Quantifying beetle-macrofungal associations in a temperate biodiversity hot spot

Beetles (Coleoptera) are often among the most abundant and diverse insects that feed on sporocarps of macrofungi, but little is known regarding their relative specialism or generalism in most communities. We surveyed >9000 sporocarps in montane hardwood forest in the Appalachian Mountains (USA) to characterize associations of mycophagous beetles and macrofungi. We used traditional metrics and network analyses to quantify relationships between sporocarp traits (mass, age, persistence, and toughness) and assemblages of adult beetles, drawing from >50 000 beetles collected over two survey years. Strict-sense specificity was rare in these associations: most beetle species were found on multiple fungal genera, and most fungi hosted multiple beetle species. Sporocarp age and fresh mass were positively associated with beetle diversity in fungi with ephemeral sporocarps (here including 12 genera of Agaricales and Russulales), but sporocarp persistence was not. In Polyporales, beetle diversity was greater in softer sporocarps than in tough or woody sporocarps. The increase of beetle diversity in aging sporocarps could not be attributed to increases in sporocarp mass or sampling point in the growing season, suggesting that age-related changes in chemistry or structure may support increasingly diverse beetle communities. Interaction networks differed as a function of sporocarp age, revealing that community-wide measures of generalism (i.e., network connectance) and evenness (i.e., variance in normalized degree) change as sporocarps mature and senesce. Beetles observed on Agaricales and Russulales with more persistent sporocarps had narrower interaction breadth (i.e., were more host-specific) than those on less persistent sporocarps, and beetles on Polyporales with tougher sporocarps had narrower interaction breadth than those on soft sporocarps. In addition to providing a large-scale evaluation of sporocarp use by adult beetles in this temperate biodiversity hot spot, this study shows that characteristics of food organisms are associated with specialism and generalism in interactions relevant to fungal and forest ecology.