Continuous assembly required: perpetual species turnover in two trophic level ecosystems

Abstract

Community assembly is often treated as deterministic, converging on one or at most a few possible stable endpoints. However, in nature we typically observe continuous change in community composition, which is often ascribed to environmental change. But continuous changes in community composition can also arise in deterministic, time-invariant community models, especially food web models. Our goal was to determine why some models produce continuous assembly and others do not. We investigated a simple two trophic-level community model to show that continuous assembly is driven by the relative niche width of the trophic levels. If predators have a larger niche width than prey, community assembly converges to a stable equilibrium. Conversely, if predators have a smaller niche width than prey, then community composition never stabilizes. Evidence that food webs need not reach a stable equilibrium has important implications, as many ecological theories of community ecology based on equilibria may be difficult to apply to such food webs.

Species interactions drive continuous assembly of freshwater communities in stochastic environments

“…The presence of internal structure led some species to persist and some species to disappear and reappear according to stochastic events, which creates the observed asynchronous fluctuations of species gains and losses. When species weakly interact, more species were included in the local communities from the regional species pool leading to highly even species distributions and low species turnover, which rendered species disappearances and (re)appearances independent events confirming previous expectations (19). The synthetic analysis also revealed that the discovered causal relationships among species traits in stochastic model communities can be utilized to closely reproduce observed biodiversity patterns, without directly inferring species interactions coefficients from empirical data.…”

“…partially or non-overlapping niches). Species fitness differences and interactions are the main drivers of assembly that hypothetically might lead to not only correlated but causally-related species gain-loss events (19). To test this cause-effect relationship, we use time-series data of benthic invertebrate communities from 66 locations across New Zealand recorded between 1990 and 2019 (Fig 1A).…”

Trophic tug‐of‐war: Coexistence mechanisms within and across trophic levels

“…Yet, when applied to more complex communities, they can lead to new insight. For example, Spaak, Millet, et al (2023) derived a null expectation for the effect of species richness, Spaak, Adler, and Ellner (2023a) and Spaak, Millet, et al (2023) applied these methods to trait-based phytoplankton and zooplankton models with non-linear species interactions.…”