Enemy‐mediated apparent competition: empirical patterns and the evidence

Abstract

2000. Enemy-mediated apparent competition: empirical patterns and the evidence. -Oikos 88: 380 -394.Apparent competition arises when two victim species negatively affect each other (− , − ) by enhancing the equilibrium density or changing the foraging behaviour of a shared natural enemy. Shared enemies can also mediate non-reciprocal ( −, 0) indirect effects, i.e. indirect amensalism, whenever one prey species is not affected by the presence of alternative prey. We review 34 studies on terrestrial and freshwater systems to evaluate the extent to which apparent competition has been perceived as a reciprocal ( −, −) or non-reciprocal ( − , 0) interaction. We found only three studies showing reciprocal effects between apparent competitors. Indirect amensalism was documented in 10 studies and could be inferred for 16 other cases (76% in total). The remaining five studies provided insufficient data to determine the form of indirect interaction. The apparent prevalence of non-reciprocal enemy-mediated interactions resembles that observed for resource-based interspecific competition. Amensal indirect effects via shared predation may result from differences in population size, nutritional value, susceptibility to attack, or asynchronous dynamics of alternative prey, or the predator's feeding preferences. Moreover, experimental protocols may confound the actual form of apparent competition through short-term observations, incomplete designs, or biased consideration of conspicuous interactions, leading to reciprocal effects being overlooked. We conclude that, at present, it is still difficult to determine the relative role of apparent competition vs indirect amensalism in natural food webs because most published studies have failed to document in full interactions via shared enemies.

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

“…The relevance of our results is supported by empirical studies revealing that low to intermediate defense levels frequently occur in nature (White, Kaul, Knoll, Wilson, & Sarnelle, 2011) and that defended prey types may outcompete undefended prey types even if they have costs for their defense (Kasada et al., 2014). The presented mechanism of indirect facilitation among prey species may provide an explanation why apparent competition, that is, an increasing density of one prey species indirectly reduces the density of the other prey species via the predator (Holt, 1977), is not always observed in nature (Chaneton & Bonsall, 2000). …”

Biological control of thrips and whiteflies by a shared predator: Two pests are better than one

“…Thus, control of whiteflies was improved by the presence of thrips, but the control of thrips was not affected by the presence of whiteflies. Such asymmetric effects of prey species on each other through a shared predator have also been referred to as indirect amensalism rather than apparent competition (Chaneton and Bonsall, 2000). Perhaps the lack of an effect of the presence of whiteflies on the population densities of thrips was caused by the low initial densities of thrips.…”

Roles of parasites in animal invasions

“…For example, freshwater community structure is radically altered when the invasive amphipod G. pulex replaces the native G. d. celticus [56]. Many studies, however, have used two-host, one-parasite systems to suggest, by extrapolation, that parasites structure biological communities or assemblages [46,57]. However, very few studies have examined the role of parasites at the community level [58] and still fewer involve invasions.…”

“…However, very few studies have examined the role of parasites at the community level [58] and still fewer involve invasions. Of the ten examples claiming parasite-mediated apparent competition (Table 1), plus 20 additional examples of enemy-(predator)mediated apparent competition available from [57], only two [13,21] involved the examination of more than a species pair and a single parasite. MacNeil et al [13] examined interactions in multiple species pairs of amphipods parasitized by the microsporidian P. mulleri, whilst Kohler and Wiley [21] describe the community implications of the complete loss of a key species, the caddisfly Glossosoma nigrior, through a catastrophic outbreak of the microsporidian parasite Cougourdella sp.…”