Interdisciplinary Minor in Global Sustainability
Senior Seminar
University of California, Irvine June
1997
With the continuously accelerating rate in the loss of wildlife habitat, there is a concern which greets the upcoming century. The problem with the presently remaining "untouched" wilderness is that large whole pieces of wild habitat end up manipulated to suit the needs of human populations. Often times mismanagement of land, besides the mere intrusion into a delicate habitat, exerts stress upon it’s state of equilibrium; so much so that certain species within an area become at risk for extinction. Depending on the stability and degree of interspecie dependency, the extinction of a couple of species of animals could lead to the gradual eventual degradation of that habitat. The focus of many leading conservation biologists within the last three to four decades has been on the study of different factors and dimensions that influence the extinction rate of different types of wild habitat. By understanding the factors and dimensions involved in the maintenance of habitat stability, conservationists may be able to more accurately explain how fragmentation effect specific types of habitat and population, and more accurately predict the effects of proposed conservation projects. Now as we enter a new era with frightening statistics on environmental destruction, conservationists are calling for quick action to slow down the rate of extinction and habitat fragmentation. The present popular proposed solution is for the construction of dispersal corridors, which will reconnect pieces of isolated habitat and reduce the rate of wildlife extinction. There is not enough available material to support this proposal but there is also not enough to prove it unworthy either. Although this is not the only solution to the problem, it is the most appealing to conservation biologists who desire to protect and reclaim wilderness quickly. Thus, ecological corridors is a critical topic of debate because it has become a popular concept taken very seriously by radical conservationists who are in a haste to implement the plan but who do not have sufficient data to prove that it might not counter their predictions and, as many skeptics fear, prove detrimental for habitat restoration.
In regions where habitat is fragmented by urbanization, cattle grazing, deforestation, etc., animals need a natural temporarily sustainable pathway for movement and migration in order to prevent either chances of inbreeding or overexploitation of prey. (1,3) Corridors act as a source of connectivity between two or more isolated habitat patches, making a natural landscape more confluent. (2) The connectivity between habitat islands is defined as the degree of animal movement and gene flow from one core area to another. (2) Keep in mind however, that both human erected and naturally occurring corridors do not benefit all resident wildlife, but only certain species, while affecting each specie differently. (3) For example, dispersal corridors promote interbreeding of individuals and groups, which is positive for species like lions, but not as positive for species of metapopulations. Interbreeding of distinct subpopulations in a metapopulation could lead to genetic homogeneity rather than increased variability. (1,3) However, metapopulations are dependent on some type of sustainable corridor to allow for surviving subpopulations to inhabit patches where extinct subpopulations have left behind. (1,3) Other species such as migrating birds can reach distant habitat islands independent of special dispersal corridors all together. (1) Notice the complicated factors and dimensions that are being dealt with in studying wildlife dynamics. This reveals how crucial it is to evaluate and reevaluate the affects of artificial implementation of corridors.
In designing a reserve with interconnecting corridors, it is essential to have as little edge affect as possible. The core area of one habitat, where human activity is prohibited to ensure maintenance of wildlife habitat, is connected to the core area of another habitat via a corridor that contains enough native resources to temporarily sustain migrating animals. (2) The entire strip of interconnected land is surrounded by a buffer zone where human activity increases by allowed degrees, but which still is able to support many species of wildlife. (2) The layer beyond the buffer zone is land primarily human oriented and only human tolerant wildlife species can be found. (2)
Theories that support the effectiveness of corridors deal a lot with connectivity dependent wildlife such as metapopulations. Theories that support the existence and necessity of habitat corridors include Richard Levin’s theory on rates of metapopulation extinction, McArthur and Wilson’s Island Biogeographical Theory (1967), and Aldo Leopold’s Law of Dispersion (1933, p 74). Richard Levin’s rates of metapopulation extinction theory states that if the recolonization rates match the extinction rates of subpopulations in a metapopulation, then the persistence of a metapopulation is possible. (1) However, the ability for a subpopulation to recolonize (‘rescue effect’) is dependent on how well the patches are connected. For example, a species of pool frog, Rana lessonae, which resides in ponds along the Baltic coast of Sweden, persists when pools are filled and die out when pools are dry. The pools become recolonized again when they are filled with water. Thus as long as these frogs migrate, the species remain at equilibrium. Should intruders drain or ditch these ponds, the viability of the species becomes threatened. ([Ecology 75: 1357-67 (1994)](1) This is a good example of the dependency a metapopulation has upon the quality of connectivity between patches of habitat. The other theories reassert the notion that connectivity is crucial to a metapopulation’s viability. By stressing the importance of connectivity to wildlife sustainability, conservationists have a foundation to argue in defense of implementation of corridors.
