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Habitat loss and fragmentation has been found to be a major factor in the decline of population of wildlife biodiversity throughout the world Harris 1984. This fragmentation is caused by environmental (climate) change, natural disaster (wildfires and floods) and increasing human development, (construction of new roads, housing development, mining activities, deforestation etc).
Isolation is also a major concern in conservation because the Species in fragmented landscapes are affected by the degree to which they are isolated in the fragments. The degree of isolation depends not only on the distance to other patches, but also on the land cover surrounding the isolated patch.
There is a growing interest in conservation and landscape planning to address the problem of habitat fragmentation and to preserve and enhance biodiversity.
Increasing the connectivity of the habitats is frequently proposed as an effective strategy to address biodiversity decline within fragmented habitats which will facilitate the easy movement of species within the habitats to find suitable climatic condition for survival or for escape in times of danger etc.
Increasing the connectivity is made possible through the creation of corridors, defined as "Avenues along which wide ranging animals can travel, plants can propagate, genetic interchange can occur, population can move in response to environmental changes and natural disasters, and threatened species can be replenished from other areas." Ninth U.S. Circuit Court Of Appeals (1990).
Numerous researchers working with corridors have noted that lack of a clear and consistent terminology, leads to confusion about the role of corridor.(Simberloff et al 1992; Benneth 1999)
If corridors are not designed to perform well defined function, the outcome may be very disappointing. The role corridor plays are derived from six ecological functions: habitat, conduit, filter, barrier, source and sink. These ecological functions have been recognized widely and adopted by different disciplines including conservation biology, wildlife management, landscape ecology and landscape planning. (Hess et al 2000)
Wildlife corridors have been identified as the solution that can be used to overcome most of the problems of fragmentation by providing linkages between the isolated habitats patches (Noss, 1993).
Corridors play a very important role in wildlife conservation and help in increasing biodiversity, through colonization (by making ways for the animals to move and colonize new areas conducive for their survival), Migration (making it easy for animals to relocate safely seasonally in search of better habitat) and Interbreeding (animals can find new mates in the new habitat so that genetic diversity can increase).
Corridors are made either on land or in water (streams and rivers) and are divided into 3 (three) categories, (Regional, Sub-regional and local) according to their width. The widths, length, design and quality is important in creating a perfect corridor. (Fleury 1997). Study has also shown that successful corridors are related to corridor attributes (the definition of the critical corridor attributes the identification of target species and a biophysical inventory of the landscape in question) and species utilising the corridor. (Bennett 1990)
In identifying and assessing wildlife habitat corridor, we have to take into consideration the type of specie the corridor is intended for. Some species are much more apt to use habitat corridors than others, depending on their migration and mating pattern. Birds and butterflies use corridors more successfully than mammals like bears. The effectiveness of a corridor depends on what specie it is meant for. (Tewskbury et al.2002). Some species are Corridor passage users (use corridors for seasonal migration), species like, large herbivores and medium carnivores while some are Corridor dwellers, (they occupy the corridor for days, months or years) species like reptiles, amphibians, birds, insects and small mammals. (Beier and Loe 1992). Whatever the specie type, corridors should be wide enough for it to be safe and for the species to be encouraged to use it. It should have everything (like soil, green grass, burrowing area etc) the species needs for survival on the journey.
The ability of the animals to move from one place to another through corridor is the conduit function of a corridor. A lot of issues have been raised in the context of conduit/ habitat, while defining the function of corridors. Some researches (Benneth et al, 1994, Downes et al, 1997) focused on habitat function assuming that if a corridor provide suitable habitat it will also enhance dispersal, while other research (Loney and Hobbs 1991, Beier and Noss, 1998) focuses on the conduit function and exclude corridors that function as habitat but without connectivity. Some of these arguments results from the confusion about the meaning and the use of the word "habitat". Habitat is defined as an area with appropriate resources like food, water, cover and good environmental condition for the survival and reproduction of species.
Corridor function could range from providing only passages to providing habitat and passage. Noss (1993) listed out the two major functions of wildlife corridors as dwelling habitats for plants and animals and as conduits for movement, while Rosenberg et al (1995) separated the habitat and conduit functions of corridors and distinguished the two based on quantitative approach. A corridor that provides for movement between patches but not necessarily reproduction is performing a conduit function while a corridor that provides resources for survival, reproduction and movement is performing a habitat function.
Barriers and filters separate and differentiate the areas opposite the corridors. (Forman, 1995). Barriers are the blockages which do not allow wildlife connectivity between the patches/habitats. Roads are examples of barriers, though they are conduits for human, they are barriers to wildlife movement. Efforts are underway to reduce the effects of roads (barrier) on wildlife movement, by creating tunnels, underpasses and bridges to serve as conduits through the roads.(Benneth et al 1995)
Filter on the other hand allows some degree of permeability and it is commonly associated with riparian zones and water quality issues. Corridors can also filter out certain species moving along them. (Forman and Gordon, 1986).This function is mostly associated with continental scale and should be considered when designing landscape linkages for continental connectivity.
WILDLIFE HABITAT CORRIDOR STUDY WORLDWIDE.
Wild life corridors have been a subject of considerable discussion amongst biologist and conservationist for a long time, with different school of thoughts arguing their advantages and disadvantages. However studies in the past few years has indicated that wildlife corridor is a critical conservation tool that can help minimize genetic isolation, offset fragmentation problems, improve animal dispersal, restore ecological processes and reduce human animal conflict.
