Population Structure of the Endangered Mitchell's Satyr, Neonympha mitchellii mitchellii (French):...

Population Structure of the Endangered Mitchell’s Satyr,Neonympha mitchellii mitchellii (French): Implications for

Conservation

JENNIFER SZYMANSKI1

U.S. Fish and Wildlife Service, Division of Endangered Species, 555 Lester Avenue, Onalaska,Wisconsin 54650

JOHN A. SHUEY2

The Nature Conservancy, Indiana Chapter, 1330 West 38th Street, Indianapolis 46208

AND

KAREN OBERHAUSER3

University of Minnesota, 100 Department of Ecology, 1987 Upper Buford Circle, St. Paul 55108

ABSTRACT.—Basic ignorance about the ecology and life history of the endangered Mitchell’ssatyr butterfly, Neonympha mitchellii mitchellii, is impeding conservation efforts. To assist withrecovery, we examined Mitchell’s satyr butterfly population structure using mark-release-recapture techniques at two sites in southwestern Michigan in 1997 and 1998. During thesingle annual flight period spanning 2 to 3 wk from mid June to late July, the sex ratiochanged over time, with males predominating early in the flight period and females in thelatter half of the season. Scott’s Method 1 and recapture decay plotting showed averageresidence times of 1.4 to 5.3 d. Males were more catchable than females, a probableconsequence of behavioral differences between the sexes. Males were commonly foundpatrolling through the vegetation in search of mates, whereas females were rarely observed inundisturbed flight. Daily population estimates were obtained by the Jolly stochastic methodand total brood size and flight area population densities were calculated using estimatedresidence rates. All population size estimates were low. Total brood size estimates ranged from164 to 372. Peak daily flight area density ranged between 70 and 159 butterflies per ha. Adultmovement was limited. Maximum range estimates were 290 m and 420 m at the two sites.Average movement distances were 37 m and 33 m. Male butterflies moved significantlygreater distances than females. Remnants of a metapopulation structure are apparent, butthe data are insufficient to differentiate between an extinction—recolonization structure anda source—sink structure. Butterfly movements between habitat patches within occupied sitesoccur, albeit rarely. Short residence times, low density and sedentary behavior make extantpopulations vulnerable to environmental stochasticity and human disturbance. Conservationactions need to consider the implications of these important life history traits if we are toreduce local extinction probabilities and recover Mitchell’s satyr butterfly.

INTRODUCTION

Mitchell’s satyr (Neonympha mitchellii mitchellii French, 1889) is one of the most imperiledbutterflies protected under the US Endangered Species Act of 1973. The subspecieshistorically occurred in five states; today it is relegated to a few discrete populations insouthern Michigan and adjacent Indiana. The primary threats that led to its federal

1 e-mail: [email protected] e-mail: [email protected] e-mail: [email protected]

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Am. Midl. Nat. 152:304–322

protective status include direct habitat loss and modification, loss of habitat to succession,wetland fragmentation and collection (U.S. Fish and Wildlife Service, 1992).

Unfortunately, Mitchell’s satyr is also one of the most poorly understood butterflies inNorth America and ignorance about the details of its ecology and life history requirementshave hindered conservation efforts (Shuey, 1997). The remaining few habitats supportingthis species are peatlands that range along a continuum from prairie/bog fen to sedgemeadow/swamp. All historical and active habitats have a sedge-dominated herbaceouscommunity, with a conspicuous deciduous or coniferous shrub component. Typical fencomplexes are rarely homogeneous systems, but rather include mosaics of open, shrubbyand forested communities and associated ecotones. Mitchell’s satyr seems to occupy areaswhere woody and herbaceous vegetation occur as a mosaic, and the butterfly favors earlysuccessional habitats dominated by sedge meadows (Shuey, 1985).

Neonympha mitchellii mitchellii is univoltine, with the flight period occurring in midsummer.Females oviposit on a variety of small (,3 cm) forbs and sedges (e.g., Thelypteris palustris andCarex spp. seedlings, Szymanski, pers. obs.) growing below dense stands of sedge. Shuey(1997) suggested that Carex stricta is the primary foodplant, but preliminary results froma foodplant study indicate that other Carex species are also acceptable to the larvae(Szymanski, 2004). Larvae undergo three molts before entering diapause in the fall and twoadditional molts in the following spring and pupate in mid-June (McAlpine et al., 1960).Although quantitative population estimates are lacking, anecdotal reports of ‘‘hundredsof butterflies’’ (Pallister, 1927; McAlpine et al., 1960) relative to the ‘‘few adult’’ sightings(usually fewer than 10 adults observed per hour) in recent years suggest N. m. mitchellii hasdeclined substantially (Shuey, 1997). These small functional populations may render thespecies at risk from problems ranging from inbreeding and genetic drift to high extinctionprobabilities due to stochastic environmental events. Further, individuals do not move greatdistances, increasing the potential for genetic deterioration and limiting potential move-ment between and within fen and sedge meadow complexes.

