Clearing of tropical forests has created a highly modified landscape where remnant patches of native flora are set in a matrix of agricultural lands and urban-residential development (Turner 1996, Williams-Linera et al 1998, Holl 1999). Deforestation of tropical forests has been rapid and extensive; between 1981 and 1990, the total loss of natural tropical forests was 154 ha, cleared at a rate of nearly 1% per year (Whitmore 1997). The vast majority of clearing in Latin America is for agricultural purposes, including both grazing and cultivation (Holl 1999). Local extinction of species in the tropics is directly related to forest clearing, which decreases total habitat area (Dale et al 1994). However, deforestation, consequently changing landscape pattern, also negatively impacts biodiversity. Fragmentation causes remnant vegetation patches to be situated in different positions in the landscape mosaic, varying in size, shape, isolation (Sunders et al. 1990), and time since excision from the continuous forest (Turner 1996). These modifying factors, in turn, indirectly influence the biodiversity of the forest patch, "and in a complex manner, the biodiversity of the collection of fragments that occupies the landscape" (Turner 1996, p. 201).
The physical effects induced by fragmentation that directly influence biodiversity include edge effects (changes in microclimate and wind damage) and isolation (habitat discontinuity and increasing time since isolation) (Godron and Foreman 1986). A better understanding of landscape fragmentation and how these physical effects impact ecological processes may provide insight into the appropriate management regimes that could control for these effects, and in turn promote the protection of tropical biota (Lamb 1997).
Effects of Tropical Forest Fragmentation
One of the most noticeable characteristics of a fragmented landscape is the significant increase in the forest edge to interior. "Edge to interior" refers to the relative amount of forest border interacting with anthropogenic clearing to the amount of forest that composes the fragment (Forman 1995). The edge of a forest patch provides a different environment than the tropical forest interior, typically having greater light availability and higher temperatures (Kapos et al 1997, Saunders et al. 1991, turner 1996, Turton 1997). Therefore, it encourages a different species composition (Kapos et al. 1997). The altered microclimate of the edge has been found to be unsuitable for some species, while promoting an increase abundance of others (Turner 1996). Lovejoy et al (1986) accredits changes in butterfly community composition in tropical forest fragments partially to the increased isolation within small forest patches. The microclimatic difference between the forest's edge and its interior limits their habitat, and resource availability, leading to local extinction of certain butterfly populations (Lovejoy et al.1986). The microclimate of forest edges, however, provides appropriate habitat for disturbance-associated species. Studies in other biomes have found that edges create "windows for invasion" for ecosystem-altering, non-native species (Brothers and Spingarn 1992).
The magnitude of microclimatic edge effects on biodiversity are strongly dependent upon the size and shape of the forest patch (Kopos 1997); "the relative importance of edges increases as fragment size decreases, and edge effects may become highly influential" (Turner 1996, 204). Small or thin fragments may experience microclimatic shifts throughout the entire patch, encouraging the presence of species better adapted to the new environment. Along with the change in microclimate, these newly established species, by altering resource availability, can potentially cause local extinction of other species within the patch.
The elimination of surrounding forest vegetation and the increase in edge causes forest patches to become vulnerable to hot, dry tropical winds. These winds damage the vegetation, creating tree falls on the edge and tree fall gaps in the forest interior (Lovejoy et al 1986). Laurence et al. (1998), in a study examining fragmentation effects on Amazonian tree communities, contend that "a sudden increase in gap-phase vegetation could help drive local extinction of disturbance-sensitive species in fragments." By opening up the forest floor to more light and as a result to higher temperatures, tree fall gaps can prevent the regeneration of shade-tolerant, moisture-adapted species, and instead promote the establishment of shade-intolerant species that do not require moist soils. Species in small or thin fragments, with higher edge to interior ratios, are particularly vulnerable to the effects of wind damage because wind has the potential to penetrate deep into a forest (Laurence 1998). Due to this, small fragments will typically have a higher proportion of their area in gaps (Laurence 1998).
Another physical effect induced by forest fragmentation is isolation of the fragment. Isolation effects on biodiversity can be examined by the degree of connectivity among patches and the time since isolation of an individual forest fragment (Saunders 1991). Many tropical fauna, such as primates, require large territories of native vegetation for their survival, isolation of small forest fragments therefore impact the survival of these species. Depending on the position of the fragment within the landscape mosaic, there can be a reduction and/ or prevention of immigration of fauna between patches, limiting colonization of species in other forest patches (Turner 1996). Studies of tropical forests have shown that many forest species will not cross even relatively small deforested zones (Dale et al. 1994). The persistence of these species in isolated patches strongly depends upon the retention of enough suitable habitat to support the local populations (Saunders 1991). Decreased movement of fauna across a landscape can limit nutrient exchange and transportation of seeds to other forest patches (Saunders 1991). Without the movement of animals from patch to patch and consequent decrease in seed dispersal, both tropical flora and fauna biodiversity are reduced at the patch and landscape level.
Time since forest patch formation is another factor of isolation that decreases the biodiversity of a forest fragment. "Upon isolation, a remnant is likely to have more species than it will be capable of maintaining, and species will be lost as the changes brought about by fragmentation take effect" (Saunders 1991, 22). As edge effects and habitat begins to change with shifts in the microclimate and with increased canopy gaps, species richness will begin to decline (Suanders 1991). An Amazonian rainforest study that examined the relation between changes in fragment avifauna and time since isolation found that four bird species showed significant declines over seven years of isolation (Bierregaard and Stouffer 1997). Extinction proneness of a particular organism also may be an influencing factor in local extinction, as some plants and animals are more resilient to the effects of fragmentation than others. For example, the study by Bierregaard and Stouffer (1997) while showing significant decline in the four bird populations showed no decrease in hummingbird populations over the seven year time period.
Conservation in the Modified Tropical Landscape
In order to protect the biodiversity of the modified tropical landscape, fragmentation effects must be recognized and controlled for. In order to do this, the ecological value of individual forest fragments should be considered and efforts should focus on the protection of flora and fauna within these individual patches. Often this is accomplished through the creation of forest reserves where the fragmentation effects are managed on a continual basis (Wiens 1994). A landscape approach to management, considering the external influences created by the surrounding matrix on the native vegetation, is taken in order to control for these effects (Saunders 1991).
Acquiring a representation of community types or ecosystems through a network of reserved fragments may potentially increase the likelihood of preservation of biodiversity at the landscape level. However, the surrounding matrix lands should not be overlooked as a contributor to this effort (Holl 1999). Restoration and recovery of lands that have been removed form agricultural could connect forest patches, potentially reducing edge effects and isolation, allowing increased habitat for fauna and improved seed dispersal.
A review of the current literature on tropical landscapes suggests that still more research on tropical fragmentation and its impact on ecological processes needs to be implemented in order to create the best conservation strategies that ensure the protection of tropical biota. The accomplishments in research so far, however, provide land managers with the foundation for effective land management plans that control for some of the consequences of fragmentation such as edge effects and isolation.
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Brothers, T.S. and A. Spingarn. 1992. Forest fragmentation and alien plant invasion of central Indiana old-growth forests. Conservation Biology 6: 91-100
Dale, V.H., S.M. Pearson, H.L. Offerman, and R.V. O'Neill. 1994. Relating patterns of land-use change to faunal biodiversity in the Central Amazon. Conservation Biology 8 (4): 1027-1036
Forman, R.T. 1995. Land mosaics: the ecology of landscapes and regions. Cambridge University Press, New York.
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