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22001166 SOUTHEASTERN NATURALIST 1V5o(1l.) :1157,5 N–1o8. 71
A Comparison of Survey Techniques for Medium- to
Large-sized Mammals in Forested Wetlands
Aimee P. Rockhill1,2, Rahel Sollman1, Roger A. Powell3, and
Christopher S. DePerno1,*
Abstract - Monitoring mammals is becoming increasingly important as state and federal
agencies develop wildlife action plans addressing increased urbanization and climatechange
impacts on plant and animal populations. We designed and implemented surveys
applicable to forested wetlands to assess detection rates, estimate species richness, compare
species distributions, and assess relative cost versus success among techniques. The survey
techniques implemented included opportunistic observations, predator calling, spotlighting,
scent stations, camera survey, and foothold trapping. Opportunistic observations produced
the highest species-richness estimate (14), and were the least expensive ($0) because they
were conducted while implementing other survey techniques. Trapping was the most expensive
technique with a cost of $61 per animal detected but provided age structure and
population estimates through mark–recapture analysis. Camera survey was relatively expensive
with a cost of $1865 for the entire study period but recorded the most detections (n =
673), which resulted in a low per detection cost ($3). Opportunistic observations and camera
surveys documented 2 species not detected by any other method. Our results indicate that,
although camera survey was a cost-effective way to detect mammals, richness and distribution
estimates could be improved by incorporating a variety of monitoring techniques
specific to forested wetlands.
Introduction
Monitoring mammals is becoming increasingly important as increased urbanization
affects their populations (Bradley and Altizer 2007, Lawler et al. 2009). Many
medium- to large-sized mammals (e.g., Ursus americanus Pallas [American Black
Bear], Lynx rufus Schreber [Bobcat], Odocoileus virginianus Zimmermann [White-
Tailed Deer], Procyon lotor L. [Raccoon], etc.; hereafter “mammals”) can serve as
indicators of ecosystem health, regulators of prey populations, prey for other mammals
and raptors, and fulfill important roles in the ecosystem such as seed dispersal
(Boddicker et al. 2002, Gaidet-Drapier et al. 2006, Sanderson and Trolle 2005).
Further, research has demonstrated the abilities of mammals to dramatically alter
vegetation composition, nutrient cycling, and plant productivity (Augustine and
McNaughton 1998, Boddicker et al. 2002, Hobbs 1996, Nowak 1991). Some mammals
are difficult to monitor due to their cryptic habits or low population densities,
resulting in few long-term monitoring efforts. Lack of information on many animal
1Department of Forestry and Environmental Resources, Fisheries, Wildlife, and Conservation
Biology, North Carolina State University, Raleigh, NC 27695. 2Current address - PO
Box 31, Galena, AK 99741. 3Department of Applied Ecology, North Carolina State University,
Raleigh, NC 27695. *Corresponding author - chris_deperno@ncsu.edu.
Manuscript Editor: Steven Castleberry
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assemblages and lack of reliable estimates of population sizes restrict managers
from implementing and evaluating land-management strategies (Caughley and
Sinclair 1994, Desbiez et al. 2010, Sutherland 2000). Hence, many state and federal
agencies have recently developed wildlife action plans that focus on mammals as
representatives for health of ecological communities (AFWA 2012, USFWS 1981).
Even so, time and cost constraints may discourage managers and landowners from
implementing mammal surveys (Sheil 2001). Therefore, landowners need information
that allows them to weigh the costs and benefits of impleme nting surveys.
The Southeast region of the United States contains over 12,000 km2 of unique,
forested wetlands important for wildlife conservation (Dahl and Stedman 2009).
However, over 650 km2 of forested wetlands in the region were lost to development
or converted to agricultural uses between 2004 and 2009 (Dahl and Stedman 2009).
Forested wetlands in some parts the Southeast have been altered by systems of
raised roads bordered by 5–10-m-wide canals. While road systems allow distinctly
greater access than available in unaltered forested wetlands, access to tracts of land
away from roads is difficult and time consuming. Further, seasonal flooding and
changing water levels prohibits use of random survey locations. Hence, forested
wetlands provide unique challenges for land managers who aim to develop longterm–
monitoring techniques for mammals.
