Direct and Indirect Effects of Resource Quality on Food Web Structure

Abstract

The diversity and complexity of food webs (the networks of feeding relationships within an ecological community) are considered to be important factors determining ecosystem function and stability. However, the biological processes driving these factors are poorly understood. Resource quality affects species interactions by limiting energy transfer to consumers and their predators, affecting life history and morphological traits. We show that differences in plant traits affect the structure of an entire food web through a series of direct and indirect effects. Three trophic levels of consumers were influenced by plant quality, as shown by quantitative herbivore-parasitoid-secondary parasitoid food webs. We conclude, on the basis of our data, that changes in the food web are dependent on both trait- and density-mediated interactions among species.

Warming and nitrogen affect size structuring and density dependence in a host–parasitoid food web

“…However, it has also been shown that plant resource quality can mediate interactions involving plants, herbivores and their parasitoids [11], by producing larger herbivore hosts, which can provide a better quality resource [12] and thus be preferred [13]. Consequently, recent studies have shown that bottom-up changes to host and parasitoid body size contributed, alongside density-mediated effects, to overall changes in food-web structure [14,15]. In addition to sizerelated host preferences, parasitoid body size may affect dispersal and search ability, whereas host body size can be inversely correlated with abundance [3], and these two factors may affect encounter rates and food-web structure [15].…”

Food web structure and biocontrol in a four-trophic level system across a landscape complexity gradient

“…They account for variation in link magnitude and energetic importance of each species in a community. Increasingly used in the last decade, these methods have been shown to provide a powerful tool with which to explore the structure of ecological communities and their responses to environmental factors that may not be revealed by analyses of species richness per se [9,[31][32][33][34]. Here, we analysed four of these quantitative metrics (generality, vulnerability, interaction diversity, linkage density) as well as the mortality rates of primary and hyperparasitoids to test the functional significance of these descriptors and their response to decline in landscape complexity.…”

Aphid–parasitoid community structure on genetically modified wheat

“…Therefore, we used measures based on Shannon information theory [12], as described in Bersier et al [13] and adapted for host-parasitoid webs [2], which use the densities of species and the frequency of interactions: (i) parasitoid diversity, which is equal to species richness when all species are equally abundant but takes on smaller values when abundances are uneven; (ii) link evenness, which equals one when all trophic links have equal frequencies and asymptotically approaches zero for highly uneven frequencies; and (iii) quantitative realized connectance, which is the observed link diversity as a proportion of maximum possible link diversity and is a measure of the complexity of the network. Since secondary parasitoids cannot be unambiguously linked to primary parasitoids [10], we calculated all the food web metrics separately for the aphid-primary parasitoid and the aphid-secondary parasitoid matrices.…”

“…The diversity and complexity of such food webs are considered to be important factors that determine ecosystem function and stability. Insect hostparasitoid systems are influenced by plant traits, which can lead to large effects on food web structure [2]. Due to their biology, interactions between aphids, primary parasitoids and secondary parasitoids can easily be quantified and have proven to be a useful system for exploring multi-trophic interactions.…”

“…For instance, changes in phloem sap composition can affect organisms that feed upon these plants including sap-feeding aphids [3]. In an aphid -parasitoid -hyperparasitoid food web, the effects of host plant traits can cascade up as far as the fourth trophic level [2], even when there are no effects on the herbivores [4].…”