Download and mimetic patterns on tropical Paciflc archipelagos

Bwbgwal Journal gthe Linnean SociGty (1996), 57: 917-926. With 2 figures
Biogeography of milkweed buttedies
(Nymphalidae: Danainae) and mimetic patterns
on tropical Pacific archipelagos
ROBERT DUDLEY's3 AND GREGORY H. ADLER213
'Department ofzoology, Universip ofTaa.s, Austin, Ix 78712, USA.
*Department ofBwbgy and Mkrobwlogy, Universip of Wuconsin-Oshkosh, Oshkosh, WZ
54901 USA
3Smithsonian Tiopical Research Institute, P.O. Box 2072, Balboa, Republic ofPanama
Received 43anq 1995, acccptulfkpublicatton 9 May 1995
Distributions of danaine butterfly species and associated mimetic patterns were compared among fAeen
archipelagos of the tropical Pacific Ocean, and withim five major archipelagos (the Bmarcks, Fiji, East
and West Solomon Islands, and Vanuatu). Using both simple and stepwise linear regression analysis,
variation in the total number of danaine species and number of mimetic patterns was assessed with
respect to island size, isolation and elevation. Relative to interarchipelago distributions, the distribution
of danaine species and number of mimetic patterns on islands within archipelagos exhibited less
dependence upon interisland distance and island area. Geographical features influencing the number of
mimetic patterns were similar to those of danaines as a whole. Analysis of residuals from stepwise linear
regression suggested that factors influencing danaine distributions were Merent from those for nondanaine butterflies. Thin result is consistent with the hypothesis of enhancement of danaine species
establishment through Miillenan mimicry, although other factors such as host plant availability and
simiiar habitat use may also be important.
01996 The b e a n Society of London
ADDITIONAL KEY WORDS: -archipelagos - buttefies - danaines - mimicry - Pacific Ocean..
C0"TS
Introduction . . .
Material and methods
Results.. . . .
Discussion . . . .
Acknowledgements .
References. . . .
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317
318
319
321
325
326
INTRODUCTION
The milkweed butterflies (Nymphalidae: Danainae) comprise a phylogenetically
well-defined clade with its centre of diversity in the tropical Indo-Pacific (Ackery 8z
Vane-Wright, 1984). Milkweed butterflies are particularly interesting because most if
not all species are unpalatable to vertebrate predators. Thus, if any similarity in wing
patterns and/or behaviour exists between two danaine species, both will potentially
00244066/96/040317
t 10 $18.00/0
317
(a1996 The Linnean Society of London
318
R DUDLEY AND G.H.ADLER
derive selective Miillerian advantage in sympatry. The sequestration of cardiac
glycosides and/or acquisition of pyrrolizidine alkaloids by danaines, and the high
degree of wing pattern similarity among many danaine species, have led Ackery &
Vane-Wright (1984)to synthesize hypotheses of distinct mimetic patterns in different
faunal regions. Each of these patterns can potentially act as a template for Mullerian
mimicry.
Although the likelihood of Miillerian mimicry among danaine species is high,
selective advantages of resemblance have rarely if ever been .demonstrated for
individual species in ecological as distinct from laboratory contexts. One means of
examining this hypothesis is to determine if danaine species distributions in specific
geographical regions are randomly composed, or whether additional ecological
factors promote presence or absence of particular species. Such factors may operate
at Werent spatial scales, and it is of particular interest to determine the scale at
which phenomena such as mimicry become important in influencing species
distributions. The numerous archipelagos of the tropical Pacific Ocean provide an
excellent opportunity to implement such an analysis, as these island groups contain
numerous danaine species and represent distinct and isolated butterfly
assemblages.
