Community composition changes as a result of habitat fragmentation: A study on tropical treeholes

Background/Question/Methods

Habitat loss and fragmentation are important causes of biodiversity loss at a global scale. Tropical forests, in particular, are experiencing accelerated rates of conversion, often resulting in isolated forest patches surrounded by a matrix of agricultural or urban areas. Metacommunity models predict that fragmentation can affect species occurrence through disrupting dispersal of either the species directly or that of dominant competitors and predators. However, such models often do not consider potential effects of fragmentation on the quality of the habitat, for example by affecting resources. We examined invertebrate colonization of artificial tree holes in Northwestern Costa Rica to compare the effects of fragmentation on the pool of colonizing species versus the quality of the detrital resources. We compared community composition in small (n=6) and large (n=6) forest patches with that of continuous forest (n=8), after 30 (n=174) and 84 days (n=188) of colonization. The detrital resources in our treholes were either typical of the site, or from a standard tree species; differences in invertebrate composition between these detrital treatments would indicate bottom-up effects of fragmentation on invertebrate colonization.

Results/Conclusions

Insect larvae colonized 94% of our tree holes, with similar overall abundances between forest patches and continuous forest. While mosquito larvae dominated the community in the first sampling period, their relative abundances decreased by the end of the experiment. Community composition differed significantly between continuous and fragmented forest, both because of differences in the pool of colonizing species as well as differences in detrital quality between habitat types. We were able to isolate the effects of colonizer pool by contrasting assembly in treeholes with identical detrital species but differing in habitat type. With standard detritus, treeholes in fragmented forest had higher larval abundances of mosquitoes, chironomids, ceratopogonids and psychodid flies, whereas continuous forest treeholes had higher larval abundances of scirtid beetles. However, when we examined treeholes containing site-specific detritus, many of these patterns reversed, with continuous forest treeholes instead having higher mosquito and chironomid abundances. We conclude that forest fragmentation affects treehole communities both through direct effects on the colonizing pool of treehole invertebrates as well as indirect effects moderated by changes in litter quality and composition.