2008 NORTHEASTERN NATURALIST 15(4):475–484 Influence of Acorn Mast on Allegheny Woodrat Population Trends in Virginia Michael T. Mengak1,* and Steven B. Castleberry1 Abstract - Neotoma magister (Allegheny Woodrat) is a medium-sized rodent associated with rock outcrops, talus slopes, caves, cliffs, and boulder fields in the central and southern Appalachians and Allegheny Plateau physiographic provinces. It is currently classified as a G3G4 species and is considered threatened, endangered, or a species of concern in almost every state in which it occurs. As part of a 12-yr study on the status and distribution of the Allegheny Woodrat in Virginia, we collected data on woodrat ecology and population demographics. Herein, we investigate the relationship between acorn production and an index of woodrat abundance for several woodrat populations in Virginia. Woodrat population size was estimated using the Lincoln-Peterson index. Acorn mast surveys were conducted by the Virginia Department of Game and Inland Fisheries from 1989 to 2002 to index mast abundance. Woodrat population estimates were positively correlated (P < 0.05) to the previous year’s mast crop index at 2 of 4 monitoring sites. Woodrat populations were not correlated to the mast crop two years prior. Acorn production alone does not appear to account for decline in woodrat populations. Range-wide declines in Allegheny Woodrats are likely due to a combination of local and landscape factors, but forest managers should consider acorn production when writing management prescriptions if woodrats are present within the management unit. Introduction Neotoma magister Baird (Allegheny Woodrat) is a medium-sized rodent associated with rock outcrops, talus slopes, caves, cliffs, and boulder fields in the Central and Southern Appalachians and Interior Highlands physiographic provinces (Castleberry et al. 2001, 2002a, 2002b; Mengak 2002a). It is currently classified as a G3G4 species (NatureServe 2006) and is considered threatened, endangered, or a species of concern in every state in which it occurs (state status: S1, S2, or S3; NatureServe 2006) except Kentucky, where it is classified as an S4 species. Widespread population declines in the northeast and western portions of its range have been reported (Balcom and Yahner 1996, Castleberry et al. 2002a, LoGuidice 2003), with woodrats extirpated from New York and Connecticut, and dramatic declines noted in New Jersey and eastern Pennsylvania (Hall 1988). Reasons for the decline are complex and potentially include landscape change (Balcolm and Yahner 1986, Ford et al. 2006), habitat modification (Castleberry et al. 2001), Baylisascaris procyonis (Raccoon Roundworm) infection (LoGuidice 2003), hard-mast decline due to Cryphonectria parasitica (Murr.) Barr (Chestnut Blight) and Lymantria dispar Linnaeus 1Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602. *Corresponding author - mmengak@warnell.uga.edu. 476 Northeastern Naturalist Vol. 15, No. 4 (Gypsy Moth) (Wright and Kirkland 2000), Quercus spp. (oak) decline, avian and mammalian predation, weather (Manjerovic 2004), and forest fragmentation (Balcom and Yahner 1996; Castleberry et al. 2001, 2002a, b; LoGuidice 2003; Mengak 2002c). Another possible factor contributing to the regional decline includes food competition with increasing populations of Odocoileus virginianus Zimmermann (White-tailed Deer), Ursus americanus Pallas (Black Bear), and Meleagris gallopavo Linnaeus (Eastern Wild Turkey) (Balcom and Yahner 1996, Handley 1991, McShea et al. 1997). It is likely that woodrat decline resulted from the congruence of several factors operating over a multi-decade time frame (Ford et al. 2006, LoGuidice 2006). Habitat change due to forest conversion of rich valleys to farmland and urban development probably restricted woodrats to ridges. Woodrats may have been forced into isolated remnant populations along the parallel ridges of the Blue Ridge and Ridge and Valley mountains. Forest clearing may have further restricted woodrat distribution to such inaccessible sites as cliff lines, caves, rock outcrops, boulder fields, and steep topography. Forest succession, fire