Standardizing large-scale, multi-species monitoring efforts is necessary to effectively
assess mammal communities (Zielinski and Kucera 1995). Reviews of
survey techniques demonstrate the need for using more than one survey type to
detect diverse species (Gompper et al. 2006, Harrison 2002, Hutchens and DePerno
2009, Lyra-Jorge et al. 2008). Further, different techniques allow for collection of
different data, and the use of 1 technique alone may limit inference about populations
of interest. For example, spotlight surveys can be used for abundance indices
or density estimates where distance sampling is possible (Edwards et al. 2000, Mc-
Cullough 1982, Naugle et al. 1996, Ruette et al. 2003), whereas automatic cameras
can provide data for calculating abundance estimates when animals are individually
marked (Carbone et al. 2001, Heilbrun et al. 2006, Karanth 1995, Karanth and
Nichols 1998, Maffei et al. 2011, Silver et al. 2004).
Land managers need a cost-effective, long-term, spatially explicit protocol
for monitoring mammal populations in forested wetlands to assess the impacts of
harvest and habitat manipulation and to ensure stable populations. Therefore, our
objectives were to assess detection rates, determine species-richness estimates,
compare species distributions , and assess cost versus success among techniques.
The survey techniques implemented were opportunistic observations, predator calling,
spotlighting, scent stations, camera surveys, and foothold trapping.
Field-site Description
From 2007 to 2010, we conducted surveys at Bull Neck Swamp Research Forest
(hereafter “Bull Neck”; Fig. 1), a 25-km2 wetland located on the south side of
Albemarle Sound in eastern North Carolina (35°56´–35°59´S, 76°23´–76°28´E).
Bull Neck is an economically self-sustaining, working forest with active, smallSoutheastern
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2016 Vol. 15, No. 1
scale timber harvests, prescribed burning, and hunting. It is owned by North Carolina
State University and managed by the Fisheries, Wildlife, and Conservation
Biology Program. Access to Bull Neck is restricted. The property is a forested
wetland with secondary and tertiary dirt roads bordered by canals and has 5 landcover
types: non-riverine swamp forest, peatland Chamaecyparis thyoides L.
(Atlantic White-cedar) forest, mesic mixed-hardwood forest, tidal cypress–gum
swamp, and tidal freshwater marsh. Based on 50-year climate records, monthly
mean temperatures ranged from 10.4 °C to 21.7 °C and rainfall averaged 126.5
cm per year (NOAA 2009).
Methods
We followed survey design suggestions by Zielinski and Kucera (1995). We
used Arc GIS to overlay a grid of 2.6-km2 cells onto a map of the property (Fig. 2).
We assigned to each cell (n = 9) 2 scent stations, 1 camera station, and 1 predatorcalling
station (Fig. 2). Live-trap locations ranged from 6 to16 traps per cell, based
on areas with abundant mammal sign. We conducted spotlight surveys across the
property on drivable roads and recorded and placed detections in the appropriate
cell post-hoc (Fig. 2). To assess cost of surveys, we recorded personnel hours for
each method, set labor cost at the then current federal minimum wage rate of $7.25
per hour, and recorded costs of supplies and equipment in 2010 dollars.
We recorded opportunistic encounters of mammals on the property while conducting
other surveys. To prevent double counting, we assigned detected animals to
1 survey technique and did not record sightings or captures as opportunistic during
Figure 1. Land-cover types on Bull Neck Swamp Research Forest and surrounding areas in
North Carolina, 2007.
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spotlighting, predator-calling, and trapping surveys when they were included as
detections for that survey. At each opportunistic encounter, we recorded the species,
date, time, location, and number of individuals. We did not quantify equipment and
labor costs for the opportunistic technique because we recorded observations while
researchers performed other survey techniques, which required no additional cost.
We made the assumption that a majority of land managers spend a considerable
amount of time on their property and could record opportunistic observations at no
additional cost.
We conducted predator-calling surveys at dawn and dusk in 1 location per cell
twice per month (Fig. 2) in June, July, and August of 2007. We concealed observers
in a portable blind and used a rabbit distress call (Primos Ki-Yi™, Flora, MS)
at 5-minute intervals as a lure, monitoring the area with binoculars for 45 minutes.
Predator calling equipment included a blind, binoculars, distress caller, and ATV
fuel and mileage. Labor costs included 2 technicians working 10 hours per month.
