Measuring Regal Fritillary Butterfly (Speyeria idalia)
Habitat Requirements in South-Central Pennsylvania:
Implications for the Conservation of an Imperiled Butterfly
Mark T. Swartz, Betty Ferster, Kevina Vulinec, and Gregory Paulson
Northeastern Naturalist, Volume 22, Issue 4 (2015): 812–829
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22001155 NORTHEASTERN NATURALIST 2V2(o4l). :2821,2 N–8o2. 94
Measuring Regal Fritillary Butterfly (Speyeria idalia)
Habitat Requirements in South-Central Pennsylvania:
Implications for the Conservation of an Imperiled Butterfly
Mark T. Swartz1,*, Betty Ferster2, Kevina Vulinec3, and Gregory Paulson4
Abstract - To understand the habitat components that contribute to the presence of populations
of a rare butterfly, we examined the abundance of critical plant-components of old
fields that support some of the last remaining Eastern Speyeria idalia (Regal Fritillary Butterfly)
subpopulations at Fort Indiantown Gap (FTIG), a National Guard training facility in
south-central Pennsylvania. We compared densities of larval-host plants (Viola spp. [violets]),
adult-nectar plants (Asclepias spp. [native milkweeds] and Cirsium spp. [thistles]),
and native, tussock-forming, warm-season bunch grasses that provide protective resting and
pupation sites in fields occupied by the butterfly and in nearby fields that were unoccupied.
We found no significant difference in violet density among sites. Fields with Regal Fritillary
Butterfly populations had significantly more nectar-plant flowering structures and greater
bunch-grass percent cover. Grassland habitat occupied by Regal Fritillaries was characterized
by a violet density of at least 1.55 plants/m2 and particular varieties of flowering
nectar-plants available throughout the June–September flight period. Bunch grasses were
also important to persistence of Regal Fritillaries; occupied sites had 20–45% bunch-grass
cover and tussock formation. Understanding the habitat needs of this rare butterfly in Pennsylvania
is vital to its restoration and reintroductions of the eastern form in the mid-Atlantic
and northeastern US.
Introduction
Butterfly populations have declined worldwide since the 1970s, largely due to
habitat destruction (Gilbert and Singer 1975, Wallisdevries at al. 2012). One increasingly
rare butterfly, Speyeria idalia (Drury) (Regal Fritillary), is a grassland
endemic that until recently (Chazal 2014) persisted at 2 locations in the eastern US
(Schweitzer 1984, 1993, 2000; Swengel 1993). The only known eastern population
of this butterfly is at Fort Indiantown Gap National Guard Training Center
(FTIG) in Annville, PA (Ferster et al. 2008). At one time, this species’ geographic
range spanned across North America from New Brunswick to New England, south
to northern Georgia, west to Colorado, and north to Manitoba (NatureServe 2014,
Swengel 1993). It was listed as a Category II species under the US Endangered Species
Act until that category was eliminated in 1996 (USFW 1996). Survival of the
1The Pennsylvania Department of Military and Veterans Affairs, Fort Indiantown Gap
National Guard Training Center, Annville, PA 17003. 2Department of Biology, Gettysburg
College, 300 North Washington Street, Gettysburg, PA 17325. 3Department of Agriculture
and Natural Resources, Delaware State University, 1200 North DuPont Highway, Dover,
DE 19901. 4Department of Biology, Shippensburg University, 1871 Old Main Drive, Shippensburg,
PA 17257. *Corresponding author - markswartz@pa.gov.
Manuscript Editor: John E. Rawlins
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Regal Fritillary depends on 3 main habitat components: host plants for caterpillars,
nectar plants for adults (Kelly and Debinski 1998), and native warm-season bunch
grasses that provide protective sites for all life stages (Ferster and Vulinec 2010).
Populations of this butterfly may have vanished as grassland-habitat components
fell below critical density (Kelly and Debinski 1998). Pesticide use (Schildknecht
1986), genetic drift (Britten and Glasford 2002), and weather (Schweitzer 1993)
can each also lead to population extinctions. The FTIG population may represent an
evolutionarily distinct group that differs from western populations in a number of
morphological and genetic characteristics (Keyghobadi et al. 2006). Habitat requirements
may differ between western and eastern populations; thus, understanding the
species’ habitat needs in Pennsylvania is vital to restoration and reintroductions of
the eastern form in the mid-Atlantic and northeastern US. At FTIG, repeated disturbance
from military activities maintains open habitats (Ferster and Vulinec 2010,
Latham et al. 2007, Warren et al. 2007). By delaying succession of woody shrubs
and trees, disturbance promotes the growth of herbaceous plants essential to the
survival of the Regal Fritillary (Latham et al. 2007). Not all open habitats at FTIG
are occupied by Regal Fritillaries, perhaps because there is between-site variation
in critical-plant densities. Conservation efforts, including habitat restoration and
reintroduction, will be successful only if habitat requirements are well understood.
For this reason, we estimated the optimal density of each critical-plant component—
larval host Viola spp. (violets), nectar plants, and bunch grasses—necessary
to support a population of this once widespread, now vanishing species, in a series
of managed grasslands in south-central Pennsylvania.