We initiated a series of field studies in 1997, 1998 and 1999 designed to fill critical gaps inour understanding of Mitchell’s satyr. These included life history studies, habitat assessmentand quantification of occupied and unoccupied habitats. Here we report on studiesdesigned to illuminate population structure at two sites in southwestern Michigan over a 2-yperiod. The objectives were to: (1) ascertain specific demographic parameters and (2)discern the spatial structure of the populations.

METHODS

STUDY SITES

We conducted field research at two sites in Berrien County, Michigan, U.S. during 1997and 1998. Blue Creek Fen (BCF) and Sarett Nature Center (SNC) are located within the St.Joseph River drainage approximately 3 km apart. Both sites occur on peat, clay and sandy soilsin oak-forested river valleys. Blue Creek Fen is a 10.4 ha linear wetland with distinct vegetationcommunities. Mitchell’s satyr has been documented in eight habitat patches at this site.Although all were occupied as recently as 1992 (Shuey et al., 1992), only three of the patcheswere occupied during this study. Access to one of the three occupied patches was denied, butthe remaining two were comprehensively surveyed. One of the two accessible patches is a 1.4ha mosaic of sedge meadow and scattered deciduous shrubs. The second patch is a 0.9 hatamarack savanna separated from the first by 230 m of thick shrub swamp and carr.

Sarett Nature Center includes a 6.8 ha peatland with suitable habitat limited to twodistinct areas. One patch is approximately 1.4 ha, and is characterized as open fen grading

305SZYMANSKI ET AL.: MITCHELL’S SATYR POPULATION2004

into thick shrub carr. The second patch, which is separated from the first by 290 m, is a small(0.2 ha) sedge opening in an otherwise closed-canopy forested wetland.

Mark-release-recapture.—During the summers of 1997 and 1998 we undertook a mark-release-recapture (MRR) study of Mitchell’s satyr. In 1997 MRR activities took place from 8July to 20 July and in 1998 from 24 June to 8 July. Observations of Neonympha mitchelliimitchellii at other locations 2 d prior to and 4 d following MRR activities, and sex-specificcapture trends (see below), suggest that the 1997 MRR program did not cover the first andlatter part of the flight season. Thus, both sexes were probably under-sampled, with initialmale emergence missed by 1 or 2 d and the female flight period shortened by 2 or 3 d in thelatter part of the season. MRR in 1998 covered the entire flight season. Each sampling daywe conducted MRR for approximately 3 h at each site walking through all suitable habitatpatches. We marked each captured butterfly with a unique number on both ventralhindwings using a fine-point, black ink Sharpie and recorded butterfly number, date, timeof day, sex, site, patch location and physical condition for each capture and recapture event.Butterfly condition was recorded as ‘‘fresh,’’ ‘‘worn’’ or ‘‘tattered.’’ Whenever possible, weidentified previously marked butterflies without handling. We marked all capture andrecapture locations with coded flags and used a Trimble GeoExplorer Global PositioningSystem (GPS) receiver to determine these capture/recapture sites in the field. We collectedat least 120 points for each location and corrected them via post-processing DifferentialGPS-using Trimble PathFinder software. Post-processing base station files from theUniversity of Indiana-Fort Wayne base station, located at 41807908.84420 North,85803953.75790 West and 221.715 m (HAE), were used (approximately 100 km from thefield sites). We calculated receiver error, using a known Geodetic marker, as 63 meters.Movement statistics were analyzed with ArcView Geographical Information System (GIS). Toensure consistency and accuracy, we used an Avenue script (a programming languageaccompanying ArcView) to calculate distance statistics.

POPULATION DEMOGRAPHY

Equal catchability.—We used two tests (Tabashnik, 1980) to discern sex-specific differencesin catchability: capture sex ratio vs. estimated sex ratio test (CES) and joint residence-catchability test (JRC). The CES test yields an average estimate of relative catchability for a setof sampling dates. Relative catchability (a) is the mean capture sex ratio divided by the meanestimated sex ratio:

for i ¼ testable sampling dates; a ¼P

n#iPn$i

3

PN �1

#iPN $i

where n ¼ the actual number of male (female) butterflies observed and N ¼ the estimatednumber of male (female) butterflies.

Actual capture data were used to calculate capture sex ratio; daily population estimates,obtained from Jolly’s (1965) stoc1hastic method, were used to calculate estimated sex ratio.Because male and female daily estimates are derived independently, estimated sex ratio isnot biased by catchability. Thus, if capture sex ratio differs significantly from estimated sexratio, the hypothesis of equal catchability can be rejected.

The JRC test plots the natural logarithm of the ratio of male recapture probability tofemale recapture probability against time. The linear regression of the plot yields anestimate of residence (k¼ eslope) and catchability (a¼ eintercept). Neither estimate should differsignificantly from zero if the sexes have equal residence rates and are equally catchable.

