The Edge
“The interface between different ecosystem types.”[1]
“The edge is where things happen.”[2]
“More than 70% of the world’s forests are within 1 km of a forest edge.”[3]
Aldo Leopold first popularized the terms edge and edge effects in his seminal 1933 book Game Management. Leopold was not the first to use the terms, he attributes prior use to both Herbert L. Stoddard and Aymer Maxwell.[4] Frederic Edward Clements used the synonym, ecotone, to describe the concept of edge as early as 1905.[5] The concept is straightforward. Whether ecosystem types as used in the quote above, or habitat patches, or types [6] Leopold’s preferred terminology, or zone[7] as used by Clements, the meaning is consistent. Forest, grassland, pond, marsh are all types of ecosystems. An animal will choose to establish residence in order to have access to multiple types, thereby assuring access, to food, water, safety, and nesting. According to Leopold if the patches are huge, the animal will choose to live at intersection of types.[8] Given smaller, contiguous patches, the animal will take up residence on the edge.[9] Leopold was a hunter and conservationist whose species of most interest was quail. He lists deer and wild turkey as species prone to edge effects as well.[10] While it is true that prey animals find safety on the edge, predators know what awaits them there as well.
There are two distinct edges on my property as shown in Figure 1. One runs north-south between the western edge of the pond and the adjacent marsh. The other runs east-west (and west-east since the animals travel in both directions) between the woods and the backyard. In my study area, deer and turkey seem to be the species that most regularly traverse the east-west edge, but the “yardmaps”[11] and photographs you will find on the kingdom/class specific pages demonstrate a great deal of species diversity on the edge.
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Figure 1. The two distinctive edges on my property, north-south along the pond, east-west along the woods. (Photo from Google Earth, topographic map courtesy of Town of Hyde Park, NY, Survey map drawn by Michael A. Dalbo)
The image in Figure 2 demonstrates how populations move along those edges. For this example, I have chosen to use deer, represented by the yellow line, and wild turkey, represented by the orange line. The species travel in both directions more or less along these lines. Note also, there are two red lines on the overhead map. The northernmost (upper part of map along the road) represents what is technically referred to as a barrier to dispersal: think roadkill. There will be more discussion on such barriers as we proceed but I want to note that it is a very important concept. The red arrow running north to south towards the lower part of the map represents the path of both species when alarmed. They basically head south and cross into the deeper forest over the low point in the stone wall, as shown in Figure 3.
Figure 2. The approximate routes of wild turkey (orange) and white-tailed deer (yellow), as well as barrier to dispersal (red at top), and alarm/exit route (red arrow).
Figure 3. The low point in the stone wall, southern demarcation of property line.
Notable features of the edges on my property can be seen in Figures 4 and 5. In Figure 4, you can see that the ground cover shows variation in the area that is most travelled. In Figure 5, you can observe animals of various species “tolerating” each other’s presence in order to take advantage of the benefits offered by the edge. Indeed, there is great diversity of species as a result of edge.
Figure 4. This is the east-west edge. Note the different coloration in the high traffic area.
Figure 5. Several different species on the pond edge, sharing the space with others of the same and different species.
