An integrative framework for stochastic, size-structured community assembly

Abstract

We present a theoretical framework to describe stochastic, size-structured 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.

Fig. 1.

We illustrate the conceptual differences between NBT and our model by using a single tree species in a forest. (A and B) Shown is the time evolution of a neutral community through one time step, undergoing stochastic birth and death processes. (C and D) Shown is the time evolution of a stochastic, size-structured community. Demographic processes are still random, but demographic rates and ontogenetic growth rates depend on size.

Fig. 2.

The species biomass distribution is sensitive both to species richness and to community size-structure. For the completely neutral community defined in the text, A, B, and C show the respective variations of the size spectrum, the SAD, and the SBD with varying species richness. Richness is parametrized by θ, which is the NBT biodiversity parameter (1, 13). We find that the shape of the size spectrum is insensitive to overall species richness, while both the SAD and SBD show increasing numbers of rare or low-biomass species with increasing θ. In constrast, D, E, and F show the variation of these three distributions with varying size structure, parametrized by the ratio of growth rate to mortality rate, $\frac{g}{d}$. With increasing $\frac{g}{d}$, the size spectrum shows increasing numbers of large individuals relative to small individuals, reflecting that more individuals have the chance to grow large before they die. The SBD shows decreasing numbers of low-biomass species relative to abundant species with increasing $\frac{g}{d}$, but the SAD is entirely insensitive to changes in size structure.