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
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2016 Vol. 15, No. 4
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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
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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
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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,
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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.
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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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