As part of a larger study examining butterfly distributions in the tropical Pacific
(see Adler & Dudley, 1994), this paper assesses the intra- and interarchipelago
biogeography of milkweed butterflies and of their mimetic patterns. Although
various examples exist of remarkable phenotypic convergence among heterogeneric
danaine species (Ackery & Vane-Wright, 1984), the possibility that such similarities
have arisen for reasons other than mimicry (see e.g. Bernardi, 1963) cannot be
evaluated statistically. However, the present study shows that the distribution of
tropical Pacific danaine butterflies and mimetic patterns relative to that of all nondanaine butterfly species is influenced by factors supplemental to geographical
features that otherwise explain most variance in species distributions. This finding
does not unequivocally confirm the hypothesis that Miillerian mimicry acts to
promote species establishment, but does suggest that a more highly resolved
assessment of species distributions and habitat overlap will demonstrate positive
associations between species of the same mimetic pattern.
MATERIAL AND METHODS
The. 15 distinct archipelagos containing danaines in the tropical Pacific were
considered in an interarchipelago analysis (archipelago definitions follow Adler,
1992). Nine geographical features characterizing each archipelago were used in the
analysis: total land area, area of largest island, distance to the nearest continental
area, the nearest larger land mass and the nearest land mass,greatest elevation, and
numbers of islands > 1000, > 500, and > 100km2;(see Adler & Dudley, 1994).
Australia, New Guinea, Asia, and large landbridge islands of the Philippines and
Sunda Shelf were considered continental (or source) areas. Five major archipelagos
(Bismarcks, Eastern and Western Solomon Islands, Vanuatu, and Fiji) were
evaluated in detail to assess intra-archipelago distributions of danaine species. For
individual islands in the intra-archipelago analysis, the following data were
determined from a variety of geographical sources: distance from the largest island
within the archipelago, distance from the nearest larger island, area, and maximum
BIOGEOGRAPHY OF PACIFIC MILKWEED BVITERFLIES
319
elevation. Derrick (1965) was particularly useful for geographical data on the Fijian
Islands, while UNEP/IUCN (1988) was helpful in determining elevational and areal
data for some of the smaller coral atolls. Jared Diamond (in Zitt.) provided
geographical data for the Bismarcks.
Distributional data for resident milkweed butterfly species, as well as definitions
and inclusion of particular species within mimetic patterns followed Ackery & VaneWright (1984). For each island or archipelago, the total number of danaine species
and the number of resident mimetic patterns were used in statistical analysis. Ackery
& Vane-Wright (1984) considered the Solomon Islands to be subdivided into two
regions (eastern and western), each with a different set of mimetic patterns. This
convention was also followed in the present analysis. The monarch butterfly Danaus
plexippus was excluded from analysis because of likely anthropogenic introductions
(see e.g. Scudder, 1875; Vane-Wright, 1993).
For both intra- and interarchipelago analyses, simple log-log and stepwise linear
regressions were used to determine the importance of aforementioned geographical
features in explaining variation in species and pattern distributions. In both analyses,
the numbers of danaine species and mimetic patterns were included as dependent
variables in separate regressions, and the geographical variables (nine for inter- and
four for intra-archipelago analyses) were included as independent variables. Each
archipelago (in interarchipelago regressions) and each separate island (in intraarchipelago regressions) represented a single observation. For stepwise regressions, a
variable was added to the model if the corresponding Pvalue was less than 0.05, and
was excluded if P > 0.10. In the interarchipelago analysis, residual from stepwise
linear regressions on geographical variables were determined for the following
categories: the total number of danaine butterflies, the number of mimetic patterns,
the number of nymphalid butterfly species less danaines, and the total of all nondanaine butterfly species (see Adler & Dudley, 1994). Residuals were then crosscorrelated to assess possible differences in danaine and mimetic pattern distributions
relative to those of nymphalids and the entire butterfly fauna of the tropical
Pacific.
RESULTS
On average, five danaine species were present per archipelago in the tropical
Pacific region (range 1-1 7, excluding archipelagos lacking danaines). The average
number of distinct mimetic patterns per archipelago was three (range 1-7). Within
archipelagos, distributions of both species and patterns were highly variable, as were
geographical data characterizing islands that comprised each archipelago (Table
1).
