but bacterial species still showed exclusion amongst species pairs, as would be expected from niche-based processes. Insects had strong habitat and species associations, indicating niche processes, except amongst closely-related species, which were randomly associated as expected from neutral dynamics. Zooplankton, between bacteria and insects in size, showed intermediate patterns in terms of species and habitat associations. Together, these results demonstrate that organisms of differing size have different mechanisms structuring their composition, and that elements of niche and neutral patterns can be found in most natural communities.