History

The current discussion on habitat corridors began with in 1962 with Frank W. Preston. Preston published an article in Ecology that outlined the species-area curve and its importance. Without diving into too much math, his equation describes the number of species that can inhabit a certain area of land based on geographic and species-specific information. Overall, it states that smaller areas support fewer species. A revolution in ecology had begun.

1967 soon arrived with another landmark publication, this time by two men, Robert MacArthur and Edward O. Wilson. The Theory of Island Biogeography expanded upon Preston’s species-area curve and added new factors, immigration and extinction. MacArthur and Wilson included a simple graph to explain them: From left to right, the immigration line decreased while the extinction line increased. In other words, as the number of species on the island increased, the number of extinctions increased and the number of successful immigrations decreased. The intersection of these two lines, they said, was the equilibrium, or normal, number of species the island could hold. A number of complex mathematical equations accompanied the graph, but the figure said it all (Quammen 1996).

These two publications did more for the study of ecology than stacks of others. Studies were founded and the theory of island biogeography was investigated more fully. Perhaps most important of all, though, was the inclusion of two lines in The Theory of Island Biogeography. “Insularity is moreover a universal feature of biogeography. Many of the principles graphically displayed in the Galápagos Islands and iother remote archipelagos apply in lesser or greater degree to all natural habitats” (MacArthur and Wilson 1967). Basically, island biogeography does not just apply to islands. Papers were published and a new question was asked, do you design a single large reserve or several small reserves? Which offers the best option for preservation? Out of this debate came the idea of habitat corridors (Quammen 1996). If we are not able to create a single large reserve (which according to the species-area curve would support a higher degree of biodiversity), can we connect the smaller ones to make larger effective area?

Urban settings such as Northfield are not condusive to large wildlife reserves and are thus perfect candidates for habitat corridors. And, as current human activities eat up still more tracts of land, fragmented and isolated populations will become increasingly prevalent (Lessica and Allendorf 1992). Habitat corridors are an effective way to preserve the robustness of the natural areas in and around Northfield.

What are corridors?

The word “corridor” is often ill-defined and used broadly. Beier and Noss (1998) provide a thorough definition of the term. A corridor is

a linear habitat, embedded in a dissimilar matrix, that connects two or more larger blocks of habitat and that is proposed for conservation on the grounds that it will enhance or maintain the viability of specific wildlife populations in the habitat blocks.

Essentially, a corridor is an area of natural habitat amongst agricultural fields, residential developments, etc., that connect other larger natural areas. Their potential functions, however go far beyond simply connecting larger natural areas to providing habitat for smaller species, acting as a filter between agricultural lands and bodies of water, or even providing for recreation (Hess and Fischer 2001). The possible benefits or uses of habitat corridors are potentially endless.

Fleury and Brown (1997) note that it is important to place the objectives for corrridor design into a larger framework. As such, this study will focus on the use of wildlife corridors to increase connectivity between larger, existing natural areas in the Northfield area. Designs can either center themselves around the protection or preservation of a specific species (Fleury and Brown 1997) or place an emphasis on preserving local habitat heterogeneity (Burbrink et al. 1998). In seeking to take a more wholistic approach, this study will pay special attention to preserving a variety of historic vegetation types.

Presettlement vegetation of Minnesota

The Northfield area is situated on the historic prairie-forest border, with the Big Woods occupying the area northwest of the Cannon River and prairie stretching to the southeast (Marschner 1974). This ecotone has fluctuated over the years, but the Cannon River acted as a relatively constant barrier between the two. The Big Woods forest type is characterized by elm (Ulmus), basswood (Tilia americana), sugar maple (Acer saccharum), ironwood (Ostrya virginiana), bitternut hickory (Corya cordiformis), butternut (Juglans cinerea), and ash (Fraxinus). Elm was by far the most common tree, making up 27% of bearing trees collected in 1850. In the southern region, however, oaks (both red and white) populated the forest in substantial numbers while bur oak, jack oak, and aspen dominated the prairie-forest border due to their adaptations to fire (Grimm 1983, 1984).

Grimm (1984) notes that the Cannon River valley is an exception to this last trend. The Cannon effectiveness as a firebreak eliminated the oak-aspen vegetation buffer, meaning that maple, basswood, and elm grew in relatively close proximity to the prairie across the river.

Purpose

The purpose of this study is to link existing or restored natural areas within the Northfield area to create a continuous landscape through which the native flora and fauna may thrive. Cooperative development of these corridors between citizens, developers, the two colleges, and local government, the Northfield community will benefit through increased beauty and ecosystem stability.