Bivariate log-log regressions suggested that the principal determinant of danaine
species abundance within and among archipelagos was island area (Table 2). By
contrast, distance and elevation effects were less pronounced. Only one of the five
archipelagos (Fiji) showed a significant relationship between number of species and
distance from largest island within the archipelago, to be contrasted with the highly
significant interarchipelago regression of total danaine species on distance. Fiji was,
of the five archipelagos considered in detail, characterized by the greatest mean
distance to the presumed source island within the archipelago (see Table 1). A
significant relationship between species number and distance to nearest larger island
R.DUDLEY AND G.H.ADLER
320
TABLE1. For the entire study area and for five tropical Pacific archipelagos, the number of
archipelagos or islane included in the analysis ( n),mean number (range) of danaine species
and mimetic patterns per archipelago or island, mean area (range) among or within
archipelagos, mean distance (range) from the continental land mass or largest island, and the
mean elevation (range) per archipelago or island
Group
tl
No. species No. patterns
Interarchipelago
15
5.3 (1-17)
3.1 (1-7)
Birmarcks
10
8.8 (2-16)
3.1 (2-5)
Eastern Solomons
10
4.6 (1-9)
2.1 (1-3)
Area
(kmp)
Distance
9700
(25-49730)
4747
(31-35742)
869
(5-4306)
1239
(13-5279)
439
(2-1 0384)
438
(1-3937)
2478
(51-5370)
149
(19-381)
116
(1-206)
186
(5-521)
255
(10-1155)
206
(3-605)
(km)
Elevation
(m)
~~
Western Solornons
21
7.6 (1-14)
3 (14)
Fiji
41
2.7 (1-5)
2 (1-3)
VanUatU
27
4.7 (1-10)
3.1 (1-6)
1221
(42743)
1011
(1-2439)
409
(761281)
811
(342743)
831
(41242)
730
(2-1890)
was found only for the Western Solomons. Interestingly, this correlation was positive
and therefore opposite to that predicted by standard island biogeography relating
colonization to distance from source. The effects of elevation on species number
(Table 1) was not significant in the interarchipelagocomparison, but was significant
for three of five intra-archipelago comparisons.
Relative to danaine species as a whole, the number of mimetic patterns within
archipelagos showed less dependence on area, with only two of the five intraarchipelago regressions significant (Table 3). However, this areal regression was
highly significant in the interarchipelago comparison. Mimetic pattern abundance
and distance to continental land mass or largest island were significantly (and
negatively) correlated only in the interarchipelago comparison. Within the Western
Solomons, distance to nearest larger island was correlated significantly (and
positively) with the number of mimetic patterns. Effects of island elevation were
sigdicant in four of the five intra-archipelago comparisons (see Table 3).
Corroboratingthe bivariate analysis, stepwise linear regressions demonstrated that
area was the primary factor in explaining interarchipelago variation in danaine and
mimetic pattern distributions (Table 4). In the intra-archipelago comparisons,
however, the relationship to area was less pronounced. In two of the five archipelagos
(Eastern Solomons and Fiji), elevation and distance to largest island, respectively,
were the strongest geographical predictors of danaine species distributions. Only in
the Western Solomons and Vanuatu did island area significantly explain variance in
distribution of mimetic patterns; elevation and distance to largest island were
implicated in the other archipelagos. In general, those geographical factors other
than area, elevation, and distance (see Material and methods) were of minimal
importance for interarchipelago distributions of both danaine species and their
mimetic patterns (Table 4).
