Eco-devo contributions to complementarity
The functional and habitat-constructing diversity that underlies coexistence, complementarity, and other ecosystem properties reflects variation in phenology; location and timing of foraging and metabolic activities; morphology, size, and placement of body parts (e.g., of coexisting root systems); and resource processing (Hooper at al. 2005). These aspects of development, physiology, and behavior are generally quite plastic and may change repeatedly within an individual's life cycle (see Chapters 3 and 4). Along with variation due to genetic diversity, intraspecific variation due to plastic trait expression is now understood to contribute to the structure and function of ecological communities, and to resource complementarity in particular (Jung et al. 2010; Burns and Strauss 2012; and references therein). This recognition represents a departure from the long tradition in ecological theory and research of treating ecological roles in a community, including patterns of resource use leading to complementarity, as the result of constitutive traits that are fixed at the species level (Ashton et al. 2010). Trait variation expressed by individual organisms is increasingly emphasized as a factor that shapes ecological interactions in general (see Section 6.1). However, as yet, the impact of context-mediated trait variation on community dynamics and structure is little understood (Miner et al. 2005).
Individual eco-devo variation in trait expression for resource-gathering organs has been experimentally shown to increase community productivity while reducing the degree to which constitutive species-level traits determine community assembly (Burns and Strauss 2012). In an unusual field study, researchers in a lowland tropical forest examined variation for critical leaf functional traits across nested ecological scales from individual leaf, to sun and shade strata within a tree canopy, to an entire tree, to species, plot, and site (Messier et al. 2010). Since trait expression was influenced by variable microclimate, insolation, and soil conditions as well as individual genotype, within-species trait variation was found to be greater than trait variation among species. Intriguingly, local habitat plots with quite different species compositions had very similar leaf trait means and distributions; this observation suggested the existence of a nonrandom assemblage based on functional diversity that did not arise at the species level (Messier et al. 2010). A different aspect of functional complementarity was studied in experimental communities that included from one to five pollinating bee species (but a constant total number of individuals). When more than a single bee species was present, seed production in a nine-species plant community increased significantly; thus, pollinator services appeared to be functionally complementary (Frund et al. 2013). As in the previous case, this community-level complementarity reflected individual eco-devo responses rather than fixed species-level traits: bees of each species changed their floral preferences depending on the presence of heterospecific bees, creating context-dependent pollinator complementarities (Frund et al. 2013).
How exactly do such individual trait adjustments contribute to complementarity at the community level? One key process is facultative character displacement, in which trait expression in co-occurring individuals is more divergent than in individuals growing alone (D. Pfennig and Murphy 2002; Burns and Strauss 2012). Such flexible, eco-devo based displacements may be far more common in natural communities than evolved, constitutive trait divergence (D. Pfennig et al. 2006). In animals, such transient displacements can include biotically induced shifts in foraging behavior, as well as changes in morphological traits involved in resource use (D. Pfennig et al. 2006). For instance, co-occurring animals may avoid direct competition by switching dietary preferences for host plants or prey items, or as pollinators may switch to plant species with less bounteous rewards but fewer other visitors. Theoretical work indicates that such individual flexibility can better promote species coexistence across a broad range of conditions than fixed traits can (e.g., Kriven 2003; additional references in Miner et al. 2005). Plastic trait expression can thus be a major mechanism for resource partitioning, and consequently complementarity, by means of shifts in resource use by individual animals (D. Pfennig et al. 2006).
Plants as well may shift patterns of resource use in response to competitive conditions (Callaway et al. 2003). Indeed, differentiation in resource use, leading to coexistence and complementarity in plant communities, may be largely due to individual plastic responses that allow plants to partition resources despite their broadly overlapping resource requirements (Ashton et al. 2010). For example, physiological plasticity in the use of different forms of a common chemical resource can mediate competitive interactions, as demonstrated in a study of four alpine herb species (Ashton et al. 2010). When grown singly, these species had the same nutrient-use patterns for different chemical forms of nitrogen, preferentially taking up nitrate, and then ammonium, over glycine. In competition, however, individuals of the competitively dominant species switched to increase their uptake of ammonium, the most available form of soil nitrogen, while the competitively inferior species continued to take up nitrate. Interestingly, the identity as well as the presence of neighbors can induce such changes in resource use (references in Ashton et al. 2010). Eco-devo responses to microsite conditions that entail changes in morphological traits (e.g., shoot height, root allocation and form, and leaf shape and structure) can also lead to fine-scale resource-use differentiation and reduced competition among plant individuals (Callaway et al. 2003). Such trait-based differentiation was found to occur, for example, among individuals in meadow communities located along a soil flooding gradient (Jung et al. 2010).
Individual plasticity in the expression of defensive traits can also contribute to community properties in both animals and plants (O. Schmitz et al. 2004). Induced defenses can reduce the amplitude of population fluctuations due to predators or pathogens, hence increasing the stability of complex ecological systems (Miner et al. 2005). For instance, sharp fluctuations in the abundance of algae, herbivorous zooplankton, and carnivorous zooplankton were much dampened when the algae at the base of this trophic system expressed an inducible defense, as compared to a system based on primary producer algae that lacked the capacity for induced defense (Verschoor et al. 2004). The expression of induced defenses can also alter competitive interactions among individuals, especially when resource stresses exacerbate the costs of defense (Callaway et al. 2003 and references therein).