However, in one study by Tormod Vaaland Burkey at Princeton University, using simple laboratory communities consisting of three trophic levels (bacteria and protozoa), he found data that contradicted McArthur and Wilson’s Island Biogeographic Theory and the ‘rescue effect’ theory. His results showed the rate of extinction for different sizes of habitat fragments as follows:
1) Unfragmented habitat persisted longer than metapopulations with dispersal corridors between subpopulations.
2) Linked metapopulation went extinct sooner than isolated subpopulations.
3) Linked metapopulations>unlinked metapopulations>unfragmented habitat.
More fragmented>less fragmented> unfragmented.
Burkey mentioned three detrimental aspects of corridor introduction to wildlife habitats:
1) "Spread of disease is one reason why metapopulations may suffer greater extinction risk than isolated patches."
2) "The stream of dispensers between patches act to synchronize the population dynamics. With this increased degree of correlation between subpopulations, the extinction process for metapopulation as a whole becomes like a random process where one draws once from a distribution rather than four times from a set of identical distribution."
3) "Another reason why dispersal corridors may not do much to enhance the persistence of metapopulations is that the subpopulations may be highly correlated anyway. This could be due to similar initial conditions after fragmentation or to any important large-scale environmental variability.
If all populations are at low densities at the same time, or go extinct at approximately the same time, the "rescue effect" would be slight at best.
Burkey concluded that "One should not conclude on the basis of this experiment that habitat corridors between patches in fragmented landscapes are useless or even detrimental to species’ viability. However, these results give reason to caution against uncritically viewing such corridors as cure-alls against the ills of fragmented populations. The establishment of wildlife corridors should not be used as an excuse for continuing to fragment natural habitats, and corridors may even be detrimental in some cases."(1)
In terms of the pros and cons for implementation of ecological dispersal corridors, the cons seem to outweigh the pros. Though this concept seems to hold the most promise for management of biodiversity and recapturing land, it is extremely expensive to construct; costing around the millions. Conservationists are limited in their financial resources, thus an investment such as this would be a shot in the dark; it’s worth questionable. While corridors seem to be a quick and easy answer and a great compromise, not enough studies have been done in dealing with the dynamics of an entire habitat, which the corridor would directly affect, to ensure a significant degree of confidence of its success. Such a study would be extremely difficult. Lastly, regulations for management of corridors are not yet strict enough to ensure that these corridor projects are implemented and maintained correctly. This could allow for special interest groups to implement corridors for nonecological uses.
Here are short lists of supporters and skeptics of the effectiveness of corridors. (3)
1. Reed Noss-editor of Conservation Biology
2. Michael Soule-UC Santa Cruz
3. Paul Beier-North Arizona University
4. Michael Beck-Endangered Habitat league
5. Dave Foreman-founder of Earth First
6. Richard Leving-Harvard School of Public Health
7. Ilkka Hamski-University of Finland
8. Per Sjoren-Gulve Genetics Centre of Uppsala.
1. Daniel Simberloff-Florida State University
2. Dennis Murphy-Stanford University
3. Hartmuth Walter-UCLA
4. R.J. (Rocky) Gutierrez-Humboldt University
5. Ted Case-UCSD
6. Tormad Vaaland Burkey-Princeton University
7. Mark Plummer and Charles Mann
For further interests, look up Wildlands Project.
Burkey, Tormod Vaaland.
http://www.sum.uio.no/~tvburkey/sd/Extinction_of_metapop.html
http://mcn.net/~amwild/col/reserve.html
Plummer, Mark L. Mann, Charles C.
http://www.discovery.org/environment/ENVIRdbEngine.php3?id=228
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