Issues Concerning Rain Forest Fragmentation
Posted October 15, 1998
By Peter Rillero Ph.D
Arizona State University West
For a variety of reasons rain forests, usually recognized for their broad range of biological diversity, have gradually been vanishing from Earth taking with them
many plant and animal species. As areas of the rain forest are removed, small fragments of forest remain and are surrounded by farms and grazing areas. This fragmentation of rain forests
may be one factor in the decline of the biomeÕs biodiversity.
Decrease in Genetic Diversity
Studies conducted on rain forest fragmentation have gained much attention in the recent years. A disturbed rain forest may recover quickly, but the variety of species it contains may
be significantly less than the number found before it was disturbed, or in a non-disturbed forest. Genetic diversity decreases when fragmentation occurs because a fragmented area may
contain only 1-10 tree species. An uncut area may have between 300-400 species of trees. When the population of a certain tree species is reduced, the lack of abundant gene variation
between trees could possibly cause inbreeding. This adverse effect could promote the disappearance of an entire species due to the absence of genetic drift. For example, one specific
species, S. globulifera, is introduced to other sites by seed dispersal from bats. After bats eat the fruits found on this tree, the seeds are passed on through their guano during travel.
It is very unlikely that bats would visit a fragmented area and release seeds due to the areaÕs small size and limited number of trees. Seeds would neither be picked up nor dropped off,
thus halting genetic drift. This could cause the species survival rate to dwindle.
Effects of a Reduced Biomass
In a 17 year study of tropical rain forests in Brazil, rain forest fragments were compared to equally sized uncut rain forest areas. The researchers measured the diameter of over 56
000 trees and calculated the biomass for the fragmented area and for the unfragmented area. The study found that the biomass of the fragmented area can be up to 36 percent less than
same size plots of unfragmented forest areas. The study suggests that the lower biomass in the fragmented areas is caused by the death of trees. The researchers suspect that increased
wind and weather extremes create a more hostile environment for the trees.
Possible Increase in Levels of Greenhouse Gases
Another issue surrounding rain forest fragmentation deals with levels of gases in the atmosphere. Due to their small size, forest fragments may not be able to absorb as much carbon dioxide
as a non-disturbed forest. This could increase the level of greenhouse gases, creating unstable atmospheric temperatures.
Researchers are focusing more of their attention on the long-term consequences of rain forest fragmentation. Studies on the reduction of genetic diversity in tree
species may help explain why altered forests have a difficult time achieving stability after many years of recovery time. These studies may also help researchers gain a clearer understanding
of the effects of high levels of greenhouse gases on the planet. With the lack of adequate absorption made by fragmented forests and the passing of time, concerns may intensify. Awareness
of each of these issues surrounding the disruption of the rain forest ecosystem is vital in determining how critical each becomes with time.
Activity:
Investigate the social, economic, and political reasons why rainforest destruction takes place. Develop a multimedia presentation to communicate your findings.
Be sure to look at all sides of the issue to develop an unbiased report. If possible use Presentation Builder or Hyperstack software to make your presentation.
References
Laurance, W.F. et al. (1997). "Biomass collapse in Amazonian forest fragments." Science, 278, pp. 1117-1118.
Pye-Smith, C. (1998). "Ragged edge: Has a crucial factor been left out of the greenhouse equation?" New Scientist, 2148, p. 12.
Williams, N. (1997). "Rain forest fragments fare poorly." Science, 278, p. 1016.
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