Vertical Stratification of Insect Species Developing in Water-Filled Tree Holes

Forest ecosystems have a distinct vertical dimension, but the structuring of communities in this three-dimensional space is not well understood. Water-filled tree holes are natural microcosms structured in metacommunities. Here, we used these microcosms as model systems to analyze how insect communities and the occurrence and abundance of individual species are influenced by biotic and abiotic microhabitat characteristics, the vertical position of the tree hole, and stand-scale habitat availability. We found that both the characteristics of water-filled tree holes and their insect communities differ significantly between canopy and ground level. Individual insect species showed contrasting responses to the vertical position of the tree holes when important environmental factors at the stand and the tree-hole scale were considered. While some species, such as the mosquito Aedes geniculatus and the beetle Prionocyphon serricornis, decreased in abundance with increasing tree-hole height, the biting midge Dasyhelea sp., the non-biting midge Metriocnemus cavicola and the hoverfly Myiatropa florea increased in abundance. Our results suggest that vertical stratification in forests is most likely driven not only by variation in tree-hole microhabitat properties, i.e., niche separation, but also by individual species traits, such as adult dispersal propensity, food preferences and mating behavior of adult stages, and interspecific competition of larval stages. Therefore, communities of insect species developing in tree holes are likely structured by competition-colonization trade-offs predicted by metacommunity theory.

Automated summary

Forest ecosystems have a distinct vertical dimension, and the structuring of communities in this three-dimensional space is influenced by various factors such as microhabitat characteristics and habitat availability. Water-filled tree holes serve as natural microcosms in which insect communities develop. This study analyzed how these microcosms and insect communities are influenced by the vertical position of the tree holes, characteristics of the water-filled tree holes, and stand-scale habitat availability. The results suggest that the vertical stratification in forests is driven by both variation in tree-hole microhabitat properties and individual species traits, leading to competition-colonization trade-offs.