Violets
Violets are the larval-host plants for all speyerid butterflies (Cech and Tudor
2005, Ferris and Brown 1981, Klots 1951). At FTIG, the only documented larvalhost
plant for the Regal Fritillary is Viola sagittata Ait. (Arrow-leaved Violet)
(Ferster and Vulinec 2010), although other violets serve as host plants throughout
the species’ range (Selby 2007, Swengel 1997). Several violet species occur at FTIG,
and V. cucullata Aiton (Marsh Blue Violet), V. lanceolata L. (Bog White Violet),
V. macloskeyi Lloyd (Small White Violet), V. pedata L. (Birdfoot Violet), Arrowleaved
Violet, and V. sororia Willd. (Common Blue Violet) have all been found in
occupied fields (TNC 2000a). Many violet species (including Arrow-leaved Violet)
are early-successional species common in open areas (Rhodes and Block 2007) that
do not compete well with taller plants. Ideal growing conditions for these violet
species can be created and maintained by moderate soil disturbances and occasional
wildfires that reduce competition (Latham et al. 2007). At FTIG, violet populations
increased fourfold following vehicle disturbance (tracked M113 armored personnel
carrier) and eightfold after a wildfire, but these changes were temporary (3 years
and 1 year, respectively; Latham et al. 2007). Lack of food-plant availability for caterpillars
may reduce genetic variability (by reducing population size), and increase
local extinctions in butterflies (Frankham and Ralls 1998, Kelly and Debinski 1999,
Saccheri et al. 1998). In midwestern fields, increased violet-population density was
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2015 Vol. 22, No. 4
positively correlated with adult Regal Fritillary female body size and abundance
(Kelly and Debinski 1998).
Bunch grasses
Bunch-grass tussocks form as the leaves of warm-season grasses die and build
up, leaving protective spaces within. The tussocks provide resting sites for adult
Regal Fritillaries during the summer (M.T. Swartz and B. Ferster, pers. observ.),
caterpillars in the winter and spring, and pupae in the late spring–early summer
(Ferster and Vulinec 2010). At FTIG, bunch-grass species include Schizachyrium
scoparium (Michx.) Nash (Little Bluestem), and to a lesser extent, Andropogon virginicus
L. (Broomsedge) (Latham et al. 2007). Both species are early-sere grasses
that rely on moderate disturbances for continued growth (Latham et al. 2007). The
time interval required for suitable tussock formation has not b een determined.
Nectar plants
Adult Regal Fritillaries drink nectar from spring- and summer-flowering
grassland forbs such as Asclepias syriaca L. (Common Milkweed), A. tuberosa
L. (Butterfly Milkweed), A. incarnata L. (Swamp Milkweed), Cirsium pumilum
Spreng. (Pasture Thistle), and C. discolor (Muhl. Ex Willd.) Spreng. (Field Thistle)
(Ferster and Vulinec 2010, Latham et al. 2007). Assessing nectar-plant abundance
is vital to understanding the ecology of Regal Fritillaries because nectar provides
adult butterflies with essential sugars for daily activities (Baker and Baker 1983).
Limitations on adult food resources are known to have adverse impacts on fecundity
and fertility in Regal Fritillaries (Wagner 1995) as well as in other species
of fritillaries (Boggs and Ross 1993, Hammond and McCorkle 1991). Elsewhere,
declines in nectar plants have been associated with declines in butterfly abundance
and diversity (Wallisdevries et al. 2012, Weber et al. 2008).
We hypothesized that Regal Fritillaries occupy fields that contain the 3 necessary
habitat components in sufficient quantities to support all life stages. We expected
to find unoccupied fields lacking (absent or at a density below some critical value)
one or more of these habitat components. Distribution and abundance patterns of
the species the Regal Fritillary depends on might explain why this butterfly has
disappeared from much of its northeastern and mid-Atlantic range. Comparing
habitat components of occupied and nearby unoccupied fields could therefore help
determine appropriate habitat-restoration efforts for reintroductions. We decided to
examine one long-monitored area (C4) to generate explanations for low butterflycounts
over years of population surveys there (Ferster and Vulinec 2010). If we
found that critical-habitat components were lacking in this area, the observed butterflies
would have likely been transients from nearby areas. These transients would
in turn not be expected to remain or survive there. The goals of our surveys were to
determine critical levels of Regal Fritillary’s 3 habitat components and to prescribe
precise habitat conservation and restoration goals, and ultimately, to increase the
probability of reintroduction success for this species.
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Field-site Description
FTIG is located in south-central Pennsylvania (40°26'13.15''N, 76°34'33.8''W),
on the Appalachian Plateau physiographic region and within the northeastern deciduous
forest biome (PAARNG 2006). The training areas consist of a mosaic of
woodland and field patches. The fields are primarily Little Bluestem–Carex pensylvanica
Lam. (Pennsylvania Sedge)-opening grassland communities (PAARNG
2006, Zimmerman et al. 2012) created by recent disturbances (Latham et al. 2007).
Ferster et al. (2008) provide a detailed field site description.
