The Understory Part 1: The White-tailed Deer

This is the first of two posts related to forest understory. This post is about white-tailed deer’s effect on understory but begins by discussing human-wildlife interactions, a topic of much current interest.

In their 2008 study, “Housing density as an indicator of spatial patterns of reported human–wildlife interactions in Northern New York,” Kretser, Sullivan, and Knuth calculated housing density as acres/hectare per unit. As described in the article, “Of particular relevance to this study was how reported human–wildlife interactions related spatially to varying intensities of development on the landscape.” While some might take issue with the manner by which they established their categories[1], urban, suburban, exurban, rural 1, rural 2, and wildlands, their findings were interesting nonetheless.[2] While the results did not indicate absolute spatial relationships, the least reported human-wildlife interactions were in urban and wildland, and there were more reported interactions in suburban and exurban housing situations.[3] Of the five species focused on individually, black bear, deer, raccoons, striped skunks, and woodchuck, “Only raccoon clustered in urban land uses and only bears clustered in wildland areas.”[4] Interactions involving the other three, deer, skunk, and woodchucks were associated with suburban, exurban, rural 1, and rural 2 housing densities.[5] The author’s focus on deer, raccoons, skunks, and woodchuck is notable because they can be viewed, using Bolger’s categorizations, as edge/fragmentation is enhanced. The authors note that those four species prefer “edges and young forests.[6] Bear accounted for 22% of the human-wildlife interactions, skunks for 19%, and deer for 16%.

Another interesting and somewhat obvious way of measuring human-wildlife interactions was demonstrated in an article by Ashley N. Schenk and Marcy J. Souza. They counted and analyzed eleven years of reports to a wildlife clinic, The University of Tennessee Veterinary Teaching Hospital, in East Tennessee. Of the 14,303 cases analyzed, 4443 were deemed, “directly or indirectly anthropogenic in origin, including cat related, dog related, hit by automobile, and other human encounters leading to trauma.”[7] What the authors refer to as “indirect” interactions with humans (cats and dogs) made up 20% of the total cases and “direct” interactions, either “human-induced trauma” or “hit-by-automobile” made up 11%.[8] The authors categorized wildlife as mammals, birds, or amphibians/non-avian reptiles but did not divide any further, for example, by species.

There is no question that a large number of human-wildlife interactions, particularly in the eastern United States, are with deer, specifically white-tailed deer. The number of interactions reported by Kretser et al. in their study in Northern New York, in a thirteen-county area including the Adirondack State Park,[9] was 165. Adams and Lindsey state that “There used to be only 100,000 deer in the United States in the early 1900s…but now they number over 30 million.”[10] Dr. Susan Stout reports that the average deer density “per forested square mile” in Pennsylvania was 35 in 2001 but adds that “in some forested areas deer population can be much higher.”[11] In the same issue of Forest Science Review Stout’s colleague Dr. Stephen B. Horsley says the populations were higher in the 1980s but were lowered due to controls put in place by the state of Pennsylvania.[12] Adams and Lindsey, based on a 1999 deer population survey by the Quality Deer Management Association, quantify populations of 1.28 to 1.5 million in Pennsylvania and up to 1 million deer in New York state.[13] In their study in the Harvard Forest in north-central Massachusetts, Edward K. Faison and his co-authors reported estimated deer densities of 4-6 per km-2.[14] One km-2 is one hectare or 0.01 square kilometers. So Faison’s reported estimate equates to 400 to 600 deer per square kilometer.

Population density is certainly of some concern in and of itself for reasons such as the car accidents reported by Schenk and Souza. Gardeners can attest to the inconvenience, as James Barilla put it “Without realizing it, I’ve been working myself into one of the great challenges of coexistence: how to keep the wild things from harvesting the food we want to eat ourselves.”[15] An article in National Wildlife magazine states that Kirby Stafford chief entomologist at the Connecticut Agriculture Experiment Station, “has found a direct link between the abundance of black-legged ticks,” also known as deer ticks, which carry Lyme and other diseases, and “the number of white-tailed deer.”[16] The symbiosis between deer and white-footed mice in the spreading of “deer” ticks is also noted.[17] Clark and Lindsey affirm this symbiosis and add oak trees (acorns), a primary food source for the mice, to the equation.[18] In the same National Wildlife article it is reported that “Richard Ostfeld, a disease ecologist at the Cary Institute for Ecosystem Studies in Millbrook, New York has linked the prevalence of black-legged ticks to habitat fragmentation and biodiversity loss.”[19]