We conducted spotlight surveys on a fixed 19.3-km route every 2 weeks for
2 consecutive nights from June to September 2007. If rain was forecasted, we
performed the survey on the next rain-free night. We repeated counts to reduce estimate
variability (McAninch 1995) and randomized the start time between 20:30
Figure 2. Grid overlay with selected locations for scent station, camera, predator calling,
trapping, and spotlight surveys at Bull Neck Swamp Research Forest, NC, 2007. The assigned
cell number is marked in the bottom right corner of each cell.
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and 02:30 hrs to ensure independence. We surveyed all secondary roads (Fig. 2) by
driving 8 km/h with 2 observers in the back of the vehicle with a 2,000,000-candlepower
spotlight. We used binoculars as needed to assist in identifying mammals
observed and recorded observer names, species observed, date, time, location,
overnight temperature, visibility, precipitation, and comments (e.g., eyeshine color,
number of individuals). Presence of canals along roads prevented measuring distances
for sightings. Equipment costs included fuel, mileage, and spotlights. Labor
costs included 3 technicians working 4 hours per night, 4 nights per month.
We trapped mammals with #1.5 Victor® Soft Catch® foothold traps (Oneida
Victor, Euclid, OH), set with a 0.91-kg pan-tension from 01 March to 09 March
2008. Trap size was selected to target medium-sized mammals (e.g., Urocyon cinereoargenteus
Schreber [Gray Fox]). We set up to 85 traps per night in locations with
animal sign (e.g., trails through vegetation, latrines) and activity based on preliminary
data from camera and scent-station surveys (Fig. 2). Costs included foothold
traps, lures, trowels, sifters, shovels, catch poles, and hatchets. In addition, we included
fuel and mileage for 2 trucks and 3 ATVs as equipment costs and 3 technicians
working 8 hour days for 10 days as labor costs. Trapped Bobcats were radio-collared
(Rockhill et al. 2011, 2013) and ear-tagged as part of a concurrent study; all animals
were released at the capture location immediately following processing.
We monitored scent stations in June, July, and August of 2007. To minimize
misdetections and to maximize visitations, we used a 0.6-m-wide scent-station
strip to connect two 1 m x 1 m scent stations placed 3 m apart on opposite sides
of secondary and tertiary roads. We cleared the stations and connecting strip of
all vegetation and used a mixture of play sand and mineral oil to preserve tracks.
A visual attractant (i.e., fake feathers, silver tassels) was stapled approximately
0.1 m from the top of an 0.8-m wooden stake placed in the center of each station
and a cotton ball was stapled to the top of each stake. Gray Fox urine was placed
on the cotton ball on one station and sardine oil on the cotton ball on the opposite
station. Stations were set and checked for 4 consecutive days each month. When
not in use, we removed stakes and scent lures from the stations. We determined if
lures used affected detections at scent stations and if species were detected more or
less than expected using contingency table analysis (SPSS Statistics for Windows;
IBM Corp. 2013) with alpha set at P < 0.05. To be consistent with standard methodologies,
we randomly selected results from one of each paired scent stations to
estimate a density index for comparison with other survey techniques. Scent-station
equipment costs included sand, mineral oil, scent, lures, stakes, cotton balls, rulers,
camera, and ATV fuel and mileage. For labor costs, we included 2 technicians
working an average of 8 hours per day for 4 days per month.
We placed 1 digital camera (Capture 3.0; Cuddeback Digital, DePere, WI)
equipped with an infrared sensor triggered by temperature and movement at 9 of
the 18 scent stations (Fig. 2). Cameras were mounted to trees at a height that placed
the sensor 0.2–0.3 m above the ground. Initial angle placement was parallel to the
road, after which we conducted walk tests from 1 m to the opposite side of the road
(~10 m) and adjusted the camera angle to maximize detections. Although cameras
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were monitored continuously for 3 years, data presented are from June, July, and
August of 2007. We programmed cameras to run 24 hours per day, taking pictures
once per minute when activity was detected. When batteries failed, we reduced
the number of camera days accordingly. Equipment costs for this survey included
cameras, USB cards, replacement batteries, download accessories, and ATV fuel
and mileage. Labor costs included 1 technician to check all cameras, download, and
record images, with all activities combined estimated to total 7 hours per month.