Some of the fields at FTIG have historically supported and/or currently support
Regal Fritillary populations. Five fields have been regularly monitored since
1998 because they support relatively large, persistent subpopulations (Ferster and
Vulinec 2010). During this time, 4 of these sites have been protected from most
military activities by a memorandum of understanding and later by a management
plan outlined in an environmental impact statement (PAARNG 2006 ). These fields
have been maintained by a haphazard combination of occasional burns, vehicle
disturbance, and manual stewardship (woody-plant removal and mowing; Ferster
and Vulinec 2010).
Methods
Specific research sites for this study were located in the training corridor and cantonment
portions of FTIG, within a 7.2-km-radius area (Fig. 1). Five of the 9 selected
sites (B12, D1, D3, R23, C4) were inhabited by Regal Fritillaries (occupied) and
monitored for their presence since 1998 (Ferster and Vulinec 2010). Four of these
sites (B12, D1, D3, R23) had been protected from most military activity since that
time. The remaining 4 sites (A1, A22, B7, B11) have not been studied in as much detail
but fall well within the grassland definitions of Zimmerman et al. (2012).
Sites that we considered unoccupied by the Regal Fritillary had historically harbored
populations or now contain grasses, violets, and nectar plants, and are within
dispersal distance of occupied fields (Ferster and Vulinec 2010). These unoccupied
fields have been maintained through irregular mowing (when mowing is required
for training, for haying in the fall, or for aesthetic purposes), occasional fires, and
vehicle maneuvering.
We chose sites A1, A22, B7, B11 for comparison because they were uninhabited
by Regal Fritillaries (unoccupied) at the time of Ferster and Vulinec’s (2010) study,
but at the time of our study, fell within the grassland definitions of Zimmerman et
al. (2012). Historic reports of Regal Fritillary occupancy exist from 3 of these sites
(A1, A22, and B7). Site B11 was forested before 2000, but when the site was then
cleared of trees, grassland plants including grasses, violets, and some nectar plants
appeared (M.T. Swartz, unpubl. data), indicating that this site may have once been
grassland as well. All 4 of these sites were included in annual presence–absence
field surveys for Regal Fritillaries established in 2000 (TNC 20 00b).
We treated the occupied fields D1 and D3 as a single site for all analyses
and fields B12 and C4 as a single site in some analyses. However, researchers
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2015 Vol. 22, No. 4
historically had treated these areas as 4 distinct sites because they were separated
by unpaved roads and/or narrow tree-lines and were designated as different military
training areas (Fig. 1). Distinguishing FTIG training fields D1 from D3, and B12
from C4 was a reasonable approach; Regal Fritillaries may not move between fields
readily. Central Iowa prairie populations responded strongly to prairie-habitat
edges—road, crop, field, and tree line—by turning toward the prairie patch (Ries
and Debinski 2001). Individuals also responded positively to conspecifics by being
more likely to stay in areas where population density was high (Ries and Debinski
2001). Prior to our study, 4 permanent population-monitoring transects had been
set up to monitor each of the 4 FTIG sites over time (Ferster and Vulinec 2010).
Later, Keyghobadi et al. (2006) demonstrated that the D1 and D3 populations were
not genetically distinct from one another. Thus, although the field-site distinctions
represented functional units to researchers and military personnel, they were not
biologically meaningful. Ferster and Vulinec (2010), therefore, merged D1 with D3
in population-size analyses. We collected plant abundance and density data separately
for D1 and D3, but combined these data for analysis (D).
Ferster and Vulinec (2010) did not consider C4 in population-size analyses because
of the small number of butterflies seen there; however, they noted the close
proximity of C4 to B12 and that 2 of the 5 butterflies caught in C4 during the 2005
mark–recapture study had been marked in B12. Keyghobadi et al. (2006) were not
Figure 1. Map of study sites at Fort Indiantown Gap, Annville, PA. Occupied sites are occupied
by Regal Fritillary (Speyeria idalia). No Regal Fritillaries were observed in unoccupied
sites during the 8 years (1998–2005) of monitoring populations.
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able to analyze C4 butterflies because they caught no Regal Fritillary butterflies
there for that study.
Ferster and Vulinec (2010) and other researchers have questioned whether C4
represented habitat that supported few Regal Fritillaries or if it merely attracted
adults from a nearby population (B12). We treated C4 data in 2 ways: (1) we merged
B12 and C4 plant data for analysis because the site designations do not represent
separate populations of the Regal Fritillary (see lumped designation in results below),
and (2) we treated them as separate sites because of the possibility that some
adult butterflies from B12 might simply wander over to C4 often enough to be seen
by researchers, but not remain there among the separate population based in C4 (see
unlumped designation in results).
Violets and bunch grass
Vegetation plots in occupied sites were originally sampled for violet and bunchgrass
densities during 5 May–25 August 2001 (Latham et al. 2007) and then again
during 28 April–19 August 2004. We assigned sampling points throughout sites
using a random-point generator in ArcView GIS 3.2 (ArcView 3.2 ESRI GIS and
Mapping Software, Redlands CA). We generated 1 point per 0.40 ha for each site.