306 THE AMERICAN MIDLAND NATURALIST 152(2)

Recapture probability, R#ij¼/#i */#iþ1 . . ./#j�1 * P#j, where/#i is the estimated Jolly residencerate for males (females) from day i to iþ1 and P#j is probability of capture for any given male(female) on day i (P#j¼# marked male butterflies recaptured / # of male marks at risk)

If male and female residence rates and catchability are equal then

/#i � /#iþ1 . . ./#j�1 � P#j ¼ /$i � /$iþ1 . . ./$j�1 � P$j

Residence rate and residence time.—The term ‘‘residence’’ (Ehrlich, 1961) rather than‘‘survival’’ (Jolly, 1965) is used because loss due to emigration cannot be distinguished fromloss due to death (Watt et al., 1977). Residence rate is the estimated fraction of individualsthat survived and stayed in the study area from day i to iþ1 (Murphy et al., 1986). Residencetime is the expected length of time (days) that an individual will remain within the studysite. Jolly’s residence estimates tend to fluctuate greatly (Watt et al., 1977); thus, we usedScott’s (1973) Method 1 (/s) and recapture decay plotting (/r) (Watt et al., 1977) to obtainestimates of residence rate. Scott’s /s is a weighted average of daily Jolly /i, and is based onresidence of all marked animals. The daily Jolly /i are weighted by the number of marks atrisk immediately following the i-th sample. These weighted daily rates were averaged toobtain a brood residence rate. In recapture decay plotting, the natural logarithm of thenumber of animals in residence d days or longer is plotted against the number of days inresidence. An estimate of mean residence (/r) is calculated as eslope. Using the formuladescribed by Cook et al. (1967), estimated residence time (days) was calculated as�(ln/)�1.

Daily and total brood population numbers and population density.—Male and female dailypopulation numbers (Ni) were calculated separately using Jolly’s (1965) stochastic method.Four estimates of total brood size were calculated. Two brood size estimates were obtainedby multiplying the RNi by the brood’s loss rate. The brood’s loss rate was calculated as 1�/as described in Watt et al. (1977). Mean residence rates derived from Scott’s Method 1 andrecapture decay plotting were used to obtain two estimates of brood loss rate. For the thirdestimate, we calculated total brood size as the RNi after subtracting the number ofindividuals expected to survive from i to i þ 1 (i.e., daily Jolly /i). Finally, we used theabsolute number of butterflies marked as a minimum brood size estimate.

Two flight area population density indices were calculated: estimated peak density andaverage daily density. Flight area was defined as the area within a patch utilized bya population and was calculated as the area of a polygon circumscribing all butterfly capturepoints within a patch. Estimated peak density was calculated as:

Estimated peak density ¼ (maxN#i þ maxN$i)/ha, where Ni is the maximum dailypopulation estimate. The estimated average daily density was calculated as:

Estimated average daily density ¼X

ðN #i þ N $iÞ=ðha�daysÞ:

SPATIAL ECOLOGY

Dispersal behavior.—We computed linear distances between capture points in ArcViewusing an Avenue script. Because butterflies do not typically fly in straight lines, thesecalculations represent minimum estimates. Vagility parameters evaluated include:

Distance moved ¼ the distance between consecutive capturesMean daily distance moved¼ the mean distance between consecutive calendar day movesRange ¼ the distance between the two most distant capturesTotal distance moved ¼ sum of all distances moved


We also calculated mean, median and maximum dispersal distances at both sites; maleand female recapture ratios; and the mean number of recapture events (average number ofrecaptures per individual).

Butterfly activity area.—We used two methods to calculate the homerange (activity area) ofmale and female butterflies throughout their adult lifespan: Minimum Area Method(Southwood, 1978) and Standard Area of Activity (Brussard et al., 1974). Southwoodsuggests that a minimum of 10 captures are necessary to calculate reasonable estimates ofhomerange for mammals, but the maximum number of recaptures for an individualbutterfly was six during this 2-y study. Given the fragility of the habitat, short residence timeand the limited dispersal tendencies of Neonympha mitchellii mitchellii, using a lower standardwas necessary. We, thus, calculated activity area estimates for all butterflies that werecaptured three or more times. Minimum Area estimates were determined by connecting theoutermost capture points and calculating the areas of the resultant polygons. The StandardArea of Activity was determined using the standard range radius (rs) for each butterfly, orthe distance between each capture point and the geometric center of all capture points:

r s ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi½�r 2

i =N � 1�q

where ri is the distance in meters between the center and each capture point and N is thenumber of measurements.

Dispersion.—Three patterns are generally used to describe the dispersion of a population;random, regular and aggregated (Southwood, 1978). The patterns of dispersion for malesand females in 1998 were calculated for each day on which at least five individuals werecaptured. To do this, we superimposed a 100 m2 grid over butterfly capture and recapturepoints and used the variance to mean ratio to calculate an index of dispersion, v2/N � 1(Southwood, 1978). v2 is calculated as s2(N� 1)/x, where N is the number of samples and s2

and x are the variance and the mean number of individuals per sample (or grid cell),respectively. The variance to mean ratio follows a Poisson series and the index of dispersionapproaches unity in randomly distributed populations. Significant departures from one(tested by v2 for n � 1 degrees of freedom) indicate regular (,1) and aggregated (.1)dispersion. To determine whether the microdistribution of males and females differed, weused the log-likelihood ratio (G) test (Zar, 1996).

RESULTS

In 1997 the earliest sighting of Neonympha mitchellii mitchellii was 4 July and the lastobservation was 28 July. In 1998 N. m. mitchellii was first and last observed on 24 June and 8July. Active times were similar between sex, site and year with butterflies observed between0700 and 1800 CDT, although activity ceased during midday on very warm (27–32 C) sunnydays. In over 200 on-site hours, adults were never observed feeding.

