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2019 NORTHEASTERN NATURALIST 26(3):537–544
The Butterflies (Lepidoptera) of an Isolated Island:
Monhegan, Maine
Ernest H. Williams*
Abstract - Repeated surveys of the butterflies of Monhegan Island, ME, from 1998 to 2018
confirmed the presence of at least 40 species. This total represents more than 42% of the
state’s recorded species—a high proportion given the island’s small size and isolation. The
rate of discovery indicates that, based on observations accumulated over time, about 50
species could be observed on Monhegan, including large migratory taxa as well as a variety
of habitat-limited sedentary residents. The absence of several mainland species can be
attributed to limited habitat and lack of some plant species. Records show changes in the
butterfly fauna over recent decades; further changes may be expected because of climate
warming, habitat modification, and stochastic events.
Introduction
Islands typically support fewer species than same-sized areas on the mainland
(MacArthur and Wilson 1967). Island residents must survive with a limited range
of habitats, and non-resident visitors are species with high dispersal ability; small
islands also experience high extirpation rates along with reduced immigration
(MacArthur and Wilson 1967). In accord with island biogeographic theory, fewer
butterfly species have been found on islands that are smaller and more distant from
the mainland than on those that are larger and closer to mainland sites (see Miller
[1984] for a study of the California Channel Islands and Dennis et al. [2008] for
studies of European islands).
The small size and isolation of Monhegan, an island 16 km off the coast of
Maine, and personal familiarity with the island spurred this study of its butterfly
fauna. In this paper, the name Monhegan refers to Monhegan Island rather than the
Monhegan Plantation, a civil entity that includes Monhegan Island and the adjacent
small island, Manana. Little information has been reported about Monhegan butterflies
previously. An early record stated the presence of Papilio glaucus L. (Eastern
Tiger Swallowtail), Pieris rapae (Cabbage White), Lycaena phlaeas (American
Copper), Phyciodes tharos (Drury) (Pearl Crescent), Nymphalis antiopa (Mourning
Cloak), and Polites themistocles (Tawny-edged Skipper) (Maynard 1909), although
the correct species would likely have been Papilio canadensis (Canadian Tiger
Swallowtail) for the swallowtail and Phyciodes cocyta (Northern Crescent) for the
crescent (as listed in Table 1).
Consequently, I surveyed the butterflies on Monhegan periodically from 1998
through 2018 to answer the following questions. How many of the species known
on the mainland (within the state of Maine) can be found on the island? What taxa
*Department of Biology, Hamilton College, Clinton, NY 13323; ewilliam@hamilton.edu.
Manuscript Editor: John Rawlins
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found on the mainland are absent on Monhegan, and can their absence be attributed
to missing host plants or habitats? How might the butterfly community have
changed over time?
Field-Site Description
Monhegan is a small island (3 km long and 1 km wide; 270 ha in area) located
16 km off the mid-coast of Maine (43.77°N, 69.32°W) and next to Manana Island
(5 ha; Fig. 1A). Monhegan’s small size limits the diversity of habitats and potential
host plants. About 70% of the island is forested (as shown in Fig. 1B), largely with
Picea glauca (Moench) Voss (White Spruce) and P. rubens Sargent (Red Spruce), in
addition to some Abies balsamea (L.) Mill. (Balsam Fir), Acer rubrum L. (Red Maple),
A. pensylvanicum L. (Striped Maple), Populus tremuloides Michaux (Quaking
Aspen), Betula papyrifera Marshall (White Birch), B. alleghaniensis Britton (Yellow
Birch), and Alnus viridis crispa (Aiton) (Green Alder) (Harris 2014, Rand et
al. 2004, Tolonen 1983). Open habitats are uncommon, although there is an interior
wet fen, and, within the village on Monhegan, some gardens and lawns. Other open
areas occur along the Underhill Trail, a wetland near the shore at Lobster Cove
Figure 1. Monhegan Island,
ME. (A) The relationship of
the island to the mid-coast
of Maine, 16 km distant.
(B) Monhegan Island, 3 km
long, showing roads and 2
wetlands, with the shaded
area indicating forest cover
(the adjacent small island is
Manana). Habitats for butterflies
are shown as: (a) Monhegan
village, (b) Underhill
trail through second growth,
(c) Lobster Cove wetland,
(d) open edges along the eastern
cliffs, and (e) open area
around the lighthouse.