For all techniques, we assessed detection rate (total number of detections per
species), species richness (number of species detected per survey method), and the
cost and effort of performing each technique in relation to detection of species.
Species distribution (percent of cells, n = 9, where each species was detected) was
quantified for camera surveys, scent stations, trapping, camera surveys + opportunistic
observations, scent stations + opportunistic observations, and all techniques
combined. We were unable to estimate species distribution from spotlight surveys,
predator calling, and opportunistic encounters due to small numbers of detections.
To assess cost versus success of each technique, we quantified total equipment and
labor costs, calculated a total monthly cost, and determined the number of monthly
detections as well as hours of labor per detection.
All animal-handling techniques were approved by the Institutional Animal Care
and Use Committee at North Carolina State University (08-012-O) and followed
guidelines provided by the American Society of Mammalogists (Gannon and Sikes
2007) and ASAB/ABS Guidelines for the Use of Animals in Research.
Results
We recorded a total of 1010 mammal detections representing 15 species across
all surveys (Table 1). Opportunistic observations accounted for the highest number
of species (n = 14), and predator calling detected the lowest number of species
(n = 2). Cameras recorded the second highest number of species (n = 12) and had
the highest number of detections (n = 653), although 63% of the detections were
of American Black Bear. No single technique detected all species identified by all
survey techniques combined (Table 1). Mustela vison Schreber (American Mink)
was only detected by opportunistic observations and Sus scrofa L.(Feral Hog) was
only detected with the camera survey. Gray Fox was the only species detected by
all surveys. Castor canadensis Kuhl (American Beaver) was detected infrequently
regardless of technique.
Trapping was successful in capturing 5 species. We had 10 total captures of 7 individual
Bobcat; 2 males (1 adult, 1 juvenile) and 5 females (1 adult, 4 juvenile). We
captured Gray Fox (7 male, 10 female), Raccoon (14 male, 3 female, 4 unknown),
Didelphis virginiana Kerr (Virginia Opossum) (7 male, 4 female, 6 unknown) and
a Canis lupus familiaris L. (Domestic Dog).
Lures used did not affect detections at scent stations (χ2 = 10.14, df = 8, P =
0.2553). We detected American Black Bear more than any other species, and over
half of the detections were in the center strip only (Fig. 3). Center-strip only detections
were observed 32.3% more than expected and would have been missed with a
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standard scent-station design (Table 2). American Black Bear was the only species
detected less than expected (-36.1%) at the urine stations while Bobcat had the
highest deviation from expected values (+50.9%) at urine stations (Table 2). Gray
Fox, Virginia Opossum, and Raccoon tended to investigate urine stations at higher
rates than expected, but their presence at one of the sections often resulted in presence
at all 3 sections of the station (Table 2, Fig. 3).
Scent stations were successful at detecting Raccoon across the entire property.
Further, distribution of species was documented more thoroughly (i.e., recorded
in more cells) with scent stations. Otherwise, detection of species across the entire
property was only possible by combining techniques (Table 3). For example,
Virginia Opossum were detected in 4 of the 9 cells with camera surveys, but a
combination of all survey techniques was required to detect them across all 9 cells.
Combining data from scent stations, cameras and live traps resulted in detection in
all cells for Black Bear, Gray Fox, Virginia Opossum, and Raccoon.
Table 1. Detection rate (total number of detections per species) by survey technique at Bull Neck
Swamp Research Forest, NC, 2007. Opport. observ. = opportunistic observations. Total number of
techniques successful at detecting each species is denoted in the far right column.
Camera Predator Scent Opport. Total
Species survey call station Spotlighting Trapping observ. techniques
American Beaver - - 1 - - 1 2
American Black Bear 408 - 35 12 - 13 4
Bobcat 17 1 8 - 10 1 5
Domestic Cat 1 - 3 - - 1 3
Domestic Dog 15 - 1 - 1 3 4
Feral Hog 1 - - - - - 1
Gray Fox 122 1 29 3 19 4 6
American Mink - - - - - 1 1
Muskrat - - 4 - - 3 2
Nutria 5 - - 4 - 4 3
Rabbit 1 - 1 1 - 1 4
Raccoon 25 - 33 1 19 9 5
River otter 1 - - - - 3 2
Virginia Opossum 14 - 26 4 17 1 5
White-Tailed Deer 43 - 1 34 - 43 4
Total captures 653 2 142 59 66 88 1010
Species richness 12 2 11 7 5 14 15
Table 2. Percent deviations between sardine, center-strip, and urine scent stations for each mammal
species estimated from contingency-table analysis based on scent-station surveys conducted at Bull
Neck Swamp Research Forest, NC, 2007–2008. Standard residuals are presented in parentheses.