Capture and recapture statistics for both sites and years are presented in Table 1. The totalnumber of individuals marked was similar between years. Recapture rates were also similarbetween years for males. We attribute the variability of female recapture rate to theundersampling of 1997 flight season. Given that the female cohort peaked later, the lack oflate flight season sampling likely biased female capture and recapture statistics.


Catchability, residence rates and residence times.—There was a bias towards the number ofmales captured both in ratio (male:female) and in captures per individual (Table 1).Average catchability of Neonympha mitchellii mitchellii favored males at both sites in 1998, but


in 1997, female exceeded male catchability at SNC. The JRC test shows that male catchabilityexceeded female catchability in 1998 (Fig. 1); a limited number of female recapturesprevented a JRC analysis of 1997 data.

The estimates of residence rate and time were similar between Scott’s Method 1 andrecapture decay plotting (Table 2). According to the JRC test, residence rates weresignificantly greater for females at BCF and SNC in 1998 (Fig. 1). Estimates of male residencetime ranged from 1.41 to 5.37 d and female residence time ranged from 2.16 to 4.03d (insufficient data prevented calculation of female residence time in 1997). The maximumobserved residence time was 8 and 9 d for males and females, respectively. Both the estimatedand observed residence times are underestimates of lifespan because it is unlikely thatindividuals were captured on the day of eclosion and died on the day of last recapture.

Daily and total brood numbers and densities.—Figures 2 and 3 show estimated Ni. Malesemerged and peaked earlier than females at both sites in both years. The sex ratio changedthroughout the flight period with males predominating early in the flight and a switch tofemale predominance in latter half of the flight season (Fig. 4, 5). In 1997 the sex ratio wasmale biased on most sampling dates, but we suspect that the sex ratio probably would havebecome female biased as observed in 1998 had MRR activities continued throughout theentire flight season.

In 1997 peak male numbers occurred on 12 July at BCF and 15 July at SNC while femalepeak numbers were observed on 14 July at BCF and 15 July at SNC (perhaps a prematureestimate given the lack of late flight period sampling). The average daily brood size estimatefor males was 26 and 49 at BCF and SNC, respectively. The female average daily brood sizeestimate was 8 at BCF and 10 at SNC.

In 1998 the sex ratio was male biased until the 6th and 7th sampling day at SNC and BCF,respectively. Peak male numbers occurred on 28 June at both sites while female peaknumbers were observed on 2 July at BCF and 1 July at SNC. Average daily brood sizeestimates for males were 27 and 25 at BCF and SNC, respectively. Female average daily broodsize estimates were 14 at BCF and 26 at SNC.

As suspected from past meander surveys, both Neonympha mitchellii mitchellii populationshave very small population sizes. The population numbers obtained from the four brood size

TABLE 1.—Mark-release-recapture statistics for N. m. mitchellii at two sites in southwestern Michigan

Year Site Sex # Marked # RecapturedRecapture

ratio# Recapture

events

Ave. #of recapsper recap,individual

Relativecatchability

1997 BCF M 83 36 0.43 51 1.36 1.32F 27 7 0.26 10 1.00Total 110 43 61

SNC M 92 41 0.45 61 1.49 0.55F 37 4 0.11 4 1.00Total 129 45 65

1998 BCF M 82 38 0.46 52 1.35 1.22F 34 15 0.44 22 1.40Total 116 53 74

SNC M 82 42 0.51 70 1.60 1.54F 62 34 0.55 47 1.26Total 114 76 117


FIG. 1.—Joint residence-catchability test, males vs. females. 1998. A y-intercept .1 ¼ catchabilitygreater for males; -slope ¼ residence rate greater for females. a. BCF: slope ¼� 0.1096 (k¼ 0.89, P ,

0.02) and y-intercept¼ 2.54 (a¼ 12.68, P , 0.001). b. SNC: slope¼�0.0437 (k¼ 0.95, P , 0.001) and y-intercept ¼ 2.11 (a¼ 8.27, P , 0.001)


estimators gave similar results (Table 3). The limited number of female recaptures woulddecrease the reliability of a population estimate based on both male and female data.Although the CES test suggests a skewed sex ratio, this was likely due to behavioral differences(as explained below) rather than an actual deviation from an equal sex ratio. Thus, wemultiplied male estimates by 2 to yield an estimate of total brood size. The 1997 total broodsize estimates ranged from 166 to 271 at BCF and 174 to 372 at SNC. The 1998 total brood sizeestimates ranged from 164 to 362 and 164 to 276 at BCF and SNC, respectively.

The flight area (¼portion of habitat used) of Neonympha mitchellii mitchellii varied betweenyears and was greater in 1998 than in 1997 at BCF (0.47 vs. 0.79 ha) and SNC (0.61 vs. 0.75ha). Similar to total brood size, flight area population density estimates were low as well(Table 3). As the true flight area is likely larger than the observed available habitat, the truepopulation densities are even lower than reported here.