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(southeastern end), along the exposed outer coast, and around the lighthouse (open
meadow is common on the small, adjacent island of Manana). These habitats are
marked on Figure 1B. Monhegan is not near the mainland coast; the distance over
water limits possible immigration, and the island is exposed to oceanic conditions.
In the years since the first human settlements on Monhegan, the vegetation has
been altered by lumbering, fire, agriculture, and the introduction of non-native plants
(Tolonen 1983). From the 1930s through the 1950s, most of the island was protected
from development, which has led to the current extensive forest cover. Indirect
changes in the vegetation took place with the introduction of Odocoileus virginianus
(Zimmerman) (White-tailed Deer) in 1955 and their subsequent elimination by 1999
(Rand et al. 2004). Further vegetative changes may occur in response to the current
warming climate. Currently, most of the island (200 ha) is undeveloped and preserved
by an island land trust (Monhegan Associates, Inc. 2018).
Methods
I compiled records of butterflies from collected voucher specimens, documentary
photographs, and direct sightings during a series of 3–5-d stays (once per
year) on Monhegan. The visits took place during 13 of the 21 years from 1998
through 2018 and occurred across the growing season from May to September.
The resultant 13 surveys provide records of the butterfly community on the island.
The surveys were not standardized observations, e.g., Pollard walks (Pollard
and Yates 1993), but resulted from frequent explorations of the island’s habitats
and extensive trails. I supplemented these data with a small butterfly collection
on display in the Monhegan Museum, and solicited additional records from other
observers who have spent time on the island (listed in the Acknowledgments). I
have noted each species in the complete list (Table 1) as recorded by voucher (V),
Table 1. The 47 species of butterflies recorded on Monhegan Island from surveys and reports through
2018. The record for each species is noted as: V = voucher (31 species), P = photograph (9 species),
S = sighting by the author (3 species), and SO = sighting reported by others (4 species). Abundance is
noted by: A = abundant, C = common, O = occasional, and R = rare. The status of each species is modified
for Monhegan from the Maine Butterfly Survey (2016): BR = breeding resident; TC = frequent
temporary colonist; RC = rare temporary colonist; RS = rare stray. The names correspond with those
used on the Maine species list (Maine Butterfly Survey 2016). [T able continued on following page.]
Scientific name Common name Record Abund. Status
Family Hesperidae
Ancyloxypha numitor (Fabricius) Least Skipper V O BR
Carterocephalus palaemon (Pallas) Arctic Skipper V O BR
Epargyreus clarus (Cramer) Silver-spotted Skipper S R RC
Euphyes vestris (Boisduval) Dun Skipper V R BR
Hesperia sassacus Harris Indian Skipper V O BR
Poanes hobomok (Harris) Hobomok Skipper V C BR
Polites mystic (W.H. Edwards) Long Dash Skipper V C BR
Polites peckius (W. Kirby) Peck’s Skipper V C BR
Polites themistocles (Latreille) Tawny-edged Skipper V C BR
Thymelicus lineola (Ochsenheimer) European Skipper V O BR
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Table 1, continued.
Scientific name Common name Record Abund. Status
Family Lycaenidae
Celastrina lucia (W. Kirby) Northern Azure V C BR
Celastrina neglecta (W.H. Edwards) Summer Azure V C BR
Cupido comyntas (Godart) Eastern Tailed Blue V C BR
Feniseca tarquinius (Fabricius) Harvester P R RC
Glaucopsyche lygdamus (Doubleday) Silvery Blue V C BR
Lycaena epixanthe (Boiduval & LeConte) Bog Copper V O BR
Lycaena hyllus (Cramer) Bronze Copper V O BR
Lycaena phlaeas (L.) American Copper V C BR
Family Nymphalidae
Subfamily Libytheinae
Libytheana carinenta (Cramer) American Snout S R RS
Subfamily Danainae
Danaus plexippus (L.) Monarch V C TC
Subfamily Heliconiinae
Boloria bellona (Fabricius) Meadow Fritillary V R BR
Boloria selene (Denis & Schiffermueller) Silver-bordered Fritillary V O BR
Euptoieta claudia(Cramer) Variegated Fritillary SO R TC
Speyeria cybele (Fabricius) Great Spangled Fritillary S O RC
Subfamily Nymphalinae
Aglais milberti (Godart) Milbert’s Tortoiseshell SO R RC
Chlosyne harrisii (Scudder) Harris’s Checkerspot V O EXT?
