Edge effects on epiphyll cover and composition in a neotropical rainforest

Background/Question/Methods

Abiotic and biotic conditions at the edges of forest fragments are often significantly different from those in the forest interior. Understanding how biotic communities within forest fragments change as a result of these edge effects is important for predicting the ecological consequences of continuing habitat fragmentation in the neotropics. Epiphylls, which are photosynthetic organisms inhabiting leaf surfaces, present a unique system for the study of edge effects. Given that they are an abundant and taxonomically diverse group of organisms in the tropics, and that they have high colonization and extinction rates, epiphylls present a useful study system for evaluating the multigenerational effects of habitat fragmentation on plant communities. To investigate if forest edges reduce epiphyll cover on leaves and to determine if there are changes in epiphyll community composition at the forest edge, we collected leaves from the forest edge and forest interior at Los Tuxtlas Biosphere Reserve, Veracruz, Mexico for three mature forest species, Rheedia edulis, Salacia megistophylla, and Poulsenia armata. We made visual estimates of ephiphyll cover using a variation of the Braun-Blanquet method, and characterized community composition at the phylum level, either Ascomycota or Bryophyta.

Results/Conclusions

Our results show that S. megistophylla had significantly (p < 0.01) less epiphyll cover at the edge than in the interior, and a similar trend was noted for epiphyll cover on both P. armata (p = 0.10) and R. edulis (p = 0.14). Community composition was found to shift consistently between the edge and interior for all three host plant species (p < 0.001 for each). Bryophyta was significantly more abundant in the interior of the forest, and was never observed without Ascomycota. This suggests that Bryophyta might only be able to survive in a subset of the habitat that supports Ascomycota, and may even be nested within the Ascomycota community at the edge. Given the importance of epiphylls for nitrogen fixation and water circulation in tropical rainforests, our data suggest that the artificial edges of tropical forest fragments may be more detrimental than previously thought, highlighting the need for further research on epiphylly and habitat fragmentation.