The effects of deforestation and forest fragmentation on a central Amazonian frog community
Tocher, Mandy Darlene
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
An investigation into the effects of deforestation and habitat fragmentation on a central Amazonian frog community was carried out on the Biological Dynamics of Forest Fragments Project, (BDFFP) based in Manaus, Amazonas, Brazil. Three aspects of deforestation were investigated: the effects of fragmentation and reserve size, the influence of matrix habitat and the effects offorest edge on frogs in 10- hectare isolates. Very few frogs species were lost from fragments (regardless of size) following fragmentation and isolation for 7-10 years. Instead, frog species richness increased, with an average increase in 10 species per fragment. The increase in species was brought about by an increase in matrix associated species, some of which were present in the absence of their preferred breeding habitat. Different frog species had varied responses to fragmentation. The abundance of Eleutherodactylus fenestratus, a terrestrial breeder, increased significantly as fragment size decreased, and its abundance was significantly higher in both large and small fragments than in continuous forest. Colostethus stepheni, a semi-terrestrial breeder, was less abundant in fragments than continuous forest. Finally, Eleutherodactylus zimmermanae and Osteocephalus sp.A did not differ significantly in abundance among fragments, or between fragments and continuous forest. Multiple regression analysis indicated that variation in litter depth and canopy cover may explain the observed increase in E. fenestratus abundance in small fragments. Breeding success of pool breeders attracted to artificial pools was variable, but there was no evidence of reduced breeding success in fragments relative to primary forest. Overall, fragmentation appeared to affect the frog community less severely than other taxonomic groups. Species richness in tropical forest remnants showed a tendency to increase as a result of fragmentation, and only one of four species exhibited lowered abundance in fragments. The effects of fragmentation and subsequent small isolated populations of frog species in fragments was alleviated somewhat by the ability of central Amazonian frogs to utilise matrix habitat. Litter frogs, wallow breeders and upland forest pool breeders all displayed an ability to permeate, and in most cases become residents of matrix habitat. Both matrix with a history of cut and burn and matrix which was only cut supported a similar frog community. Thus, for central Amazonian frogs isolates were not truly isolated due to the ability of frogs to utilise matrix habitat. Litter frogs in 10- hectare isolates displayed an independence from edge related abiotic and biotic gradients. This independence was true for frog species richness, abundance and composition. Consequently, forest fragments are in effect larger for frogs than for other taxa who are constrained by edge effects to exist only within the core of large fragments. Colostethus stepheni was one species found to be negatively affected by habitat fragmentation. Abundance of Colostethus stepheni was significantly lower in fragments and matrix habitat compared to primary continuous forest. This species was also found to be sensitive to edge, with higher abundances recorded as distance from the edge increased. The community level approach adopted in this study, as opposed to intensive investigation of single species, may well have overlooked other' sensitive' species which show more subtle responses to habitat fragmentation than that of Colostethus stepheni. The ability to use matrix and the relative independence from edge related phenomena accounts for the resilience of central Amazonian frogs in a disturbed and fragmented landscape. However, with respect to matrix habitat, forest surveyed in this study has not been repeatedly cut or cut and burnt (with the exception of pasture-land where the frog community is depauperate). In other areas of the Amazon, away from the experimental plots of the BDFFP, matrix habitat is destroyed more frequently. More work is needed to determine the ability of central Amazonian frogs to utilise matrix habitat which is disturbed more frequently.