Junonia coenia Huebner Common Buckeye P R TC
Limenitis archippus (Cramer) Viceroy V R RC
Limenitis arthemis (Drury) White Admiral V O TC
Nymphalis antiopa (L.) Mourning Cloak P C BR
Nymphalis I-album (Boisduval & LeConte) Compton Tortoiseshell SO R RC
Phyciodes cocyta (Cramer) Northern Crescent V C BR
Polygonia interrogationis (Fabricius) Question Mark P C TC
Polygonia comma (Harris) Eastern Comma P R RC
Vanessa atalanta (L.) Red Admiral V C TC
Vanessa cardui (L.) Painted Lady P C TC
Vanessa virginiensis (Drury) American Lady V C TC
Subfamily Satyrinae
Coenonympha inornata (W.H. Edwards) Inornate Ringlet V C BR
Cercyonis pegala (Fabricius) Common Wood Nymph V C BR
Lethe eurydice (L.) Eyed Brown V O BR
Megisto cymela (Cramer) Little Wood Satyr V C BR
Family Papilionidae
Papilio canadensis Rothschild & Jordan Canadian Tiger Swallowtail V C BR
Papilio polyxenes Fabricius Black Swallowtail SO R RC
Family Pieridae
Colias eurytheme (Boisduval) Orange Sulphur P A TC
Colias philodice Godart Clouded Sulphur P C BR
Phoebis sennae (L.) Cloudless Sulphur P R RS
Pieris rapae (L.) Cabbage White V A BR
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photograph (P), or sighting (S). I based the acceptance of a sighting record on unlikely
confusion of the species reported, but all sighting records are noted as such
because no confirmation exists for these records. I submitted all records and documentary
photographs to the Maine Butterfly Survey (2016), and deposited a few
vouchers with the Maine State Museum. The records are available through the
Maine Butterfly Survey (2016). I deposited all other vouchers with the McGuire
Center for Lepidoptera and Biodiversity, University of Florida, Gainesville, FL.
Scientific and common names for species including authors and generic combinations
correspond with those used on the Maine species list (Maine Butterfly
Survey 2016) and are provided in Table 1 of this paper. The species accumulation
analyses were calculated following Soberon and Llorente (1993) and, for the
Chao incidence equation, Gotelli and Colwell (2011).
Results
A grand total of 47 species have been reported from the island: 31 by voucher
specimens, 9 by photographs, with 7 by sight only (Table 1). I made 3 of the sight
records and 4 were reported by others. This total contrasts with 118 species reported
for the state of Maine (Maine Butterfly Survey 2016). Given the island’s limited
area compared to the state as a whole (equal to 0.003% of the state’s area), the total
for Monhegan was higher than I expected (40 species with confirmed documentation;
Table 1). Diversity was highest in the middle of the summer, with more than
17 species observed each visit during surveys from mid-June to mid-August.
I observed 10 species during the initial survey, and the total increased to 40 by
the 13th survey in 2018 (Fig. 2). This total includes 37 documented records (vouchered
specimens and photographs) plus personal sightings of Epargyreus clarus
(Silver-spotted Skipper), Libytheana carinenta (American Snout), and Speyeria
cybele (Great Spangled Fritillary). The number of possible species expected to occur
on Monhegan can be estimated by analysis of the rate of species accumulation.
Using a Chao equation (bias-corrected form) for replicated incidence data (Gotelli
and Colwell 2011), richness of the Monhegan butterfly community is projected
to be 48 species (standard deviation of the estimate = 7.1; Fig. 2). A logarithmic
model of the species discovery rate (Soberon and Llorente 1993) estimated that the
total number of species would keep increasing to 46 species. These 2 analytical approaches
suggest that records of butterflies on Monhegan could reach a total of ~50
species. If observations by others over the past 3 decades are added to the survey
results, the current total stands at 47. This does not mean, however, that 47 species
could be found on the island in any 1 year; about 27 species appear to be residents,
and an additional 6 species reach the island regularly each year as migrants or temporary
colonists.
The species on Monhegan include residents, regular migrants from the mainland,
and infrequent migrants from southern states. The most commonly encountered
butterfly was the Cabbage White, a resident species seen throughout the island,
especially in the village, where gardens with Brassicaceae host plants are present.