Utilizing standardized residuals from stepwise linear regressions on area, elevation
and distance to continental land, an interarchipelago comparison showed no
relationship between residuals for all danaines versus those for all non-danaine
butterfly species, whereas residuals derived for non-danaine nymphalids were
positively related to residuals for all non-danaine butterflies (Fig. 1). These results
BIOGEOGRAPHY OF PACIFIC MILKWEED BWITERFLIES
32 1
suggest that factors influencing the distribution of milkweed butterflies are Herent
from effects that influence distribution of non-danaine nymphalids across archipelagos. Residuals from stepwise regressions on aforementioned geographical factors
were also calculated for the number of danaine species within the largest mimetic
pattern found on each archipelago. Across all archipelagos, these latter residuals
were strongly correlated with those for all danaine species (Fig. 2), suggesting that
species membership in the largest mimicry pattern does not promote archipelago
establishment beyond that indicated for danaine butterflies as a whole.
DISCUSSION
A primary problem in biogeographical studies is adequacy of distributional
information. In this regard, the milkweed butterflies of the tropical Pacific are wellstudied (Ackery & Vane-Wright, 1984). Carpenter (1953), for example, presented
detailed distributional data for the major danaine genus Euploeu in the tropical
Pacific, while D'Abrera (1990) treated comprehensively the entire Melanesian
TABLE
2. Bivariate log-log regressions of number of danaine species on area, distance to nearest
larger archipelago (interarchipelago analysis) or island, distance to continental land mass
(interarchipelagoanalysis) or to largest island, and elevation among fifteen archipelagos and
within five archipelagos. n is the number of archipelagos or islands in a regression, c is the
intercept, and z is the slope
Independmt vatiable: a m
Group
n
C
Interarchipelago
15
-0.67
Bismarcks
10
0.38
Eastern Solomons
10
0.22
Western Solomons
21
0.06
Fiji
41
0.25
Vanuatu
27
0.13
Indcpmdmt variable: distance to continental land mass or to kqmt
Group
n
c
15
2.29
Interarchipelago
10
1.04
Bismarcks
Eastern Solomons
10
0.63
21
0.55
Western Solomons
41
0.78
Fiji
27
0.48
Vanuatu
Independent variable: distance to tlcMcst k n p amhipelup or island
Group
n
C
Interarchipelago
15
-0.50
10
1.03
Bismarcks
Eastern Solomons
10
0.56
21
0.26
Western Solomons
41
0.44
Fiji
27
0.29
Vanuatu
IfidGPendet vatiable: elevation
Group
Interarchipelago
Bismarcks
Eastern Solomons
Western Solomons
Fiji
Vanuatu
n
15
10
10
21
41
27
c
-0.22
-0.35
-0.53
-0.36
0.10
0.17
2
0.37
0.18
0.18
0.31
0.10
0.22
?
R
0.81
0.66
0.69
0.76
0.32
0.60
0.01
0.04
0.03
0.01
0.06
0.01
islands
z
?
R
-0.55
-0.10
-0.04
-0.20
-0.35
0.04
0.68
0.18
0.01
0.27
0.14
0.05
0.01
0.64
0.78
0.49
0.03
0.79
z
1.79
-0.11
-0.003
0.40
-0.04
0.20
z
0.26
0.41
0.46
0.41
0.13
0.15
?
0.39
0.04
0.0001
0.47
0.009
0.10
R
0.02
0.62
0.99
0.01
0.57
0.11
?
R
0.22
0.29
0.58
0.25
0.12
0.08
0.77
0.14
0.01
0.02
0.03
0.16
R DUDLEY AND G. H. ADLER
322
butterfly fauna. Reasonable confidence may therefore be placed on the interarchipelago data set; deviations by several species com the total species count will
have little effect on the overall statistical analysis. Danaines (as well as ithomiine
butterflies; see Beccaloni & Gaston, 1995) are also much more apparent to human
collectors than are other butterflies, but this effect is unlikely to influence the
residuals analyses (Figs 1, 2) for interarchipelago distributions. Similarly, danaines
and unpalatable butterflies generally are known to possess tougher bodies and
greater longevities, although neither of these qualities are likely to change results of
the interarchipelago analyses.
