Why intraspecific trait variation matters in community ecology

Abstract

Natural populations consist of phenotypically diverse individuals that exhibit variation in their demographic parameters and intra- and inter-specific interactions. Recent experimental work indicates that such variation can have significant ecological effects. However, ecological models typically disregard this variation and focus instead on trait means and total population density. Under what situations is this simplification appropriate? Why might intraspecific variation alter ecological dynamics? In this review we synthesize recent theory and identify six general mechanisms by which trait variation changes the outcome of ecological interactions. These mechanisms include several direct effects of trait variation per se and indirect effects arising from the role of genetic variation in trait evolution.

Figure 1

Equilibrium abundance of three consumer genotypes (B: asexual, dark bars; sexual, light bars) and their respective three prey resources (A). In (A) a horizontal line indicates the carrying capacity of all three resources (K = 1000). All parameters are identical across consumer-resource pairs (r = 1.5, a = 0.01, b = 0.05, d = 0.15) except that P₂ (sexual genotype Aa) has attack rate a = 0.001.

Figure 1

The effects of individual predator variation in attack rates (a) and handling times (b) on rates of prey consumption. In both figures, the feeding rate f -- a function of an individual’s attack rate and handling time -- is plotted as a solid black line. For a dimorphic population consisting of individuals with low and high trait attack rates (or handling times), the average feeding rate f̄(a) is highlighted in blue and the feeding rate of the average individual f(ā) is highlighted in red. For any composition of individuals with low and high trait values, the average feeding rate lies on the thick dashed line. When the feeding rate is a concave function of the attack rate, the average feeding rate is lower than the feeding rate of the average individual. Conversely, feeding rate is a convex function of handling time and so variance in handling means the average feeding rate is higher than the feeding rate of the average individual.