Increased consumer density reduces the strength of neighborhood effects in a model system

Associational Effects and Indirect Interactions-The Dynamical Effects of Consumer and Resource Traits on Generalist-Resource Interactions

Trophic interaction modifications occur in food webs when the direct or indirect interaction between two species is affected by a third species. These behavioral modification effects are often referred to as associational effects. Changes in focal resource availability and consumption by a generalist herbivore can affect a range of outcomes from resource exclusion to multiple resources coexisting with the focal plant species. Here, we investigate the indirect interaction between a focal and alternative resource mediated by a generalist consumer. Using theoretical approaches we analyse the conceptual link between associational effects (both resistance and susceptibility) and the theory of apparent competition and resource switching. We find that changes in focal resource traits have the potential to affect the long-term outcome of indirect interactions. Inclusion of density-dependence expands generalist life-histories and broadens the range where, through associational effects, the availability of alternative resources positively influence a focal resource. We conclude that different forms of associational effects could, in the long-term, lead to a range of indirect interaction dynamics, including apparent competition and apparent mutualism. Our work aims to connects the theoretical body of work on indirect interactions to the concepts of associational effects. The indirect interactions between multiple resources need more thorough investigation to appreciate the range of associational effects that could result from the dynamical interaction between a generalist consumers and its focal and alternative resources.

Climate affects neighbour-induced changes in leaf chemical defences and tree diversity-herbivory relationships

Associational resistance theory predicts that insect herbivory decreases with increasing tree diversity in forest ecosystems. However, the generality of this effect and its underlying mechanisms are still debated, particularly since evidence has accumulated that climate may influence the direction and strength of the relationship between diversity and herbivory.We quantified insect leaf herbivory and leaf chemical defences (phenolic compounds) of silver birch Betula pendula in pure and mixed plots with different tree species composition across 12 tree diversity experiments in different climates. We investigated whether the effects of neighbouring tree species diversity on insect herbivory in birch, that is, associational effects, were dependent on the climatic context, and whether neighbour-induced changes in birch chemical defences were involved in associational resistance to insect herbivory.We showed that herbivory on birch decreased with tree species richness (i.e. associational resistance) in colder environments but that this relationship faded as mean annual temperature increased.Birch leaf chemical defences increased with tree species richness but decreased with the phylogenetic distinctiveness of birch from its neighbours, particularly in warmer and more humid environments.Herbivory was negatively correlated with leaf chemical defences, particularly when birch was associated with closely related species. The interactive effect of tree diversity and climate on herbivory was partially mediated by changes in leaf chemical defences.Our findings confirm that tree species diversity can modify the leaf chemistry of a focal species, hence its quality for herbivores. They further stress that such neighbour-induced changes are dependent on climate and that tree diversity effects on insect herbivory are partially mediated by these neighbour-induced changes in chemical defences.

Natal-habitat experience mediates the relationship between insect and hostplant densities

For some animals, the habitat which they first experience can influence the type of habitat which they select later in life and, thus, potentially their population distribution and dynamics. However, for many insect herbivores, whose natal habitat may consist of a single hostplant, the consequences of natal hostplant experience remain untested in landscapes relevant to the adult, which may select not only among plants, but among plant patches. As a first step towards understanding how natal hostplant experience shapes patterns of insect feeding damage in landscapes relevant to adults, we conducted partially caged field experiments with diamondback moths that were reared on either mustard or collard plants and then allowed to choose among and within patches of plants that varied in plant density and composition. We predicted that natal hostplant experience would interact with patch size and composition to influence the number of diamondback moth offspring and feeding damage per plant. As predicted, when moths were reared on collards, we found more offspring on and damage to collard plants in four-collard patches than in two-collard patches (i.e., resource concentration), but no difference when moths were reared on mustards. Contrary to predictions, we found no difference in the number of offspring on or damage to mixed plant patches compared with two- or four-collard plant patches regardless of natal hostplant type. Our research suggests that prior hostplant experience has complex consequences for how insects and their feeding damage are distributed in patchy environments and highlights the need for future research in this area.