Wildlife/Biodiversty corridors are a strip of land that has been reserved as a natural habitat in a developed area. They are usually used to connect between otherwise isolated habitats. The corridors are thought to allow plants and animals to disperse, or migrate, from one habitat to another (Queensland Department of Main Roads 2000). These corridors have been used as an option in conservation planning to help reduce the effects of habitat fragmentation (Laurance and Laurance 1999). If the corridors actually work is a point that is to be discussed in this paper. Animals are often seen in the corridors but there is a lack of evidence as to whether they are being used as intended, as a corridor for movement (Collinge 1998). Studies including animals, insects and plants will be looked at.
Uses and Definitions
There are different uses and definitions for the corridor, as stated by Hess and Fisher (2001). Figure 1 below shows the different uses of essentially the same strip of land.
Figure : Uses of corridors (Hess and Fisher, 2001)
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A conduit corridor is designed for organisms to move from one side to the other but not actually live within the corridor. A habitat corridor is designed to act as exactly that, a habitat for organisms to live and reproduce within. Vegetation and buffer strips adjacent to waterways are designed to filter out and stop pollutants entering those waterways, these strips of land are either a filter or barrier corridor. A source corridor is a well designed habitat corridor where the reproduction rate of organisms within the corridor is greater than the mortality rate. And a sink corridor is the opposite of a source corridor, where an organism's mortality rate is higher than the reproduction rate within the habitat. (Hess and Fisher 2001.)
When designing these corridors for use it is important to design to maximize the usability of the corridor to suit a number of species (Queensland Department of Main Roads 2000). Figure 2 below illustrates the difference of movement patterns of organisms in relation to the corridor type.
Figure : Expected movement rates of animals in corridors as a consequence of variations in the quality and type of connectivity. (Queensland Department of Main Roads 2000).
Studies of Wildlife/Biodiversity Corridors
A study of 10 species of birds, numbering about 450 birds total, conducted in Southern Costa Rica within an area where only 4.6% was covered in riparian strips showed that the corridors were used by some of the species as a travel path up to 38% of the time. Some of the 10 species studied showed no sign of using the corridors at all (Sekercioglu 2009). This could be a troubling sign as the species that do not use the corridors may die off.
Tree dwelling animals of tropical rainforests in Northern Queensland were the focus of another study that produced similar findings to the Costa Rican bird study. It showed that some species used the corridors for movement and habitat and some did not use them at all, rather staying in the primary rainforest areas. There are several factors that could cause these results. The sleeping patterns of some species could be a reason for them to use a corridor or not. If a species requires a hollowed out tree to sleep in it would be more likely to find this in an older rainforest rather than in a corridor that has relatively younger trees, however a species that sleeps in branches may be more likely to use the corridor. Mammals that were more likely to cross over open land had higher colonisation rates, where as mammals that strictly stayed in the tops of mature trees, which are not common in regrown corridor strips, were rarely seen using the corridors. (Laurance and Laurance, 1999.)
Figure 3 below shows the results of a study done by Haddad, Bowne, Cunningham, Danielson, Levey, Sargent and Spira, 2003, that shows corridor use by different organisms including animals, insects and plants. In the study it showed that one rodent species, P. polionotus, frequently travelled by the corridor between connected patches where as another rodent species', S. hispidus, movement was not affected by the presence of corridors at all.
Figure : Corridor Effect (Haddad, Bowne, Cunningham, Danielson, Levey, Sargent and Spira 2003)
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From the studies looked at for animals there was no definitive answer as to whether these corridors work or not. Some animals will use the corridors and some will not, depending on there sleeping habits or the type of vegetation within the corridor among other reasons.
Insects are a very large and diverse group to study when it comes to wildlife/biodiversity corridors. A study involving the Junonia coenia butterfly, pictured below in figure 4, delivered results that showed corridors that were short distances made no difference to the route taken to the colonisation area by the butterfly. At longer distances, however, the butterfly was twice as likely to travel via a corridor (Haddad 2000). The corridors also acted as a habitat for the butterfly. This was shown, 'evidence supporting use of corridors as habitat by the J. coenia was that after five days, 13% of recaptured butterflies initially released within corridors were recaptured within a corridor' (Haddad 2000).
Insect species presence and abundance within corridors was directly related to vegetation cover and the complexity of hedgegrows. Davies and Pullin 2006, found this by doing a review of various journals relating to the effectiveness of hedgegrows between woodland habitats. The movement of insect species increased with dense vegetation and was inhibited when there were gaps within hedgegrows.
Angold, Sadler, Hill, Pullin, Rushton, Austin, Small, Wood, Wadsworth Sanderson and Thompson 2006, concluded there was little evidence that movement of insect species was greater when corridors were available. For insects in particular it seems quality of habitat is more the reason for population and diversity as corridors were used as habitats by insects and not just for movement.
Figure : Junonia Coenia butterfly (Uniprot 2010)
The strongest evidence of Wildlife/Biodiversity corridors effectiveness was with flora. A study conducted in Germany by Wehling and Diekmann (2009) found that hedgegrows connecting forests contained up to 77% of the same plant species as the forests they were connected to, including some endangered species. The species of plant and their location within the hedgegrow depended on the distance out from the connecting forest and the age of the hedgegrow. As the distance form the connecting forest increased, the density of the hedgegrows decreased. This was due to a number of reasons. The soils in the corridors differed to the forest soils, as the hedgegrows became less dense the species of plant found were 'tougher' as they saw more sunlight and the soil was also found to be dryer.
A direct link to a species richness in patches of habitat that were connected as opposed to patches of habitat that were isolated was found in experiments done by Damschen, Haddad, Orrock, Tewksbury and Levey (2006). At the start of the experiment the species richness was no different between connected and isolated patches of habitat, but by the end of the 5 year experiment they found that there was about 20% more plant species in the habitats connected by a corridor then in the isolated patches.
Effects of Design on Wildlife/Biodiversity Corridors
According to Horn (2003) the use and needs of the Wildlife/Biodiversity corridor should be identified before design. 'To be effective for all species (including bird and reptile fauna), a corridor should by its structure and floral composition, provide all the elements of food, shelter, and protection from predators.' (Horn 2003).
Width of a corridor produces different edge effects, therefore width is an important factor in the design of corridors. Light and wind affect habitat quality, also predation may increase on corridor users if the corridor is too wide.
It was found that species richness in a corridor decreased as a corridor length increased. Figure 5 below shows the ecological value of a corridor in relation to its length. For a species to be able to disperse along a corridor it needs to be a certain length and within a certain proximity to existing habitat. Figure 5 can be used as a guideline in identifying the best locations for wildlife/biodiversity corridors. 'In summary corridor length, and proximity to native vegetation, combined with width, are crucial to the effective dispersal of native animals.' (Horn 2003).
Figure : Value of Corridors, developed by Carruthers and Smith 1996 (Horn 2003)
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Most of the studies concluded that wildlife/biodiversity corridors were beneficial in some way. Not always for the intended use, movement, or species though the corridors do provide habitat for at least some species. If planned well these corridors can be an effective environmentally sustainable design feature. They should not be relied on solely however, maintaining as much natural habitat when designing should be the main concern.