We used a Trimble Total Station 5700 GPS unit (Trimble Navigation Ltd., Sunnyvale,
CA) to locate points in the field. We placed a 2-m2 census quadrat at each point
and calculated the collective rosette density for all violet species. We considered
the violet species collectively because Regal Fritillary larvae reportedly consume
multiple species of violets (Selby 2007, Swengel 1997), although we found very
few species other than Arrow-leaved Violet at FTIG sites (M.T. Swartz and B. Ferster,
pers. observ.). We visually estimated bunch-grass abundance as a proportion of
groundcover and estimated cover as 0%, 1%, or the nearest multiple of 5%. In 2004,
we re-evaluated occupied sites and established new sample-points for unoccupied
sites not sampled in 2001. There was a difference in sampling effort in occupied
areas over time. In 2001, 244 of 265 plots were sampled and in 2004 all sites were
sampled. To compensate for the difference in number of sites sampled between
years, we only used the 244 plots sampled in both years for time-series comparisons
(2001 vs 2004). We sampled a total of 107 plots in unoccupied sites in 2004.
Nectar-plant identity
Nectar plants utilized by Regal Fritillaries in occupied fields were identified at
FTIG over 8 years of butterfly population surveys from 1998–2005 (Ferster and
Vulinec 2010). Researchers established permanent survey routes—Pollard-walk
transects (Pollard and Yates 1993)—in each occupied site and walked them weekly
during the adult flight period each year to generate Regal Fritillary population estimates
and to identify nectar plants favored by butterflies. Researchers noted the
gender and location of each adult Regal Fritillary sighted as they walked routes at a
steady pace during a narrow range of environmental conditions. Methods and most
results of the Regal Fritillary population surveys are reported elsewhere (Ferster and
Vulinec 2010). When we encountered Regal Fritillaries foraging on flowers during
our surveys, we recorded the plant species and report the results here (Table 1).
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Nectar-plant abundance
At each site, we counted and determined the number of flowering stems or
heads/ha (density) for each of the 5 primary nectar plants—Common Milkweed,
Butterfly Milkweed, Swamp Milkweed, Pasture Thistle, and Field Thistle. Surveyors
were spaced 2 m apart and they moved through the field in a line at a similar
pace while counting nectar plants. We used flagging tape to mark the edges of each
pass through a field to avoid double counting. For milkweeds, we counted flowering
stems, but we counted flowering heads on each thistle because we thought that this
method would more accurately estimate nectar abundance of perennial plant species
with very different types of inflorescences (e.g., umbels vs. composite heads)
and large variation in the number of flowering heads/plant in a population. In order
to catch peak flowering of both the early-flowering Common Milkweed and Butterfly
Milkweed and the later-flowering Swamp Milkweed, Pasture Thistle, and
Field Thistle, we surveyed each field twice during the field season. We conducted
nectar-plant surveys between 21 June and 20 September 2005. We counted plants
over a total of 37 days.
Analysis
We used the Mann-Whitney U-test with the Dunn-Sidak correction for multiple
comparisons (Sokal and Rohlf 1995) to detect the difference in violet density (rosettes/
m2), bunch-grass abundance (% cover), and nectar-plant density (# flowering
stems and heads/ha) between occupied sites and unoccupied sites. We employed
Table 1. Nectar-plant use by the Regal Fritillary (Speyeria idalia) as observed over 8 years (1998–
2005) of data collection along permanent Pollard-walk transects in S. idalia-occupied sites at Fort
Indiantown Gap, Annville, PA. We made a total of 1032 S. idalia nectaring observations during which
we identified 14 nectar-plant species. Three Cirsium (thistle) species—C. discolor (Muhl.) Spreng.,
C. pumilum (Nutt.) Spreng., and the exotic C. vulgare (Savi) Ten.—were lumped here because early
data collection did not distinguish the species. See text for information about our treatment of Wild
Bergamot.
# of nectaring
Plant species Common name observations % nectaring
Achillea millefolium L. Common Yarrow 4 0.39%
Apocynum spp. Dogbane 12 1.16%
Asclepias incarnata L. Swamp Milkweed 8 0.78%
Asclepias syriaca L. Common Milkweed 50 4.84%
Asclepias tuberosa L. Butterfly Milkweed 510 49.40%
Centaurea nigrescens Willd. Knapweed 45 4.36%
Chrysanthemum leucanthemum L. Ox-Eye Daisy 1 less than 0.01%
Cirsium spp. Thistles 395 38.28%
Cichorium intybus L. Blue Chicory 1 less than 0.01%
Dianthus armeria L. Deptford Pink 2 less than 0.01%
Monarda fistulosa L. Wild Bergamot n/a n/a
Solidago spp. Goldenrod 2 less than 0.01%
Vernonia noveboracensis (L.) Michx. New York Ironweed 1 less than 0.01%
Pycnanthemum spp. Mountain Mint 1 less than 0.01%
Total 1032 100.00%
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the Wilcoxon signed-rank test to examine changes in violet density (rosettes/m2)
and bunch-grass abundance (% cover) from 2001 to 2004. We chose non-parametric
tests because each treatment (occupied and unoccupied) had ≤5 replicates. Therefore,
we reported the median and inter-quartile range (Q1–Q3) rather than the mean
and standard deviation. All data analyses were conducted in the statistical software
SigmaPlot 11.0 (Systat Software, San Jose, CA).
