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Prescribed Burning Affects a Measure of Fitness in Ctenus hibernalis (Araneae: Ctenidae) at Oak Mountain State Park, Shelby County, AL
T. Jeffrey Cole and Robert A. Hataway

Southeastern Naturalist, Volume 15, Issue 4 (2016): 646–652

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Southeastern Naturalist T.J. Cole and R.A. Hataway 2016 Vol. 15, No. 4 646 2016 SOUTHEASTERN NATURALIST 15(4):646–652 Prescribed Burning Affects a Measure of Fitness in Ctenus hibernalis (Araneae: Ctenidae) at Oak Mountain State Park, Shelby County, AL T. Jeffrey Cole1 and Robert A. Hataway1,* Abstract - Fire-suppressed forests in Oak Mountain State Park (OMSP; Shelby County, AL) have undergone experimental prescribed burning as a means to restore the open canopy architecture and diverse understory characteristic of Pinus palustris (Longleaf Pine) communities. Populations of a ground-hunting spider, Ctenus hibernalis, in the forests of OMSP were studied in order to examine the effect of restoration efforts on populations of understory arthropods. Study sites included regions burned 1 year prior and 5 years prior, as well as a region that has experienced 2 decades of fire suppression. No individuals of C. hibernalis were found in the region burned 1 year prior. There was no significant difference in the total number of spiders in the fire-suppressed region and the region burned 5 years prior, although the body mass of the spiders in the region burned 5 years prior was significantly greater than those in the fire-suppressed region. These results suggest that increased resource availability related to prescribed burns leads to increased spider fitness. Introduction Pinus palustris Mill. (Longleaf Pine) ecosystems, historically the dominant ecosystems in the southeastern United States, depend on frequent fires to maintain understory biodiversity and the open forest architecture that fosters Longleaf Pine recruitment (Bruce and Bickford 1950). Following European colonization, particularly in the 20th century, forest fires were suppressed (Van Wagtendonk 2007). The resulting accumulation of leaf litter and understory shrubs constrains Longleaf Pine germination (Garren 1943). This additional fuel can cause fires to be hotter when they do occur (Youngblood et al. 2005). Recently, prescribed burning has become a common practice in an attempt to restore the composition and structure of Longleaf Pine communities (Bradstock et al. 1998). Effective burns remove leaf litter, downed logs, and competitive fire-intolerant shrubs, opening the understory for the growth of grasses and other light-dependent herbaceous vegetation (Beckage and Stout 2000, Outcalt and Brockway 2010, Ware et al. 1993). Forest-fire suppression has dramatically altered forest ecosystems in the dwindling montane Longleaf Pine woodlands across the southeastern United States (Shumway et al. 2001). This change is especially evident in the largest state park in Alabama, Oak Mountain State Park (OMSP, ~4047 ha [~10,000 ac]), Shelby County, AL. Prescribed burning designed to simulate historical periodic forest fires and alleviate forest-fire suppression has taken place in over 40.5 ha (100 ac) of experimental forest plots in OMSP at various frequencies over the past decade. 1Department of Biological and Environmental Sciences, Samford University, 800 Lakeshore Drive, Birmingham, AL 35229. *Corresponding author - rahatawa@samford.edu. Manuscript Editor: Jason Cryan Southeastern Naturalist 647 T.J. Cole and R.A. Hataway 2016 Vol. 15, No. 4 Species diversity and population dynamics of flora and fauna within an ecosystem can be used to monitor the progress of restoration efforts by comparing sites affected by the prescribed burns to regions undergoing fire suppression (Malumbres-Olarte et al. 2013, Matthews and Spyreas 2010, Rey Benayas et al. 2009). Since faunal succession depends on complex variables other than vegetation return (e.g., ecological and landscape composition, surrounding gene-pool structure, connectivity, and biotic factors), studying the forest fauna is a more inclusive tool for monitoring restoration than floristic surveys alone (Brudvig 2011, Grimbacher and Catterall 2007, Majer 2009). Arthropod communities are useful as indicators of community response to restoration measures because they are very sensitive to ecological changes due to their short life cycles, rapid growth rates, and ubiquity (Brand 2006, Kremen et al. 1993). A recent study conducted in a Piedmont forest in northern South Carolina, under burning circumstances similar to OMSP, found that spider families were impacted differently. Numbers of spiders were either not affected or quickly recovered after the burn events (Vickers and Cullin 2014). However, research has not been conducted on the population dynamics of an individual species. Ground-hunting spiders, such as Ctenidae (wandering spiders), are common arthropods studied in population ecology (Marc et al. 1999). They are suitable organisms to monitor community responses to restoration due to their position as generalized predators of soil arthropods. This aspect of their life history makes them sensitive to changes in leaf-litter depth, grassy biomass, and shrub abundance (Uetz and Unzicker 1975). Changes in the microhabitat affect the spiders’ food supply as well as protective cover from predators (Wheater et al. 2000). Fire suppression significantly reduces the amount of grasses in woodlands, thus depleting the preferred open understory habitat for these spiders, which provides space for burrowing and shelter from predators as well as a diverse array of prey organisms (Major et al. 2006, Martin and Major 2001, Pinzon et al. 2013, Ryndock et al. 2012). The ground-hunting spider Ctenus hibernalis Hentz is a generalist predator endemic to the southeastern United States that has been collected primarily in Alabama and can be found throughout OMSP (Bradley 2012, Peck 1981). With its large home range, this species is an ideal candidate for monitoring the response of predatory arthropods to restoration efforts. The aim of this study was to monitor the density of 3 geographically distinct populations of C. hibernalis in OMSP. By comparing the population size and body mass of C. hibernalis in 2 burn treatments of different frequencies and 1 fire-suppressed site, we hoped to gain insight as to whether the initiation of prescribed burning has been successful in opening and restoring the microhabitat of this species. Methods We established three 10 m x 