Evolution to alternative levels of stable diversity leaves areas of niche space unexplored

Abstract

One of the oldest and most persistent questions in ecology and evolution is whether natural communities tend to evolve toward saturation and maximal diversity. Robert MacArthur’s classical theory of niche packing and the theory of adaptive radiations both imply that populations will diversify and fully partition any available niche space. However, the saturation of natural populations is still very much an open area of debate and investigation. Additionally, recent evolutionary theory suggests the existence of alternative evolutionary stable states (ESSs), which implies that some stable communities may not be fully saturated. Using models with classical Lotka-Volterra ecological dynamics and three formulations of evolutionary dynamics (a model using adaptive dynamics, an individual-based model, and a partial differential equation model), we show that following an adaptive radiation, communities can often get stuck in low diversity states when limited by mutations of small phenotypic effect. These low diversity metastable states can also be maintained by limited resources and finite population sizes. When small mutations and finite populations are considered together, it is clear that despite the presence of higher-diversity stable states, natural populations are likely not fully saturating their environment and leaving potential niche space unfilled. Additionally, within-species variation can further reduce community diversity from levels predicted by models that assume species-level homogeneity.

Complex life cycles drive community assembly through immigration and adaptive diversification

“…These two communities can be viewed as alternative stable states where the former is not reachable through gradual evolution starting with a single ancestral species. That gradual evolution in two‐dimensional trait spaces need not lead to globally stable communities has also been reported by Rubin et al (2021). In their Lotka‐Volterra competition model the competition coefficient and carrying capacity are functions of a two‐dimensional trait vector.…”

Adaptive diversification and niche packing on rugged fitness landscapes

“…While we will not fully comment on higher dimensional landscapes, the general results we focus on here largely stem from the relative curvature of the carrying capacity and competition functions and are thus likely replicated in higher dimensions. However, multi-dimensional evolutionary dynamics are notoriously complex and can result in cyclic and chaotic dynamics (Doebeli and Ispolatov, 2014, 2017; Rubin et al, 2021). How these complex evolutionary dynamics interact with rugged carrying capacity landscapes remains unexplored and could prove insightful to both the peak-shift and adaptive dynamics theories.…”

“…While we will not fully comment on higher dimensional landscapes, the general results we focus on here largely stem from the relative curvature of the carrying capacity and competition functions and are thus likely replicated in higher dimensions. However, multi-dimensional evolutionary dynamics are notoriously complex and can result in cyclic and chaotic dynamics Ispolatov, 2014, 2017;Rubin et al, 2021).…”

“…), all communities have a globally stable equilibrium and are therefore indifferent to initial population sizes (Hernández-García et al, 2009; Rubin et al, 2022). Likewise, as we only consider 1-dimensional phenotypes and symmetric competition, evolutionary dynamics always result in a stationary equilibrium (for discussions of non-stationary evolutionary dynamics see Doebeli and Ispolatov, 2014, 2017; Rubin et al, 2021).…”

Maximal ecological diversity exceeds evolutionary diversity in model ecosystems

“…As nicely summarised by Edwards et al (2018), communities not at an ESS will experience selective pressure towards the ESS (however, see Doebeli & Ispolatov, 2014;Doebeli et al, 2017;Rubin et al, 2021, for discussions of non-equilibrium evolutionary dynamics). On evolutionary timescales, super-saturated communities (those with a larger diversity than the ESS) will often experience the extinction of certain species and collapse to the ESS (e.g.…”