Results
Violets
We did not detect significant differences in violet density between occupied and
unoccupied sites over time (2001 vs. 2004) (lumped: Z = -1.60, df = 2, P = 0.44;
unlumped: Z = -1.83, df = 3, P = 0.24; Table 2) or in 2004 (lumped: U = 3.00, df = 1,
P = 0.61; unlumped: U = 4.00, df = 1, P = 0.53; Table 3). In 2001, occupied sites
had slightly higher violet densities than in 2004 (Table 2), regardless of site lumping.
Site C4 showed the greatest decrease in violet density over time, dropping from
5.37 plants m2 in 2001 (Latham et al. 2007) to 1.82 plants m 2 in 2004.
In 2004, an unoccupied site (B7) had the highest overall violet density (4.53
violet rosettes/m2; Table 3). Violet densities in the remaining unoccupied sites were
below those of occupied sites. Violet density in occupied sites ranged from 1.55 to
2.67 violet rosettes/m2 (median = 1.60 violet rosettes/m2).
Bunch grass
We did not detect significant differences in bunch-grass abundance between occupied
and unoccupied sites in 2004 when lumped (U = 0.00, df = 1, P = 0.10), but
the difference was nearly significant when unlumped (U = 0.00, df = 1, P = 0.06)
(Table 3). Bunch-grass abundance in C4 was well within the range of what we
observed in the other occupied sites and was greater than the abundance recorded
Table 2. Violet density (VD; rosettes/m2) and bunch-grass abundance (BG; % cover) in Regal Fritillary
(Speyeria idalia)-occupied fields in 2001 (Latham et al. 2007) and 2004. One occupied site (C4)
was known to attract individuals from a nearby occupied site (B12) (Ferster and Vulinec 2010) and,
therefore, may be used as a single site by S. idalia. To examine this possibility, we considered these
data two ways—(1) lumping (LUMP): sites C4 and nearby B12 into one replicate and (2) treating each
site as separate replicates (UNLUMP). Ranges shown are all quar tile 1–quartile 3.
Violet density (rosettes/m2) Bunch-grass cover (% cover) P-value
Site (n = 244) 2001 2004 2001 2004 (VD/BG)
C4 (n = 42) 5.37 1.82 16.79 19.32
B12 (n = 35) 3.99 3.69 13.32 20.11
B12/C4LUMP (n = 77) 4.73 2.67 15.21 19.68
R23 (n = 56) 2.19 1.60 34.40 43.25
D (n = 111) 1.92 1.55 22.57 24.14
MedianLUMP 2.19 1.60 22.57 23.07 0.44/0.44
(range) (1.99–4.10) (1.48–2.40) (17.50–31.44) (20.53–38.21)
MedianUNLUMP 3.09 1.71 19.68 21.59 0.24/0.24
(range) (2.06–4.68) (1.52–2.76) (15.06–28.49) (19.72–33.16)
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Table 3 (continued on following page). Violet density (rosettes/m2), bunch-grass abundance (% cover), total nectar-plant density (flowering stems and
heads/ha), and individual nectar-species densities (flowering stems or heads/ha) in Regal Fritillary (Speyeria idalia)-occupied (O) and -unoccupied (U)
sites during the study period. One occupied site (C4) has been known to attract individuals from a nearby occupied site (B12) (Ferster and Vulinec 2010)
and, therefore, may be considered by S. idalia as one site. To examine this possibility, these data were considered two ways: (1) lumping (L) sites C4 and
nearby B12 into one replicate and (2) treating each site as separate replicates or “unlumped” (UL). Q = Quartile, n = # of plots. *denotes a significant difference.
Monarda fistulosa (Wild Bergamot) was not included in total nectar-plant density analysis.
Unoccupied (n = 107; total = 43.32 ha) Occupied (n = 265; total = 90.32 ha)
Resource categories A1 A22 B11 B7 C4 B12 C4/B12L D R23
n 46 24 27 10 42 35 77 132 56
Size (ha) 18.62 9.72 10.93 4.05 17.00 14.16 31.16 53.41 22.66
Violet density 0.92 0.18 0.38 4.53 1.82 3.69 2.67 1.55 1.60
Bunch-grass abundance 4.04 0.01 0.06 2.53 19.32 20.11 19.68 24.14 43.25
Total nectar-plant density 37.54 32.62 0.00 72.62 1562.98 1790.55 1666.41 605.53 4174.62
Nectar plant density by individual species
Common Milkweed (flowering stems/ha) 1.99 13.48 0.00 0.49 15.56 344.89 165.22 3.06 28.0
Butterfly Milkweed (flowering stems/ha) 0.70 13.89 0.00 13.59 27.43 123.85 71.25 19.95 22.55
Swamp Milkweed (flowering stems/ha) 0.05 3.91 0.00 11.36 1.89 9.81 5.49 0.09 0.07
Pasture Thistle (flowering heads/ha) 1.72 1.34 0.00 5.68 20.50 5.46 13.67 77.89 136.10
Field Thistle (flowering heads/ha) 33.08 0.00 0.00 41.50 1497.60 1306.58 1410.80 504.53 3987.85
Wild Bergamot (flowering stems/ha) 0.00 420.68 0.00 0.00 0.00 6.58 2.98 0.00 40.77
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Table 3, continued from previous page.
