Habitat Fragmentation and Degradation

             In addition to wholesale habitat destruction, many habitats are being negatively affected by fragmentation.   Habitat fragmentation not only reduces the area of available habitat but also can isolate populations and increase edge effects. Habitat fragmentation may also make habitats more susceptible to other forms of habitat degradation such as overexploitation of valuable species, the introduction of exotic species, and contamination.  Understanding the possible consequences of habitat fragmentation has become of great concern to conservation biologists, since almost all natural habitats have become fragmented at some scale. 

Costa Rica represents a prime example of the need to understand the consequence of  habitat fragmentation since the majority of remaining natural habitat exists in isolated parks and reserves.  Are these parks large enough?  What type of species are most effected by habitat fragmentation?  Can knowledge about the influence of habitat fragmentation be used to improve the conservation of protected areas or mitigate its negative effects?

The rate of tropical deforestation exceeds fifteen million hectares annually. Human-induced deforestation is not a new phenomenon: since the beginning of an agrarian society, indigenous peoples have always harvested the forests to raise their crops.  The main difference between these ancient practices and current deforestation is the difference in scale and rate of increase.  In the past, indigenous peoples generally slashed and burned small patches which quickly grew back upon abandonment.  The result were patches of fields in a larger forested landscape.  Current trends are the opposite, in which we are left with remnant forest patches in a sea of severely altered and degraded landscape.  In this lecture, we will present some concepts of forest ecology, emphasizing the importance of these ecosystems on a local and planetary level.  We will then discuss how habitat fragmentation, and in general, degradation, can severely compromise the function of tropical landscapes, and inquire into the future of tropical landscapes in the face of continued fragmentation.

Tropical forests, especially rainforests, are one of the most productive and diverse ecosystems on the planet.  The high productivity is due to a  very tight coupling between production and decomposition: “turnover” rates are very high, and minerals are recycled quickly and efficiently.  Part of this is a result of high biodiversity: an incredible host of organisms have adapted to form a tight interconnecting loop coupling production and decomposition.  There are many mutualistic, symbiotic and parasitic relationships which take advantage of this energy flow (e.g. lianas, bromeliads, mycorhizae).  As a result, tropical forests are very productive despite typically having poor soils. 

This production is very beneficial globally insofar as it sequesters large amounts of carbon dioxide in the trees and soils of the forest. Production is very important on a local scale for the crops and medicines that humans can harvest from the forests.  Biodiversity is a related issue in terms of potential genetic material that can be used to generate disease-resistant crops, or cures for diseases.

When the larger forested ecosystem is destroyed, patches of remnant forest remain.  There are many issues related to the ecology of forest patches.  A very important concept is that of species-area curves, which simply states that, all other things being equal, a greater number of species can survive in a larger area.  However, nothing is equal.  Habitat range sizes are not constant throughout the plant and animal world.  Some species may be perfectly capable of surviving in a remnant forest patch –many others may not.  A forest patch is not the same as a piece of original forest: edge effects may now encroach, or even traverse the whole patch.  Weedy species may invade.   Some species’ populations may become separated, leading to inbreeding depression.  Whatever the combination of biotic and abiotic changes, the forest patches generally can no longer sustain the production or biodiversity that it once had as part of the larger forest.

What can we do?  Current efforts are aimed at somehow linking remnant forest patches such that genetic information can cross from one patch to another.  Animals that require larger home ranges can travel from one fragment to another while remaining within the natural forest matrix that it requires.  Biological corridors also help educate the public, giving them an ideal example for understanding the interconnectedness of organisms (including ourselves!) and their natural environment.

References

Laurance, W.F., Bierregaard, R.O. (1997) Tropical forest remnants: ecology management and conservation of fragmented communities.

Laurance, W.F. (1991)  Edge effects in tropical forest fragments – application of a model for the design of nature reserves.  Biological Conservation  57:205-219.

Laurance, W. F. (1998)  Tropical forest fragmentation and greenhouse gas emissions.  110: 173-180.

Turner I. M. (1996)  Species loss in fragments of tropical rain forest: a review of the evidence.  Journal of Applied Ecology 33: 200-209.

Didham, R. K. et al (1998)  Beetle species responses to tropical forest fragmentation.  Ecological Monographs  68: 295-323.

Estrada, A. et al (1999) Tropical rain forest fragmentation, Howler monkeys (Alouatta palliata), and Dung Beetles at Los Tuxtlas, Mexico.  American Journal of Primatology  48:253-262.

Kellman, M.R. et al (1998) Structure and function in two tropical gallery forest communities:  implications for forest conservation in fragmented systems.  Journal of applied Ecology 35:195-206.

Stevens, S. M., Husband, T.P.  (1998) The influence of edge on small mammals: evidence from the Brazilian Atlantic forest fragments.  Biological Conservation 85:1-8.  

For further reading:

Bierregaard, R. O. Jr., T.E. Lovejoy, V. Kapos, A.Augusto dos Santos and R.W. Hutchings.  1992.  The biological dynamics of tropical rainforest fragments.  Bioscience.  42(11): 859-866.

Murcia, C. 1995.  Edge effects in fragmented forests: implications for conservation.  Tree. 10(2): 58-62.

Noss, R.F. and B. Csuti. 1997.  Fragmentation.  Pgs. 269-304 In: G.K. Meffe and C.R. Carroll, (eds.).  Principles of Conservation Biology.  Sinauer Associates, Inc. Sunderland, Massachusetts.

Orions, G.H.  1997.  Global biodiversity I: patterns and processes.  Pgs. 87-122 In: G.K. Meffe and C.R. Carroll, (eds.).  Principles of Conservation Biology.  Sinauer Associates, Inc. Sunderland, Massachusetts.