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Why equalising trade-offs aren’t always neutral

Lindsay A Turnbull, Mark Rees, and Drew W Purves


Life history trade-offs have been invoked to reconcile observed species differences with the fitness equalisation required by neutral theory. This is an appealing explanation for the dramatic seed size variation observed within guilds of otherwise similar plants: under size-symmetric competition, where resource capture is proportional to mass, the outcome of competition should be insensitive to how different species partition reproductive output. However, we show that, under perfectly size-symmetric competition, stochastic variation in seed rain leads to the exclusion of all but the smallest-seeded species. Thus stochasticity in seed arrivals, a process that has previously been supposed to guarantee drift, leads to deterministic competitive exclusion. A neutral outcome is possible under one special case of a more general equalising framework, where seed mass affects survival but not competition. Further exploration of the feasibility of neutral trade-offs is needed to understand the respective roles of neutrality and niche structure in community dynamics.


Publication typeArticle
Published inEcology Letters

Newer versions

Lindsay A Turnbull, Christopher D Philipson, Drew W Purves, and AndOthers. Plant growth rates and seed size: a re-evaluation, Ecology, Ecological Society of America, June 2012.

Drew Purves and Lindsay Turnbull. Different but equal: the implausible assumption at the heart of neutral theory, Journal of Animal Ecology, British Ecological Society, 3 August 2010.

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