In many observational studies at the landscape scale, a positive relation is found between the probability of colonization of a suitable habitat patch and the density of landscape elements considered to function as corridors (e.g., Pahl et al. 1998: Vos and Stumpel 1996; Bright et al 1994). Verboom et al 1990, in their study of red squirrel found that the probability of a suitable habitat patch depended on the number of hedgerows surrounding a woodlot within 200-600m. In another study by Grashof-Bokdam 1997, the probability of occurrence of holly appeared to be higher with an increasing number of hedgerows within a range of 1000m in an agricultural landscape. Other fragmentation factors and habitat quality were accounted for in both studies.
In other studies by Dmowski et al (1990) and Dunning et al. (1995), they found out that patches that are actually connected by corridors had a higher colonization probability than in patches that were not connected. Hass (1995) in his study of American Robin found that the average number of dispersal events between pairs patches connected by corridors was 2.50 but only 0.17 between unconnected patches.
Machtans et al (1996) and Desrochers et al (1997) studied the use of linear forest strips between forest birds before and after harvesting of adjacent forest. The use of strip increased and the movement rates through the forest was significantly lower, indicating the use of these strips as dispersal corridors.
Haddad (1999) in his study with two butterfly species, showed that corridor increase interpatch movement rates.
Species presence in corridor does not provide evidence that that corridors function as dispersal route between habitat patches. Most studies on the use of corridors are based on measurement of animal movement by radiotelemetry and direct observations over mark-recapture techniques. (Bright and Morris 1991).
Wauters et al 1994; Beir 1995 and Bennett 1999, gave more convincing evidence of use of corridor for dispersal from observation of movements through corridors from one habitat to another, in combination with avoidance of surrounding landscape types.
###Modelling approaches to predict corridor effectiveness.###
Dispersal models give the effectiveness of corridors and they require spatially realistic models to investigate the effectiveness of the corridor. These spatial models are based on information provided by Geographical Information System, commonly presented in a grid (raster) and vector format.
Van Dorp et al (1997) used a grid-based model to investigate the efficacy of linear landscape elements as corridors for perennial grassland species with short-range seed dispersal. They concluded that linear elements were not effective because estimated dispersal rates were low.
Johnson et al (1992) used simulation method in combination with empirical observation to quantify the importance of corridor continuity and to estimate the distance dependant dispersal mortality for two forest birds. They found that one of the species relied much more on corridor continuity than did the other species.
In their own study, Grashof et al (1997) developed a vector based colonization model for forest plants to evaluate the effects of alternative landscape configuration on colonization success of secondary forest habitat and found that colonization improved by adding corridors to the landscape.
Tischendorf et al (1997) used a CRW (correlation random walk)-movement model to investigate the impact of corridor width and movement attributes on the probability of successful corridor passage and found that with stronger movement autocorrelation, individuals covered longer distances within corridors.
Singleton et al (2002) carried out a study on how to evaluate regional-scale large carnivore (Wolves, wolverine, lynx and grizzly bears) habitat connectivity in Washington using GIS weighted- distance and least cost corridor analysis. In their study, they developed dispersal habitat suitability models for the four focal species based on literature review and then developed a large carnivore model, based on the parameters identified for the species-specific models, to provide a single generalization of the landscape patterns identified by the species-specific models. They compiled the GIS data sets representing landcover class, roads, highways, human population density and topographic characteristics. They based their analysis on the idea that resistance to movement can be mapped by assigning each cell in a map a relative weighted distance or cost of moving across that cell, where the cell cost is determined by the characteristic of the habitat. Cells with good characteristics (forested landcover, low road density, less human activity) have low movement cost while cells poor characteristics (high human population density, high road density and agricultural cover) have high movement cost. Their analysis produced an explicit map of estimated landscape permeability and expected linkages, created from a consistent analysis across the region. The study provides an important first step for regional conservation planning, however the study has some limitation associated with the scale and accuracy of the base data and the hypothetical, untested nature of the landscape permeability model.
Walker et al (1997) delineated best corridors routes in Northern Rockies using ARC/GRID and Montana gap analysis (relating to vegetation cover), by deriving habitat model for the species and combining it with road density data to create kilometer-scale cost surface of movement. They performed a least cost path analysis to locate broad potential corridor routes for wildlife. Though the method has advantage in terms of ease of computation and interpretation, it also has disadvantage, because delineating a least cost path could be much more computationally demanding due to the number of possible cell-path combination in a large region and sometimes the least cost path may not be the least resistance path.
Clevenger et al (2002) used GIS-Generated, expert-based models to identify linkage across a major transportation corridor. The model was based on empirical habitat data and expert information (opinion and literature based) developed in a multicriteria decision making process. The empirical model was used as a yardstick to measure the accuracy of the expert based model. They validated the performance of the models with an independent data set. The test showed that the expert literature model was consistently more similar to the empirical model than the expert opinion based models. The expert based technique advantage is that an assortment of GIs tools designed for model building purposes are readily available today and easy to use, however the empirical method has some short coming in that the model predicted annual habitat selection and did not take seasonality into account. This model differs from the ones mentioned above, because they are spatially explicit and in local scale and not in regional or large scale.