An experimental test of the importance of isolated trees for frog diversity in tropical landscapes

Isolated trees are conspicuous in fragmented landscapes and are often considered 'keystone structures', due to their significant ecological role despite occupying small areas. They are also regarded as 'biodiversity foci', because they may support higher abundance and species richness than nearby open areas. However, these suggestions have not been tested experimentally yet. Using experimental ponds and amphibians as a model system, we tested the importance of isolated trees for biological communities in a biodiversity hotspot, the Atlantic Forest. We built and sampled 28 artificial ponds, 10 near the edge of a continuous forest, nine beneath isolated trees and nine in open pasture. To test for differences in amphibian community structure among experimental treatments, we quantified community abundance, local species richness and community composition. We also tested whether the abundance of the four numerically dominant species in the ponds (Physalaemus signifer, Leptodactylus latrans, Rhinella ornata and Scinax aff. x-signatus) differed among treatments. Finally, we compared total (landscape-level) species richness among treatments. Ponds located beneath isolated trees and near forest edges had a higher community abundance and abundance of P. signifer and R. ornata, as well as a higher total (landscape-level) species richness than ponds located in open pasture. Ponds beneath isolated trees had a similar community composition to ponds near the edge of the continuous forest, whereas open pasture communities had a markedly different community composition. Our results provide experimental evidence that isolated trees contribute to increase total (landscape-level) species richness and community abundance of anurans in ponds in deforested areas, making local communities more similar to those found near forest edges. We recommend protecting and planting isolated trees across deforested areas, particularly near natural ponds, as well as restoring ponds near isolated trees. These relatively low-cost actions can contribute substantially to increasing the abundance and species richness of anuran communities in fragmented landscapes.