Extinctions of interactions: quantifying a dung beetle–mammal network

Abstract

Loss of large‐bodied mammals across the globe through hunting, habitat degradation, and fragmentation is one of the most significant anthropogenic impacts on the environment. Cascading effects of these extinctions through ecosystems have been little studied, although correlative studies have revealed co‐extinctions in closely linked groups, with implications for ecosystem structure and function. Despite playing important roles in seed dispersal and hence seedling recruitment, mammals have been largely neglected in network studies. Similarly, the role of secondary seed dispersers, such as ants and dung beetles, has been largely unexplored. Most dung beetles rely on mammal feces for feeding and breeding and provide a suite of important ecosystem functions and services. While dung beetle community responses to environmental change have been widely investigated, studies quantifying the network of associations between dung beetle and mammal species are lacking. By developing the first quantitative mammal–dung beetle networks, we address several important knowledge gaps contributing to the understanding of how interactions in networks involving mammals and secondary insect seed dispersers are structured. We use the resulting quantitative interaction networks to model mammal species extinction scenarios to further explore the consequences for dung beetle populations, and the extent to which networks change the strength of interactions through resource switching. Dung beetle feeding and breeding networks did not differ significantly in structure and showed high nestedness and low levels of trophic specialization. Simulations suggested that mammal extinction scenarios based on mammal body mass and mammal dung volume will impact dung beetle populations to a greater extent than random scenarios of mammal loss. Thus, despite their generalist feeding preferences, realistic mammal extinction scenarios have the potential to negatively impact the dung beetle community, which may have consequences for ecosystem functioning.

Co‐declining mammal–dung beetle faunas throughout the Atlantic Forest biome of South America

“…Yet a network approach is rarely attempted in studies of consumer-detritivore interactions. Studies on the ecological interactions between dung beetles and mammals suggest that the structure of dung beetle feeding and breeding networks did not differ significantly and showed high degree of nestedness and low levels of trophic specialization (Raine et al 2018, Raine andSlade 2019). On the other hand, the reintroduction of folivore primate (Alouatta guariba) in a previously defaunated Atlantic Forest site restored lost ecological interactions and promoted secondary seed dispersal, two years after the release of the first individuals (Genes et al 2019).…”

“…Model simulations have suggested that size-biased extinction scenarios for large mammals (and consequently the total amount and diversity of dung resources) will have detrimental effects on dung beetle populations (Raine et al 2018). The distribution of mammal body mass also influenced the size structure of dung beetle assemblages.…”

“…At all scales, the networks revealed a high degree of connectance and nestedness, . Studies on the ecological interactions between dung beetles and mammals suggest that the structure of dung beetle feeding and breeding networks did not differ significantly and showed high degree of nestedness and low levels of trophic specialization (Raine et al 2018, Raine andSlade 2019). Acronyms in the figure are C: connectance; M: modularity; and N: nestedness.…”

“…The pervasive depletion of mammal populations in the Anthropocene has led to wholesale changes in community structure and ecosystem functioning worldwide (Estes et al 2011, Raine et al 2018. Dung beetles represent one of the greatest expressions of detritivore food webs, and often depend on mammalian fecal resources.…”

“…The distribution of mammal body mass also influenced the size structure of dung beetle assemblages. Model simulations have suggested that size-biased extinction scenarios for large mammals (and consequently the total amount and diversity of dung resources) will have detrimental effects on dung beetle populations (Raine et al 2018). Large-bodied mammals are precisely those that first succumb to human overexploitation (Ripple et al 2019), and are therefore missing from ~70% of the relictual mammal assemblages across the Atlantic Forest (Bogoni et al 2018).…”