Abiotic conditions differ from canopy to understory in rain forests. Many taxa are more diverse or abundant in the canopy and/or have vertically stratified species distributions. Water-filled tree holes are common aquatic habitats in many neotropical forests, yet little is known of factors influencing their biodiversity. I compared community parameters (species richness, abundance, and composition) of tree hole macroorganisms at different heights (canopy, midstory, understory) in the forest of Barro Colorado Island, Panama. Data from multiple censuses of 40 artificial tree holes during three wet seasons (1995—1997) revealed a small but significant decrease in the average number of species present with increasing height above the ground. Species richness and abundance were greater in larger holes. Similar patterns were observed in 206 natural tree holes. Of seven top predator species, one was not found in holes > 7 m above the ground; four other species occurred only in understory holes (ca. 1 m above the ground). Chemical properties of tree hole water did not differ with height, but canopy tree holes dried out more frequently and were thermally less stable than midstory and understory holes. Harsh thermal conditions and higher disturbance frequency may be responsible for the decline in species richness with height.
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