Competition among small individuals hinders adaptive radiation despite ecological opportunity

Abstract

Ontogenetic diet shifts, where individuals change their resource use during development, are the rule rather than the exception in the animal world. Here, we aim to understand how such changes in diet during development affect the conditions for an adaptive radiation in the presence of ecological opportunity. We use a size-structured consumer–resource model and the adaptive dynamics approach to study the ecological conditions for speciation. We assume that small individuals all feed on a shared resource. Large individuals, on the other hand, have access to multiple food sources on which they can specialize. We find that competition among small individuals can hinder an adaptive radiation to unfold, despite plenty of ecological opportunity for large individuals. When small individuals experience strong competition for food, they grow slowly and only a few individuals are recruited to the larger size classes. Hence, competition for food among large individuals is weak and there is therefore no disruptive selection. In addition, initial conditions determine if an adaptive radiation occurs or not. A consumer population initially dominated by small individuals will not radiate. On the other hand, a population initially dominated by large individuals may undergo adaptive radiation and diversify into multiple species.

Complex life cycles drive community assembly through immigration and adaptive diversification

“…If, however, one of the life stages has a wider feeding niche than species with a simple life cycle, then an ontogentic niche shift can result in lower species richness. A similar finding has been recently reported by ten Brink & Seehausen (2022), who show that adaptive radiations in adult traits can be impeded if juveniles feed on a single resource.…”

“…The common finding from these models is that two species can coexist if one is more regulated at the juvenile stage and the other more regulated at the adult stage. A very different approach has most recently been taken by ten Brink & Seehausen (2022), who study adaptive radiations in species with a complex life cycle under the assumption that small individuals feed on a shared resource while large individuals have access to multiple resources on which they can specialize. They find that an ontogentic niche shift can hinder diversification.…”

Consequences of life-cycle complexity to the potential for evolutionary branching

“…To the best of our knowledge, this is one of the few theoretical explorations of the role of complex life-cycles on biological diversification – but see Saltini et al (2022, Chapter 4) and ten Brink and Seehausen (2022). The importance of understanding this phenomenon cannot be overstated, as the majority of life on Earth undergoes developmental programs that encompass niche shifts throughout their life-spans.…”

Complex life cycles drive community assembly through immigration and adaptive diversification

“…In particular, in the wake of Robert MacArthur seminal work (MacArthur, 1972;MacArthur & Levins, 1967), theorizing on these latter questions has been dominated by models describing the population dynamics of unstructured populations (e.g. Abrams, 1986;Dieckmann & Doebeli, 1999;McPeek, 2022;Taper & Case, 1992).…”