Forest structure and heterogeneity increase diversity and alter the composition of host-parasitoid networks

Antagonistic host-parasitoid interactions can be quantified using bipartite and meta networks, which have the potential to reveal how habitat structural elements relate to this important ecosystem function. Here, we analysed the host-parasitoid interactions of cavity-nesting bees and wasps, as well as their abundance, diversity, and species richness with forest structural elements from 127 forest research plots in southwestern Germany. We found that parasitoid abundance, diversity, and species richness all increase with host abundance, a potential mediator between parasitoids and forest structure. Both parasitoid abundance and diversity increased with stand structural complexity, possibly mediated by the abundance of hosts. Additionally, parasitoid abundance increased with increasing standing deadwood and herb cover. The bipartite networks of host-parasitoid interactions showed higher connectance with increasing standing deadwood, herb cover, and host abundance. Analyses of interactions within the host-parasitoid metanetwork revealed that increasing host abundance and decreasing canopy cover diversify the suites of interactions present at the plot level. These results demonstrate that forest structural elements can improve the stability and resilience of host-parasitoid networks by promoting parasitoids and diversifying interactions in ecological networks. Methods Data were gathered by sampling cavity-nesting bees and wasps using trap nests. The abundance and parasitism events in the trap nests were recorded during nest opening, with individuals reared for species identification.