Network properties of an epiphyte metacommunity

1. Trophic relationships are often depicted as networks, in which species are connected by links representing predatory interactions. Network analyses have also been applied to non-trophic species interactions, such as pollination and seed dispersal mutualisms, to summarize community-level patterns in mutually beneficial interactions. However, the processes responsible for common network properties such as degree distributions (the number of interactions maintained by each species in the network) and nestedness (the tendency for specialists to interact with perfect subsets of the species interacting with generalists) are poorly understood. 2. I evaluated the network properties of a novel type of species interaction, commensal interactions between epiphytes and their host trees. I quantified the occurrence of 19 vascular epiphyte species on seven host tree species in a metacommunity of epiphytes in New Zealand. I then developed null models to test whether observed network properties result from the frequency of species interactions, which is a commonly cited but rarely tested explanation for degree distributions and nestedness. 3. Results showed that degree distributions of both epiphyte and host tree species were consistent with a null model that randomly populated hosts with epiphytes. Several null models that test for nestedness by randomly linking pairs of interacting species showed significantly higher nestedness in the real community. In fact, the observed level of nestedness was among the highest yet recorded for any type of ecological interaction. A second null model that randomized species interactions according to their overall frequencies of occurrence in the community confirmed support for nestedness. 4. Results indicate that the degree distributions were consistent with randomized expectations; the number of interactions maintained by both epiphyte and host tree species appears to be determined by their frequency of occurrence in the community. However, interaction frequencies could be ruled-out as an explanation for nestedness. Nested epiphyte-host interactions may instead result from a predictable sequence of epiphyte succession, wherein earlier colonists ameliorate environmental conditions within host trees for later recruiting species.