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 We also show that parameters such as th..., For the meta-analyses, we compiled raw data from unpublished or published community studies where possible. If data was available as a figure only we used the web-based tool WebPlotDigitizer (https://automeris.io/WebPlotDigitizer/) to extract the data points. We also used Pearson’s correlation coefficients (r) from published studies reporting the effects of tree-hole size or detritus amount on the abundance and richness of organisms. Furthermore, t, F and χ2 values were converted to correlation coefficients in some instances. Correlation coefficients between tree-hole size or detritus amount, respectively, and organism abundance or richness, respectively, were then used as effect sizes in four meta-analyses after transformation to Fisher’s z. We ran additional analyses using partial correlation coefficients for those studies for which we had raw data on both detritus amount and size to control for the effects of size on detritus amount. Data on separate groups of organisms or response v...,