A Threat To Wildlife And Bio-Diversity
Thesis Statement: The acceleration and diversification of human induced disturbances upon natural ecosystems during the past decades has contributed to wildlife habitat fragmentation. The changes in land use have driven wildlife managers to reconsider the benefits previously attributed to the Edge Effects on wildlife diversity
Habitat fragmentation has been recognized as a major threat
to the survival of natural populations and to the functioning of ecosystems. The reduction of large continuous habitats to small and isolated remnants affects the abundance and species composition of various Taxa. Some possible factors contributing to this decline include changes in food and cover availability, microclimatic effect, evolution of predation, loss of genetic variation, and lack of recolonization following local extinctions. Ultimately, habitat loss and fragmentation are processes that isolate small populations, which have higher extinction rates that may lead to a reduction in biological diversity. The acceleration of the land acreage consumption by human activities to the detriment of natural areas has revealed dramatic changes in the land uses during the past decades. Previously thought to be beneficial to wildlife habitat and diversity, the edge effects have been reevaluated by wildlife managers.
A. Wildlife habitat
In a formal sense, wildlife habitat can be defined as an area with the combination of resources such as food, cover, water, substrate, topography, temperature, precipitation, and security that promotes occupancy by individuals of a given species and allows them to survive and reproduce (Morrison et al., 1992). Usually, the quality and extent of an animal's habitat in large measure governs its ability to survive, and loss of habitat appears to be the greatest single threat to wildlife in the United States. The major factor in wildlife population decline in most areas around the world is the outright loss of habitat and/or the fragmentation of existing habitat into parcels too small or too isolated to support viable wildlife populations (Morrison et al., 1992).
B. Habitat fragmentation
The fragmentation of contiguous areas of homogenous habitat affect the habitat quality of wildlife in particular ways. By altering and modifying the vegetative cover, the quality and variety of the food base has changed. The consequent fluctuations of temperature and moisture levels have created changes in microclimates and in availability of cover. These profound disturbances of the ecosystem balance have brought species together that normally have little contact, and thus have increased rates of parasitism, competition, disease, and predation.
C. Ecotone, or Edge Effects
Usually an animal must rely on two or more plant communities to satisfy its essentials needs (cover, food, water and breeding sites); therefore, the contact zone or region where two different ecosystems (such as marsh and oak woods) come together, called an ecotone or edge, is as a general rule viewed as beneficial to both density and variety of any given species (Owen and Chirras. 1990). In fact, such edge zones usually show higher species diversity and productivity than areas away from them. In some cases, particular species are adapted specifically to edge habitat. According to Cox (1993), "Game management practices, geared to early or middle successional species, have often encouraged the increase of edge habitats by cutting of clearings or corridors in continuous stands of forest or brush land "(p.34). But from a conservation standpoint, extensive edge habitat may not always be desirable, because it often supports a high population of predators.
D. Loss of habitat
The effects of predation may extend well into the interior of fragmented woodlots
and cause regional extinction of forest interior species. In recent decades, as many once contiguous forests have been cut into ever-smaller pieces by timber harvests and changes in the land use. The increase of predation and parasitism on nest dwelling forest birds show a "higher decline in the abundance and richness of passerine birds associated with forest interiors" (Reese and Ratti. 1988). Increasing edge habitats can also increase the numbers of herbivores, such as white tailed deer, that may permeate and over-browse forest interior areas and deplete habitat suitability of fragmented forests for other species during breeding or nesting seasons. Edge effects are not only confined to forests but occur in all landscape transitions. Fragmented prairie patches also suffer reduced nesting success because adjacent woodlands shelter predators; not surprisingly, migratory herds cannot adapt to fragmentation and suffer similar fates (Sauer.1998)
II. Changes in the land use:
A. Rural and urban interface
The extensive development of housing projects at the margin and in the rural areas blocks or disturbs the movement of large herds throughout the matrix of the landscape. The residential type of landscape has favored some ornamental vegetative features more suitable for some species than for others. The changes in the use of the land have profoundly changed the rural and urban interface everywhere in the United States. For several decades the process of sub-urbanization has dramatically altered the landscape, reducing the acreage of natural areas to give the right of way to shopping centers and subdivisions. Simultaneously structural developments have occurred to provide these new centers of economic activities with adequate infrastructures of transportation, creating an increase of patchiness in the landscape. Each parking lot, road and "hardened access" represents far more than a physical barrier to the original migration routes of wildlife; they cause a destruction of habitat suitability and a profound disruption in the ecosystem cycles. The unbroken forest expanse has been replaced by small islands, each with little or no forest interior. The remnant of the original matrix virtually occurs within the fabric of developed land and has experienced, according to Sauer (1998), "rapid changes in environmental conditions, including major alterations in the hydrologic cycle; soil disturbance from vegetation clearance; increased erosion and trampling; and air and water pollution "(p.11).