There are two effects of deer overpopulation that Stout, Horsley, Faison, et al., Adams and Lindsey, and the authors of an additional journal article entitled “Does Ungulate[20] Foraging Behavior in Forest Canopy Gaps Produce a Spatial Subsidy with Cascading Effects on Vegetation” all agree upon. The first is the alteration and destruction of already marginalized habitats. The result of white-tailed deer browsing in places where there is an overpopulation of deer is possibly irreparable damage to the shrub and seedling layer. Moreover, this damage to the seedling layer ultimately affects the ability of the forest to reforest so all forest layers, including the canopy, are impacted. Stout notes that “Many areas clearcut in the 1960s did not regenerate into forest…unless they were fenced to exclude deer.” [21] She additionally notes the deer-induced exclusion of many flowers and shrubs as well as the absence of “saplings of sugar maple, white ash, and pin cherry.”[22] And she quotes Horsley as saying, “In the long term, deer have the capability of changing forest ecology, by changing the direction of forest vegetation development.”[23] Faison and his co-authors note the challenges to the regrowth of eastern hemlock forests already stressed by the presence of the hemlock woolly adelgid as compounded by ungulate, especially deer, browsing.[24] In the aforementioned article by Betsy Tahtinen et al. on ungulate browsing and canopy gaps, it was found that deer show a preference for smaller gaps, tend to overbrowse these gaps, and “Given the importance of canopy disturbances and gaps to the perpetuation of forest ecosystems, localized and/or heterogeneous impacts may be magnified as forests turn over.”[25] Adams and Lindsey reiterate and add “The heavy and constant browsing of urban white-tailed deer can destroy entire vegetative communities (e.g. forest understory) upon which other species depend for their survival.[26]

[1]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): 286. “For this study, we classified the land uses as follows: urban = 0.00–0.16 ha per unit (0.00–0.40 acres per unit); suburban = 0.17–2.00 ha per unit (0.41–5.00 acres per unit); exurban = 2.01–16.00 ha per unit (5.01–40.00 acres per unit); rural1 = 16.01–40.00 ha per unit (40.01–100.00 acres per unit); rural2 = 40.1–404.60 ha per unit (100.01–1000.00 acres per unit) wildlands≥404.70 ha per unit (>1000.00 acres per unit).” [2] Ibid., 286. [3] Ibid., abstract, 282. [4] Ibid., 289. [5] Ibid., Table 2., 289. [6] Ibid., 289. [7] Ashley N. Schenk and Marcy J. Souza, “Major Anthropogenic Causes for and Outcomes of Wild Animal Presentation to a Wildlife Clinic in East Tennessee, USA, 2000-2011.,” abstract, Journal of Wildlife Rehabilitation 34, no. 2 (May 2014): 7. [8] Ibid., 9. [9] Kretser, et.al., 284. [10] Adams and Lindsey, 335. [11]Susan L. Stout, “The Forest Nobody Knows,” Forest Science Review 1 (Winter 2004): 4, accessed February 18, 2016, http://www.fs.fed.us/ne/newtown_square/publications/FSreview/FSreview1_04.pdf. [12] Stephen B. Horsley, “Canary in the Coal Mine—A Short History of Northern Pennsylvania Forests and Their Deer Herd,” Forest Science Review 1 (Winter 2004): 4, accessed February 18, 2016, 3. http://www.fs.fed.us/ne/newtown_square/publications/FSreview/FSreview1_04.pdf. [13] Adams and Lindsey, 339. [14] Edward K. Faison et al., “Functional Response of Ungulate Browsers in Disturbed Eastern Hemlock Forests,” Forest Ecology and Management 362 (February 15, 2016): 178, accessed March 8, 2016, http://www.sciencedirect.com/science/article/pii/S0378112715007331. [15] James Barilla, My Backyard Jungle : The Adventures of an Urban Wildlife Lover Who Turned His Yard into Habitat and Learned to Live with It (Cumberland, RI, USA: Yale University Press, 2013), 56-7, http://site.ebrary.com/lib/alltitles/docDetail.action?docID=10687939. [16] James Marinelli, “The Tick Predicament,” National Wildlife, April-May 2016, 14. [17] Ibid., 14. [18] Adams and Lindsey, 342, 344. [19] Marinelli, 14. [20] Ungulate refers to the now divided between odd-toed and even-toed taxonomical order Ungulata of hoofed mammals and includes wild animals such as deer, elk, moose, and wild boar as well as domesticated animals such as cows and pigs. [21] Stout, 4. [22] Ibid., 5. [23] Ibid., 5. [24] Faison, et al., 182. [25] Betsy Tahtinen et al., “Does Ungulate Foraging Behavior in Forest Canopy Gaps Produce a Spatial Subsidy with Cascading Effects on Vegetation?,” Forest Science 60, no. 5 (October 2014): 826, accessed January 19, 2016, http://search.proquest.com/docview/1609269915?accountid=40999. [26] Adams and Lindsey, 341.