How are dung beetle species affected by deer population increases in a cool temperate forest ecosystem?

Using plant–herbivore–decomposer trophic chains as an example, we have tried to clarify the key roles of multitrophic interactions in species diversity. The interactions included two‐link (herbivore–decomposer and decomposer–plant) and three‐link (decomposer–herbivore–plant) chains within a community. Specifically, we investigated how sika deer abundance impacted dung beetle populations via dung supply and vegetation changes by surveying deer and beetle abundance and community composition monthly in Japan. The forest sites were similar in canopy cover, but differed in the presence (sites A and B) or absence (sites C) of an understory and in the abundance of deer (rare at site A, moderate at sites B and C, and common at site D). Site D was patchy grassland. Beetle species fell into two groups based on whether they were more abundant at sites with more dung or at sites with an understory. We suspect that the type of dung usage and/or beetle body size affected this finding. First, one beetle group was more strongly affected by vegetation cover than dung supply, and they were mostly dwellers. The other group was affected by dung supply more than vegetation cover and comprised mostly tunnelers. Dwellers may be strongly negatively affected by decreased understory vegetation because of dung drying. Second, large beetle species were positively affected by decreasing vegetation cover and increasing dung supply; understory vegetation may negatively affect mobility in larger species. Our results suggested that increased deer abundance had both positive and negative as well as direct and indirect effects on the dung beetle community by increasing the dung supply and changing the vegetation structure, respectively. Moreover, dung beetle species responded differently depending on their ecological requirements and body sizes.