B. Structural Developments
Therefore the amount of interior forest habitat and the species it supports is constantly diminishing. In many areas, forests and shrublands remain only as relic fragments of the natural habitat, surrounded by farmland and suburban residential areas. This transformation of continuous forest into scattered woodlots has a major impact on the forest flora and fauna. The microclimate of the forest is greatly modified, especially near the woodlot edge. Isolation from the other woodlots reduces the interconnection of forest species, and increases the likelihood of extinction of the remnant population. The structural features of these isolated patches make them unable to offer suitable and sustainable habitats to any given forest dwelling species. Therefore, when these species (already under stress) decline it is difficult for them to relocate and avoid extinction.
C. Forest Fragmentation: The Impacts
Finally fragmentation or loss of habitat due to suburban development also marks, as indicated by Cooksey (1997), "a change in land ownership from one owner to many as the land area is parceled. The results of this loss of contiguous forest and stable land ownership are potentially negative for a forest ecosystem's ability to protect water quality, to provide diverse and suitable habitat, and as a viable economic resource that provides recreation, forest products and amenities"(p.1).
III. The Edge Effects
A. Type of vegetative cover
As human populations continue to expand, fragmented areas increasingly dominate the landscape. One effect of the increasing fragmentation is the obvious increase of edge effects. For decades, the ecotone or border zone between distinct ecosystems has been regarded as a habitat generally beneficial to wildlife. The effects of the human disturbances (usually clear-cut harvesting) on the structure of the vegetative cover are dramatic to both horizontal and vertical arrangement. The composition and arrangement of plant species, their relative abundance at specific times of the year, and their structural features are so profoundly modified that the habitat suitability for a given specie may disappear. The natural successional patterns are disrupted and their degradation can lead to the elimination of entire communities. Effects of wildlife disturbance mirror those previously mentioned for vegetation: loss of habitat, changes in the structure and composition of the remaining habitat, changes in population proportions, and the introduction of disease and competing exotic species.
B. Changes in micro-habitats
Birds, reptiles, and amphibians are excellent indicators of environmental degradation due to edge effects. Some ongoing research tends to assess and "quantify the manner and extent to which forest fragmentation influences amphibian community structure of vernal pool ecosystems". According to Johnson (1999),"the stressors associated with increased fragmentation (e.g., decreasing canopy cover, altered hydrological patterns, increased isolation of habitat patches and increased UV radiation) will result in changes in the species composition and relative abundances of amphibian communities. Thus, woodland amphibians should provide robust ecosystem indicators of the effects of forest fragmentation that can be applied across geographic regions"(p.1).
C. Impact on wildlife habitat (cover, food, migration)
Many studies have shown that nests of birds in small woodlots, in the eastern United States, are subject to much heavier predation rate than those in larger tracts. Predators from adjacent open habitat, such as crows, skunks, jays and grackles are able to penetrate into the edge of a forest area, and in the case of small woodlots, the entire tract might become accessible to such species. Reese and Ratti (1998) noted that, "as a result the richness and abundance of bird guild decline much faster from large to small woodlots than from large to small areas within a continuous forest"(p.78). Similarly, researchers have observed a stronger decline of forest interior dwelling species, insectivorous, predatory, and migratory species, than for other species. Therefore, as large sections of continuous forest are broken into smaller tracts, birds, and other wildlife species experience a loss of habitat for nesting, breeding, and foraging as well as an increase of predatory risk.
D. New threats: endangered species, invasive, and exotics
The rapid growth of urban development into the coastal forest of British Colombia is making it difficult for the bald eagle to maintain a stable population. In this case, habitat fragmentation is the reduction and isolation of livable forest, which makes up the niche of the bald eagle. As deforestation destroys the mature forest, the eagle's nests are also destroyed, setting back their reproductive cycle. They will have to spend time and energy relocating to a suitable habitat and rebuilding their nest. This added stress may result in the reduction of eagle's brood size and delayed hatching, thus limiting chances of survival of the fledges. Overall, the results of recent studies provide evidence that increased forest fragmentation is associated with greater variability of the species richness of forest bird communities over time. Ultimately, habitat fragmentation commonly creates small islands of contiguous habitat surrounded by areas of unsuitable habitat that isolates population, limits the genetic interchange, and causes the localized extinction of some population. As noted by Sharpe and Hendee (1992), "the maintenance of the traditional migration corridors/routes providing sufficient visibility and escape terrain or cover, is critical to the survival of large, mobile wildlife population" such as bighorn sheep, elk, mule deer, and bears (p.243).
