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22001166 SOUTHEASTERN NATURALIST 1V5o(3l.) :1551,3 N–5o2. 23
Development of a Biologically Centered Habitat-Monitoring
Technique: SPIDER Transect Method
Stacy L. Hines*
Abstract - Overabundance of Odocoileus virginianus Zimmermann (White-tailed Deer)
can have negative effects on woody vegetation. I developed the SPIDER transect method
to quantify an area impacted by deer overbrowsing. I compared area evaluated and time
expended (effort) with the traditional belt-transect method. The SPIDER transect method
had 3× less transects that were at least 20× longer and evaluated an area 50× larger (350 ha)
with 50% less effort compared to the belt-transect method (6 ha). The quantifiable area is
an advantage of the SPIDER method that is not obtained using the traditional belt-transect
method; in this study, woody vegetation in a 304.5-ha area around a park campground exhibited
overbrowsing. The SPIDER transect is a wildlife-centric, efficient method that could
be beneficial for prescribing and evaluating management recommen dations.
Introduction
Overabundant species can modify ecosystem processes and limit resources
(Healy et al. 1997). Overabundance of Odocoileus virginianus Zimmermann
(White-tailed Deer; hereafter Deer) can interfere with nutrient cycling, simplify
vegetation communities, and have negative effects on other species (Barrett and
Stiling 2006, Cote et al. 2004, Rooney 2009). Management of Deer densities is
important to avoid these negative consequences on the abiotic and biotic components
of habitat. However, Deer density estimates are often inaccurate, especially
in forested areas. Therefore, researchers have proposed monitoring Deer impacts on
vegetation to prescribe and evaluate management recommendations (Aldous 1944,
Healy et al. 1997, Morellet et al. 2001).
Belt transects are widely used for evaluating woody vegetation in forested areas;
established protocols include sampling all vegetation along 50–100-m transects until
the species area curve levels off (e.g., no new species identified with additional
transects; Andrews 1974, Barbour et al. 1987). However, biologists need better
methods for assessing relative population size because established vegetationmonitoring
methods can be time consuming and sample only a small portion of the
landscape (Aldous 1944, Morellet et al. 2001). Vegetation monitoring completed
at smaller scales may not accurately portray impacts of species because utilization
of vegetation communities across the landscape can be heterogeneous (Healy et al.
1997). Development of monitoring methods based on vegetation use by individual
species would provide insight into the species’ impacts and better guide management
decisions.
*Department of Biology, University of North Carolina at Greensboro, Greensboro, NC
27402. Current address - Caesar Kleberg Wildlife Research Institute, Texas A&M University-
Kingsville, MSC#218, Kingsville, TX 78363; stacylhines@bellsouth.net.
Manuscript Editor: Alvin Diamond Jr.
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The objective of this study was to compare vegetation assessments (area evaluated
and time expended) using the traditional belt-transect method (following
established protocols) with a new method I developed to evaluate the impacts
of overabundant deer on woody vegetation that was biologically centered, easily
replicated, and required little effort and equipment. The Spherical Primary and
Intermediate Directional Examination of Resources (SPIDER) transect method is
biologically centered because the vegetation community is systematically evaluated
in a circular pattern radiating outward from a central location—emulating Deer activity
surrounding a feed site (Brown and Cooper 2006, Cooper et al. 2006, Doenier et
al. 1997) or core area of use (Kilpatrick and Stober 2002, McNulty et al. 1997, Oyer
and Porter 2004). A qualitative comparison of these methods is reported to illustrate
how established vegetation-monitoring techniques may accurately describe the vegetation
community, but may not depict a species use of the vegetation community.
Field-Site Description
Morrow Mountain State Park is located in the Piedmont region of North Carolina
(1919 ha; 35°22'N, 80°5'W) and has been open to the public since 1939 (NC
State Parks 2009). The park is part of the Uwharrie Mountains located in the eastern
temperate forest ecoregion. Canopy vegetation is dominated by Quercus spp.
(oaks), Carya spp. (hickories), and Pinus virginiana Mill. (Virginia Pine). Understory
vegetation consists of species such as Acer rubrum L. (Red Maple), Ilex opaca
Aiton (American Holly), and Cornus florida L. (Flowering Dogwood).
