Vertical stratification of rainforest collembolan (Collembola: Insecta) assemblages: description of ecological patterns and hypotheses concerning their generation

Abstract

We describe a complex vertical stratification of collembolan assemblages from rainforest leaf litter samples and identify distinct assemblages associated with forest floor, lower canopy and upper canopy samples. Leaf litter samples were collected from the forest floor and deposits of leaf litter suspended in epiphytes in the canopy of a subtropical rainforest site at Lamington National Park in southeast Queensland. The patterns of relationship among assemblages of Collembola extracted from these samples were examined using a variety of analyses of a matrix of similarities between samples. The results of ANOSIM analyses showed that forest floor, lower canopy and tipper canopy samples formed discrete groups. These results permit a discussion of these groups as three distinct collembolan assemblages. Analysis of the dissimilarities between these assemblages revealed a gradient of similarity from the forest floor through the lower to the upper canopy. This gradient represents a more complex vertical stratification than has previously been identified in rainforest canopy arthropods. We suggest that limitations on the dispersal of some forest floor species into the canopy may be responsible for this pattern. We also identify a second gradient of similarities among these assemblages. We show that dissimilarity among samples from forest floor is significantly lower than dissimilarity among samples from within the lower canopy, and that the level of dissimilarity between samples from within the upper canopy is significantly higher again. We suggest that dispersal barriers and higher probabilities of extinction in upper canopy collembolan colonies may be responsible for higher heterogeneity of species composition and abundance among samples from the upper canopy. We outline a number of testable hypotheses aimed at determining the importance of these processes in producing the patterns we have observed.