Species Sardine Center-strip Urine
Black Bear +6.5 (+0.28) +32.3 (+1.59) -36.1 (-1.81)
Bobcat -19.5 (-0.31) -37.5 (-0.67) +50.9 (+0.93)
Gray Fox -17.4 (-0.69) -1.3 (-0.06) +14.4 (+0.66)
Opossum -0.6 (-0.02) -11.8 (-0.5) +11.8 (+0.51)
Raccoon +19.6 (+0.54) -16.3 (-0.96) +16.2 (+0.47)
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Trapping had the highest overall cost ($4024) and highest cost per species
($805; Table 4). Due to high equipment and labor costs, total cost per capture was
$61. Trapping targeted specific mammals, which resulted in low species richness
(n = 5), and was labor intensive, requiring a high number of hours (1.21) per species.
Scent-station surveys were relatively inexpensive ($369/month), had the
second lowest cost per species detected ($34; Table 4), and the second lowest total
cost per detection ($8) and hours of labor needed per species observed (0.68).
Of all techniques that allowed calculation of detection rates, camera survey had
Figure 3. Total number of detections at sardine oil, center, and Gray Fox-urine scent stations
at Bull Neck Swamp Research Forest, NC, 2007. The dashed portion of each bar represents
the number of detections that were exclusively recorded at that section of the scent station.
Table 3. Distribution of mammal species detections by the most successful of the survey techniques
and combinations of techniques used at Bull Neck Swamp Research Forest, NC, 2007.
Camera Scent
surveys + stations + All
Camera Scent Opport. opport. opport. techniques
Species Trapping surveys stations observ. observ. observ. combined
American Beaver 0 0 11 11 11 22 11
Black Bear 0 89 89 56 100 100 100
Bobcat 44 56 56 11 56 56 78
Domestic Cat 0 0 22 11 11 33 22
Domestic Dog 11 33 11 22 33 33 44
Gray Fox 78 78 78 78 78 78 88
Muskrat 0 0 33 22 22 44 33
Nutria 0 22 0 11 22 11 22
Rabbit 0 11 11 11 11 22 22
Raccoon 67 67 100 56 67 100 100
Virginia Opossum 67 44 89 11 56 89 100
White-Tailed Deer 0 67 0 89 100 100 67
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the lowest cost per detection ($3; Table 4), and most of the cost accrued was for
equipment and initial setup ($1865), resulting in a cost per species observed of
$56. Monthly maintenance and data processing was inexpensive ($51) compared
to other techniques and required only 0.01 hours of labor per detection. Opportunistic
observations had no costs and documented 1 species (Mink) not detected
with any other technique (Table 1).
Opportunistic observations, which add no extra cost, combined with cameras detected
all species and together resulted in the most total detections (Tables 1, 3, 4).
Opportunistic observations combined with scent stations detected all but Feral Hog
and had the second most total detections, again without adding cost. While camera
surveys combined with opportunistic observations constituted the best detection
rate, scent stations combined with opportunistic observations was the most cost
effective (Tables 1, 3, 4).
Discussion
Through this study, we compared various survey techniques and assessed
each technique for performance in terms of estimates of detection rates, species
richness, and species distribution, and for cost-effectiveness. Further, we provided
additional species-specific information on the study property, Bull Neck.
Nine mammal species had been documented on the property prior to our survey,
and this study documented 6 additional species (Bobcat, Feral Hog, American
Mink, Myocastor coypus (Molina) [Nutria], Felis catus (L.) [Domestic Cat], and
Domestic Dog). Although we could not compare detection rates for any single
species across all survey techniques, we could compare detection rates across an
area within a survey technique, assuming no spatial variation in detection probability.
Interestingly, no techniques produced consistently high or low estimates
for all species in the same cells, indicating that spatial detection varied by technique
and a combination of techniques would be necessary to accurately record
presence and distribution of species on a property (Table 3).
