Aquatic islands in the sky: 100 years of research on water-filled tree holes

Water-filled tree holes are unique ecosystems that may occur high up in
tree crowns like aquatic islands in the sky. Insect larvae, mesofauna and
other organisms colonize the waterbodies and feed on the accumulating
detritus. Water-filled tree holes are not only important habitats for
these species but have been used as model systems in ecology. Here we
review more than 100 years of research on tree-hole inhabiting organisms
and show that most studies focus on selected or even single species (most
of which are mosquitoes) whereas only few studies examine groups other
than insects, especially in the tropics. Using a vote counting of results
and a meta-analysis of community studies, we show that the effects of
tree-hole size and resources on abundance and richness were investigated
most frequently. Both were found to have a positive effect, but effect
sizes were modulated by site-specific environmental variables such as
temperature or precipitation. We also show that parameters such as the
height of the tree holes above ground, tree-hole density, predation and
detritus type can be important drivers of organism abundance or richness
but are less often tested. We identify several important research gaps and
potential avenues for future research. Specifically, future studies should
investigate the structure, functions and temporal dynamics of tree-hole
food webs and their cross-system interactions, for example with
terrestrial predators that act as a connection to their terrestrial
surroundings in meta-ecosystems. Global observational or experimental
tree-hole studies could contribute pivotal information on spatial
variation of community structure and environmental drivers of community
assembly. With a better understanding of these unique aquatic habitats in
terrestrial ecosystems, natural and artificial tree holes can not only
serve as model systems for addressing fundamental ecological questions but
also as indicator systems of the impacts of environmental change on
ecosystems.