SPATIAL ECOLOGY

Dispersal behavior.—Neonympha mitchellii mitchellii is a relatively sedentary butterfly, withboth sexes moving a median distance of less than 30 m (Table 4). Likewise, median rangeand median total distance estimates were less than 50 m. The limited number of femalerecaptures prevented statistical comparisons between sexes in 1997. Male and female 1998distance estimates significantly differed in all movement categories. The trend in both yearswas that males had a greater number of longer movements than females. This generalpattern of dispersal is illustrated in Figure 6 with 1998 data from BCF for all four movementcategories.

Butterfly activity area.—Homerange estimates for males in 1997 and 1998 and for females in1998 are presented in Table 5. For all estimates, the Standard Area of Activity results weresmaller than those produced by the Minimum Area method. Homerange estimates did notsignificantly differ between sexes at SNC for either Minimum Area (H¼0.24, P¼0.63, df¼1)or Standard Area of Activity (H ¼ 0.03, P ¼ 0.86, df ¼ 1) methods. Estimates at BCF didsignificantly differ between males and females (Minimum Area: H ¼ 7.81, P ¼ 0.005, df ¼ 1;Standard Area of Activity: H¼9.36, P¼0.002, df¼1); however, the female sample size at BCFwas relatively small. Homerange estimates for males between the two sites did not significantlydiffer (Minimum Area: H¼1.51, P¼0.22, df¼1; Standard Area of Activity: H¼0.40, P¼0.53,df¼ 1). Using estimates calculated by the Minimum Area method, in 1997 the average maleused 15% of the available habitat at BCF and 5% at SNC. In 1998 BCF males and females used

TABLE 2.—Residence rates & residence times for N. m. mitchellii at two sites in southwestern Michigan./s ¼ mean residence rate calculated by Scott’s (1973) Method 1. /r ¼ mean residence rate calculatedfrom recapture decay plots. (ln/)�1 ¼ mean expected residence (days)

Estimated residence rates & residence timesMedian

daysMax. daysobserved

#ObservabledaysYear Site Sex /s �(ln/r)

�1 /r �(ln/r)�1

1997 BCF M 0.60 1.96 0.59 1.89 2.0 6 13F * * * * 3.0 5 13

SNC M 0.83 5.37 0.63 2.13 2.0 8 13F * * * * 1.0 2 13

1998 BCF M 0.62 2.09 0.49 1.41 2.0 6 15F 0.63 2.16 0.78 4.03 2.0 8 15

SNC M 0.66 2.40 0.57 1.78 2.0 7 15F 0.78 4.02 0.65 2.30 2.0 9 15


4% and 0.10% of the available habitat, respectively. At SNC, the average male butterfly used3% and the average female butterfly used 5% of the available habitat. Although some of oursample included animals captured only three times, we believe that the results of the rangeand total distance moved calculations suggest these homerange estimates indicate thatindividual butterflies use a small portion of the available habitat at the study sites.

Dispersion.—At BCF, there were six sampling dates in which male and female dispersionestimates could be obtained for 1998. At SNC, there were seven and eight testable samplingdays for females and males, respectively. Males and females, at both sites, on most samplingdates occurred in an aggregate spatial pattern (Table 6). The microdistributions of malesand females did not significantly differ at either site (BCF: G¼ 50.05, df¼ 38, P¼ 0.09; SNC:G ¼ 62.65, df ¼ 67, P ¼ 0.63). In other words, males and females seem to use the sameportions of the available habitat.

FIG. 2.—Daily population estimates, male vs. female. 1997. Dashed line represents the standard errorsfor male Ni. Insufficient data prevented standard error calculations for female Ni


DISCUSSION


Both catachability tests suggested that males were more likely to be captured thanfemales, and these tests concur with the behavioral patterns we observed. Male Neonymphamitchellii mitchellii meandered (probably in search of mates) through the vegetation whilefemale N. m. mitchellii spent almost their entire lives perching. Females were rarely capturedin undisturbed flight. Furthermore, the frequency of females observed low in the vegetationsuggests that mated females spend very little time flying or being visible. The implications ofthis female behavior are twofold. First, since capture statistics influence population size anddensity estimates, as well as residence rates and longevity estimates, it is important toaccount for unequal catchability in ascertaining the demographic characteristics of N. m.mitchellii. Second, devising a conservation strategy for N. m. mitchellii requires a solid under-standing of its autecology. The secretive behavior of female N. m. mitchellii, however, impedesthe study of oviposition and larval activity in situ.

