Corridors

Corridors are continuous strips of habitat connecting areas of habitat that would otherwise not be connected. They are one of the major ways in which we attempt to improve habitat connectivity. However, corridors serve many purposes that are related to connectivity, from facilitating the movement ofindividuals, enhancing social structure (e.g., for primates), population viability (e.g., for spotted owls in the Pacific Northwest of the US), and community structure to improving ecosystem properties. Furthermore, corridors can be natural (e.g., riverine vegetation) or constructed (e.g., wildlife crossings on roads), deliberate or inadvertent (e.g., road verges), and vary in scale from highway crossings to ambitious international projects such as connecting the Greater Yellowstone area in Montana and Idaho to the Yukon and Alaska with a forest corridor to facilitate the movement of large mammals. Constructed corridors are often greenbelts or greenways in urban and suburban areas that serve recreational purposes, enhance ecosystem functioning (e.g., groundwater infiltration), or are low-value areas, such as those subject to flooding.

When first conceived of, corridors were viewed as being nonselective pathways that facilitate movement. The reality is that certain species may benefit from corridors of a particular type, depending on width, length, and habitat composition within them, the surrounding habitat matrix, and what they are connecting; all of these things are measured relative to the requirement of the species and individuals in question. Corridors may also be selective in terms of factors like the ages, social status, or other traits of the individuals dispersing. Increasing connectivity through the use of corridors is not always good. For example, weedy plants are often spread along highway corridors, introducing non-native invasive species that are selected for factors like rapid generation times, high movement capacity, being able to benefit from disturbance (e.g., open ground for germination), and high competitive ability. The spread of a disease, pathogen, predator, dominant competitor, or non-native invasive species may also occur and impact either particular species or whole communities and ecosystems. Nonetheless, the negative effects of isolation on population viability, genetic diversity, species diversity, and community structure are generally severe so that the weight of evidence suggests that improving connectivity is usually desirable. It is clear that long-term studies of the community-wide effects of altering habitat connectivity are especially desirable. Figure 3 shows an aerial view of a long-term ecological experiment where Nick Haddad and colleagues manipulated the connectivity of pairs of habitat patches consisting of clearings within a pine forest in South Carolina. The experiment controls for the amount of habitat edge and the increase in habitat area caused by having corridors by using winged patches that are of similar area to patches connected by corridors except that the corridors do not connect to other patches. Nick Haddad and colleagues showed that connecting patches using corridors increased the interpatch movement rate of a diverse

Figure 3 False color aerial photograph showing a large-scale habitat fragmentation experiment conducted by Nick Haddad and colleagues. Three treatments are shown: pairs of patches connected by a habitat corridor (100 m by 100 m patches connected by a 25 m by 150 m corridor; an unconnected patch (of 137.5 m by 100 m) and winged patches that have the same total habitat area (100 m by 100m, with two 25 m by 75 m wings). The amount of edge habitat is also broadly similar in a connected patch and a winged patch.

Figure 3 False color aerial photograph showing a large-scale habitat fragmentation experiment conducted by Nick Haddad and colleagues. Three treatments are shown: pairs of patches connected by a habitat corridor (100 m by 100 m patches connected by a 25 m by 150 m corridor; an unconnected patch (of 137.5 m by 100 m) and winged patches that have the same total habitat area (100 m by 100m, with two 25 m by 75 m wings). The amount of edge habitat is also broadly similar in a connected patch and a winged patch.

suite of taxa, including butterflies, small mammals, and bird-dispersed plants. In the same study system, Ellen Damschen and colleagues demonstrated that by the end of a 5-year period after initiating the experiment the connected patches contained an average of 20 more plant species than unconnected patches.

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