suppression, re-establishment of Whitetailed Deer, and changes in forest canopy due to introduced pests further altered the landscape. Restricted to isolated refugia, Allegheny Woodrats experience low gene flow even among subpopulations isolated by as little as 3 km (Castleberry et al. 2002c). Acorn production is well known to be episodic among many oak species common to the Appalachian forest (Beck 1977, Greenberg 2000, Wolff 1996). Numerous studies have documented a positive correlation between acorn production and abundance of many wildlife species including Whitetailed Deer (McShea and Rappole 1992, Wentworth et al. 1992), Peromyscus maniculatus Linneaus (Deer Mouse) (Gashwiler 1979), birds (Smith 1986), tree squirrels (Nixon and Hansen 1987), and other small mammals (Wolff 1996). Wentworth et al. (1992) found that condition indices of White-tailed Deer showed a one-year lag period with acorn crop abundance in the southern Appalachian Mountains of north Georgia. Thus they also found a decline in condition indices following years of poor acorn production. Oak mast crops serve as a major determinant of the overall community structure and function in Appalachian forests (McShea 2000). A general model of the ecological connections between mast, mice, deer, and humans was presented by Ostfeld et al. (1996). Mast crops likely play a significant role in the ecology of the Allegheny Woodrat, but no published studies are available on the specific relationship. Woodrats (including Allegheny Woodrats) are known to cache food items in the fall for overwinter use (Poole 1940). Manjerovic (2004) did not find a correlation between hard mast and woodrat capture index. However, Castleberry et al. (2002a) found that presence of hard mast in the woodrat diet tracked acorn production and availability. The relationship between chestnut mast and Allegheny Woodrats was the subject of at least one review (Wright and Kirkland 2000) that implicated the decline 2008 M.T. Mengak and S.B. Castleberry 477 of Castanea dentata (Marsh.) Borkh. (American Chestnut) in woodrat population decline. Although oaks have replaced the chestnut as dominant forest trees, the frequency and volume of the acorn crop does not rival former American Chestnut mast (Diamond et al. 2000). We hypothesized a positive correlation between oak mast and woodrat population size. Our objective was to investigate the relationship between acorn production and woodrat abundance for four woodrat populations in Virginia. Study Areas As part of a 12-yr study on the status and distribution of the Allegheny Woodrat in Virginia, we collected data on woodrat ecology and population demographics. Although woodrat distribution was examined by trapping at over 130 sites in 27 counties, the focus of the long-term study was to examine annual trends in woodrat abundance at selected locations. Fieldwork occurred from June 1990 through October 2001. In this paper, we focus on trapping in autumn (September–October) of each year. Four sites were chosen for analysis in this study based on proximity to oak mast survey sites conducted by personnel with Virginia Department of Game and Inland Fisheries (VDGIF). All sites were located in the Jefferson-George Washington National Forest in western Virginia. The Bath County site (38o10'N, 79o45'W) at 658 m elevation consisted of a cave system and cliffs along a slope above a firstorder unnamed stream. The overstory vegetation was composed of Tsuga canadensis (L.) Carr. (Eastern Hemlock), Tilia americana L. (American Basswood), Fagus grandifolia Ehrh. (American Beech), Carya ovata (Mill.) K. Koch (Shagbark Hickory), Pinus strobus L. (Eastern White Pine), and Acer rubrum L. (Red Maple). The Giles County Site (37o22'N, 80o37'W) was a cliff and associated rock breakdown at 1280 m elevation. Overstory vegetation consisted of various oaks species, Betula lenta L. (Sweet Birch), Liriodendron tulipifera L. (Yellow Poplar), and Sorbus americana L. (Mountain Ash). The Rockbridge County site (37o34'N, 79o28'W) was an approximately 2.5-ha boulder field known locally as Devil’s Marbleyard. Boulders were large, averaging approximately 3–4 m diameter. Elevation was 610 