FIG. 3.—Daily population estimates, male vs. female. 1998. Dashed lines represent the standard errorsfor male Ni; solid lines female Ni


This study (and other concurrent monitoring efforts) documented the length ofNeonympha mitchellii mitchellii flight season. In the 2 y of this study, the flight period spanned14 to 26 d within a 6-wk period from mid June to late July. Considering the limited dispersaltendencies of this species, survival rates are likely to closely approximate residence rates.Although there was variability in the estimated residence rates, butterfly longevity appearsshort. Scott’s (1973) Method 1 and recapture decay plotting resident time estimates indicatethat average longevity is 1.4 to 5.3 d, with 9 d as the maximum residence time observed. Anobvious implication of such a short life span is the narrow window in which they canreproduce. In years of environmental extremes (e.g., very rainy, cool or very hot, dryweather), N. m. mitchellii’s reproductive success may be greatly diminished (as documentedin other butterflies, see Tabashnik, 1980). Prior to this study, there were no quantitative

FIG. 4.—Estimated sex ratio over time. 1997. Sex ratio is 1 along the dashed line. Points above the lineindicate male predominance; points below female predominance


population estimates for Mitchell’s satyr. Previous walk-through surveys suggested that itsabundance had declined from historical levels, and the results of this 2-y study confirm thatN. m. mitchellii occurs at very low population densities at the study sites. At BCF, N. m.mitchellii presence had been recently documented in eight patches (Shuey et al., 1992), butonly three of these patches were occupied during this study. The species’ absence from fiveof these eight patches suggests that the population is declining. The low densities and totalbrood size estimates we obtained invoke small population biology concerns at both BCF andSNC. At such low densities, N. m. mitchellii is vulnerable to extinction due to environmental,demographic and genetic stochasticity (Pulliam and Dunning, 1994). Thus, managementobjectives must focus on increasing the extent of suitable habitat within the wetland andmaintaining the habitat suitability of currently occupied patches. Another consequence of

FIG. 5.—Estimated sex ratio over time. 1998. Sex ratio is 1 along the dashed line. Points above the lineindicate male predominance; points below female predominance


low population densities is that collection or any other temporary disturbance that increasesadult mortality, which may be inconsequential at higher population numbers, couldnegatively impact these populations. This is particularly germane if the actions areconcentrated in the latter part of the flight when females are most abundant.

TABLE 3.—Flight Area Population Density (#observed/ha) & Estimated Total Brood Size for N. m.mitchellii at two sites in southwestern Michigan. Total brood size calculated using: (1) brood loss,RNi *(1�/), (2) daily Jolly survival rates, R(Niþ1� (Ni */i),(??) and (3) # of butterflies marked. For totalbrood size calculations, male estimates were multiplied by 2

1997 1998

BCF SNC BCF SNC

Total Brood EstimatesWatt (1 � /r) Total Brood Size 268 372 362 276Scott (1 � /s) Total Brood Size 271 174 266 228Jolly (/i) Total Brood Size 262 316 234 218Minimum Total Brood Size 166 184 164 164

Brood Density EstimatesAverage Daily Population Density 35/ha 66/ha 58/ha 40/haEstimated Peak Population Density 70/ha 131/ha 159/ha 91/ha

TABLE 4.—Dispersal statistics for N. m. mitchellii at two southwestern Michigan sites. All distances inmeters. Median values are given, with range and sample size indicated in parentheses. Letters indicatesignificantly different values between variables. 1997 data insufficient for statistical analysis. 1998 datawere analyzed by Friedman’s test to determine if differences exist; significant results were further testedby Mann-Whitney U test (P , 0.05)

BCF SNC

Table 4a. Distance moved between consecutive captures.

1997 M 15.17 (2.06–290.04, n ¼ 48) 20.32 (1.55–420.64, n ¼ 52)F 15.15 (9.84–48.70, n ¼ 4) 21.38 (6.07–36.69, n ¼ 2)

1998 M 22.12d (0.74–148.35, n ¼ 54) 24.90c (0.84–89.21, n ¼ 112)F 10.38a (1.41–54.11, n ¼ 19) 21.90b (5.56–88.27, n ¼ 39)

Table 4b. Mean daily distance moved

1997 M 15.62 (3.68–290.04, n ¼ 29) 14.45 (1.55–420.64, n ¼ 26)F 9.84 (9.84–9.84, n ¼ 1) 21.38 (6.07–36.69, n ¼ 2)

1998 M 21.75b (6.90–118.55, n ¼ 21) 25.59c(0.84–89.21, n ¼ 40)F 13.52a (3.34–44.50, n ¼ 8) 15.28a (5.73–58.34, n ¼ 15)

Table 4c. Range

1997 M 17.02 (3.68–290.04, n ¼ 32) 23.12 (2.53–420.64, n ¼ 35)F 23.99 (15.08–48.70, n ¼ 4) 21.38 (6.07–36.69, n ¼ 2)

1998 M 26.53c (5.69–148.35, n ¼ 40) 23.19c (0.67–89.21, n ¼ 43)F 14.19a (1.41–54.11, n ¼ 16) 23.95b (5.67–88.27, n ¼ 30)

Table 4d. Total distance

1997 M 19.66 (3.68–622.88, n ¼ 34) 23.12 (2.53–431.26, n ¼ 35)F 24.69 (15.45–48.70, n ¼ 3) 21.38 (6.07–36.69, n ¼ 2)

1998 M 31.62c (5.69–153.99, n ¼ 40) 28.13c (0.84–177.89, n ¼ 43)F 14.79a (1.41–54.11, n ¼ 16) 22.84b (5.73–88.27, n ¼ 30)


SPATIAL ECOLOGY

The results of the vagility analysis indicate that most individuals stay within the natalhabitat patch. Although detection of dispersal capability is limited with MRR methodology(Tabashnik, 1980), a high recapture rate of and a tendency for individual butterflies tounderutilize available habitat suggest that MRR results reflect Neonympha mitchellii mitchellii’s

