The conservation of epiphyte-phorophyte networks in a changing world
Vascular epiphytes establish obligate, commensalistic interactions with their host plants (phorophytes). The fact that epiphytes have long life cycles, limited dispersal and depend on phorophytes, makes them one of the most vulnerable plant growth forms. Anthropogenic disturbances such as deforestation for land use conversion to agriculture and grazing threatens their long term conservation. Estimates indicate that epiphytes represent the 10% of all vascular plant species. They provide important resources for many taxa and their disappearance will cascade through the ecosystem. The objective of this study was to evaluate the impact of anthropogenic disturbances on epiphyte-phorophyte networks in a western tropical dry forest of Mexico using two approaches; first, comparing epiphyte-phorophyte networks of old-growth conserved forest, and of secondary forests; and second, simulating several disturbance scenarios of different type, frequency and magnitude. We constructed epiphyte-phorophyte networks for three successional stages (early, intermediate and late successional stages) using three sites per stage, and compared species richness and abundance, and several network metrics. Disturbances were simulated on all networks by removing phorophytes from the network according to different regimes of disturbance, and registering the number of Tillandsia species that remained connected to the network.
The networks involved 13 Tillandsia species (Bromeliaceae) and 98 woody species (phorophytes). Epiphyte species richness and abundance, network size and the number of links were significantly higher at conserved forests. Nestedness was also higher in conserved forest, and differences among successional stages could be explained by species abundances. We found a low level of specialization of epiphytes on woody species, and specialization did not differ among successional stages. Simulations show that removing the most generalist phorophytes causes the highest impact on the Tillandsia community. The second worst scenarios for the maintenance of Tillandsia individuals and species in the network are the selective logging of abundant woody species, and of large woody individuals. The random removal of phorophytes had the lowest impact on the networks. Additionally, the effects of these disturbance regimes on networks from sites under ecological succession were more severe for the Tillandsia community than on networks from conserved forest, showing the vulnerability of the system once a disturbance has occurred. Our results suggest that keeping large remnant phorophytes as sources of epiphyte propagules can aid in maintaining more diverse epiphyte communites in managed forests.