m. Surrounding the boulder field was a second-growth forest composed mostly of Quercus prinus L. (Chestnut Oak), Q. rubra L. (Northern Red Oak), Q. coccinea Muenchh. (Scarlet Oak), P. virginiana Mill. (Virginia Pine), and Red Maple. The Allegheny County site (37o44'N, 80o07'W) was a linear outcrop (≈500–600 m in length) along a forested ridge line at 994 m elevation. Dominant canopy trees were various species of oak and hickory, Virginia Pine, and American Beech. Methods We set 40 Tomahawk (No. 201) collapsible live traps (40.6 x 12.7 x 12.7 cm) baited with one-half an apple at each site for 2 consecutive nights 478 Northeastern Naturalist Vol. 15, No. 4 during each trapping session. Trapping protocol was identical at all sites throughout the entire study. Traps were checked each morning before 1100 h. Captured animals were ear-tagged, weighed to the nearest 2.5 g, sexed, examined for reproductive and overall body condition, and released at the site of capture. Only autumn (September–October) trapping results were used in the correlations reported here. All trapping conformed to standards of the American Society of Mammalogists in effect at the time of the study (ASM 1987). Oak mast surveys were conducted independent of this study by biologists and technicians of the VDGIF in late summer to determine an index of mast abundance (D. Martin, VDGIF, Staunton, VA, pers. comm.). Mast index data from 1989 to 2002 were obtained for comparison with the woodrat trapping data. A mast index was calculated by VDGIF personnel by surveying 20 permanently marked Red Oak (n = 10) and Qercus alba L. (White Oak; n = 10) trees at each site. Sites were distributed throughout the state. Each September, acorns were counted on 10 randomly selected branches from each tree. A mast index was constructed based on the mean number of acorns per branch. The index was scaled as: 0 = failure, 1–5 = poor, 6–20 = fair, 21–35 = good, and ≥36 = excellent. Wolff (1996) compared the VDGIF data with acorns counted on his trapping grid at the Mountain Lake Biological Station and concluded that the VDGIF mast index was positively correlated with his mast survey plots. McShea (2000) conducted a more detailed study of mast production near Front Royal, VA and similarly found close agreement to the VDGIF dataset. Perry and Thill (1999) concluded that visually surveying acorns could be an effective method of evaluating production and could be superior to mast traps if observer bias is controlled. VDGIF attempted to survey each site with the same biologist or technician each year. We used 1–3 mast survey sites (20 to 60 trees) located ≤8 km from each woodrat monitoring site. When multiple mast survey sites were available, we computed a mean acorn crop value from the mast crop survey data at each mast survey site and used the mean index value in our statistical analysis. Woodrat population size was estimated using the Lincoln-Peterson index (Williams et al. 2002). We used the Spearman correlation coefficient to determine the relationship between mast crop index and Allegheny Woodrat population index (SAS 1999). Statistical significance was set at alpha = 0.05. Because the Bath County location lacked a significant oak component in the overstory, we used this site as a negative control. For each site and for three mast-dominated sites combined, correlations were conducted between the woodrat population index and the mean acorn crop index for the current year, previous year, and two-years previous. Results Acorn crops were rated as good or excellent at three of the four sites in 1989, 1996, and 2000, and at all four sites in 1991 (Fig. 1). Good to excellent years were generally followed by poor or fair years. Woodrat population estimates varied over the time period examined (Fig. 2) from 2008 M.T. Mengak and S.B. Castleberry 479 a high of 21 individuals at a monitoring site to a low of zero individuals (Mengak 2002c). The woodrat population index for three sites combined was positively correlated to the previous year’s mast crop index (P = 0.002, r = 0.578; Table 1). Additionally, 2 of the 3 individual monitoring sites, Allegheny County (P = 0.0008, r = 0.9550) and Rockbridge County (P = 0.0465, r = 0.7143), were positively correlated to the previous year’s mast crop index. Figure 1. Oak-mast production index at four long-term Allegheny Woodrat monitoring sites in Virginia surveyed from 1989–2001 by Virginia Department of Game and Inland Fisheries. Figure 2. Allegheny Woodrat population estimates (± 1 S.E.) at four long-term monitoring sites in the Appalachian Mountains, VA, 1990–2001. 