Predicting Coexistence in Species with Continuous Ontogenetic Niche Shifts and Competitive Asymmetry

Abstract

A longstanding problem in ecology is whether structured life cycles impede or facilitate coexistence between species. Theory based on populations with two discrete stages in the life-cycle indicates that coexistence requires at least one species to shift its niche between stages and that each species is a better competitor in one of the niches. However, in many cases, niche shifts are associated with changes in an underlying continuous trait like organism size and we have few predictions for how the conditions for coexistence are affected by this type of ontogenetic dynamics. Here we develop a framework for analyzing species coexistence based on Integral Projection Models (IPMs) that incorporates continuous ontogenetic changes in both the resource niche and competitive ability. We parameterize the model using experimental data from Trinidadian guppies and make predictions about how niche shifts and competitive symmetries allow or prevent species coexistence. Overall, our results show that the effects of competition on fitness depend upon trait-mediated niche-separation, trait-mediated competitive asymmetry in the part of the niche that is shared across body sizes, and the sensitivity of fitness to body size. When all three conditions are considered, we find multiple ecological and evolutionary routes to coexistence. When both species can shift their niche with increasing body size and competition for resources among the species and sizes is symmetric, then the species that shifts its niche to a greater degree with ontogeny will competitively exclude the other species. When competitive ability increases with increasing body size, then the two species can coexist when the better competitor shifts its niche with body size to a lesser degree than the weaker competitior. This region of coexistence shrinks as the better competitor increasingly shifts its niche with increasing size. When both species shift their niches with size, but each is a better competitor on resources used by smaller or larger individuals, then the model predicts an alternative stable state over some range of niche shifts. We discuss how our results provide new insights into species coexistence and the evolutionary consequences of size-structured interspecific competition.

Causes and consequences of ontogenetic dietary shifts: a global synthesis using fish models

“…Werner & Hall, 1988). For organisms with distinct life stages, such as aquatic insects and amphibians, these shifts are typically abrupt and consist of complete switches between separate niches following metamorphosis (Claessen & Dieckmann, 2002;Bassar, Travis & Coulson, 2017). Most organisms, however, exhibit less-abrupt shifts in niche utilisation, but ODSs may nonetheless manifest as relatively distinct changes in prey choice or diet composition associated with shifts in habitat use during ontogeny, as is often seen in fish (Fig.…”

Environmental Perturbations and Transitions Between Ecological and Evolutionary Equilibria: An Eco-Evolutionary Feedback Framework

“…In more complicated models, interaction coefficients can be replaced with functions, and the population size of interacting species replaced with information on population structure (Inouye 2001, McCoy and Bolker 2008, Bassar et al 2017. Such formulations are useful as empirical observations have revealed that the outcomes of interactions between two individuals are often dependent upon their phenotypic trait values (Bolnick et al 2011).…”

“…When individual specialisation is determined by phenotypic traits, this can result in competitive interactions between individuals varying as a function of the distance between their trait values. Such trait-mediated interactions either within or between speciescan have significant impacts on the diversity of resources that are utilised by a species, patterns of resource use, community dynamics, selection, and evolution (Roughgarden 1972, Bassar et al 2017.…”

“…Key causes of mortality or limiting resources such as the risk of predation or food availability can be included into the function as additive effects, or in interaction with the phenotype Rees 2006, Coulson 2012). Traitmediated effects can also be included in the function (Bassar et al 2016, Bassar et al 2017.…”

“…However, the underlying theory required to make each link has been developed. For example, within the IPM framework, Adler et al (Adler et al 2010) and Bassar et al (Bassar et al 2017) have developed models of interacting species, Coulson et al developed models that include single locus genotype-phenotype maps (Coulson et al 2011 and Coulson et al (Coulson et al 2017) incorporated quantitative genetic genotype-phenotype maps into models, Schindler et al developed two sex models that incorporated flexible breeding systems and patterns of mate choice (Schindler et al 2013), a number of authors have incorporated density-dependence, environmental stochasticity and age-structure into models (Childs et al 2004, Ellner and Rees 2006, Coulson et al 2011, Coulson 2012, Simmonds and Coulson 2015, Ellner et al 2016), Bassar et al included trait-mediated interactions (Bassar et al 2016), Vindenes added demographic stochasticity (Vindenes and Langangen 2015), and Smallegange incorporated mechanistic growth rules as a function of energy budgets (Smallegange et al 2017). In addition to adding in the various feedbacks and links in figure 5, a number of authors have shown how numerous quantities of interest to biologists can be calculated from models, and the dynamics of these quantities explored using sensitivity and elasticity analysis.…”