Interspecific competition frequently affects the performance and co-existence of species. Most models of competition assume that species’ populations are phenotypically homogenous, such that species have fixed values for traits that influence competitive ability (Tilman 1982; Holt et al. 1994; Chesson 2000). In reality, single populations often exhibit variation for multiple traits, and a combination of conceptual and mathematical theory suggests that intraspecific variation for competitive ability can affect species co-existence (Aarssen 1983; Abrams 2006; Urban 2006; Vellend 2006). Therefore, predicting the effects of competition on species interactions may rely on knowing the distribution of traits within species and the ecological effects of these traits on competitors. Here we describe a series of experiments that examined whether genetic variation in a native plant can affect competition with neighbouring plant species.
Recent research at the intersection of community ecology and evolutionary biology hypothesizes that heritable variation and evolution of ecologically important traits can affect ecological processes and patterns within communities (Whitham et al. 2003; Urban & Skelly 2006; Johnson & Stinchcombe 2007). Most research has focused on the ‘bottom-up’ effects of genetic variation in basal resource populations on consumer communities (Maddox & Root 1987; Dungey et al. 2000; Johnson & Agrawal 2005; Wimp et al. 2005), and how evolution in resource (Yoshida et al. 2003) and predator populations (Fussmann et al. 2003) shape predator–prey dynamics. Outside of extensive research on the evolution of character displacement (Schluter 2000), the effects of genetic variation and evolution on the ecology of competing species has received little attention compared to the study of multitrophic effects.
Genetic variation is predicted to affect the ecological outcome of competition when: (i) a population genetically varies in response to competition, indicating the potential for an evolutionary response to selection by competitors (Shaw & Platenkamp 1993; Cahill et al. 2005); or, (ii) genetic variation in a species’ traits affects the fitness of competing species by depleting or preventing access to shared resources (Aarssen 1989). Theoretical models indicate that these mechanisms can lead to greater or lesser co-existence among species compared to models that ignore genetic variation and evolution (Urban 2006; Vellend 2006). Most empirical evidence for the role of genetic variation and evolution in competitive interactions comes from plants. For example, Trifolium repens exhibits fine-scale adaptation to competition with specific plant species (Turkington & Harper 1979), and even to specific genotypes within competing species (Aarssen & Turkington 1985). Recent studies also show that variation among plant genotypes can influence the growth, biomass and density of competing plant species (Taylor & Aarssen 1990; Proffitt et al. 2005; Fridley et al. 2007; Lankau & Strauss 2007). Moreover, genetic variation in traits can lead to intransitive competitive interactions due to overlap in the competitive ability of co-existing plant species (Taylor & Aarssen 1990; Whitlock et al. 2007), which can increase co-existence within plant communities (Aarssen 1989; Laird & Schamp 2006; Lankau & Strauss 2007). Together, these results show that genetic variation in competitive ability can influence the performance of individual plants and potentially regulate diversity.
Further research is needed to understand whether genetic variation in competitive ability is ecologically important in nature. Most experiments have been conducted in pots where environmental variance is kept to a minimum to assess the potential effects of genetic variation on competitive interactions (Aarssen & Turkington 1985; Taylor & Aarssen 1990; Fridley et al. 2007). However, both the genotype and the environment determine a plant's phenotype, and environmental variation in the field may dampen the ecological effects of genetic variation. A combination of experiments from controlled growth environments and the field would provide the strongest test for the ecological importance of genetic variation. Studies that additionally manipulate ecological factors of known importance to competition (e.g. resource availability) provide further insight into the relative importance of genetic variation.
Here, we examine the hypothesis that genotype identity of a native plant (common evening primrose, Oenothera biennis) is an important factor affecting the performance and diversity of neighbouring plant species. Our first objective was to determine whether O. biennis competes with an exotic grass species Bromus inermis, with which it commonly co-occurs in nature. Once we established the nature of competitive interactions, we used greenhouse experiments to address three specific questions: (i) is there genetic variation in O. biennis for traits that can influence interspecific competitive interactions? (ii) Does genetic variation in plant traits of O. biennis affect the performance of B. inermis? and, (iii) what is the relative importance of plant genotype of O. biennis vs. variation in soil fertility in affecting the performance of B. inermis? Based on our results from pairwise competition greenhouse experiments, we then tested whether O. biennis genotype affects the community-wide performance and diversity of naturally occurring plants in the field.