MedianU MedianLO MedianULO P-value
Resource categories (Q1–Q3) (Q1–Q3) (Q1–Q3) (L/UL)
Violet density 0.65 (0.28–2.73) 1.60 (1.56–2.40) 1.71 (1.58–2.76) 0.61/0.53
Bunch-grass abundance 1.30 (0.04–3.29) 24.18 (20.81–38.48) 22.15 (19.72–33.72) 0.10/0.06
Total nectar-plant density 35.58 (16.81–55.08) 1666.41 (870.75–3547.57) 1676.79 (1084.26–2982.61) 0.06/0.03*
Nectar plant density by individual species
Common Milkweed (flowering stems/ha) 1.24 (0.25–7.74) 28.05 (9.31–130.93) 21.81 (9.31–186.47) 0.45/0.27
Butterfly Milkweed (flowering stems/ha) 7.15 (0.35–13.74) 22.55 (20.06–59.08) 24.99 (21.25–75.64) 0.23/0.14
Swamp Milkweed (flowering stems/ha) 1.98 (0.03–7.64) 0.09 (0.08–4.14) 0.99 (0.08–5.85) 1.00/1.00
Pasture Thistle (flowering heads/ha) 1.53 (0.67–3.70) 77.89 (29.73–121.55) 49.20 (12.98–107.00) 0.23/0.27
Field Thistle (flowering heads/ha) 16.54 (0.00–37.29) 1410.80 (731.10–3343.59) 1402.09 (905.46–2742.73) 0.23/0.14
Wild Bergamot (flowering stems/ha) 0.00 (0.00–105.17) 3.29 (0.00–15.13) 2.98 (1.49–21.88) Not tested
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2015 Vol. 22, No. 4
at any unoccupied site (Table 3). We found no significant change in bunch-grass
density over time regardless of site lumping (Table 2).
Nectar plants
We observed 1032 adult Regal Fritillaries nectaring on 14 different floweringplant
species over the 8 years of surveys (Table 1). Thistle species were not
distinguished during early data collection, but later observations included 3 species—
the native species Field Thistle and Pasture Thistle and less frequently, the
exotic C. arvense (L.) Scop. (Canada Thistle)—which we lumped for our analyses.
We rarely observed Regal Fritillaries feeding on most nectar-plant species
(less than 0.01%); most documented nectaring was on the native thistle and milkweed
species for which we later determined density.
Median nectar-plant density was not significantly different between occupied
and unoccupied sites when we lumped occupied sites (U = 0.00, df = 1, P = 0.06)
but was significantly different when we analyzed occupied sites separately (unlumped)
(U = 0.00, df = 1, P = 0.03) (Table 3). Site R23 had an overall nectar-plant
density greater than twice that of the next-highest site (Table 3). Occupied sites had
more than 45 times the number of total stems and heads/ha than unoccupied sites
(Table 3). Site C4 had 1562.98 stems and heads/ha and the second-highest density
of Field Thistle flowering heads (Table 3).
All but 1 nectar-plant species were more abundant in occupied than in unoccupied
sites (Table 3). The density of Field Thistle heads in unoccupied sites was
only ~1% of what we observed in occupied sites, and the density of Pasture Thistle
heads in unoccupied sites was only 2–3% of the density found in occupied sites.
Butterfly Milkweed flowering stems in unoccupied sites comprised ~30% of the
abundance of occupied sites, and Common Milkweed flowering-stem abundance in
unoccupied sites was only 5–6% of that found in occupied sites. Only Swamp Milkweed
was more abundant in unoccupied sites; however, these data were most likely
skewed by the presence of this species in large numbers at one unoccupied site, and
was nearly absent altogether from 2 occupied sites (Table 3). Researchers planted
Monarda fistulosa L. (Wild Bergamot) in occupied sites (M.T. Swartz unpubl. data,
TNC 2005) and it occurs naturally at one unoccupied site. Thus, we counted this
species at each site, but did not include it in the statistical analysis (Table 3). Three
unoccupied sites (A1, A22, and B7) contained nectar plants that never flowered;
therefore we did not count them because butterflies did not use them. Site C4 (occupied)
was more similar in flowering stem and head abundance to occupied areas
than unoccupied areas for all nectar plant species.
Discussion
Occupied sites had nearly statistically significantly more flower structures (i.e.,
more nectar) and bunch grasses than unoccupied sites. Violet density was also higher
in occupied sites than in 3 of the 4 unoccupied sites, though we found no statistically
significant difference. Violets were most abundant in unoccupied site B7, suggesting
that high violet abundance alone does not support a Regal Fritillary population.
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M.T. Swartz, B. Ferster, K. Vulinec, and G. Paulson
2015
823
The enigmatic site, C4, harbored all 3 critical Regal Fritillary habitat components—
larval-host plants, adult-nectar plants, and protective bunch grasses—at
abundance levels similar to occupied sites. C4 was in close proximity to occupied
sites B12 and R23 (Fig. 1) and caterpillars have been found on violets there, yet this
site harbored only a few individual adults each year of the long-term monitoring
study that began in 1998 (Ferster and Vulinec 2010). Our data did not allow us to
explain the low number of adult occurrences here. This site would be optimal as an
experimental plot for future research to test population establishment in areas where
habitat components have been restored and that are within dispersal distance from
an existing population. Future studies may allow us to examine other environmental
factors that impact survivorship of late juvenile stages, thereby decreasing adult
population density. The effects of disease and microclimate must also be examined.