Deer density in the park was estimated at 30–34 Deer/km2 (K. Knight, NC Wildlife
Resources Commission, Albemarle, NC, pers. comm.). Feeding of wildlife is
prohibited in NC state parks (North Carolina General Statute § 113-35: 15A NCAC
12B .0201 [c]). However, park visitors have been feeding Deer in the campground
area of the park for more than 20 years (T. McCree, retired park superintendent,
Morrow Mountain State Park, Albemarle, NC, pers. comm.). Food provided to Deer
by humans tends to congregate Deer and sometimes leads to increased population
densities in such areas. As Deer populations become overabundant, browse lines
become more noticeable (Cooper et al. 2006). For these reasons, distinct browse
lines are evident in the campground area of the park, and not seen in other areas of
the park although Deer populations are present (Fig. 1).
Methods
From February to April, just prior to spring green-up, I completed vegetation
assessments of woody species in the campground area at Morrow Mountain State
Park (North Carolina State Parks Research Activity Permit No. R09-03). I did not
include herbaceous, non-woody vegetation in evaluations because Deer diets consist
predominately of woody species during winter (Aldous 1944, Johnson et al.
1995, Swihart and Picone 1998). I chose the center of the main campground road
as the central point of vegetation assessments because along this road most human
feeding of Deer occurred and Deer were usually observed there every evening
(S. Hines, 1.5 years of pers. observ.).
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SPIDER transect method
I developed the Spherical Primary and Intermediate Directional Examination of
Resources (SPIDER) transect method, described herein, to evaluate the impacts of
Deer on woody vegetation. I named the transect method SPIDER for the 8 “legs”
(number of transects) and developed an acrostic from each letter which best described
the method. A central location is determined by the researcher and study
question of interest, but could be identified by core area of use, feed source, water
source, bedding site, etc. Resources are examined along 8 transects which radiate
outward from the central location, as in the spokes of a wheel (a “spherical-like”
shape), following the primary (N, S, E, W) and intermediate (NE, SE, NW, SW)
magnetic directions (while it is understood the shape is not exactly a sphere, spherical
was the best term that began with “S” and described a circular-type shape).
I used the SPIDER method to quantify the area impacted by Deer overbrowsing.
Woody vegetation is most prone to Deer browsing within 30–60 cm from the forest
floor (Cote et al. 2004). If the browse-line height is higher than the range where
woody vegetation is most susceptible to Deer browsing, then overbrowsing would be
indicated. For this study, overbrowsing was indicated when the browse-line height
was greater than 55 cm. I used 55 cm as the cutoff because it was the average browseline
height of woody vegetation in an area (2.4 km from the campground) known to
have active Deer population present, but without the influence of human feeding.
Browse-line height of woody vegetation deer consumed, identified by the unique
cut plane left from Deer browsing on twigs (Morellet et al. 2001), was measured
from the forest floor to the first-encountered horizontal, twig-sized limb (≤2 cm
Figure 1. Distinct browse line in the campground area of Morrow Mountain State Park,
Albemarle, NC.
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in diameter). One browse-line height measurement was taken at 50-m increments,
along each of the 8 transects, within 5 m of the transect line. The 50-m increments
were paced. Calibration of stride and number of steps per 50 m was determined before
measurements began; the same individual completed all sampling. I recorded
all points using a GPS. No pre-determined length (distance from central starting
point) was used for transects because the objective was to quantify the area of
overbrowsing. Therefore, browse-line height measurements continued along each
transect every 50 m until browse-line height was within typical range most prone to
deer browsing. The end point for each transect was established when 3 consecutive
measurements were 55 cm or less or the park boundary/lake was encountered.
Transect points and corresponding browse-line height at each point were mapped
using ArcMap (ArcGIS software version 10.0; Redlands, CA). Browse-line heights
were divided into 3 categories: ≤55 cm, 56–100 cm, and ≥101 cm. The extent of
the area of Deer overbrowsing was the area contained within the perimeter defined
by the furthest point (from the center outward) along each transect at which the
browse-line height of that point was ≥56 cm, but does not contain the points when
3 measurements in a row were ≤55 cm. In the case a boundary was encountered
before 3 measurements in a row were ≤55 cm, the area within the perimeter would
not contain the furthest point (or last 2 points) on the transect if the browse-line
height was ≤55 cm at those points.
Traditional vegetation assessment method: belt transect
Along the campground road, I evaluated woody vegetation species within belt
transects according to established vegetation-assessment protocols. I assessed
woody vegetation across twenty-six 1 m × 50 m belt transects, 13 on each side of the
road, starting 5 m from the road (to alleviate affects from routinely mowed grass),
and 50 m apart (Andrews 1974). Within every 1-m2 block of the 50-m2 transect, all
woody vegetation was identified and browse-line height (cm) was measured. The
number of species were plotted against increasing number of transects to determine
if an adequate number of transects had been completed to be representative of the
woody vegetation community (Barbour et al. 1987).
