What shapes galling insect–parasitoid interaction networks on closely related host plants?

Abstract

Ecological networks describe community structure by focusing on how nodes are connected by links (interactions). Related species have higher probabilities of sharing links if interactions depending on phylogeny and/or species are functionally similar; however, if and how this modularity impacts higher trophic levels is still an open question. We aimed to evaluate the effect of different structuring factors on an observed bipartite network of galling insects and associated parasitoids on a system with two closely related and recently diverged plants, where host‐tracking could be underway. We hypothesized that gall structure, galler taxonomy and host plant identity could contribute, from a higher to a lower degree respectively, to network modularity by generating a selective pressure on parasitoids. Samples were collected between May 2015 and June 2017 in the forest areas of Porto Alegre, southern Brazil. Overall, we found 4629 galls of three monophagous and four oligophagous galling species; 664 parasitoid individuals were obtained and classified into 37 morphospecies. We constructed four networks with the collected data by organising nodes according to the tested factors, that is, gall structure, galler taxonomy, host plant identity and gall–plant interaction. All networks were significantly modular, and for three out of four analysed factors (gall structure, galler taxonomy and gall–plant interaction), modularity levels were at least intermediate (at about Q = 0.4). Barriers to or preferences for certain interactions appear to be responses to multiple concomitant factors. However, galler taxonomy and host plant identity are somehow stronger determinants for the parasitoids in terms of attacked hosts.