Swamp Milkweed may not be essential for Regal Fritillary survival at FTIG
where other nectar sources overlap in flowering phenology. The low abundance of
Swamp Milkweed in D and R23 (Table 2) suggests that although Regal Fritillaries
use this nectar plant (Table 1), a population can persist as long as other nectar
sources are available. Swamp Milkweed was more abundant in old-fields at Gettysburg
National Park, which historically supported Regal Fritillaries (Schildknecht
1986) and where reintroduction efforts failed in 2005 (Ferster 2005).
Common Milkweed blooms in early June and is used by this butterfly at FTIG
and elsewhere (Opler and Krizec 1984, Swengel 1993). Its flowering corresponds
to the first male Regal Fritillary emergences each year at FTIG (Ferster and Vulinec
2010). Therefore, although in low abundance in some fields at FTIG, Common
Milkweed is an important resource for the butterfly. We recommend that habitat
restoration and reintroduction efforts include Common Milkweed as a nectar plant.
Butterfly Milkweed, is a low-growing, perennial plant that flowers during July–
August (Rhoads and Block 2007) and is used frequently by nectaring Regal Fritillaries
(Table 1). It was abundant at 2 of the unoccupied sites, but apparently cannot
support Regal Fritillary populations alone. Butterfly Milkweed is an important
habitat component that provides nectar resources during mid-summer when other
nectar plants are not blooming.
Farmers, ranchers, and other land stewards often target thistles for removal.
To that end, multiple exotic thistle species appear on noxious plant lists for many
states (PDA 2003). However, 2 native species are important butterfly nectar-plants
(Table 1). Pasture Thistle has a short flowering period at FTIG during the summer
season (B. Ferster, pers. observ.), but Regal Fritillaries are frequently observed
nectaring on these flowers when they are in bloom (Table 1). All but 1 occupied
site (B12) had higher Pasture Thistle flower-head counts than the unoccupied sites.
The occupied sites had the highest abundance of the 2 longest-flowering nectar
plants, Common Milkweed and Butterfly Milkweed. Perhaps these species can act
to bridge the gap in flowering phenology filled by Pasture Thistle in other fields.
Stable Regal Fritillary populations (Ferster and Vulinec 2010) were associated
with violet densities of at least 1.55 violets/m2, bunch grasses between 20% and
45% cover, and maintenance of milkweeds and thistles at species-specific densities
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2015 Vol. 22, No. 4
(Table 3). Common Milkweed is essential to butterfly survival but does not need
to be present at high abundance. Butterfly Milkweed and Field Thistle comprise
much of the nectar-plant community for adult butterflies. Wild Bergamot has also
been reported to be an important Regal Fritillary nectar plant (Huebschman 1998).
However, this nectar-plant species was not found in occupied fields at FTIG until
land stewards planted it in some occupied fields in 2003, 2004, and 2005 in an
effort to increase nectar-plant abundance (TNC 2005). Butterflies have been seen
nectaring on this species; however, it represents a small proportion of nectar plants
in these fields, and populations of Regal Fritillaries persisted at FTIG without Wild
Bergamot. Although it is apparently not an obligate nectar plant for Regal Fritillaries,
we recommend that Wild Bergamot be included in restorations because so few
nectar-plant species are used by this rare butterfly; however, we cannot suggest an
appropriate planting density.
Keyghobadi et al. (2013) suggest that the remaining Pennsylvania Regal Fritillary
population has a haplotype that is genetically distinct from the western
haplotype that includes the isolated (and probably extinct) Virginia population
(Chazal 2014). Their study, which included museum specimens collected from
now-extinct populations and fresh specimens from surviving populations, supports
earlier findings by Williams (2001a, b) who suggested subspecies designation for
the 2 groups (S. i. idalia for the eastern subspecies, and S. i. occidentalis for the
western subspecies), with a transition zone centered on western PA, OH, WV, and
VA. Keyghobadi et al. (2013) found little evidence to support a prolonged period of
isolation between the eastern and western groups. They suggested that genetic and
morphological differences resulted from strong selection in different parts of the
range where butterflies occupy different habitats. They cite Opler and Krizek (1984)
and Swengel (1997) to support the hypothesis that Eastern Regal Fritillary populations
preferred wet meadows and swampy areas. Swengel’s (1997) study examined
the western form and habitat and host-plant partitioning among violet-feeding
fritillaries (Euptoieta, Speyeria, Boloria). Although the study mentions populations
“outside the prairie”, it does not indicate that this refers to eastern populations
specifically. At FTIG, Regal Fritillary habitat consists mostly of dry fields where
the necessary vegetative habitat components can be found. Arrow-leaved Violet,
the violet used as the larval host plant at FTIG is a species of “dry woods, fields, and
edges” throughout Pennsylvania according to Rhoads and Block (2007), who also
report that Butterfly Milkweed is found most often in “dry fields, roadsides, and
shale barrens.” Among the important nectar plants we have seen Regal Fritillaries
feeding on at FTIG, only Swamp Milkweed is a plant of “swamps, floodplains, and
wet meadows” (Rhoads and Block 2007). It is likely that adults will nectar at wet
meadows, but it is unlikely they oviposit there, or that caterpillars could survive in
this habitat. Reintroduction efforts in eastern locations should focus on dry fields
for habitat restoration, and wet meadows should be restored to increase nectar-plant
availability for populations established in nearby dry field sites. Indeed, habitat partitioning
(Keyghobadi et al. 2013) should be reconsidered as an important selective
pressure contributing to subspecific integrity.
