As the human population continues to increase exponentially it becomes our responsibility to determine how our actions and mere presence affects the surrounding environment. Habitat fragmentation quickly is becoming one of most apparent and destructive manifestations of human ecological disturbance. Habitat fragmentation can be defined as the fragmentation of a previously continuous habitat (Kelt,2000) as well as a shift in landscape composition from late successional to early successional communities. Succession is the gradual change in community structure and abiotic factors over time. This process is important for the successful colonization and survivorship of all the organisms in the ecosystem. The stage of late succession is characterized by stable, and often interdependent, species relationships. Early successional stages on the other hand are subject to constant change and are characterized by short life spans and high reproductive output of it’s inhabitants. Therefore, the forest remnants, created by fragmentation, often show changes in the behavior, distribution, and number of indigenous species, as well as changes in abiotic factors. Consistent results have been presented by researchers who conclude that fragmentation generally leads to the extinction of a species since communities in these disrupted areas relax to a diversity that can be supported by the decreased environmental area (Kelt, 2000). The break in continuity also increases susceptibility to invasion by exotic or foreign species.
Forest remnants most often occur in areas that were formerly uninhabited and then became exploited by land developers. This trend is seen in Central and South American countries that are economically dependent on farming. In order to limit deforestation Brazil passed a law that required each landowner to maintain at least 50 % of the land as a forest. While this law has good intentions it results in the creation of isolated tracts of land (Kricher, 347). The past 40 years has also seen an enormous increase in logging companies in Central and South America. Farmers in these countries began selling off their land when crops failed and livestock couldn’t be sustained. Logging companies not only wipe out large areas of rainforests, they also disturb and divide the surrounding ecosystems with the roads that are constructed, and the pollution that is spewed into the air or dumped into the water supplies. Types of fragmentation can range in scale from progressive subdividing, which results in scattered patches, to widespread clearing (Kelt, 2000). Once created the fragments are then further exploited as local residents strip the trees for firewood and use the remnants for grazing areas. These examples further disrupt the environment and lessen the chance that the native species will be able to persist.
The Theory of Island Biogeography, proposed by MacArthur and Wilson, has long been an important part of conservation biology. This theory outlines the relationship between ecosystem area and species richness & abundance. It states that more species are sustained in large habitats that are close together, or connected, and are shaped so that there is more interior area and less edge. The Theory of Island Biogeography is used to predict, quantitatively, the number of species that will likely be found in a given area. It has been seen, on islands, that the number of species increases logarithmically with area (Gascon et al., 1998). A major challenged faced by wildlife biologists is understanding how fauna will respond to habitat fragmentation. Based on these previous findings it was therefore hypothesized that the level of species diversity in forest fragments would follow many of the same predictions made by MacArthur and Wilson, since habitat remnants are in effect similar to islands (Newell, 1999). In many cases the Theory of Island Biogeography held strong. Douglas Kelt, in his studies of fragmented rainforests in Chile and Argentina, found that species richness in these remnants was uniformly low. When the data was plotted these areas also displayed a flat species-area relationship (i.e. slope of near zero), which is much like the relationship found on a small island. Similar results have been found in studies involving many bird species and primates. Various researchers have shown that virtually all taxa are sensitive to area and distance effects (Kircher, 348, Newell, 1999). The result is often a noticeable decrease in biodiversity, though there are exceptions. Some conservation biologists have proposed that the Theory of Island Biogeography has not lived up to it’s initial promise (Gascon, 1998). The reason is that the theory doesn’t address the influences of edge effects and the surrounding matrix, both of which greatly effect forest remnant and their inhabitants. Some predominant edge effects are tree mortality and general loss of biomass, as well as human manipulation. “Many tropical rainforests are physiologically and morphologically specialized for growing in large forest tracts in which environmental changes are buffered by the dense canopy cover and closely spaced neighboring trees. When forests are fragmented these protective barriers break down and tree mortality occurs” (Laurence, 1998). The surrounding matrix becomes a foreign environment which can effect forest dynamics and the survivorship of the native organisms. Since the area of the ecosystem has decreased some organisms are forced to dwell on the edge. Unless they are able to adapt, it is inevitable that they will begin to die out. Eventually the regional species pool, which is necessary for recolonization to occur, will dry up and that species will go extinct. There are some interesting deviations though. Some taxonomic groups actually show an increase in species richness compared to their richness before isolation (Gascon, 1998). Small, non flying mammals and some amphibians have shown an increase in reproductive fitness and number of species in fragmented forests (Kircher, 348, Gascon, 1998).