By contrast, it is likely that existing knowledge of species distributions within
tropical Pacific archipelagos is much less complete. Small islands within archipelagos
are generally much less fiequently visited than are main islands, and presence of
isolated transients may also confound characterization of resident populations. The
distributional data presented by Ackery & Vane-Wright (1984) refer primarily to
major islands within archipelagos. Such islands may well act as metapopulation
sources for smaller islands that transiently and over variable time scales are colonized
by individual danaines. In the absence of continuous sampling for species presence
TABLE
3. Bivariate log-log regressions of mimetic patterns on area, distance to nearest larger
archipelago (interarchipelago analysis) or island, distance to continental land mass
(interarchipelago anal+) or to largest island, and elevation among fifteen archipelagos and
within five archipelagos. n is the number of archipelagos or islands in a regression, e is the
intercept, and z is the slope
IndcpmdsntVfniab&:maO
Group
Interarchipelago
Bipmarch
Eastern Solomons
Western Solomons
Fiji
VanUatU
n
c
15
10
10
21
41
27
-0.37
0.28
0.06
-0.01
0.22
-0.05
Z
0.24
0.07
0.11
0.185
0.04
0.22
va7iabb: &taw to CmJinnrkrl land mass m to lalpi kka?kis
Group
n
c
2
InterarChipelagO
15
1.15
-0.34
Bismarch
10
0.70
-0.12
Eastern Solomom
10
0.54
-0.15
Western Solomons
21
0.29
0.14
Fiji
41
0.50
-0.10
VanUatU
27
0.48
0.04
Ifui+dmt vlniabk: &taw to Mamst kngeradiphgo O r k W
Group
n
c
Z
Interarchipelago
15
1.05
-0.26
Bumarch
10
0.47
-0.004
Eastern Solomom
10
0.37
-0.09
Western Solomom
21
0.14
0.22
Fiji
41
0.33
-0.04
VanUatU
zndcpmdsntvarwblc:slcvation
Group
Interarchipelago
BismardLs
Eastern Solomom
Western Solomom
Fiji
Vanuatu
27
0.11
0.20
?
R
0.78
0.59
0.55
0.69
0.20
0.60
0.01
0.07
0.10
0.01
0.23
0.01
?
R
0.62
0.51
0.52
0.03
0.09
0.05
0.01
0.16
0.15
0.51
0.07
0.79
?
-
0.22
0.m1
0.83
0.33
0.02
0.10
R
0.08
0.97
0.45
0.01
0.46
0.11
n
C
Z
?
Fc
15
10
10
21
41
27
0.17
4.36
-0.59
-0.26
0.16
-0.19
0.16
-0.28
0.36
0.45
0.05
0.22
0.19
0.66
0.63
0.21
0.05
0.16
0.11
0.01
0.01
0.04
0.19
0.04
BIOGEOGRAPHY OF PACIFIC MILKWEED BUTTERFLJES
323
TABLE
4. Stepwise linear regressions of loglo number of danaine species or mimetic patterns
among and within tropical Pacific archipelagos
No. danaine species
Group
Interarchipelago
n
15
Bismarck
Eastern Solomons
Western Solomons
Fiji
Vanuatu
No. mimetic pattens
Group
Interarchipelago
Bismarck
Eastern Solomons
Western Solomons
Fiji
Vanuatu
P
R
0.79
F
23.2
0.01
10
10
21
41
27
Regression model
1.02t0.21 log,, area-0.52 log,, distance
to continental land
0.09t0.27 log,, area
-0.49*0.45 log,, elevation
4.019tO.33 loglo area
0.78-0.19 log,, distance to largest island
0.13-0.22 log,, area
0.64
0.48
0.56
0.14
0.33
10.9
6.5
22.9
5.6
12
0.03
0.05
0.01
0.05
0.01
n
15
10
10
21
41
27
Regression model
-0.37+0.24 log,, area
-0.36t0.29 log,, elevation
-0.67+0.40 log,, elevation
-0.04t0.20 log,, area
0.51-0.11 log,, distance to largest island
-0.05+0.22 log,, area
12
F
R
0.61
0.61
0.60
0.69
0.13
0.33
20.6
9.4
10.6
16.8
5.2
11.6
0.01
0.03
0.03
0.01
0.05
0.01
on small islands, the distributional data synthesized by Ackery & Vane-Wright (1984)
should be regarded as sufficient for an initial biogeographical analysis. Additional
geographical surveys, particularly of smaller islands, would advantageously supplement existing distributional information for tropical Pacific butterflies.
