Habitat loss, fragmentation and isolation have been found to be major factors in the decline of population of wildlife (biodiversity) throughout the world (Harris 1984). Wildlife is defined by the 1969 endangered species act as any living species, from invertebrates to vertebrate, butterflies to bears, while wildlife habitat is defined as the physical and biological features on the range in which species exist. Fragmentation is caused by environmental (climate) change, natural disaster (wildfires, floods) and increasing human development (construction of new roads, housing development, mining activities, deforestation etc). Isolation which is often linked with fragmentation in the issue of biodiversity is 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. Isolated populations have a higher probability of inbreeding as genetic exchange between them decreases which can lead to extirpation or local extinction. (Forman et al., 2003)
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Connectivity plays a very important role in biodiversity. Increasing the connectivity of the habitats have been found to be effective in addressing biodiversity decline within fragmented habitats which will facilitate the ease of movement of species within the habitats to find suitable climatic condition for survival or for escape in times of danger etc. Enhancing connectivity is made possible through the creation of corridors, defined by Forman and Godron (1986), as â€œnarrow strips of land which differ from the matrix on either sideâ€Â and by the Ninth U.S. Circuit Court Of Appeals (1990), as â€œAvenues 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 areasâ€Â. Some conservation biologist limited the definition of corridors to conduits that allow passage of only one native species. (Noss 1991)
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.
Corridors 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 may be isolated strips but they are usually attached to a patch of similar vegetation.
FUNCTIONS OF A CORRIDOR
Corridors are defined based on their structure or their function. Corridor function could range from providing only passages to providing the 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.
Forman (1995) identified 6 major functions of corridors as follows:
1. CONDUIT: The ability of the animals to move from one place to another through corridor is the conduit function of a corridor. Conduits provide for movement and survivorship but do not necessarily provide reproduction of specie between other habitat patches but increases actively increases landscape connectivity for the focal organism.
Fig 1a: Conduit
2. HABITAT: Defined as an area with suitable resources like food, water, cover and good environmental condition for the survival and reproduction of species. Habitat provides either temporary or permanent habitation. Habitat passively increases landscape connectivity between focal organisms.
Fig 1b. Habitat.
3. BARRIER: Barriers are the blockages which do not allow wildlife connectivity between the patches/habitats. Organisms and materials cannot pass through or cross the corridor. Roads are examples of barriers, though they are conduits for human, they are barriers to wildlife movement.
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Fig 1c. Barrier
4. FILTERS: Prohibits or impedes the flow of energy or species across. It 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.
Fig 1d. Filter.
5. SOURCE : Organism and material originate from the corridor. It is a habitat in which Local reproduction exceeds mortality.
Fig 1e. Source.
6. SINK: Organism and materials that enter the corridor are destroyed. It is a habitat in which mortality exceeds reproduction.
Fig 1f. Sink.
These last two functions (Source and Sink) are described less frequently than the first four functions.
CHARACTERISTICS OF CORRIDOR
Corridors are made either on land or in water (streams and rivers) and are divided into 3 (three) categories according to their width; Regional, Sub-regional and local. Regional corridors are wide (>500m) landscape connections between large important areas of habitat. Sub-regional corridors are large (>300m) corridors that provide connectivity for a wide range of species habitat, while Local corridors are smaller (<50m), less defined linkages that provide local connection of remnant patches of vegetation and landscape features. The widths, length, design and quality are 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 utilizing the corridor. (Bennett 1990)
Barrett and Bohlen (1991) classified corridors into Five (5) classes based on their origin. (Planted, disturbed, regenerated, environmental resource and remnant). The suitability of a land as a wildlife corridor is determined by many factors like its length, the topography and vegetation, adjacent human activities and the specie of interest it is meant for. (Harrison 1992). The most important determinant is the specie of interest. Different species have different behaviours in relation to long distance movements (Beier et al 1998) and these behaviors have to be taken into consideration when evaluating the suitability of a land as a corridor. 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. 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). The effectiveness of a corridor depends on what specie it is meant for. (Tewskbury et al, 2002). 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.
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 have indicated that wildlife corridor plays a vital role in biodiversity conservation in the following ways;
Enhances the movement of individual species through disturbed landscape on regular basis, seasonally or at different stages of the life cycle.
Increase immigration rates of isolated habitat, thereby enhancing the genetic variation and reducing the risk of extinction and inbreeding depression.
Provides refuge and shelter (habitat) for many species moving through the landscape.
Facilitate the continuity of natural ecological processes in developed landscapes.
Maintains higher species richness and diversity.
Provide ecosystem services such as maintenance of water quality, reduction of erosion and suitability of hydrologic cycles.
Helps re-establishment following local extinction.
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.
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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 while, Haddad (1999) in his study with two butterfly species, showed that corridor increase interpatch movement rates.
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.
A simulation method in combination with empirical observation was used by Johnson et al (1992) 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.
Studies on regional scale was done by Singleton et al (2002) 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.
In their own study, 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.
In another study of biodiversity gain by Angold et al (2005), they examined the biodiversity in urban habitat in Birmingham (England) with the aim of, (i) analyzing the extent to which flora and fauna utilize the urban greenways both as corridors and as habitat. (Ii) To examine the effects of habitat fragment size and connectivity on the ecological diversity and the distribution of individual species. (iii) To understand the ecological characteristics of the biota of cities model. They used a combination of field surveys of plants and beetles, genetic studies of butterflies, modeling the anthropochorous nature of the floral communities and spatially explicit modeling of mammal species. In their studies, they found that green corridors make little difference on the diversity of plants and bettles but provide valuable habitat for them, especially on river corridors and railway land. They also found no evidence that wetland beetle diversity is greater on or near the green corridor and no evidence that corridors are necessary for dispersal of butterflies, while for the invertebrates, the quality of the habitat appears to be a significant factor in the usage of the green corridor. This finding indicates the importance of identifying a target species or group of species for urban greenways intended as dispersal route ways rather than as habitat.
Bailey. S (2005), in her study of how to increase connectivity in fragmented landscapes, investigated the evidence of biodiversity gain in the woodlands in Britain (UK). Her study indicated a lack of firm empirical evidence that species increase following attempts to increase connectivity in fragmented woods. She further stated that biodiversity losses are most likely as a result of the amount of regional habitat loss rather than fragmentation as suggested by other studies. (Harrison, 1994; Fahrig, 1998; Rosenberg et al. 1997). She further suggested that expansion of existing woodlands and establishment of new ones next to mature woodlands may increase survival probabilities of existing populations thus increasing the probability of dispersal (Forman, 1995; woodland trust, 2000; Watts et al, 2005). Isolation effects, extinction rates and edge effects in fragments can all be reduced by increasing the quality of the matrix (Caroll et al., 2004). Also buffering existing woodlands and enhancing matrix quality is likely to yield conservation benefit quickly than forest network development (Walker et al, 2004; Donald et al, 2006).