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.
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.
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.
of Corridor Width
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).
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)
1. Optimal corridor vegetation design (Fleury and Brown 1997)
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.
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.
2. A serial system versus a maximally connected system
3. The three arrangements of habitat and corridors, necklace,
spider, and loop (Pickett and Cadenesso 1995)
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.