The primary result for the distribution of danaine species is the reduced influence
of island area and distance within archipelagos, in contrast to the significant effects
of these variables on interarchipelago distributions (Table 2). Geographical factors
influencing interarchipelago distributions of danaine species also differed somewhat
from those for all butterflies. For the latter category (n = 26 archipelagos), distance
to the nearest land entered a stepwise regression model first, followed by total land
area (Adler & Dudley, 1994). For danaines (n = 15 archipelagos), area entered the
regression model first, followed by distance to continental land (Table 4). In part,
failure to observe intra-archipelago areal effects on danaine distributions may reflect
-
.
-2.54
-2.5 -2
-
21
-lmn
-
.
-
t
1.5
2
; - - - -
1.4
1,
0
-1.5 -1
-.5
I
0
.5
1
L
standardized residuals,
non-danaine butterflies
Figure 1. Residuals from stepwise linear interarchipelagoregressions (Table 4) for danaine species (0)and
non-danaine nymphalids (0)versus residuals for all butterfly species. Data for the latter two categories
were taken from Adler & Dudley (1994).The regression between residuals for non-danaine nyrnphalids
and for all non-danaine buttedies was highly significant (y = 0.87X,8 = 0.76, P = O.OOOl), whereas the
comparable regression for danaine species was not significant (? = 0.02, P = 0.65).
R.DUDLEY AND G.H.ADLER
324
standardized residuals,
all danaines
Figure 2. Residuals from stepwise linear interarchipelago regressions for the number of danaine species
in the largest mimicry pattern versus residualsfor all danaine species. The significant correlation between
these two sets of residuals (y = 0.64x, ? = 0.92, P = O.OOOl), suggests that danaine species in the largest
mimicry pattern behave as a random subset of all danaines in the context of interarchipelago
distribution.
a sampling bias. Only major islands greater than 1 km2were included in the intraarchipelago analysis; variance in area among archipelagos was substantially greater
than that within archipelagos (Table 1). Many archipelagos, however, are comprised
of up to 300 smaller islands (e.g. the Fijian archipelago),some of which are only a few
hectares in area and have not been evaluated biotically. Had the entire range of
island sizes within archipelagos been included, much stronger species-area
regressions would probably have been obtained. The significant effects of elevation
on some intra-archipelago distributions of danaine species and patterns (Tables 2
and 3) may in part reflect the exclusion of high elevation species (altitudinal
range > 1000m) from the numerous low-lying islands found within most archipelagos (see Table 1).
In the interarchipelago comparison, effects of distance on danaine species and
pattern distributions are not surprisingly significant (Tables 2 and 3) in light of the
mean interarchipelago distance of approximately 2550km (Table 1). As with area,
however, the effects of distance on intra-archipelago distributions are much less
pronounced. Within four of the five archipelagos studied, distance to nearest larger
or to the largest island is not a statistically significant factor influencing species
numbers in a multivariate analysis (see Table 4), even though the mean interisland
distance for these four archipelagos ranges from approximately 100 to 200 km (Table
1). Similarly, only in Fiji is the distribution of mimetic patterns significantly
dependent on the distance to the nearest larger island (Table 4). Reduced effects of
isolation on intra-archipelago distribution have been previously reported for birds in
the Solomon Islands (Diamond & Mayr, 1976), but it is nonetheless surprising for
buttexflies that distance effects are of so little consequence. In birds, for example,
airspeeds and capacity for long-range powered flight is substantially higher.