FIG. 6.—The distribution of male vs. female movement for N. m. mitchellii at BCF in 1998. Solid barsrepresent females and stripped bars represent males


dispersal behavior at these sites reasonably well. Range and total distance are the best indicesof dispersal capability. The site averages for total distance moved were 37 and 33 m at BCFand SNC, respectively. Range estimates were also limited. The greatest range estimates(which, incidentally, were consecutive calendar day captures) were 290 m and 420 m at BCFand SNC, respectively. However, these two movements were anomalous. With the exceptionof a 285 m move by the same animal at BCF, all other range movements at both sites wereless than 150 m and greatest mean range estimates were 27 m at BCF and 24 m at SNC.

Both the BCF and SNC long-distance dispersal events were inter-patch movements bymales. The vegetation between the two patches at BCF is primarily shrub thickets, with little

FIG. 6.—Continued


intervening suitable habitat. At SNC, an open path (mostly boardwalk) is maintainedbetween the two occupied patches, but there is no intervening suitable habitat. Thus, itappears Mitchell’s satyr is capable and willing to disperse through unsuitable habitat.

Females moved significantly less than males. These differences in intersex dispersalstatistics can be explained by the differences in male and female flight patterns. Femaledispersal was short and uninterrupted whereas male flight was longer and interrupted bycon- and heterospecific interactions. Females were never observed meandering through thevegetation as typically observed with males. When observed in undisturbed flight, the femalewas either engaged in a brief interaction with another butterfly or making a direct shortmovement to oviposit. Given that adult Neonympha mitchellii mitchellii were not observed tofeed during this study, limiting flight activity to mating and ovipositing is an advantageousbehavior for energy conservation and avoiding predation.

Although the populations at both BCF and SNC occupied all the seemingly suitablehabitat within a patch, homerange estimates indicate that the average individual uses only

TABLE 5.—Average area (ha) of individual activity for N. m. mitchellii at two southwestern Michigan sitesin 1998. Mean values are given, with range and sample size indicated in parentheses. Values followed bythe same superscript are not significantly different at the P , 0.05 confidence level. Data tested byKruskal-Wallis test

Site Minimum Area Method Standard Area of Activity

BCF Male (1997) 0.0705 (0.0012–0.5601, n ¼ 12) 0.0279 (0.0038–0.1060, N ¼ 7)Male (1998) 0.0325a (0.0015–0.1845, n ¼ 9) 0.0150a (0.0031–0.0500, n ¼ 10)Female (1998) 0.0008b (0.0005–0.0010, n ¼ 2) 0.0050b (0.0040–0.0050, n ¼ 2)

SNC Male (1997) 0.0302 (0.0009–0.1378, n ¼ 7) 0.0149 (0.0030–0.0294, n ¼ 13)Male (1998) 0.0412a (0.0005–0.0601, n ¼ 13) 0.0120a (0.0060–0.0260, n ¼ 12)Female (1998) 0.0297a (0.0003–0.0959, n ¼ 9) 0.0170a (0.0030–0.0280, n ¼ 8)

TABLE 6.—Daily Index of Dispersion, v2/N � 1, estimates for N. m. mitchellii at two southwesternMichigan sites in 1998. v2/N � 1 significantly .1 indicates aggregate dispersion

Date

Male Female

v2/N � 1 p Date v2/N � 1 p

BCF26 June 1.5 0.02 30 June 1.31 0.0927 June 1 0.47 1 July 1.01 0.4428 June 1.54 0.018 2 July 1.43 0.0429 June 1.54 0.018 4 July 2.13 ,0.00130 June 1.47 0.03 5 July 1.14 0.261 July 0.97 0.52 7 July 1.43 0.09

SNC25 June 1.20 0.13 29 June 1.35 0.0326 June 1.53 0.003 30 June 1.22 0.1127 June 1.52 0.004 1 July 1.54 0.00328 June 1.48 0.006 2 July 1.26 0.0729 June 1.40 0.016 3 July 1.07 0.3330 June 0.78 0.91 5 July 1.17 0.161 July 1.15 0.19 6 July 1.96 ,0.0012 July 1.47 0.008


a small proportion of this habitat. Intrinsic barriers (sensu Erhlich, 1961) to dispersal mayprovide an explanation for this behavior, especially for males, which seem to fly constantly,yet go almost nowhere. The short lifespan and the restricted environmental conditionssuitable for activity limit the time available for butterflies to reproduce and disperse theireggs. Thus, most Neonympha mitchellii mitchellii butterflies may confine activity to their natalmicrohabitats where suitable habitat is assured. Limited time to mate and disperse eggs hasbeen implicated as a limiting factor for butterfly populations by other lepidopteraresearchers (e.g., Dempster, 1971; Bink, 1985), and seems plausible for N. m. mitchellii as well.