480 Northeastern Naturalist Vol. 15, No. 4 The woodrat population index was not correlated to the current year or twoyears’ prior mast crop at any of the sites. The woodrat population index was not correlated to any measure of mast crop at the Bath County site. Discussion Population density of many oak-mast-eating mammals is often closely tied to acorn mast crop (McShea 2000, Nixon and Hansen 1987, Wolff 1996). Small mammals respond to oak-mast abundance by increasing reproductive output (Wolff 1996). White-tailed Deer shift their movement to occupy oak-dominated stands during years of mast abundance (Ostfeld et al. 1996). Late-fall White-tailed Deer diets consist of little else besides acorns (76% volume, 100% occurrence) in years of abundant mast crops (Harlow et al. 1975). Woodrats, although certainly mast consumers (Castleberry et al. 2002a, Wright and Kirkland 2000), are not acorn obligates. Thus, they were able to maintain high population levels at the sites in our study that are not dominated by oaks (Bath County) or have a variety of food resources (Giles County). Furthermore, acorns contain abundant amounts of energy in the form of lipids (Wright and Kirkland 2000) and carbohydrates (Goodrum et al. 1971) and nutrients such as calcium, phosphorous, and vitamins (Goodrum et al. 1971). However, the cyclical nature of acorn corps may negatively impact woodrat populations. Acorns are more perishable than nuts from other trees such as hickory and American Chestnut (Wright and Kirkland 2000). The tannic acid content of acorns lowers their digestibility by wildlife. Acorns contain less Vitamin A, for which a deficiency may negatively impact an animal’s night vision, than chestnuts (Wright and Kirkland 2000). Table 1. Correlation coefficients (r), significance (P), and sample size (n) reported as years of annual fall monitoring data for oak mast and woodrat capture index at four long-term monitoring sites in the Appalachian Mountains, VA, monitored from 1990–2001. Current year Previous year Two-year Site mast mast previous mast Three sites combined r 0.0041 0.5785 0.0973 P 0.9841 0.0020 0.6437 n 26 26 26 Giles County r -0.5818 0.3909 -0.3333 P 0.0604 0.2345 0.3466 n 11 11 10 Allegheny County r 0.1982 0.9550 0.1081 P 0.6701 0.0008 0.8175 n 7 7 7 Rockbridge County r 0.5238 0.7143 0.2036 P 0.1827 0.0465 0.6287 n 8 8 8 Bath County r 0.1865 0.0886 -0.3816 P 0.5829 0.7956 0.2766 n 11 11 10 2008 M.T. Mengak and S.B. Castleberry 481 Across the three combined sites in our study, woodrat abundance was positively correlated (P = 0.002) with the previous year’s mast index (Table 1). Correlations at individual sites provide further insight into the relationship. The Bath County site, which did not show a significant relationship (P = 0.7956), was dominated by a northern hardwood forest with few mast-producing trees (mostly hickory). In the almost total absence of oak mast, woodrat diets at this site likely consisted of green vegetation, soft mast, fungi, and other items (Castleberry et al. 2002a). Although present at the Giles County site, oaks were only one component of an overstory composed of a variety of tree species. As such, woodrats had access to a variety of mast and non-mast producing trees along with seasonal abundance of Rubus spp. (blackberry) and Vaccinium spp. (blueberry) fruits. The sites where strong correlations between woodrat abundance and mast availability in the previous year were detected (Rockbridge and Allegheny County sites) had oak-dominated overstories. However, the Allegheny Woodrat does not seem to be under the same influence of acorn production as other small mammals in this environment (McShea 2000, Wolff 1996). Our results (Table 1) and those of Castleberry et al. (2002a) suggest