Discussion

Habitat corridors are a unique and intriguing solution to the problems presented by urbanization and fragmentation. Since publication of The Theory of Island Biogeography, the question of the applicability of island biogeography to the mainlands has been debated. Furthermore, some have questioned the lack of consistent experimental data on the importance of habitat corridors and similar forms of connectivity (Simberloff et al. 1992). This doubt, however, is not shared by all in the scientific community (Noss 1987, Beier and Noss 1998). In fact, Noss (1987) states that many of the possible disadvantages of corridors can be mitigated through larger corridor widths and smart ecological zoning. It should be emphasized that corridors are not a replacement for a system of larger reserves but a complement. By definition, corridors rely on a matrix of existing but discontiguous natural areas to realize their full potential. Setting aside reserves of significant size should always remain a priority.

Habitat corridors serve many purposes within an ecological framework beyond providing a conduit for movement. Corridors can also act as a habitat, a barrier or filter, or a source of effects on the surrounding landscape (Fleury and Brown 1997), such as microclimate alterations. While it may seem counter-intuitive that habitat corridors can act as barriers or filters, or even should act as such, the reality is quite the opposite. Properly designed and well placed corridors can filter harmful effects of urbanization or agriculture from environmentally sensitive areas. Such harmful effects can include but are not limited to herbicides, pesticides, fertilizer, or even urban sprawl itself. Corridors’ impact on urban sprawl is an interesting concept and will be explored in more depth later on.

Depending on width, corridors can also function as habitat for some species. Different species have different requirements for suitable habitat, so where a certain piece of land may serve as a corridor for wolves, it may also provide certain bird species with necessary core habitat.

Beyond habitat requirements, corridor width is perhaps one of the most important concerns in the design phase. Just as species have different requirements for habitat, so too do they have different requirements for corridor width. In general, the wider one can make a corridor the better (Fleury and Brown 1997). However, due to cost and space restrictions in an urban setting, compromises have to be made. Fleury and Brown (1997) note that length to width may be more important that width alone, but do not provide any guidelines. Harrison (1992) outlines a set of minimum corridor widths based on the requirements of large mammals as they often require the widest corridors. In designing a corridor for a particular large mammal, that mammal is designated the keystone species. By meeting the keystone species’ requirements, other species using the corridors will be adequately supported (as their requirements tend to be lower). In this region, white-tailed deer may be considered the keystone species for corridor design, having a recommended minimum corridor width of 600 m. This width, however, is based on permanent occupation and its validity has been questioned (Lindenmayer and Nix 1993). In our case, corridors are designed primarily for movement of the keystone species and not occupation. Thus, the minimum corridor width in the Northfield area can in fact be smaller than Harrison’s recommendations. Other suggestions for minimum corridor width have been put forth for birds and medium sized mammals, ranging from 2 m to 40 m (Fleury and Brown 1997).

Minimum width is not the only factor determining corridor quality. Fleury and Brown (1997) found that corridor design and quality is often just as important. Birds, small, medium, and large sized mammals all have different requirements. (Fig. 1)

The edge effect is present the length of the corridor, a fact which can be used as an advantage. Edge does not simply have detrimental effects; beyond changes in microclimate, varying distances from the edge encourage different vegetation types and patterns. This variation in vegetation will provide the necessary difference in habitat to accommodate the different species requirements.

Habitat quality is further influenced by the type and degree of connections (Pickett and Cadenasso 1995, Jordán 2000). Reserves arranged and connected in a serial fashion are less desirable when compared to a ring of patches with corridors connecting the circumference and the diameters (Fig. 2). The maximally connected system is provides more avenues for movement and is better insured against the loss of one or more corridors. There are three typical arrangements of habitats and corridors, the necklace, the spider, and the loop (Fig. 3).


Figure 2. A serial system versus a maximally connected system (Jordán 2000)	Figure 3. The three arrangements of habitat and corridors, necklace, spider, and loop (Pickett and Cadenesso 1995)

Of these, Jordán (2000) recognizes that the necklace is unreliable for migration and the spider only benefits central species or populations. The loop is better suited to migration in terms of reliability, as the loss of one corridor does not completely cut off one patch. While each arrangement has its own advantages and disadvantages, the best system is that which creates the maximum number of connections.