Ecology and conservation of fragmented tropical forests

Panama City, Panama-Today the world’s tropical forests are not only being cleared at an extraordinary rate, they are also increasingly being divided into fragments that can rapidly lose their original rich biodiversity. At the 2002 meetings of the Association for Tropical Biology, hosted by the Smithsonian Tropical Research Institute (STRI), Panama, this conservation crisis was addressed in a symposium bringing together concerned investigators from throughout the tropics.

Organized by William Laurance of STRI and Pierre-Michel Forget of the Museum d’Histoire Naturelle, Paris, the session included 24 presentations that examined the causes, scope, and consequences of fragmentation in studies from 12 countries on five continents. The range of organisms included was equally broad, extending from mosses to forest trees, and from dung beetles to lemurs. The following are a sampling of the results reported at the meeting.

Tom Lovejoy, of H. John Heinz Center for Science, who initiated the first large scale experimental studies of tropical forest fragmentation in the Brazilian Amazon, the Biological Dynamics of Forest Fragments (BDFF) Project, presented an overview of what has been learned so far and suggested directions for the future. He pointed out that although species declines small fragments may be rapid, larger fragments of 100 hectares or more retain their diversity much longer, enabling us to take steps towards restoration before losses become irreversible.

William Laurance described the rapid changes in forest composition that take place in small forest fragments, based on data collected by the BDFF over the past 22 years. These changes are driven by greatly accelerated tree mortality near forest edges, causing species typical of old growth to decline and disturbance-tolerant species to increase. As a result, the composition of different fragments becomes more similar over time, causing a loss of tree diversity in the fragments taken as a whole even though the diversity decline in individual fragments is limited.

In contrast, data from a naturally fragmented forest in India showed that fragments serve as reservoirs for tree genetic diversity. Rajani Kanth of the University of Agricultural Sciences, Bangalore, presented work with collaborators B. Tambat, G. Ravikanth, U. Shankaar, and K.N. Ganeshaiah, demonstrating that although individual small fragments of shola forest in the Western Ghat mountains are low in genetic diversity, collectively they contain more diversity than large fragments, emphasizing the value of small fragments for conservation.

Jean Paul Metzger of the Universidade São Paolo, Brazil, reported on studies of the effects of fragment size and connectivity on species survival, community richness, and regeneration processes in the Atlantic forest of Brazil. Detailed surveys of trees, vertebrates, and butterflies showed fragmentation affected species richness and diversity for all study groups, and also affected important ecological processes, but some of these effects were unexpected. For example, tree communities in small isolated fragments turned out to be richer in than those in larger connected ones.

Surrounding land use can have important effects on species persistence in fragmented landscapes. Luis Miguel Renjifo of the Insituto Alexander von Humboldt, Colombia, compared the relative abundances of 113 species of birds in forest fragments surrounded by either pastures or exotic tree plantations, to those in plots surrounded by continuous forest. More than 65% of the species showed significant differences in abundance depending on the surrounding type of land use. Some species persisted better in patches surrounded by plantations than in those surrounded by pasture, suggesting that such a land use could be used as a management tool as a complement to habitat protection and restoration.

Future climate change due to global warming may have catastrophic effects on species restricted to fragmented habitats. Steve Williams of James Cook University, Australia, presented an analysis of the potential effects of climate change on the vertebrate fauna of the wet tropics of Australia, especially the 72 species endemic to the region. While a conservative scenario of a 1E C temperature rise would result in the direct loss of only one endemic, a perhaps more likely rise of 3.5E could cause the loss of 50% of the endemic fauna.