Maxwell, Stoddard, and Leopold were not the first people to notice edge effects. Native Americans, notably in southern New England, engaged in documented controlled burning as far back as the 17th century, the purpose of which was to manipulate forest understory with the intention of eliminating pests and encouraging desirable species, in particular game species. The ecological effects were many and varied and noted by the first English settlers in the region, most specifically the increase in animal populations in response to increased forage.[12] It has also been noted that when the practice stopped, largely because the English settlers replaced the Native Americans in that region, the English settlers began to deforest the region and domestic forage animals replaced their wild counterparts.[13]
Certainly, edge effects can provide aesthetic, functional, and philosophical value. Writing in 2005, Karen Harper and her colleagues, acknowledging Leopold’s early work, explained that the thinking of researchers about edge effects had evolved. Forest managers had, from the time Leopold wrote Game Management, accepted that edge effects supported biodiversity and larger populations of edge species. Forestry management practices were designed to increase the amount of edge. In more recent decades, negative effects have been observed and researchers have been more focused on edge influence, the deleterious effects on forest health caused by an increase in edge.[14] In his 1997 work on the function of greenways, Jonathan M. Labaree provided specifics on the damage and its causes, “Edge effects are particularly acute where a cultivated field borders a native forest or woodlot. Increased light and wind at the forest's edge leads to drier soils, more light-favoring species, and more blowdowns. Species that are adapted to the moist soils and shady environment of the forest interior are unable to survive in edge habitat.”[15] Biologists and ecologists now realize that edge effects in the present day are a symptom of habitat fragmentation which is of great concern and endangers numerous species of both flora and fauna. While edge is still diverse, because of the link to fragmentation, it threatens rather than encourages biodiversity.[16]
It is important to note that edge is not only on the periphery. For example, a clearing in the middle of a forest is surrounded by edge. The relationship between habitat fragmentation and edge creation can be explained further using geometry. A forest shaped like a doughnut has concentric circles of edge. Consider, for example, a square block of land of a single type as a hypothetical habitat: the edge is the perimeter of that block of land. If that block of land is then divided by a transect longitudinally and again latitudinally, the result will be four patches of habitat. There will be no more or less habitat in total, but there will be 50% more edge, along the transects. If the transect is a road, or highway, or other barrier which renders the habitat patches no longer connected, those patches are fragmented. Leopold’s affirmation of smaller habitat patches relied on their being adjacent or adjoining. It is the barriers that drive fragmentation.
In brief, the more fragmentation the more exposed edge. In Figure 6 and 7, you can see the increase in edge between 1995 and 2016 as a result of the completion of the planned anthropogenic habitation of the study area. In suburban and rural areas, because properties tend to be spread out, even more edge is created. Animals are forced to live in smaller and smaller patches, connectivity between habitat fragments is limited, and all these forces act prohibitively on the ability of both individual members of a species and their genes to move between patches. Biodiversity is diminished. Non-native species are given greater opportunities to move in, survive, and even thrive. These factors also combine to increase human-wildlife interactions.
Figure 6. Overhead photograph showing edge (blue) in 1995 (photo from Google Earth/U.S. Geological Survey).
Figure 7. Overhead photograph showing edge in (blue) in 2016. (photo from Google Earth).
In their 2015 article entitled “Habitat fragmentation and its lasting impact on the earth’s ecosystems,” Nick M. Haddad and his co-authors compare numerous longitudinal studies on fragmentation and use some novel methods such as “analyz[ing] the world’s first high-resolution map of global tree cover to measure the magnitude of forest fragmentation.”[17] And the results are staggering as almost, “20% of the world’s remaining forest is within 100 m of an edge [and] More than 70% of the world’s forests are within 1 km of a forest edge.”[18]
In their paper entitled “Breeding Bird Abundance in an Urbanizing Landscape in Coastal Southern California,” Bolger, Scott, and Rotenberry measured approximate population density of twenty common bird species using that density to hypothesize the bird’s response to fragmented versus non-fragmented habitat. As a result, they were able to separate the birds into three categories: edge/fragmentation insensitive, those species the population of which did not change as a result of urbanization or encroachment upon their natural habitat; edge/fragmentation enhanced, those species whose population increased when habitat was encroached upon, or were able to form a symbiosis with humans; and edge fragmentation/reduced, those species those species whose population was decreased as a result of human encroachment and urbanization.[19] These categories can obviously be and have been applied to other species. For example, as noted previously, coyote are edge/fragmentation enhanced, as are raccoons and white-tailed deer.[20] These animals are also sometimes referred to as generalist or human-adapted.[21] Animal species that thrive by predating on nesting birds, such as skunks, blue jays and domestic cats tend to be edge fragmentation enhanced as well.[22]
I will continue to look with amazement at the diversity I am so lucky to be exposed to on a daily basis. Now that I understand at least part of the ecological mechanisms driving and simultaneously threatening that diversity, especially on a global scale, I also understand the proliferation of Citizen Science efforts, and understand the value. More on that on the next page: here.