that Allegheny Woodrat populations consume oak mast heavily when available, but they are opportunistic foragers and can persist in areas where oak mast is low or non-existent. In the Ridge and Valley Province of Virginia, acorns were not found in the diet from fall 1997 through summer 1998 (Castleberry et al. 2002a) following a generally poor mast crop in 1997 (Fig. 1). By fall 1998, a fair to good mast year, acorns made up 15% of the diet (Castleberry 2000, Castleberry et al. 2002a). All hard mast combined made up less than 10% of the woodrat diet from fall 1997 to spring 1998, but rose to 22.9% and 21.9% in the summer 1998 and fall 1998 diets, respectively (Castleberry et al. 2002a). Woodrats generally consume green vegetation as their primary dietary components and opportunistically consume hard and soft mast. Acorns are seasonally important in the diet of the Allegheny Woodrat (Castleberry et al. 2002a) and other species such as White-tailed Deer, Sciurus carolinensis Gmelin (Gray Squirrel), Eastern Wild Turkey, and small mammals. Episodic abundant production of acorns can positively affect woodrat populations (Table 1). However, poor acorn production does not seem to account for decline in woodrat populations, as other acorn-using species are not experiencing similar population declines. Woodrats change their foraging rates and home-range size in response to timber management activities (Castleberry et al. 2002b). Woodrats, along with other mammal species, consume acorns when available, and forest managers should manipulate timber stands to favor good acorn producers (Goodrum et al.1971, Greenberg 2000). Range-wide declines are likely due to a combination of local and landscape factors (Balcolm and Yahner 1996, Castleberry et al. 2002c, Ford et al. 2006, LoGuidice 2003, Wright and Kirkland 2000). Although Manjerovic (2004) found a correlation between captures and weather, she noted the correlation might be due to other factors that impact 482 Northeastern Naturalist Vol. 15, No. 4 body reserves. She noted that mast was not a limiting factor, but might be critical for overwinter survival. Supplemental feeding has been anecdotally shown to dampen population fluctuation in one study (Mengak et al., in press). Abundant oak-mast crops may contribute to better overwinter survival and higher reproduction in woodrats. Woodrat body size (Castleberry et al. 2006), reproductive output (Mengak 2002b), longevity (Mengak et al. 2002), and specific habitat requirements suggest that the Allegheny Woodrat functions within its environment similar to other rodents such as Gray Squirrel, Tamiasciurus hudsonicus Erxleben (Red Squirrel), and Glaucomys volans Linnaeus (Southern Flying Squirrel) and not like the typical r-selected small mammal. With a longer life span, lower reproductive output, and more specialized habitat requirements than other small mammals, woodrats will require different management strategies to ensure their survival and persistence in their isolated habitats. Additional, manipulative studies may be needed to further examine the relationship of mast production and woodrat population response. Acknowledgments This study was funded by the Virginia Department of Game and Inland Fisheries (VDGIF) Pittman-Robertson Federal Aid to Wildlife Restoration Project WE99R, the US Forest Service (USFS) Jefferson-George Washington National Forest, National Park Service (NPS) Shenandoah National Park, MeadWestvaco Corp., Georgia- Pacific Corp., and the Virginia Academy of Sciences. We gratefully acknowledge the support of R. Reynolds (VDGIF), P. Griep (USFS), and S. Klinger (USFS). D. Martin (VDGIF) supplied the acorn mast datasets. Numerous students from the Environmental Science Program, Ferrum College and K. Mengak, C. Mengak, and L. Mengak assisted with field trapping and monitoring activities. K.V. Miller and D. Osborn reviewed earlier drafts of this manuscript. M.J. Conroy provided advice on woodrat population estimation. Two anonymous reviewers provided useful suggestions for improving the manuscript. Literature Cited American Society of Mammalogists (ASM). 1987. 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