Intraspecific Variation and Species Coexistence

Abstract

We use a two-species model of plant competition to explore the effect of intraspecific variation on community dynamics. The competitive ability ("performance") of each individual is assigned by an independent random draw from a species-specific probability distribution. If the density of individuals competing for open space is high (e.g., because fecundity is high), species with high maximum (or large variance in) performance are favored, while if density is low, species with high typical (e.g., mean) performance are favored. If there is an interspecific mean-variance performance trade-off, stable coexistence can occur across a limited range of intermediate densities, but the stabilizing effect of this trade-off appears to be weak. In the absence of this trade-off, one species is superior. In this case, intraspecific variation can blur interspecific differences (i.e., shift the dynamics toward what would be expected in the neutral case), but the strength of this effect diminishes as competitor density increases. If density is sufficiently high, the inferior species is driven to extinction just as rapidly as in the case where there is no overlap in performance between species. Intraspecific variation can facilitate coexistence, but this may be relatively unimportant in maintaining diversity in most real communities.

Why intraspecific trait variation matters in community ecology

“…A modest number of theoretical studies have investigated how intraspecific trait variation affects population dynamics [31][32][33][34], interspecific competition [35][36][37], and predator-prey or host-parasitoid systems [38][39][40]. These studies consider variation in diverse traits, including (1) traditional phenotypes like size or morphology; (2) emergent traits like competitive ability [41], prey attack rate [39], or vulnerability to enemies [42]; and (3) fitness-related traits like fecundity or survival [33].…”

“…For instance, mean clutch size can fluctuate through time because the individuals who (randomly) succeed in reproducing in different years happen to have different expected fecundity, or because clutch size varies randomly among individuals with equal expected fecundity. Demographic variance (and trait sampling in general) will be most pronounced in small populations, and can affect long-term population dynamics [32,33,78], population persistence [79], and species coexistence [37].…”

“…As each highlighted mechanism is quite general, intraspecific trait variation can have nearly ubiquitous ecological effects. These effects can cascade throughout a community of interacting species and substantially alter equilibrium densities [46] and change conditions for population stability or multispecies coexistence [31,37].…”

Intraspecific genetic variation and competition interact to influence niche expansion

“…The degree of individual variation in niche use has significant implications for population ecological and evolutionary dynamics (see Bolnick et al 2003 for a review). For instance, individual niche variation can substantially alter population dynamics (Lomnicki 1978;Kendall & Fox 2002) and species coexistence (Lichstein et al 2007). Depending on other conditions such as niche-based assortative mating (Snowberg & Bolnick 2008), niche variation can allow populations to undergo subsequent adaptive diversification and speciation (Dieckmann & Doebeli 1999).…”

The evolution and ecology of individual specializations amongst a group of dietary generalists.

“…These ecological parameters that influence IS are likely to change over time, and populations can move between being composed of individual specialists versus of individual generalists (Tinker et al 2008). Second, individual specializations may increase species coexistence by decreasing the interaction strength among competing heterospecifics (Lichstein et al 2007;Bolnick et al 2011). This increase in species coexistence can decrease the likelihood that discrete polymorphisms will develop because more ecologically similar species in a community will utilize the ecological opportunity that developing polymorphisms require (Losos 2010).…”