Variation in the effectiveness of biotic defence: the case of an opportunistic ant-plant protection mutualism

Abstract

Benefits to plants in facultative ant protection mutualisms are highly variable. This allows examination of the sources of this variation and the mechanisms by which ants protect plants. We studied opportunistic interactions between ants and an extrafloral nectary-bearing vine, Dioscorea praehensilis, during 3 different years. Variation in plant protection among years was striking. Several factors affected the effectiveness of the biotic defence. Stems recently emerged from the underground tuber were self-supporting, contacting no other plants and encountering few foraging ants. Stems then became lianescent, and contact with supporting plants greatly increased ant recruitment. Both species and number of ant workers influenced the effect of ants on the major herbivore, the chrysomelid beetle Lilioceris latipennis. Protective actions included limitation of oviposition (reduction in the number of eggs laid on the plant) and predation, leading to increased larval mortality. The probability of successful predation was strongly dependent on larval size. If temporarily low ant-patrolling activity allows larvae to grow beyond a critical size, their mechanical (thick integument) or chemical (plant-derived compounds in a fecal shield) defences become more effective against ants. Secondary metabolites derived from the host plant thus appear to be important for the anti-predator mechanisms of this beetle, being necessary for its survival and reproduction on a host plant that actively recruits ants as a biotic defence against herbivores.

Defensive effects of extrafloral nectaries in quaking aspen differ with scale

“…Thus, patterns of egg predation support our Wndings that EFNs mediate a diVuse, rather than targeted, defense against ALM damage on individual leaves. This Wnding contributes to earlier work showing that predators can reduce herbivory on a plant by increasing herbivore mortality in response to EFN expression (Bentley 1977;Di Giusto et al 2001).…”

“…This "optimal" EFN frequency will not be constant across time and space as the relative costs and beneWts of EFNs depend on environmental conditions, including the identities and abundances of herbivores and predators (Barton 1986;Di Giusto et al 2001;. Phenotypic plasticity of aspen EFN frequency through induction in response to herbivore pressure (Wooley et al 2007) may allow genets to approximate this optimum under diVerent conditions.…”

Key factors affecting the predation risk on insects on leaves in temperate floodplain forest

“…Another issue usually discussed is the most suitable bait (Kaspari & Joern, 1993;Berger et al, 2006). For generalist predators like birds and ants, size (Lang et al, 1999;Di Giusto et al, 2001;Mänd et al, 2007;Remmel & Tammaru, 2009;Remmel et al, 2011), palatability (Skelhorn & Rowe, 2006;Lundgren et al, 2009), mobility (Eubanks & Denno, 2000 and colour (Church et al, 1997) are the main characteristics determining the probability of a bait being attacked. The most widely used baits in studies in tropical forest are termites, which are highly palatable prey for ants and all the other important predators of herbivorous insects in the tropics (Olson, 1992;Leponce et al, 1999;Novotny et al, 1999).…”

A selection mosaic in the facultative mutualism between ants and wild cotton

“…For example, in protection mutualisms between ants and plants, variation can arise from changes in the abundance of partners (Di Giusto et al 2001), the identity of partners (Horvitz & Schemske 1984), the availability of alternative resources for partners (Gaume et al 1998) or the identity and abundance of plant consumers (Barton 1986). Therefore, the net outcome of these interactions often shifts along a continuum from mutualism to parasitism.…”