Microhabitats with emergent plants counterbalance the negative effects of fish presence on diving beetle (Coleoptera: Dytiscidae) diversity in urban ponds

Aquatic plants are important prey refuges for aquatic organisms, and their species richness is positively related with aquatic invertebrate species richness. Yet, it is unclear how the quantity of refuges, i.e. aquatic vegetation cover, affect aquatic invertebrate assemblages and their habitat use under different levels of predation risk (e.g. in the presence or absence of fish), nor at different scales (i.e. microhabitat and pond scales). Here, we investigated how provision of refuges affects diving beetle (Dytiscidae) species richness and abundance in the presence and absence of fish predators. We sampled diving beetles (Dytiscidae) with 1-L activity traps and estimated emergent plant cover at both the pond and microhabitat scales. We found that plant cover affected dytiscid populations differently: At the pond scale, dytiscid presence was positively correlated with increasing plant cover, both with and without fish, indicating the importance of emergent plants for aquatic biodiversity. At the microhabitat scale, dytiscid species richness and abundance were positively correlated with increasing plant cover in ponds with fish, but there was no such relationship in ponds without fish, emphasizing that the level of predation risk can alter prey species’ use of prey refuges. Our findings provide evidence that the availability of both vegetated and non-vegetated microhabitats can benefit aquatic invertebrates. We suggest maintaining variation in the provision of emergent plant cover, to retain high habitat heterogeneity in urban ponds and to enhance freshwater biodiversity.