Results
SPIDER transect method
Browse-line height was measured on 170 individual saplings and trees (range:
16–37 individuals per transect) except for Pinus spp. (Appendix A). The length
of the 8 transects from the central starting location ranged from 1000 to 1750 m
(Fig. 2). The S, SE, E, and NE transects continued until the park boundary/lake was
encountered. The N, NW, W, and SW transects continued until 3 measurements in a
row were ≤55 cm. The N transect was the longest; terrain was mostly flat with undulating
hills (largest elevation change was 12 m over a distance of 100 m). The SW
transect was the only transect where the browse-line height of woody vegetation
gradually decreased along the transect (100 m beyond central starting location, the
browse-line height was ≥101 cm for 300 m, decreasing to 56–100 cm, mostly, for
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Figure 2. Map of study site in Morrow Mountain State Park showing the browse-line height
of woody vegetation along SPIDER transects, the area centered on the campground that
is impacted by deer overbrowsing (304.5 ha; area within polygon), and the area evaluated
using traditional belt transect method (6.13 ha; black filled blocks). For SPIDER survey
points, white circles and triangles represent places woody vegetation was consider overbrowsed
(to ≥56 cm above the ground), and black triangles represent places exhibiting
typical deer browsing (≤55 cm).
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350 m, and then was ≤55 cm). There were 1 or 2 consecutive measurements along
4 of the 8 transects where the browse-line height of woody vegetation was ≤55 cm,
but immediately followed by measurements ≥56 cm, ranging from 50–500 m in
length along the transect. Surrounding the campground area, woody vegetation
in 304.5 ha exhibited a browse-line height ≥56 cm, with most of the area (except
for 23 ha surrounding the SW transect) exhibiting a browse-line height ≥101 cm.
Traditional vegetation assessment method: belt transect
There were 257 woody plants of 19 species identified in the campground area
(Appendix A). No new species were identified after 19 transects, therefore 26
transects were an adequate number of transects to be representative of woody vegetation
community according to established protocols (Fig. 3). The total area within
the perimeter of the belt transects was 6.13 ha (Fig. 2). Browse-line height could
not be measured on all 257 woody plants because in forests, trees self-prune lower
branches. Browse-line was within reach and measured on 170 individual saplings
and trees, of which 113 individual saplings and trees were species browsed by Deer.
The average browse-line height for all woody species (n = 170) and woody species
browsed by Deer (n = 113) was 122.1 ± 8.8 cm and 131.2 ± 6.2 cm, respectively
Figure 3. Number of woody vegetation species identified plotted against the increasing
number of belt transects evaluated. One of the protocols for the belt-transect method
necessitates the addition of transects until the species area curve levels off (e.g., no new
species identified with additional transects) to ensure an adequate number of transects were
completed to be representative of the vegetation community (Barbour et al. 1987), which
occurred after 19 transects in this study.
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(mean ± 1 SE; n = 26). The entire area within the belt transects (6 ha) exhibited
Deer overbrowsing.
Comparison of SPIDER and belt-transect methods
The objective of the SPIDER transect method was to quantify the area impacted
by Deer overbrowsing, and evaluation continued until there was a measurable
difference of the impact Deer had on woody vegetation (Table 1). In contrast,
according to established protocols, the objective of the traditional belt-transect
method is to describe the vegetation community and evaluation continued until
no new vegetation species were identified with the addition of more transects. The
SPIDER transect method had 3× less transects, although these were at least 20×
longer and evaluated an area 50× larger with 50% less time invested compared to
the belt-transect method. Browse-line height was measured on a similar number of
individual saplings and trees using both methods. However, due to differences in
objectives and protocols between methods, 100% of individuals evaluated using
the SPIDER transect method were species browsed by Deer compared to approximately
67% of individuals evaluated using the traditional belt-transect method.
Table 1. Qualitative comparison of Spherical Primary and Intermediate Directional Examination of
Resources (SPIDER) transect and traditional belt transect methods of vegetation assessment; specifically
examining deer use of resources (browse line height of woody plants) following established
protocols for each method.
Vegetation assessment method
Qualitative comparison SPIDER transect Traditional belt transect
Objective1 Evaluate impacts on woody Evaluate the woody vegetation
vegetation community from deer; community; specifically to
specifically to quantify the area adequately describe the
impacted by deer overbrowsing. vegetation community.
