Rainforest fragmentation affects reptiles and amphibians
Rainforest fragmentation affects reptiles and amphibians
A technical paper by J. Nicolás Urbina-Cardona
February 20, 2008
RAINFOREST HERPETOFAUNA AND FOREST FRAGMENTATION: EFFECTS ON DIVERSITY, COMPOSITION, ABUNDANCE AND MICROHABITAT RELATIONSHIPS
Deforestation of tropical ecosystems is one of the major threats to biological diversity (Picture 1). Anthropogenic activities transform tropical environments into semi-natural landscapes generating a great amount of forest edge that limits with pastures and agricultural lands. Between-patch isolation and within-patch edge effects modify animal community structure and movements. Within the remnants, changes in microclimate structure and composition of the herpetofauna, reduce population viability and increase herpetofauna' vulnerability to extinction. Amphibian and reptile populations inhabiting remain forest fragments are likely to be affected by the loss of suitable microhabitats (e.g. reproduction, forage and refugee) due to factors such as edge effects. The species that tolerate and use the pasture to disperse and reproduce are less affected by forest fragmentation. In contrast, some forest interior affine species are the most vulnerable to extinction due to their sensibility to edge effect. During my Doctoral Dissertation I evaluated the edge effect on amphibian and reptile's diversity in a tropical rainforest in Los Tuxtlas region (Veracruz, Mexico) and their relationship with microhabitat dynamics.
Deforestation of tropical ecosystems is one of the major threats to biological diversity (Picture 1)
Eighteen ecotones of 250 x 50 m were set in six tropical rainforest fragments. Each ecotone was divided into three habitats: matrix (delimited within the first 50 m of the pasture), forest edge (delimited within the first 20 m of forest) and forest interior (placed beyond the first 50 m of forest from the edge) (Picture 2). For each habitat we traced a line perpendicular to the forest edge and installed three permanent transects 50 m in length and parallel to the rainforest edge at each habitat. The whole study includes 18 ecotones containing 162 transects. On each transect I measured 12 environmental variables describing the microclimate, vegetation structure, topography and distance to forest edge and streams to evaluate the main correlation patterns among microhabitat variables and herpetofaunal species composition and diversity. Each transect was sampled six times (form June 2003 to April 2005) by two persons at day and night for a total effort of 1296 man hours (Picture 3, 4).
After six field seasons, 2835 individuals of 24 amphibian species (12 in pasture, 8 in secondary forest, 19 in forest edge and 20 in forest interior) and 1498 individuals of 42 reptile species (12 in pasture, 10 in secondary forest, 31 in forest edge and 32 in forest interior) were captured. I found that amphibian and reptile composition were different in pasture than in forest edge and interior. The surrounding pasture matrix was dominated by Lithobates berlandieri, Leptodactylus fragilis, Syrrhophus leprus and Sceloporus variabilis, while forest edge and interior were dominated by Craugastor rhodopis and Anolis uniformis. I detect two pasture species (Ameiva undulata and S. variabilis) that could invade the forest interior using the microhabitat created under the canopy gaps. I determined that Pseudoeurycea orchimelas, Ollotis valliceps (Picture 5), Craugastor vulcani (Picture 6), Imantodes cenchoa (Picture 7), Scincella cherriei and Plestiodon sumichrasti were truly forest interior species and their disappearance may be an indication of habitat degradation within a fragment, or that a fragment is not large enough to exclude edge effects. Towards the pasture the number of species of amphibians and reptiles with terrestrial habitats increased, as did the amphibians with reproductive mode of eggs and larvae in water. Towards the interior of the forest the number of species of reptiles with larger size and arboreal habitats increased, as did the amphibians with direct embryo development and with fosorial and arboreal habitats.
I founded that seasonality and habitat type (pasture, forest edge or forest interior) had a strong effect on reptile's abundance and composition, while seasonality, habitat type and edge orientation had a strong effect on amphibian's abundance and composition. During wet season the combination of canopy cover, temperature and leaf litter cover best explained amphibian and reptile pattern; during dry season the combination of distance to streams, temperature, leaf litter depth and leaf litter cover best explained amphibian and reptile pattern (Picture 8).
Determining the distribution of amphibians and reptiles across the pasture, forest edge and forest interior gradient and their relationship with microhabitat will allow for the creation of robust tools for conservation and management of the herpetofauna in highly fragmented tropical landscapes. This work provides robust data on natural history of Los Tuxtlas lowlands herpetofauna, complements the edge effect theory on tropical environments and propose concrete local management and conservation strategies for the amphibians and reptiles. I propose to maintain in the Los Tuxtlas Biosphere Reserve the forest remnants in the lowlands that have gentler slopes and a deep cover of leaf litter, a dense understorey, and high relative humidity and low temperature, to buffer the effects of edge related environmental changes and the invasion of species from the matrix. I suggest that high connectivity among habitats will facilitate the dispersal of edge species between forest patches and that habitat restoration projects must consider edge orientation and seasonal movements of species to ensure high habitat quality in degraded landscapes (Picture 9). Future research efforts most focus on the mechanisms that determine amphibian and reptile's distribution in rainforest interiors to better understand their seasonal fluctuations, population ecology and their threats to extinction.
For further information consult:
Urbina-Cardona, J.N., Olivares-Pérez, M. & Reynoso, V.H. 2006. Herpetofauna diversity and microenvironment correlates across the pasture-edge-interior gradient in tropical rainforest fragments in the region of Los Tuxtlas, Veracruz. Biological Conservation 132:61-75.
Urbina-Cardona, J.N. 2006. Tropical amphibian diversity and edge effect. FROGLOG 77: 1-2.
Urbina-Cardona, J.N & Reynoso, V.H. 2005. "Recambio de anfibios y reptiles en el gradiente potrero-borde-interior en la Reserva de Los Tuxtlas, Veracruz, México". Capítulo 15. En: Halffter, G., J. Soberón, P. Koleff & A. Melic (eds.) "Sobre Diversidad Biológica: El significado de las Diversidades Alfa, Beta y Gamma". CONABIO, SEA, DIVERSITAS & CONACyT. Volumen 4. Editorial Monografías Tercer Milenio, Zaragoza, España. Pp:191-207. (In Spanish)
J. Nicolás Urbina-Cardona
Zoology Museum, Sciences Faculty, Universidad Nacional Autónoma de México
Web page: http://nurbina77.googlepages.com/home