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M.T. Swartz, B. Ferster, K. Vulinec, and G. Paulson
2015
825
Intact ecosystems provide goods and services (such as food, pollination, nutrient
cycling, carbon sequestration, etc.) that diminish as these ecosystems are modified
by human activity. Low levels of invertebrate diversity can be an indicator of ecosystem
degradation because these organisms occupy a wide variety of functional
niches (Kremen et al. 1993). The disappearance of the Regal Fritillary across much
of its historic range likely indicates that land-use practices since the 1970s, or perhaps
earlier, have degraded grassland ecosystems (Ferster and Vulinec 2010). The
butterfly is even absent from grasslands that are protected and managed for other
wildlife, perhaps an indication that management practices are insufficient for effectively
maintaining biodiversity in these ecosystems. Many butterfly species have
evolved in close association with their host plants and are mono- or oligophagous
and thus highly dependent on particular plants (Ehrlich and Raven 1964, Fordyce
2010). Indeed, declines in larval host-plants and/or adult nectar-plants have been
associated with many declining butterfly populations (Pleasants and Oberhauser
2013, Schultz and Dlugosch 1999, Severns and Warren 2008, Wallisdevries et al.
2012), and unless butterflies can switch host plants and/or nectar plants to more
common species, they may be destined for extinction (Severns and Warren 2008).
Conservation efforts aimed at butterflies must focus on maintaining the particular
plant communities that butterflies rely on, or they will fail. Successful efforts to
manage habitats for butterflies may contrast with methods used in conservation
for species that are less tied to particular plants (i.e., mowing at 2–3 year intervals
for grassland birds; Johst et al. 2006, Rothbart and Capel 2006). Mowing in unoccupied
fields prevents tussocks from forming, decreases bunch-grass density and
percent cover when it is being grown from seed, and promotes growth of weedy,
cool-season grasses (Fedewa and Stewart 2011). Mowing regimes used in management
for hayfields (i.e., twice per year) do not support most grassland-specialist
butterflies (Johst et al. 2006; Swengel 1996, 1997). Mowing may also encourage
cool-season grasses and other invasive vegetation by reducing competition from
bunch grasses (Fedewa and Stewart 2011). Few butterflies may survive certain
management and land-use strategies, reducing butterfly population persistence. In
this study, we determined the densities of important plant species necessary for
the persistence of Regal Fritillary populations. This information combined with the
understanding of how these plants respond to various types of disturbance (Latham
et al. 2007) can be used to formulate effective management strategies for both stewardship
of existing populations and restoration of grasslands for successful Regal
Fritillary reintroduction efforts in the mid-Atlantic and Northeast.
The habitat that currently supports the rare Regal Fritillary Butterfly was accidently
and haphazardly established and maintained, and only recently managed
for butterflies and other biological elements (PAARNG 2006). Determining how
best to mimic these conditions for conservation and reintroduction are questions we
continue to explore and understand.
Soil disturbance such as that caused by track vehicles (Latham et al. 2007), or
perhaps bison (Ferster and Vulinec 2010), is more effective at increasing violet
and bunch-grass densities than mowing. We know that 5 years after such a soil
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2015 Vol. 22, No. 4
disturbance, grasslands can support a Regal Fritillary Butterfly population (Ferster
and Vulinec 2010), but we do not know how long this “good habitat” lasts. Given
too much time without further management, violet populations, tussock-forming
grasses, and nectar plants are shaded out as taller perennials and then woody plants
become established and reduce essential habitat-components below some critical
density. Mowing may extend the health of established grasslands that would
otherwise succeed into forests by reducing taller perennials and woody plants, but
the frequency of mowing and how long this treatment will successfully maintain
habitat must both be established.
Acknowledgments
Fred Habegger, Denise Johnson Watts, Rattiford P.E. Jones, Andrew Mehring, Dr. Walter
E. Meshaka Jr., Dave Zapotok, and Lindsay Zemba all enthusiastically assisted with fieldwork
and data entry. Dr. Larry Klotz, Dr. Walter E. Meshaka Jr., Burnetta Swartz, Virginia
Tilden, Dr. Richard Stewart, Ann B. Swengel, and anonymous reviewers provided invaluable
comments to earlier drafts of this manuscript. The Pennsylvania Army National Guard
(especially range-control personnel) granted safe access to fiel d sites and vehicles for field
use. This project was sponsored by the PAARNG (Cooperative Agreement # DAHA36-01-
2-9001). Funding for this project was also provided by the Pennsylvania Department of
Military and Veterans Affairs (DMVA). The information presented here does not necessarily
reflect the position or the policy of the US government, and no official endorsement should
be inferred.
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