DOI: 10.5061/dryad.08kprr55f
title: Aquatic islands in the sky: 100 years of research on water-filled tree holes

description: 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.
description: 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 compiled raw data from unpublished or published community studies where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 χ<sup>2</sup> 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 variables from one study were used as separate data points. Annual mean temperature and annual precipitation were used as reported by the studies. If this information was not available from the publication, temperature and precipitation data were extracted from the WorldClim database (version 1.4, www.worldclim.org). For those studies that used several sites, averages of the climate data between the sites were used. If several sites per study were used, these were never more than 300km apart and climates did not differ much.