Intraspecific competition drives increased resource use diversity within a natural population

Abstract

Resource competition is thought to play a major role in driving evolutionary diversification. For instance, in ecological character displacement, coexisting species evolve to use different resources, reducing the effects of interspecific competition. It is thought that a similar diversifying effect might occur in response to competition among members of a single species. Individuals may mitigate the effects of intraspecific competition by switching to use alternative resources not used by conspecific competitors. This diversification is the driving force in some models of sympatric speciation, but has not been demonstrated in natural populations. Here, we present experimental evidence confirming that competition drives ecological diversification within natural populations. We manipulated population density of three-spine sticklebacks (Gasterosteus aculeatus) in enclosures in a natural lake. Increased population density led to reduced prey availability, causing individuals to add alternative prey types to their diet. Since phenotypically different individuals added different alternative prey, diet variation among individuals increased relative to low-density control enclosures. Competition also increased the diet-morphology correlations, so that the frequency-dependent interactions were stronger in high competition. These results not only confirm that resource competition promotes niche variation within populations, but also show that this increased diversity can arise via behavioural plasticity alone, without the evolutionary changes commonly assumed by theory.

Figure 1

Prey responses to stickleback density manipulations. (a) Zooplankton and (b) benthic invertebrate biomass. (c) Zooplankton and (d) benthic invertebrate diversity, measured with Levins' D. Samples immediately outside each pair of enclosures are presented to indicate the natural base-line state. The different lines represent different pairs of enclosures and their controls.

Figure 2

Tests of whether density manipulation resulted in resource competition among sticklebacks. (a) Relative stomach content mass as a measure of foraging rate. (b) RNA/DNA ratio as a measure of current growth rates in sticklebacks (Ali & Wootton 2003; Dahlhoff 2004). Results for wild-caught fish are presented to indicate the natural base-line state. The different lines represent different pairs of enclosures.

Figure 3

Effect of population density on diet variation among sticklebacks. (a) Mean diet overlap (IS) between individuals and their population's total diet distribution (Bolnick et al. 2002), comparing paired low- and high-density populations in enclosures. Note that high diet overlap corresponds to low diet variation and vice versa. The mean diet overlap for wild-caught (control) fish in the same lake is presented for comparison. (b) Population diet breadth measured with Levins' D and (c) mean individual diet breadth. Note that the y-axis scale in (c) is smaller than in (b). The different lines represent different pairs of enclosures.

Figure 4

Effect of competition on the linkage between phenotype variation and diet variation. The diet/morphology relationship was consistently stronger in high-density enclosures. The value for wild-caught control fish is provided for comparison. The different lines represent different pairs of enclosures.