While the amount of forest interior habitat is being reduced, hundreds of new plant and animal species have been introduced to most of the American landscape, both intentionally and accidentally. Some of these so called invasive-exotics, such as Norway maple, kudzu, Japanese honey-suckle, and knotweed are spreading very aggressively in the absence of natural controls, and they profoundly disrupt ecosystems, which are already stressed. According to Sauer (1998), " many species of the native plants are listed of special concern, and 15% are endangered or threatened"(p.64). Similarly, diseases and pests introduced from abroad have found "unresisting hosts" in North America, using edges and contact zones as a sure right of way for penetration and propagation, and consequently are shifting the composition of the forest community (Sauer, 1998).
IV. Conservation of wildlife diversity
A. Challenges encountered by wildlife managers
Until recently, wildlife managers viewed the edges, or ecotone, resulting from fragmentation as a desirable wildlife habitat because of what appeared to be a high floral and faunal richness associated with edge environments. But in recent decades, foresters and land managers came to the conclusion that fragmentation leads to the degradation of global forest health. Because of the work of several researchers, it is now accepted that a higher incidence of exotics and invasive weeds are "colonizing" fragmented tracts already more vulnerable to insect and disease out-breaks or attacks. Therefore, it appears that the whole forest health and diversity, in terms of its ecological function, might be seriously compromised by an excessive fragmentation. The challenges encountered by land and wildlife managers are multiple and complex when looking at a more global or holistic management approach. Land managers need to look at the whole system to understand how fragmentation affects ecosystem health. Some key conservation strategies propose to "maintain the full range of successional and climax communities in an appropriate balance and distribution pattern", and to protect highly specialized species as well as their habitat (Cox, 1993).
B. A holistic approach: biological corridors
Given that extensive fragmentation of habitat now exists, there has been considerable interest in providing corridors for wildlife to travel between habitat patches, thus minimizing isolation. These routes hold the possibility of a physical connection between islands, helping to reduce extinction rates and increase colonization rates, therefore, preserving biodiversity. The biological corridors include various landscape features such as streamsides, riparian zone, shelterbelts, or fencerows and may be an increasing component of conservation plans. The primary ecological function of these linear patches is to increase population persistence by allowing "continued exchange of individuals between patches, by increasing gene flow, and by increasing recolonization rates of unoccupied patches (Rosenberg, 1997).
Conclusion: Preservation of the land and its attribute
The fragmented landscape, no matter how well managed, cannot functionally replace larger sites. Continuous forested tracts offer the greatest opportunities for the preservation of bio-diversity. Nevertheless, fragments are essential pieces of the landscape, serving not only as "recreational greenways", but also as refuges in densely populated areas, for migratory birds. With fragmentation we are not just losing animal species, we are losing plant species as well. Plants and animals have coevolved over millennia and plant reproduction is inextricably tied to wildlife. Where wildlife is impoverished, many plants have no means of effective reproduction and survival. In the decades to come, improving conditions for native wildlife and preserving bio-diversity in fragmented landscapes will bring some of the greatest challenges to land managers.
Cooksey, R. (1997). Forest fragmentation in the East: Does it impact resources and
economic health. Forestry notes, USDA Forest Services. http://www.nacdnet.org/forestrynotes/november97/fragment.htm
Cox, G. W. (1993). Conservation ecology: Biosphere & biosurvival. WCB, CA:
San Diego State University.
Johnson, L., Johnson, C., Boone, R., and Gross, J. (grant 1999-2002). The effects of
forest fragmentation on community structure and metapopulation dynamics of
amphibians. National Center for Environmental Research. EPA Grant # R827642
Morrison, M. L., Marcot, B. G. and Manon, R. W. (1992). Wildlife-habitat relationship.
University of Wisconsin Press, Madison.
Owen, O. S. and Chiras, D. D. (1990). Natural Resource Conservation: An ecological
approach. (5th ed.) Mac Millan Publishing Company, N.Y.
Reese, K. P. and Ratti, J. T. (1988). Edge Effects: A concept under scrutiny.
Trans. N. Am. Wildl Nat. Res conf 53.127-136.
Rosenberg, D.K. and Noon, B.R. (1997). Biological corridors: Form, function and
efficacy. Bioscience, 47, 677.
Sauer, L. J. & Andropogon Associates. (1998). The once and future forest: A guide
to forest restoration strategies, Island Press, Washington.D.C. / Covelo. Calf.
Sharpe, G. W. and Hendee, C. (1992). Introduction to forest and renewable resources,
(6th ed.). Mc Graw Hill INC. N.Y.