Does diversity beget diversity? The case of host specificity of vascular epiphytes

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The canopy of tropical rainforests hosts an enormous, still unknown number of animal and plant species. The evolution and maintenance of this species richness is subject to intensive research. For the diversity of herbivorous insects, a direct relationship with the diversity of their host tree species has been postulated. The same might apply for the very species-rich group of vascular epiphytes, i.e. vascular plants growing on trees without parasitizing them. However, to test this hypothesis it is necessary to quantify the host specificity of this group. In spite of the many studies on this topic we are still far from this goal. State of the art is that epiphytes are usually capable of colonizing any tree species in a forest but that the observed epiphyte abundances are often unequally distributed among host species. However, due to a number of methodological issues such observed biases are not an unambiguous demonstration of host specificity: When analyzing distributional data statistically artefacts might arise from, e.g., disregarding the clumped occurrence of epiphytes. Moreover, there are barely any unambiguously interpretable study approaches that focus directly on the individual fitness of epiphytes. Finally, given the daunting number of epiphyte and tree species in tropical rainforests, the usually applied floristic approach is inadequate to predict associations on the basis of general rules. The objective of the proposed project is to address all these issues. Subproject 1 investigates, with the aid of a computer model, the effect of spatial autocorrelation of conspecific epiphytes on the results of host specificity tests. To improve the analysis of empirical datasets, a null model that considers this effect will be developed. Subproject 2 (a systematically designed field study) compares epiphyte abundances and species composition among tree species. Comparably large sample sizes will permit to reliably detect distributional biases. Subproject 3 investigates host specificity on the level of fitness parameters (germination, survival and growth rates) with the aid of experimental manipulations. Finally, subproject 4 leaves the level of species interactions, which have been the focus of previous studies, and turns onto the interaction of epiphyte and tree traits (trait matching). The long-term goal is to explain distributional patterns mechanistically and to develop general rules for the correlation of traits. A decisive precondition for the success of the project is the choice of the field site in Panama. There are already two temporally replicated datasets from a previous project, describing the spatial distribution of ca. 20,000 epiphytes within a 0.9 ha forest plot. Comparing the distributional patterns from these datasets to the results of the proposed project will allow to evaluate how suitable such datasets are for the investigation of host specificity.