The patch connectedness and occupancy in the recent past suggest that the BCFpopulation may have functioned either as a metapopulation or source-sink population.If a metapopulation, then Neonympha mitchellii mitchellii’s absence from seemingly suitablepatches and the lack of subsequent recolonization of recently occupied areas suggest thatthe population may be contracting. Alternatively, it is possible that these vacant patches areonly occupied in years of high butterfly density (i.e., a source-sink dynamic) and, thus, thechange in occupancy reflects normal fluctuations in butterfly numbers. As the currentpopulation structure at BCF is unclear, but suggestive of a declining metapopulation,further research is needed. The current population structure at SNC is much lesscomplicated; N. m. mitchellii occurs in all suitable habitat patches and the distance betweenthe habitable patches provides little opportunity for dispersal. Thus, extant patchpopulations are seemingly independent demographic units. Anecdotal accounts of theSNC wetland complex a few decades ago indicate that suitable habitat was more abundantthan today (C. Nelson, SNC Director, pers. comm.). So, here too, N. m. mitchellii may haveoccurred as a metapopulation.

The importance of determining the spatial structure of these populations is not merelyacademic. The factors that influence the persistence, and consequently management, ofa population vary among the different structures. The persistence of a source-sinkmetapopulation, for example, depends on the viability of the source population. Thesmaller, occasionally occupied sinks contribute little, if anything, to the long-termpersistence of the metapopulation (Harrison, 1991). Thus, only the factors influencingthe viability of the source population (e.g., patch area and population densities) (Harrison,1991) are of management concern. Conversely, in an extinction-recolonization structure, alllocal populations are vulnerable to extinction and, thus, influential in the persistence ofthe metapopulation. In this type of structure, the factors controlling extinction andrecolonization rates (e.g., number of occupied and unoccupied patches, patch size, patchpopulation densities, arrangement of patches and corridor availability) (Harrison, 1991;Neve et al., 1996) are of management concern. Spatial population structure can also havelegal implications. The Endangered Species Act, for example, provides protection againstharm to the listed species but leaves unoccupied sites vulnerable to modification. Toconserve source-sink populations only the core populations require protection, but if allpopulations within metapopulations are vulnerable to extinction (as occurs in extinction-recolonization dynamics), it is necessary to protect the landscape matrix of occupied andunoccupied patches. In the context of our two study sites, it is prudent, given that thepresent data are insufficient to differentiate between these two population models, to adopta conservative approach. That is, the safest management strategy is to protect all occupiedand unoccupied patches within the wetland complexes.

Regardless of population structure, the restricted dispersal tendencies of Neonymphamitchellii mitchellii, coupled with current habitat conditions, greatly influences themanagement needs at both sites. Given the low population densities and the limitedhabitat patch area, the continued survival of N. m. mitchellii at these two sites appears


contingent on movement, particularly female dispersal, between suitable patches. Theresults of this study, however, suggest that N. m. mitchellii only rarely disperse throughunsuitable habitat. Furthermore, despite being strongly tied to its habitat, which is animportant determinant of corridor utility (Schultz, 1998), the sedentary behavior of femaleN. m. mitchellii limits the value of corridors for this species. At BCF and SNC, distancesbetween suitable patches, even if dispersal corridors were created, are seemingly too great toprovide for any meaningful movement between patches. Thus, the immediate managementstrategy should be to maintain the existing habitat patches and to restore additional earlysuccessional fen habitat patches with corridor connections throughout the wetlandcomplexes. It is important that such habitat patches are closely arranged so that inter-patch corridor length is minimized.

SUMMARY OF CONSERVATION IMPLICATIONS

Populations of Neonympha mitchellii mitchellii at the two study sites occur at very lowdensities, and the BCF population may be declining or contracting. Consequently, thesepopulations are potentially vulnerable to environmental stochasticity and humandisturbance. To guard against further population decline, the extent of suitable habitatwithin the wetland areas should be increased to influence population size and habitatconnectivity, long-term protection and management of the wetlands and surroundinguplands should be secured and human disturbance should be minimized. The sedentarytendencies of the N. m. mitchellii require that habitat restoration minimize inter-patchcorridor length to restore habitat patch connectivity. Furthermore, management effortswithin occupied patches should be spatially and temporally subdivided considering thelimited recolonization potential at both sites. In minimizing human disturbance, it isparticularly important to avoid activities in later half of the flight when females are mostabundant. Finally, the short adult lifespan of N. m. mitchellii and low fecundity also rendersthe species susceptible to environmental extremes. Microhabitat diversity may be importantin buffering populations from such environmental conditions. Long-term monitoring isessential for assessing the health of these populations.

Acknowledgments.—We thank Chuck Nelson for his interest in endangered species conservation andaccess to his site for our research. Mary Rabe, Mike Penskar, Denny Albert, Kim Chapman, WillMcKinnon and Tony Reznicek for their advice on project design. Mary Mitchell for her patient GISinstruction and assistance. A special thanks to Kim Mitchell, Lori Scinto, Christi Delorea, Jenny Wilson,Mark Paulson and Laura Ragan for field assistance. Neil Darlow for data entry and Carmen Thomas forher statistical analysis support and her reliable counsel. Preliminary versions of the manuscript wereimproved by the constructive comments of Francie Cuthbert and Mary Wilson.

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SUBMITTED 12 AUGUST 2003 ACCEPTED 11 MARCH 2004


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