Epiphyte-phorophyte network structure in a fragmented tropical dry forest in western Mexico

Background/Question/Methods The biodiversity and level of endemism of tropical dry forests (TDF) in Mexico is high for many taxa and have been affected dramatically by human activities. The Chamela-Cuixmala Biosphere Reserve is one of the few natural reserves that protect TDF in Mexico. Around the reserve, vegetation cover has been fragmented and has been transformed to secondary TDF because of human disturbance. Habitat fragmentation affects mainly plants, but not all the growth forms are equally affected.  Epiphytes  represent 10% of the vascular plant species of the planet and attain their highest richness in the Neotropics.  Epiphytes might be particularly vulnerable to forest
fragmentation but very little is known about such anthropogenic disturbance on the survivorship of this important life form. The aims of this study were to evaluate the specialization of the interaction between Tillandsia (Bromeliceae) and their phorophytes using a network approach, and to assess how the Tillandsia community is affected by human disturbance in the TDF of Mexico.  We studied three successional stages (early, intermediate and mature) of TDF forest according to the time elapsed since human disturbance. We studied three sites by condition, where all the woody individuals with diameter at breast height (DBH) greater than 2.5 cm were identified to species and all Tillandsia individuals on them were recorded and identified to species. We constructed Tillandsia-phorophyte interaction networks and assessed the role of neutrality and phorophyte DBH in explaining the interaction networks.

Results/Conclusions We found 13 Tillandsia species and 55 tree species. Tillandsia species richness and abundance increased along the successional gradient, where mature forests showed the greatest richness and abundance. Tillandsia-phorophyte interaction networks occurred between 23 woody species and 13 Tillandsia species. The networks lost complexity along the successional gradient, the number of interactions per species, either phorophytes or Tillandsia, decreasing from early successional sites to mature forest sites. Preliminary analyses showed that both, phorophyte DBH and neutrality, help to explain the Tillandsia-phorophyte network structure.  Our data showed the importance of remnant trees for maintaining Tillandsia diversity in disturbed TDF.  We identified two endemic species, T. eistetteri and T. makoyana, as the most vulnerable ones of the Tillandsia community. These species suffered a drastic decrease in the number of individuals in fragmented conditions.