Food Selection by Herbivores and Neighbourhood Effects in the Evolution of Plant Defences

Abstract

A number of studies have reported how neighbouring plants may influence herbivory on palatable or unpalatable plants. Such neighbourhood effects can have important evolutionary consequences as they may either promote the evolutionary stability of plant defences or, alternatively, select against the fixation of plant defences and instead promote a stable polymorphism of palatable and unpalatable plants. These consequences depend on whether the difference in herbivore damage between unpalatable and palatable plants is smaller or, alternatively, greater when the neighbours are unpalatable instead of palatable. Such relations can arise when the neighbourhood effects are non-parallel among palatable and unpalatable plants. We outline two basic situations of non-parallel neighbourhood effects and illustrate how they can come about. A detailed dissection of these interactions reveals that there are several qualitatively distinct mechanisms that promote either evolutionary stability of plant defences or alternatively polymorphism. Our classification of mechanisms can be used to clarify and explain observations obtained in the field of plant-herbivore interactions and predator-prey interactions, both at the population and the community level.

Associational effects against a leaf beetle mediate a minority advantage in defense and growth between hairy and glabrous plants

“…This pattern has been reported as a type of associational effect; specifically, our example corresponded with the 'neighbor contrast defense' (Bergvall et al 2006;Rautio et al 2012) because we observed less herbivory on the defended phenotype (hairy plants, in our case) when surrounded by undefended individuals. We also found a reciprocal minority advantage in biomass production for the hairy and glabrous plants, which might serve as a potential mechanism to maintain the polymorphism through negative frequency-dependent selection.…”

“…However, it was still possible that density as well as frequency of hairy and glabrous plants might result in the pattern of leaf damage, because our experiment with two neighborhood conditions could not determine the relative importance of density-and frequency-dependent damage on the two plant types (but see Kim and Underwood 2015). We should also note that when herbivores respond nonadditively to plant density (i.e., group effects; Rautio et al 2012) it may lead to similar patterns of damage for an individual plant as those caused by associational effects (see also Hambäck et al 2014 for a theoretical consideration). In future studies, the density and the frequency of the two plant types should be manipulated simultaneously (i.e., response surface design; Damgaard 1998;Inouye 2001) to examine potential interactions between the associational and the resource concentration/ dilution effects on plant damage Hambäck et al 2014).…”

“…As a type of associational effect, Bergvall et al (2006) introduced the term 'neighbor contrast defense and susceptibility' to denote, respectively, the minority advantage or disadvantage in herbivory on a given plant type. Additionally, these preceding studies suggest that herbivore perception on contrasting diets is a pivotal mechanism for that type of associational effect (Bergvall et al 2006;Rautio et al 2012). However, empirically, little is known about whether and how associational effects mediate frequency dependence in defense and growth for defended and undefended plants within a species (Wise et al 2009).…”

“…Although associational effects are investigated as a type of interspecific interaction (e.g., Root 1973;White and Whitham 2000;Callaway et al 2005), the effects can also be applied to anti-herbivore defense polymorphism within a single species (Tuomi et al 1999;Rautio et al 2012). Based on evolutionary game theory, several authors discuss how defended and undefended plants coexist under associational interactions (Sabelis and de Jong 1988;Till-Bottraud and Gouyon 1992;Tuomi et al 1999).…”

“…Some researchers have noted the crucial role of such 'associational effects' in determining the coexistence of multiple plant types because the associational effects may mediate the local frequency or the density dependence of herbivory Kim and Underwood 2015). Although several terminologies have been proposed for associational effects (reviewed by Barbosa et al 2009;Rautio et al 2012;Underwood et al 2014), we followed Underwood et al's (2014) definition which is ''Associational effects occur when consumer effects on individuals of one resource organism type, at a given density of that type, are a function of the neighborhood composition of other resource types at particular spatial scales. ''…”