An integrative framework for stochastic, size-structured community assembly

Abstract

We present a theoretical framework to describe stochastic, sizestructured community assembly, and use this framework to make community-level ecological predictions. Our model can be thought of as adding biological realism to Neutral Biodiversity Theory by incorporating size variation and growth dynamics, and allowing demographic rates to depend on the sizes of individuals. We find that the species abundance distribution (SAD) is insensitive to the details of the size structure in our model, demonstrating that the SAD is a poor indicator of size-dependent processes. We also derive the species biomass distribution (SBD) and find that the form of the SBD depends on the underlying size structure. This leads to a prescription for testing multiple, intertwined ecological predictions of the model, and provides evidence that alternatives to the traditional SAD are more closely tied to certain ecological processes. Finally, we describe how our framework may be extended to make predictions for more general types of community structure. ignited an ideological debate in community ecology, challenging the viewpoint that deterministic forces play the dominant role in shaping patterns of biodiversity (2-9). NBT proposes that community-level patterns are primarily determined by the effects of demographic stochasticity, and that a detailed knowledge of the traits of and interactions between individuals comprising the community is irrelevant. It has been argued that the predictions of NBT are uninformative of process (10-12), and it has been demonstrated that the assumptions underpinning NBT are often manifestly violated: in the tropical forests where the theory has found striking success (1,13,14), there is a huge variation in demographic rates of individuals (15,16). Although there has been much debate (6,17,18) about the importance of neutral, stochastic processes, one common principle has emerged (17-20): the need for a unified, theoretical framework both to quantify the effects of demographic stochasticity relative to other forces, and to generate a broader range of predictions more closely tied to process.In this article, we integrate the effects of demographic stochasticity with ontogenetic variation in the size of individuals (21), and allow the demographic rates of individuals to explicitly depend on their size. We assume that this variation in demographic rates depends on size alone, and is not linked to species identity, an approach that is closely related to the philosophy of allometric scaling theory (8,(22)(23)(24): individuals of a given size play by the same rules, regardless of species identity. Given the strong evidence that demographic rates in nature are correlated with size (8, 16), this synthesis of size variation with demographic stochasticity may be thought of as adding a crucial extra layer of biological realism to NBT.From these ingredients we derive a functional differential equation to describe ecological communities, and use its analytical solution to answer three key questions. First, are our predict...

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