This butterfly was the only species seen on all 13 visits to the island. The sulphurs,
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E.H. Williams
2019 Vol. 26, No. 3
both Colias philodice (Clouded Sulphur) and C. eurytheme (Orange Sulphur), were
also common, especially around the village. I observed 5 strong-flying, large (>5
cm wingspan) species on more than half the visits, both throughout the summer, and
in many places on the island: Danaus plexippus (Monarch), Vanessa atalanta (Red
Admiral), V. cardui (Painted Lady), V. virginiensis (American Lady), and Polygonia
interrogationis (Fabricius) (Question Mark). All 5 species are migratory and readily
reach Monhegan; the first 3 are especially known for widespread mass movements,
while the latter 2 migrate more regionally (Cech and Tudor 2005). Although
much less common, 3 additional strong-flying species that have been recorded on
Monhegan migrate northward to Maine from southern states late in the summer,
occasionally in large numbers: Phoebis sennae (Cloudless Sulphur), Junonia coenia
(Common Buckeye), and Libytheana carinenta (American Snout). Three small
(less than 3 cm wingspan) resident multivoltine species were among the most frequently
observed taxa: Lycaena phlaeas (American Copper), Phyciodes cocyta (Northern
Crescent), and Coenonympha inornata (Inornate Ringlet).
Discussion
The most conspicuous taxonomic gap in these records is the absence of species
of Theclinae (hairstreaks) and Callophrys (elfins). Quercus (oaks) and Pinus (pines)
are missing on the island, explaining the absence of several of these species but not
Satyrium liparops Leconte (Striped Hairstreak) or Strymon melinus (Hübner) (Grey
Figure 2. The species
accumulation curve for
butterflies on Monhegan
determined from
3–5-d surveys conducted
once per year
during 13 y from 1998
to 2018. The diamonds
show the accumulated
species totals. The x
shows the estimate of
total species richness
determined from the
Chao equation for replicated
incidence data
(see text).
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2019
543
Hairstreak), both of which use a range of other woody host plants. The absence of
elfins is best explained by the absence or limited occurrence of their specific host
plants. Other species that seem possible but have not yet been recorded include
Speyeria atlantis (W.H. Edwards) (Atlantis Fritillary), Lethe anthedon (A. Clark)
(Northern Pearly-eye), Erynnis icelus (Scudder & Burgess) (Dreamy Duskywing),
and several Polygonia spp. (commas). Continued observation would likely add to
the list already accumulated.
Given the northern latitude (43˚77'), it is not surprising that for pairs of closely
related species, the species on Monhegan is the more northern representative.
In particular, all confirmed records of Phyciodes species have been of Northern
Crescent, rather than Pearl Crescent. In addition, all tiger swallowtails have been
Canadian Tiger Swallowtail rather than Eastern Tiger Swallowtail.
Island faunas change over time due to changes in land use as well as periodic
immigration and extirpation. The habitats available for sustaining populations of
different butterfly species on Monhegan have been altered over time, and other
changes have taken place, too. Numerous butterfly species go through population
explosions and then seemingly disappear; thus, abundance can be quite variable
over time (Cech and Tudor 2005). Thymelicus lineola (European Skipper) was
introduced to North America in the early 20th century (Cech and Tudor 2005) and
is present on Monhegan. Chlosyne harrisii (Harris’s Checkerspot) was observed
through 2008 but has not been observed since, despite an abundance of its host
plant, Doellingeria umbellata (Mill.) Nees (Flat-topped White Aster). This butterfly
is known for being uncommon and very local (Cech and Tudor 2005), and
its recent absence may be a local extirpation. With the overall decline of Danaus
plexippus (Monarch) in eastern North America (Brower et al. 2012), the abundance
of this species on the island has also diminished. In sum, a butterfly community of
at least 33 species appears to be observable on the island in any 1 year despite the
small area, isolation, and changes in land use. The results presented here provide a
baseline for continued study of this dynamic island system.
Acknowledgments
I am grateful to Bryan Pfeiffer for records and thoughts about butterflies on Monhegan;
to Doug Hitchcock, Jackie Sones, and Bill Thompson for observational records; to the
Monhegan Associates, Inc., for use of the island trail map; to Bryan Pfeiffer, Chris Briggs,
Sharon Williams, and 2 anonymous referees for helpful comments on the manuscript; and
to Michael and Jeanne Beck for support. This report is dedicated to the memory of Noreen
Flynn Beck, who first invited me to Monhegan and encouraged me t o keep returning.
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