Camera and scent-station surveys were the most effective techniques for surveying
mammals in forested wetlands; both techniques recorded the majority of
species and when used together detected all but one species (i.e., Mink). Scent
stations were relatively economical and required minimal implementation efforts
Table 4. Labor hours and monthly costs for each survey technique used to detect mammal species at
Bull Neck Swamp Research Forest, NC, 2007–2008. All costs are in U.S. dollars ($).
# of # of Hours of
Total cost species detections labor per
Survey technique Equipment Labor Monthly observed per month detection
Opportunistic observations 0 0 0 14 29 0.00
Spotlighting 166 1044 403 7 20 0.81
Trapping 2284 1740 4024 5 66 1.21
Scent station 411 696 369 11 47 0.68
Camera survey 1865 152 672 12 224 0.01
Predator calling 268 435 234 2 1 15.00
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for establishment and monthly monitoring (Conover and Linder 2009); nonetheless,
that technique may produce low detection rates for some species (i.e., Bobcats; Harrison
2002). Cameras had high upfront cost, though the relatively low operational
expenses result in relatively economical costs for long-term monitoring efforts (Dajun
et al. 2006, Lyra-Jorge et al. 2008, Nichols et al. 2011, O’Connell et al. 2011,
Silveira et al. 2003). We detected few aquatic mammals with these 2 techniques,
and surveys designed specifically for their detection would be an ideal inclusion to
mammal-monitoring protocols (O’Connell et al. 2011).
Although not as effective as camera and scent-station surveys, other methods
provided useful data that were either species specific or expanded distributional
data. Spotlight surveys were effective for detecting White-Tailed Deer but were
not a realistic option for density estimates due to low detection rates and extensive
canals posing logistic difficulties (Focardi et al. 2001, McCullough 1982, Naugle
et al. 1996). Because distance sampling is a benefit of spotlight surveys and roads
bordered by canals are characteristic of managed coastal wetlands, this survey technique
is not recommended in forested wetlands. Although predator calling allowed
us to detect elusive carnivores, the low numbers of individuals and species observed
prevented us from estimating species richness, distribution, and detection rates.
While implementing trapping may be too expensive for annual use, it increased our
knowledge of the abundance and distribution of furbearers. We, therefore, recommend
including trapping surveys when feasible, or obtaining data from trappers.
Opportunistic encounters resulted in the highest species richness estimates at the
lowest cost.
Managers have come to rely nearly exclusively on camera surveys for monitoring
species abundance and distribution (Ahumada et al. 2011, O’Connell et al.
2011). Our results are consistent with previous studies that report the need to employ
multiple survey techniques to monitor species richness and composition accurately
(Gompper et al. 2006, Harrison 2002, Hutchens and DePerno 2009, Lyra-Jorge et
al. 2008). No single technique is ideal for surveying medium- to large-sized mammals
(Gompper et al. 2006). For example, scent stations had a high rate of missed
detections, as shown through the center-strip data, whereas cameras were less efficient
at detecting smaller mammals (e.g., American Mink, Ondatra zibethicus (L.)
[Muskrat]). While cameras may be appropriate for monitoring the distribution or
populations of some species, we caution against depending solely on camera surveys
to make inferences on mammal presence or absence, distribution, and richness; we
suggest that a number of techniques be used for maximum accuracy.
A lack of information on the time and cost constraints associated with monitoring
mammals in forested wetlands has limited land managers from implementing surveys
(Sheil 2001). We urge land managers to implement a combination of survey techniques
to provide the greatest amount of information at the lowest cost. If sufficient
funds and resources exist, we recommend a combination of all survey techniques
to produce the greatest amount of information. Realistically, budget and time constraints
limit land managers from implementing all survey techniques reported. If
budget limited, scent stations are sufficient in providing baseline data on species
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distribution and diversity (Conover and Linder 2009). If adequate upfront funds exist,
we suggest combining camera surveys with recorded opportunistic observations;
assuming managers spend adequate time on the property in addition to monthly camera
checks. In general, managers will need to decide which techniques best meet their
objectives while understanding the limitations of each technique.
Acknowledgments
This project was funded from the Bull Neck Swamp Research Fund, Fisheries, Wildlife,
and Conservation Biology Program, and the Department of Forestry and Environmental Resources
at North Carolina State University. We are thankful to all technicians and volunteers
that assisted with field work, photo interpretation, and data en try.
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