Antagonistic interaction networks are structured independently of latitude and host guild

Abstract

An increase in species richness with decreasing latitude is a prominent pattern in nature. However, it remains unclear whether there are corresponding latitudinal gradients in the properties of ecological interaction networks. We investigated the structure of 216 quantitative antagonistic networks comprising insect hosts and their parasitoids, drawn from 28 studies from the High Arctic to the tropics. Key metrics of network structure were strongly affected by the size of the interaction matrix (i.e. the total number of interactions documented between individuals) and by the taxonomic diversity of the host taxa involved. After controlling for these sampling effects, quantitative networks showed no consistent structural patterns across latitude and host guilds, suggesting that there may be basic rules for how sets of antagonists interact with resource species. Furthermore, the strong association between network size and structure implies that many apparent spatial and temporal variations in network structure may prove to be artefacts.

Tree phylogenetic diversity promotes host–parasitoid interactions

“…Albeit limited by the relatively small size of our networks, this may indicate that network structure can be independent of the environment (but see [9]). In this regard, our results conform to a global meta-analysis [61], which demonstrated that host-parasitoid networks are generally specialized, independent of geography, scale, taxa or, host-parasitoid system. That a phylogenetically diverse tree community supports parasitoids and parasitism agrees with earlier evidence that 'healthier' ecosystems [8] are characterized by high parasitism [62].…”

Vertical canopy gradient shaping the stratification of leaf‐chewer–parasitoid interactions in a temperate forest

“…For instance, connectance decreased steadily from the lowest to the highest trees, regardless of tree species, which might be due to changes in the prevalence of specialists or generalists (Dunne, 2006). Moreover, generality was highest and $H_2^{\prime }$ was lowest in the tallest tree species, suggesting lower specialization in the upper canopy (Morris et al., 2014). High generality, recorded in the tallest tree species ( Fraxinus spp.…”

“…To assess the changes in leaf‐chewing host–parasitoid food web structure in a vertical canopy gradient, we calculated standard metrics that characterize the complexity and structure of the entire food web, and which reflect the degree of network specialization (Morris, Gripenberg, Lewis, & Roslin, 2014; Tylianakis, Tscharntke, & Lewis, 2007). We focused on quantitative metrics that reflect interaction network properties and that are more robust against variation in sampling intensity, matrix size, and symmetry than qualitative ones (van Veen et al., 2006).…”

“…Poisson distribution and link function log were used when the values of the food web metrics were integers, and inverse Gaussian distribution and link function log were used when the food web metrics were real numbers. As matrix size (i.e., the total number of interactions recorded between individuals) might affect many of the metrics (Morris et al., 2014; Rodriguez‐Girones & Santamaria, 2006), we controlled for matrix size (which varied from two to 219 interactions) in our models. First, we regressed the food web metrics on a common logarithm of matrix size.…”

“…The partial effect of each explanatory variable was tested using LR analysis of deviance with the chi‐square statistics. Food web metrics were calculated in the package bipartite (Dormann et al., 2009), using the empty.web  =  false option to account for hosts that were present but not parasitized (Morris et al., 2014). …”