Cyclic assembly trajectories and scale-dependent productivity-diversity relationships

Empirical studies show that the relationship between species diversity and productivity can be strongly scale dependent. At the local scale of population dynamics the relationship is commonly unimodal or humped, but it is frequently monotonically increasing at larger spatial scales. Such contrasting patterns can occur if beta diversity (compositional dissimilarity among sites) also increases with productivity, but mechanistic reasons for this remain unresolved. Here we use theoretical models to explore the influence of ecological assembly on the diversity of multitrophic communities along gradients of productivity. We show that assembly dynamics can generate scale-dependent patterns consistent with those observed in nature; while unimodal productivity-diversity patterns are produced at the scale of local communities, positive monotonic patterns emerge when diversity is measured at larger spatial scales (among communities). This occurs due to increases in beta diversity with productivity. Our results suggest that increases in beta diversity, and resultant scale dependency, can depend on three vital and interactive elements: the presence of more than one consumer trophic level, a greater propensity for cyclic compositional change at high productivities, and the stochastic nature of species dispersal and invasion history. Key words: assembly; beta diversity; heteroclinic cycles; invasibility; metacommunity; permanence; productivity-diversity relationships, scale-dependent; scale; species diversity; species turnover.