10 m quadrats in each of 3 burn treatment plots in OMSP: plots that were burned 1 year prior (33.35763°N, 86.710452°W), 5 years prior (33.36974°N, 86.691518°W), and in a plot that has undergone fire suppression for at least the past 2 decades (33.35798°N, 86.70749°W), for 9 total quadrats. Quadrats were located in the understory of south-facing slopes of predominantly Southeastern Naturalist T.J. Cole and R.A. Hataway 2016 Vol. 15, No. 4 648 oak/hickory/Longleaf Pine forest at a similar elevation (~230 m). Quadrats within a treatment plot were a minimum of 20 m apart. All sampling sites were at least 100 m from a road to prevent any edge effect that could influence predation rate or other parameters (Cady et al. 1980). We collected Ctenus hibernalis individuals at night by spotlighting the reflective tapetum within their eyes (Beccaloni 2009). We sampled each quadrat within each site 5 times between the months of June and July 2014, at least 1 hour after dark for 1 hour. This time period allowed for an exhaustive search of the entire quadrat. Adults were hand collected and visually identified to species based on morphological characteristics, counted, weighed, and released. We postponed sampling if there was any precipitation within 24 hours prior to collecting. The total number of individuals that were collected per quadrat in one hour of sampling determined population densities. At each sampling, we recorded National Weather Service weather data such as temperature, average wind speed, moon phase, moon position, amount of recent precipitation, humidity, and cloud cover to be used in multiple regression models to account for confounding factors. We measured leaf-litter depth in all 9 quadrats was measured in 9 places to compare leaf-litter removal between the regions. All statistical analysis was conducted using R version 3.1.1 (R Core Team 2014) statistical programming language. We used a Mann-Whitney U-test to detect differences in spider abundance and spider biomass between the burn treatments. We created multiple linear regression models using all environmental and human factors as independent variables in the MuMIn package in R to predict total spiders caught, and then evaluated the models using Akaike Information Criterion (AIC) techniques. Finally, we regressed spider mass and total number of spiders caught against one another to test for associations between individual size and population density. Results We collected a total of 315 individuals of Ctenus hibernalis in the fire-suppressed region and the region burned 5 years prior. The density of C. hibernalis in the region burned 1 year prior was too small to be quantified, finding 0 spiders in the quadrats and less than 5 outside of the quadrats. This region was removed from all subsequent analyses. There was no significant difference in population density between the plot that was burned 5 years prior and the fire-suppressed region as determined by a Mann-Whitney U-test. Of the 127 linear models, created using all environmental and human factors, the model with the highest AIC score did not predict population density (AIC = 210.58, weight = 0.10, P = 0.558). The mean ranks of the number of individuals per 100 m2 quadrat of the burned and fire-suppressed region were 12.07 ± 6.44 SD and 12.27 ± 8.81 SD, respectively (n = 30, U = 104, P = 0.7352, r = 0.064; Fig. 1A). Spider masses were significantly greater in the region burned 5 years prior than in the fire-suppressed region as determined by a Mann-Whitney U-test. The mean ranks of the individual masses within the burned and fire-suppressed region were 0.17 ± 0.12 g SD and 0.12 ± 0.09 g SD, respectively (n = 202, U = 3310, P < 0.0001, Southeastern Naturalist 649 T.J. Cole and R.A. Hataway 2016 Vol. 15, No. 4 r = 0.30; Fig. 1B). There was no significant relationship between the average mass of spiders and total number of spiders caught per quadrat. This result suggests competitiveness of large individuals does not influence population density negatively. Discussion There was no significant difference in the total number of spiders in the firesuppressed region and the region burned 5 years prior, although the body mass of the spiders in the region burned 5 years prior was significantly greater than those in the fire-suppressed region. Spider mass is positively correlated with fecundity and overall fitness (Nicholas et al. 2011, Skow and Jakob 2003). In our study, the differences in spider mass between sites suggests that the individuals living in the region that had been burned 5 years prior have higher fecundity and overall potential individual fitness than individuals in the fire-suppressed region. Additionally, the lack of correlation between individual masses and population density supports the prediction that spiders in the region burned 5 years ago have more available resources. Figure 1. (A) Average individuals caught per 10- m2 quadrat between treatment regions. (B) Average mass of spiders caught per treatment region. Asterisks indicate significance (P < 0.001). Bars indicate standard error. Southeastern Naturalist T.J. Cole and R.A. Hataway 2016 Vol. 15, No. 4 650 Previous studies on spider community changes in response to fire restoration in both the piedmont forest in northern South Carolina and the open oak woodlands in northern Mississippi found changes in the community structure that varied according to family, with open-habitat specialists’ abundance increasing with the restoration of fire regimes (Ryndock et al. 2012, Vickers and Cullin 2014). The community in the South Carolina study had recovered 1 year after burning whereas the data presented here, with no individuals being found, suggests that the region that was burned 1 year prior has yet to recover from that burn. This difference could perhaps be due to variation in the intensity of the burns. Future experiments will be necessary to determine the relationship between arthropod populations and understory vegetation in relation to the prescribed burn regime. Despite the expectation that population densities in the burned region would be greater than in the fire-suppressed region, we were unable to detect a significant difference in population density among sites, except for the fact that no spiders were found in the plot burned 1 year prior. Acknowledgments This research was supported by a REU grant from the NSF (REU award number 1327466). For providing helpful comments, we thank L.J. Davenport, Erynn Maynard, and Mary Anne Sahawneh. 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