Food web structure changes with elevation but not rainforest stratum

Abstract

Changes in species richness along elevational gradients are well documented. However, little is known about how trophic interactions between species and, in particular, the food webs that these interactions comprise, change with elevation. Here we present results for the first comparison of quantitative food webs in forest understorey and canopy along an elevational gradient. Replicate quantitative food webs were constructed for assemblages involving 23 species of cavity‐nesting Hymenoptera and 12 species of their parasitoids and kleptoparasites in subtropical rainforest in Australia. A total of 1589 insects were collected using trap nests across 20 plots distributed at sites ranging from 300 to 1100 m a.s.l. Insect abundance, insect diversity and parasitism rate generally decreased with increasing elevation. Food web structure significantly changed with elevation. In particular, weighted quantitative measures of linkage density, interaction evenness, nestedness (weighted NODF) and potential for enemy mediated interactions (PAC) decreased with increasing elevation, and network specialisation (H2′) increased with increasing elevation, even after controlling for matrix size; but there was no change in weighted connectance. Changes in forest type and temperature along the elevational gradient are likely to be, at least partly, responsible for the patterns observed. We found no significant differences in insect abundance, insect diversity or parasitism rate between canopy and understorey. Furthermore, there were no differences in food web structure between strata. These results contribute further evidence to studies revealing changes in food web structure along natural environmental gradients and provide information that can potentially be used for predicting how communities may respond to climate change.

Tree phylogenetic diversity promotes host–parasitoid interactions

“…A decline of parasitism towards higher elevations is likely, as the frequency of trophic interactions decreases polewards [59] and local elevation gradients reflect large-scale climatic gradients. To our knowledge, only two recent studies from subtropical Australia have addressed this topic and found a decrease of parasitism with elevation [18,46], which we support. Higher parasitism at lower and warmer lower elevation might be caused by higher attack rates of parasitoids, as demonstrated in controlled warming experiments [60].…”

“…The properties of such interactions can mathematically be quantified by various indices describing the structure of species interaction networks [16,17]. Thus, relating network properties to the environment can provide insight on how trophic interactions are influenced by changing habitat conditions [9,18,19].…”

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

“…Moreover, no previous study focused on leaf‐chewing assemblages. Inconsistent results obtained by previous studies suggest that the vertical patterns of parasitism are guild‐, latitude‐, or site‐dependent (Chaij et al., 2016; Morris et al., 2015; Paniagua et al., 2009; Sobek, Tscharntke, Scherber, Schiele, & Steffan‐Dewenter, 2009). The parasitism rate decreased as the season progressed, in accordance with another study on leaf‐chewing Lepidoptera in a temperate forest (Le Corff, Marquis, & Whitfield, 2000).…”

“…Thus, studying herbivore–parasitoid communities in forest canopies could enhance our understanding of their structure and diversity (Stireman, Cerretti, Whitmore, Hardersen, & Gianelle, 2012). Moreover, investigating differences in ecological communities along habitat gradients could help us to recognize the factors that determine community structure (Morris, Sinclair, & Burwell, 2015). …”

“…However, only three studies have investigated the vertical stratification of host–parasitoid food webs (Chaij, Devoto, Oleiro, Chaneton, & Mazía, 2016; Morris et al., 2015; Paniagua, Medianero, & Lewis, 2009). Therefore, more studies on the quantitative food webs of herbivorous insect–parasitoid communities in forest canopies are needed to understand differences between forest types and the particularities of various herbivorous guilds.…”