Figure 8. View of the edge from above.
Figure 9. The edge in winter.
Figure 10. Video: Deer on the Edge. Deer crossing the southern edge of my property. This compilation does not show every deer that crossed the southern edge of the property, but is a sampling from June 10 through September 24, 2016. Fawns at 1:56. Deer calls courtesy of Macaulay Library at the Cornell Lab of Ornithology and soundbible.com.
Notes
[1]Karen A. Harper et al., “Edge Influence on Forest Structure and Composition in Fragmented Landscapes,” Conservation Biology 19, no. 3 (June 2005): 771.
[2] Akiko Busch, The Incidental Steward: Reflections on Citizen Science (New Haven: Yale University Press, 2013), 91.
[3] Nick M. Haddad et al., “Habitat Fragmentation and Its Lasting Impact on Earth’s Ecosystems,” Science Advances 1, no. 2 (March 20, 2015): 2, accessed March 10, 2016, http://advances.sciencemag.org/content/1/2/e1500052.full, doi:10.1126/sciadv.1500052.
[4] Aldo Leopold, Game Management (1933; repr., Madison: University of Wisconsin Press, 1986), 131.
Leopold is referring here to two of several works where the term edge was used in the same context before him. Those works are, Aymer Maxwell, Partridges and Partridge Manors (London: Adam and Charles Black, 1911), 46, 47, 219. ; Herbert L. Stoddard, The Bobwhite Quail: Its Habits, Preservation and Increase (Charles Scribner’s Sons, 1931), 21, 190, 213, 215, 363.
[5] Frederic Edward Clements, Research Methods in Ecology (University Publishing Company, 1905), 187, accessed April 9, 2016, Google Books.
[6] Leopold, 132.
[7] Clements, 187.
[8] Leopold, 129.
[9] Ibid., 131.
[10] Ibid., 131.
[11] I am borrowing this terminology from the Yardmap Citizen Science project being conducted by the Cornell Lab of Ornithology which you can find out about here, www.yardmap.org/, or on our Citizen Science page.
[12] William Cronon, Changes in the Land: Indians, Colonists, and the Ecology of New England, first revised (New York: Hill and Wang, 2003), 51.
[13] Ibid., 108
[14] Harper, et al., 769.
[15] Jonathan M. Labaree, “How Greenways Work: A Handbook on Ecology,” The National Park Service Rivers and Trails Conservation Assistance Program and QLF/Atlantic Center for the Environment, October, 1997, chapter 3, accessed January 25, 2016, http://www.americantrails.org/resources/greenways/NPSintroGrnwy.html.
[16] William Cronon, “Why Edge Effects?,” Edge Effects, October 9, 2014, http://edgeeffects.net/why-edge-effects/.
[17] Nick M. Haddad et al., 2.
[18] Ibid., 2.
[19] Douglas T. Bolger, Thomas A. Scott, and John T. Rotenberry, “Breeding Bird Abundance in an Urbanizing Landscape in Coastal Southern California,” Conservation Biology, 1997, 417.
[20] Clark E. Adams and Kieran J. Lindsey, Urban Wildlife Management, 2nd ed. (Boca Raton: CRC Press, 2010), 228.; Busch, 94-5.
[21] Heidi E. Kretser, Patrick J. Sullivan, and Barbara A. Knuth, “Housing Density as an Indicator of Spatial Patterns of Reported Human–wildlife Interactions in Northern New York,” Landscape and Urban Planning 84 (January 1, 2008): 283.
[22] Adams and Lindsey, 152.
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