One of the most amazing aspects of the rainforest is not only the amazing amount of biodiversity but the interdependence that is seen between the many species. These plants and animals have coevolved over the years and now maximize their survivorship through symbiotic relationships. These relationships result in certain organisms taking on roles as keystone species. A keystone species is a species that has a disproportionate effect on food web interactions therefore their presence, or lack there of, effects the success of multiple trophic levels. One example of a keystone species is the fig tree (Ficus spp.) found in the tropical rainforests of China. The trees form a symbiotic relationship with fig wasps in which the fig trees benefit by increased flowering and seed dispersal while the wasps are dependent upon the trees as sites for larvae maturation (Yang, 1999). Da-rong Yang et al. discovered that fragmentation occurring in these tropical rainforests directly affected the biodiversity and survivorship of the fig trees. The decrease in ecosystem size, and the resulting fragmentation effects, decreased the Ficus population and researchers discovered a dramatic decrease in the number of fig wasps in the remnant. This study is simply another example of the many ways in which the well developed natural order of the rainforests are disrupted by habitat fragmentation, leading to decreased population fitness and the possibility of extinction.
Abiotic factors, which in turn impact biotic factors, are also effected by tropical rainforest fragmentation. Forest remnants in the Amzonian basin, when tested, showed an increase in greenhouse gas emissions versus similar sections of rainforest that were left in their natural, continuos state (Laurence, 1998). This change not only increases the effects of global warming it also decreases the output of one of the most productive ecosystems on the planet. No terrestrial ecosystem accomplishes more photosynthesis than the rainforests (Kircher, 45). The increase in carbon dioxide (the most prevalent greenhouse gas) emission combined with a decrease in oxygen production affects the normal atmospheric gas ratios and also decreases the availability of oxygen to the animals and plants in the fragmented ecosystem, further perpetuating the cycle. The greenhouse effect can also be localized in that area thereby raising the environmental temperature and further disrupting the environmental equilibrium of the now fragmented rainforest (Laurence, 1998).
Statisticians predict that the world population will double in the next 41 years. With 12 million people inhabiting the Earth it will be more important than ever to find a balance between sustaining the human race and conserving the environment we exploit. Forest fragmentation is a serious problem that is becoming an inevitable part of our expansion. Though it may be difficult to stop entirely there are ways to minimize the disruption that fragmentation has on these ecosystems. Researchers have discovered that the spatial pattern of clearing can have a major impact of fragmentation effects. “Clearing patterns that minimize fragmentation effects, such as the quantity of habitat edges and isolated patches, are to be strongly preferred over those that do not” (Laurence, 1998).
Certain types of clearing or cutting patterns have also been seen to stimulate succession in the disrupted areas (Kircher, 346). Conserving valuable environmental resources, like the rainforests, is not only our responsibility it is also necessary for our continued existence on this planet.
References:
Kelt, D. 2000. Small mammal communities in rainforest fragments in Chile.
Biological Conservation. 93(3): 345-358.
Kircher , John. (1997). A Neotropical Companion. New Jersey, Princeton University Press.
Laurence, W. 1998. Tropical rainforest fragmentation and greenhouse gas emissions.
Forest Ecology and Management. 110: 173-180.
Gascon, C.L. 1998. Ecological impacts of forest fragmentation in central Amazonia.
Zoology (Jena). 101(4): 273-280.
Newell, G.R. 1999. Responses of the tree kangaroo to loss of habitat within a tropical
rainforest fragment. Biological Conservation. 91: 181-189.
Yang, D. & Li, C. 1999. The effects of fragmenting of tropical rainforest on the species
structure of fig wasps and fig trees. Zoological Research. 20(2): 126-130.
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