Apparently for danaine butterflies, a combination of powered flight and ambient air
motions is sufficient within several hundred kilometers to overcome over time the
distributional effects of distance on species numbers. It is clear that long distance
dispersal over oceans is possible in many butterflies (Williams,1930; Holzapfel &
BIOGEOGRAPHY OF PACIFIC MILKWEED BUTTERFLIES
325
Harrell, 1968; Fox, 1978; Farrow, 1984), including danaines (see e.g. Urquhart &
Urquhart, 1976, 1979).
The distribution of danaine mimetic patterns generally showed dependence on
geographical features similar to that for danaine species. In part, this effect may
simply reflect definition of individual mimetic groups as non-overlapping subsets of
all danaine butterflies, although the extent to which such discrete categories are
perceived by potential predators is unclear. Existing criteria for assigning species to
mimicry groups rely not on quantitative similarity of wing shape, colour and flight
behaviour, but rather on qualitative human synthesis of visual features. Furthermore,
the actual predators of the tropical Pacific danaine fauna are unknown, as are the
character and range of sensory modalities actually utilized during predation. It is
likely, however, that birds and lizards are the principal predators of butterflies. Given
their presumed unpalatability and overall morphological similarity, all danaines can
potentially act as Miillerian co-mimics (Ackery & Vane-Wright, 1984). It is therefore
appropriate to evaluate not only aforementioned mimetic patterns but also all
danaine species as a single mimetic pattern in any analysis of the potential
consequences of mimicry. This conclusion is supported by the finding that, within
the mimetic pattern containing the greatest number of species on each archipelago,
there is no enhancement of species presence beyond that established for the total
count of danaine butterflies (Fig. 2). A quantitative assessment of wing colour and
shape in danaines would usefully evaluate the correspondence of particular species to
emergent groupings that potentially confer mimetic advantage.
Danaine butterflies appear to be distributed differently from non-danaine species
in an interarchipelago comparison (Fig. l), suggesting that factors supplemental to
geographical features are acting to influence danaine distributions. Such factors that
would differentially promote species establishment may include danaine longevity
and toughness, shared host plants, similar use of habitat, and/or selective advantages
associated with Miillerian mimicry. This last effect would potentially act to promote
species establishment either within particular mimetic patterns or for danaine
butterflies generally. The finding that danaine distributions differ from those of
butterflies in general does not unequivocally demonstrate that Miillerian mimicry
acts to promote species establishment, but it does suggest that a more highly resolved
assessment of species distributions and habitat overlap will demonstrate positive
associations among species of the same mimetic pattern or of overall morphological
similarity. At present, data are unavailable to examine in detail the hypothesis that
species establishment is promoted by common habitat or hostplant use. Given the
marked differences in distribution between danaines and butterflies as a whole, it is
possible that selective advantages of mimicry conferred to individuals may occur
through predator generalization of all danaine species (Ackery & Vane-Wright,
1984).Membership in specific mimicry groups, however, may be more important in
determining local abundance on particular islands or archipelagos, which may also
vary in quantity and character of insectivorous predators. Field observations and
population data are therefore necessary to evaluate the contributions of the
aforementioned ecological factors to danaine species distributions.
ACKNOWLEDGEMENTS
We thank the Smithsonian Tropical Research Institute for logistical support and
R.DUDLEY AND G. H.ADLER
326
J. Diamond for providing geographical data for the Bismarcks. E. Pianka kindly
commented on the manuscript. The constructive comments of R.I.Vane-Wright
and an anonymous reviewer are gratemy appreciated.
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