Competition–defense tradeoffs and the maintenance of plant diversity

Abstract

Ecologists have long observed that consumers can maintain species diversity in communities of their prey. Many theories of how consumers mediate diversity invoke a tradeoff between species' competitive ability and their ability to withstand predation. Under this constraint, the best competitors are also most susceptible to consumers, preventing them from excluding other species. However, empirical evidence for competition-defense tradeoffs is limited and, as such, the mechanisms by which consumers regulate diversity remain uncertain. We performed a meta-analysis of 36 studies to evaluate the prevalence of the competition-defense tradeoff and its role in maintaining diversity in plant communities. We quantified species' responses to experimental resource addition and consumer removal as estimates of competitive ability and resistance to consumers, respectively. With this analysis, we found mixed empirical evidence for a competition-defense tradeoff; in fact, competitive ability tended to be weakly positively correlated with defense overall. However, when present, negative relationships between competitive ability and defense influenced species diversity in the manner predicted by theory. In the minority of communities for which a tradeoff was detected, species evenness was higher, and resource addition and consumer removal reduced diversity. Our analysis reframes the commonly held notion that consumers structure plant communities through a competition-defense tradeoff. Such a tradeoff can maintain diversity when present, but negative correlations between competitive ability and defense were less common than is often assumed. In this respect, this study supports an emerging theoretical paradigm in which predation interacts with competition to both enhance and reduce species diversity.meta-analysis | resource limitation | predation | species diversity I dentifying processes that maintain species diversity in the face of competitive exclusion is a key goal of ecology (1). Because consumers can alter the outcome of competition between their prey, consumer-based mechanisms are commonly invoked to explain species coexistence (2-4). Many empirical (5-7) and theoretical studies (8-10) have suggested that consumers maintain species diversity when predation differentially harms superior competitors. For example, Lubchenco (3) showed that snail herbivory increased algal diversity in tide pools only when preferred prey were also the competitive dominant. Similar requirements for consumers to maintain diversity of their prey have been formalized in mathematical models: when competing species share both resources and consumers, coexistence is possible only if the prey species that are superior competitors for resources are also less resistant to predation (9, 10).However, a large gap has developed between the empirical evidence supporting this theoretical tradeoff and its application to explain how consumers regulate real communities. For the many studies that have invoked a tradeoff between competitive ability and defense aga...

Not attackable or not crackable—How pre‐ and post‐attack defenses with different competition costs affect prey coexistence and population dynamics

“…Several studies indicated that there is often no interspecific trade‐off between defense and competitiveness as a higher resource supply adversely affected defended plants (Lind et al., 2013; Viola et al., 2010). In fact, Lind et al.…”

Herbivore preference drives plant community composition

“…Although many studies have experimentally manipulated abiotic or biotic limiting factors (e.g., through nitrogen addition or herbivore exclusion) to identify filters driving the assembly and composition of plant communities and to search for trade-offs between plant strategies (Viola et al 2010, Lind et al 2013), only few have provided insight into the underlying ecological mechanisms (HilleRisLambers et al 2012). Scha¨dler et al (2003) related the response of 13 herbaceous plant species to invertebrate herbivore exclusion with plant palatability based on herbivore performance of a generalist slug and the generalist house cricket, but found no relationship between the palatability of plants and their cover change due to herbivore exclusion.…”

“…However, also generalist herbivores can have stabilizing effects on plant communities when they switch host or food plant species and consume disproportionally whichever species is most abundant (Murdoch 1969, Chase et al 2002. Other theories on how herbivores can promote coexistence require a trade-off between the vigorous growth of plants and their defense against consumers, assuming that defense is costly and constrains investment in other important traits (Coley et al 1985, Herms and Mattson 1992, Viola et al 2010, Kempel et al 2011, Lind et al 2013. If the plants growing most vigorously in a community are also the least defended ones, herbivores promote coexistence by selectively feeding on more vigorously growing and hence less defended plant species (Pacala andCrawley 1992, Carson andRoot 1999), thereby reducing average fitness differences between species, which is considered as an equalizing mechanism (Chesson 2000).…”

Floodgate Operations and Fish Communities in Tidal Creeks of the Lower Fraser River (British Columbia, Canada)

“…These zero values are potentially important features of the data, so we added the minimum non-zero value for that variable to every observation before calculating the log response ratio. This method of adjustment has been used as a conservative estimate of the log response ratio in data where species were not detected in some samples (Viola et al 2010). We also computed log response ratios for the richness, biomass, and number captured for the four most commonly sampled taxa: three-spined stickleback (Gasterosteus aculeatus), pumpkinseed sunfish (Lepomis gibbosus), prickly sculpin (Cottus asper), and juvenile minnows (Cyprinidae).…”