Rule objective was met Evaluation along transect continued Evaluation of transects
continued until 3 measurements in (number used) continued until
a row were ≤ the height woody no new vegetation species were
vegetation is most susceptible identified with the addition of
to deer browsing. more transects.
Number of transects 8 26
Length of transects (m) 1000–1750 50
# of individual saplings and trees2 170 170
Individuals browsed by deer 170 113
Total area within perimeter of 350 6.13
transects (ha)
Area exhibited overbrowsing( ha)3 304.5 6.13
Total time invested (days) 4 9
Time in field (days) 3 6
Time for data entry/analysis (days) 1 3
1The objective is a brief summary of the protocol for each method, specifically what each method
evaluates.
2Number of individual woody plants in which browse line height was measured.
3Overbrowsing is defined as browse-line height ≥56 cm, or higher than the height that woody species
are most susceptible to deer browsing.
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Discussion
The SPIDER transect method is a wildlife-centric assessment of how a
wildlife species impacts the vegetation community, whereas the traditional belttransect
method is a vegetation-centric evaluation of the vegetation community.
The SPIDER transect method is more rapid and quantifies the area Deer negatively
impact, which is not obtained using the traditional belt-transect method.
In addition, use of the SPIDER transect provided information regarding how
the species utilized resources across the landscape. For example, Deer heavily
browsed on woody species further from the central feed site when topography
was flat or gently sloping (North transect). Furthermore, Deer utilization of
woody vegetation was heterogeneous across the landscape (e.g., there were small
patches within the area impacted by Deer overbrowsing where browse-line height
was within typical range).
The SPIDER transect method is not intended to replace other vegetation-assessment
techniques, but can be quicker for assessing browsing impacts with fewer,
albeit longer, transects focused on evaluating resources impacted by the species of
interest. A primary application of this method is for species that concentrate activity
around a core area of use (feeder or man-made feed plot, water source, bedding/
denning site, etc.) and there is a definable change in the use of resources (browseline
height, abundance of a food resource highly selected for, standing crop of
annual forbs, etc.). Length of transects would not be pre-defined, but determined
from in-field evaluation using pre-established criteria to indicate that the species
has changed its use or impact on resources. However, the length of transects could
be fixed, for example based on the area of core use, home-range size, or study
enclosure. An advantage of the SPIDER transect method is the quantifiable determination
of the area impacted by particular wildlife species, with the assumption
that resources between transect lines are impacted in a similar manner as defined
along transect lines. Thus, the SPIDER transect could be beneficial for prescribing
and evaluating management recommendations.
Acknowledgments
I would like to thank A.E. Hershey for the use of equipment; J. Amoroso, B. Beck, E.
Beverly, and G. Queen for their assistance with ArcMap; K. Knight for assisting with field
research; and T.E. Fulbright, D.G. Hewitt, and S.L. Webb for reviewing this manuscript.
Funding for this project was provided through the Advisory Council with the University of
North Carolina at Greensboro’s Dean of Undergraduate Studies. Publication funding was
provided by Caesar Kleberg Wildlife Research Institute (CKWRI) at Texas A&M – Kingsville;
this is CKWRI publication number 15-113.
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Appendix A. Woody vegetation species identified in the belt transects conducted in the
campground area at Morrow Mountain State Park, Albemarle, NC. One of the protocols
for the belt transect method requires the identification of all vegetation species within each
transect to ensure an adequate number of transects were completed to be representative of
the vegetation community (see Fig. 3; Barbour et al. 1987).
Common name Scientific name
American Hazelnut Corylus americana Walter
American Holly Ilex opaca Aiton
Chalk Maple Acer leucoderme Small
Chestnut Oak Quercus montana Willd.
Eastern Redcedar Juniperus virginiana L.
Flowering Dogwood Cornus florida L.
Loblolly Pine Pinus taeda L.
Mockernut Hickory Carya tomentosa (Lam.) Nutt.
Muscadine Vitis rotundifolia Michx.
Red Maple Acer rubrum L.
Shagbark Hickory Carya ovata (Mill.) K. Koch
Shortleaf Pine Pinus echinata Mill.
Sourwood Oxydendrum arboretum (L.) DC.
Sweetgum Liquidambar styraciflua L.
Virginia Pine Pinus virginiana Mill.
Yellow Poplar Liriodendron tulipifera L.
Oak Species Quercus sp.
Unknown 1 -
Unknown 2 -