Biological Consequences of the Proposed 24th Street Corridor on the Big Woods Natural Area
College of St. Benedict/St. John�s University
Collegeville, MN 56321
(320) 363 � 2782; firstname.lastname@example.org
(this essay was written July 16, 2004 in response to the APO plan to put a road through the Big Woods Natural Area, St. Cloud, MN)
By way of introduction, I am a professor of Biology (plant science) in the joint Biology Department of the College of St. Benedict/St. John�s University. I am a former resident of St. Cloud and for nine years I lived near the Big Woods (18th St. S). My children attended Oak Hill School. In addition I was one of the scientific members of the St. Cloud Environment and Development team when the Big Woods property was originally planned for development. In short, I am familiar with the property.
Let�s imagine that the 24th St. Corridor is constructed along the north side of the Big Woods Natural Area as is currently being proposed. In this document I will address the question, �What are the likely biological consequences of this road?� Obviously the road will decrease the size of remaining �Big Woods.� However, what is not so obvious is that by decreasing the size of the parcel, the road will: (1) isolate the woods further from any other forested areas especially along the north side of the property; (2) increase the proportion of edge, or proportion of perimeter to area; and (3) influence the microclimate of the newly exposed portion. Let�s look at the implications of each.
The road will increase the isolation of the Big Woods from any surrounding natural areas. It is analogous to increasing the size of a moat around a castle and it will act as an effective barrier to separate the woods from other similar habitat in the area. In addition to providing for less habitat for foraging and fewer refuges from disturbance, the isolation will likely: (a) make it more difficult for reproduction by both plants and animals since they will be more reproductively separated from other individuals; (b) decrease the ability of native species to recolonize/colonize the area; and (c) temporarily increase the concentration of deer and other animals since they will be clustered in a smaller area. In turn, this could result in increased competition and predation. One potentially positive outcome of the isolation of the Big Woods is that it could decrease the spread of pathogens or some invasive species since it will be more difficult to move into the Big Woods from outside. Not only will these changes impact the Big Woods, but also the trees and property to the north of the proposed road.
2. Edge & 3. Microclimate Effects
By decreasing the size of the Big Woods, the proportion of the perimeter of the property relative to its area will increase. To explain, consider a square that is one meter by one meter. It has an area of 1 m2 (length x width) and a perimeter of 4 meters. The ratio of the perimeter to the area (= perimeter divided by area) is 4. If we remove half of the square, we are left with a rectangle that is 0.5 x 1.0 meters. The area and perimeter of the remaining piece decreases to 0.5 m2 and 3 meters, respectively. However, the perimeter/area ratio increases to 6. Thus, as the Big Woods (or any object) gets smaller, there is a greater proportion of edge relative to the interior. This is important because it means that a much smaller fraction of the interior is free from any effects from the edge.
The �edge effect� will result in: (1) increased sunlight into areas currently receiving very little light. This will favor sun-loving species (which will mostly be weeds) and the decline of shade-tolerant ones; (2) increased temperature since the increased solar radiation will warm up the edge areas to a greater degree than shaded interior areas. Among other things, the increased temperature will affect the decomposition rate of the leaf litter and will increase water loss from the edge sites making these areas overall hotter and drier, once again favoring weeds to the detriment of shade-tolerant plants that generally do not fare well in dry conditions; (3) increased wind which, in turn, will also decrease the humidity and increase the rate of drying and desiccation. In addition, wind damage and windthrows are more likely; (4) nutrient cycles will become altered � some nutrients will increase while others will decrease. This will be the result of increased surface water, increased runoff, increased salt deposition (especially during winter), and increased pesticide exposure and nutrient deposition (e.g., phosphorus) from lawn fertilizers in runoff; and (5) increased soil erosion.
Impact on Species Diversity
Studies of islands suggest that the size of an island is inversely related to the number of species that exist on the island. Thus, there are usually more different species on larger islands than smaller ones. If the Big Woods becomes smaller, based on these studies of island biogeography, we would expect the species diversity of the site to decrease.
Scientists sometimes observe an increase, though usually temporary, in species diversity. For example, bird diversity has been shown to increase in woodlots that were fragmented. This occurs because the remaining area is used as a refuge and because of the edge effect described above. However, over the long-term the general scientific consensus suggests that species diversity decreases.
There is no doubt that a road along the north edge will impact the �Big Woods.� The question that remains to be answered by our politicians and planners is whether the potential impacts are worth the potential advantages for a proposed connector road.
References: (the following references were consulted in the preparation of this document)