Showing posts with label shade. Show all posts
Showing posts with label shade. Show all posts

Monday, November 29, 2010

Keeping Some of the Pieces, The Importance of Shade in Cacao Plantations

Cacao and coffee are shade crops that provide habitat for plants and animals dependent upon tropical forest. Unlike corn, they enhance biodiversity in agricultural landscapes locally but they may also have a more global role when they are cultivated in regions of high endemism suffering heavy deforestation. Sulawesi is one of those places that still conceals many undescribed species, but like many other places the forests are being logged and converted into human landscapes. Deforestation and subsequent land-use changes are rampant in the tropics and will eventually force the species that survive the upheaval to use altered habitats such as agro-ecosystems and urban areas that tend to be warmer, drier, brighter and less structurally complex than natural forests.
Southeast Asian amphibians and reptiles are among the most poorly studied and the most threatened vertebrates  (estimated at 30% and 31%, respectively based on IUCN 2008 data). Tropical amphibians and reptiles are highly sensitive to habitat modifications and climate change, making mitigating the effects of land-use change on herpetological diversity in Southeast Asia a conservation priority. Wanger et al. (2010) studied a land-use modification gradient ranging from primary forest, secondary forest, natural-shade cacao agro-forest, planted-shade cacao agro-forest, to open areas in central Sulawesi, Indonesia. They determined species richness, abundance, turnover, and community composition in all habitat types and related these to environmental correlates, including canopy cover and thickness of leaf litter. Human disturbances create environments that favor some species over others. Lizards and snakes for example, thermoregulate by basking in open patches of sun and it may be better to have several (or many) small openings in the canopy than just one large open patch. Therefore, many small openings in the canopy may be a better predictor for species richness and abundance of lizards. Frogs, on the other hand avoid direct exposure to the sun and a closed canopy may be a predictor of their diversity and abundance. Wanger and colleagues used Bayesian model selection to identify the best environmental predictors for amphibian and reptile species richness and abundance, including the lacunarity index (a measure of the degree of gaps) to measure canopy heterogeneity. Their results show that amphibians in Sulawesi were more negatively impacted by land-use changes than reptiles. Amphibian species richness and abundance declined as disturbance increased from pristine forest to open areas, while reptile species richness peaked in natural-shade cacao agro-forest between mildly (secondary forest) and strongly (planted-shade cacao agro-forest) disturbed habitats. Studies done in the Neotropics produced similar responses of amphibians and reptiles to disturbance in humid forests.  Thus conserving species of amphibians and reptiles in tropical environments may be best done long-term by allowing shade trees to rejuvenation on cacao plantations and allowing leaf litter to accumulate.

Wanger, T. C.,  D. T. Iskandar, I. Motzke, B. W. Brook, N. S. Sodhi, Y. Clough and T. Tscharntke. 2010. Effects of Land-Use Change on Community Composition of Tropical Amphibians and Reptiles in Sulawesi, Indonesia. Conservation Biology 24: 795–802.

Changing Vegetation & the Herpetofauna

Sceloporus undulatus
Managing forests (logging, reforestation, clearing brush, using herbicides, etc) can have unforeseen consequences for wildlife. As vegetation goes through succession it creates a variety of changing micro-environments that favor some species over others, so as the stages of succession change the animal populations change with them. This concept has been long known and its application to rattlesnakes was noted in Philip Tome’s 1845 book, Thirty Years a Hunter. Tome lived in northeastern Pennsylvania and described frequent encounters with Timber Rattlesnakes (Crotalus horridus).  Tome described the early setters burning forests to control rattlesnakes. Of course, in the long term this opened up the canopy and provided more basking sites. Jäggi and Baur (1999) linked the decline of Viper aspis populations to changes in vegetations, and other authors have made similar reports of changes in snake populations to changes in vegetation.

Kevin Shoemaker and James Gibbs of the State University of New York now report that the Eastern Massasauga (Sistrurus c. catenatus) persists as two isolated populations at the eastern edge of the species’ geographic range, and those two populations are threatened by the increasing density of woody plants. They found microhabitat temperatures were substantially lower at the closed-canopy site, where catenatus selected the warmest available basking sites.
At an open-canopy catenatus selected basking sites that afforded greater cryptic cover. And, they recommend the experimental reduction of shrub cover to improve basking habitat at the closed-canopy site. But they caution that management efforts to reduce shrub cover for basking should maintain adequate cryptic cover, (that is the grasses, leaf litter, rocks, and other cover) used by the snakes to conceal themselves.

In another paper, Charlotte Matthews and colleagues (2010) report that the recent use of prescribed fire and fire surrogates to reduce fuel hazards has spurred interest in their effects on wildlife. They conducted studies to determine the impact of “reducing fuel” in forests on amphibian and reptile populations in areas that were twice burned, had the understory cut mechanically, and in an area that had the understory cut mechanically followed by two burns. They trapped the herpetofauna using pitfall and funnel traps. The study was done at the Green River Game Land, Polk County, North Carolina. The results revealed that salamanders were reduced in number in the twice burned, mechanically opened habitats – possibly because this site had reduced leaf litter.  While these same habitats supported larger numbers of lizards and snakes, undoubtedly due to the increased number of basking sites and the newly created thermal gradients.

Jaggi, C., and B. Baur. 1999. Overgrowing forest as a possible cause for the local extinction of Vipera aspis in the northern Swiss Jura mountains. Amphibia-Reptilia 20:25–34.

Matthews, C. E., C. E. Moorman, C. H. Greenberg, and T. A. Waldrop. 2010. Response of Reptiles and Amphibians to Repeated Fuel Reduction Treatments. Journal of Wildlife Management 74(6):1301-1310.

Shoemaker, K. T. and J. P. Gibbs. 2010. Evaluating basking-habitat deficiency in the threatened Eastern Massasauga Rattlesnake. Journal of Wildlife Management 74(3):504-513.

Tome, P. 1854. (1989 reprint) Pioneer Life; or, Thirty Years a Hunter. Baltimore: Gateway Press.