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Distribution of reptiles and amphibians on the islands of eastern Lake Michigan: summary and analysis.

ABSTRACT

We studied the island biogeography of reptiles and amphibians in the Lake Michigan Archipelago, which is composed of twelve major islands in eastern Lake Michigan. We created an updated distributional record of reptiles and amphibians in the Lake Michigan Archipelago, quantitatively compared our distribution data with a commonly used data set published in 1948, analyzed the distributions in light of the theory of island biogeography, and compared species diversity of the Archipelago to that of the mainland and other island groups in the Great Lakes. A total of 10 reptile and 11 amphibian species are documented in the Lake Michigan Archipelago. Our updated distribution data are a qualitative improvement over the 1948 data set. There is a positive relationship between island area and the number of both amphibian species and reptile species present, but no relationship between the latter variables and island distance to the mainland. The Lake Michigan Archipelago appears to be less diverse both in general and in relation to the adjacent mainland than other island groups in the Great Lakes. Possible reasons for some of our findings are discussed. This research underscores the incompleteness of commonly used distribution data for the Lake Michigan Archipelago, and raises several questions for future study.

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INTRODUCTION

The study of insular flora and fauna has played a dominant role in shaping biological theory. In effect, modern biological study began with the work of Darwin (1859) and Wallace (1869) whose work on insular organisms helped lead them independently to the theory of evolution by natural selection. The subdiscipline of biogeography, formalized by Wallace (1880), has more recently stimulated a great deal of research. This began with the equilibrium theory of island biogeography (MacArthur and Wilson 1967). The equilibrium theory led indirectly to metapopulation biology (Hanski and Simberloff 1997) and directly to the unified neutral theory of biodiversity and biogeography (Hubbell 2001). In general, it is hard to overemphasize the importance of islands to the study of biology.

[FIGURE 1 OMITTED]

The island group focused upon in this study is the Lake Michigan Archipelago. This archipelago is composed of twelve major and several minor islands located in eastern Lake Michigan. The islands vary widely in surface area and distance to the mainland (Table 1; Figure 1). The Archipelago is adjacent to the Leelanau Peninsula in the south and the Straits of Mackinaw in the north, a straight north-south distance of ca. 90 km. The islands with the highest elevation (Beaver, High, South Fox, and North and South Manitou) were first present in glacial Lake Algonquin ca. 9000-10,000 yr. B.P. (years before present). From 4000 to 8000 yr. B.P. (Lake Chippewa stage), water levels were lower. The northern islands (Beaver, Garden, High, Hog, Squaw, Trout, and Whiskey) were all exposed and connected to the mainland via Waugoshance Point, while the Manitou Islands were connected via the Leelanau Peninsula. Whether or not the Fox Islands were ever connected to the mainland is unknown. Lake levels then rose again during the Lake Nipissing stage (3000-4000 yr. B.P.), and all but the highest areas (parts of Beaver, Garden, High, North and South Fox, and North and South Manitou) were inundated. Water levels then gradually fell to that of modern-day Lake Michigan (Dietrich 1978; Hatt et al. 1948).

Land vertebrates likely reached the islands via land bridges during the Lake Chippewa stage or by rafting across the Lake since that time (Hatt et al. 1948). Because the islands were connected to the Lower Peninsula of Michigan and because surface currents run from the Lower Peninsula to the islands (Harrington 1895), land vertebrate populations on the islands are likely derived from those on the Lower Peninsula (but see Placyk and Gillingham 2002). The land vertebrates (amphibians, reptiles, birds, mammals) of the islands have long attracted the attention of biologists, but the only attempt at a comprehensive survey of the entire Archipelago was that of Hatt et al. (1948). Although many updates of species distributions in the Archipelago have appeared since that time, Hatt et al. (1948) is generally used as the only reference for the Archipelago in published analyses and summaries of island biogeography. This in effect amounts to the use of an incomplete, if not incorrect, data set.

Recently, Placyk and Gillingham (2002) summarized literature on the distribution of reptiles and amphibians in the northern portion of the Lake Michigan Archipelago (the Beaver Archipelago). Here we add to this work, creating an updated account of the distributions of reptiles and amphibians in the entire Lake Michigan Archipelago by including the Fox and Manitou islands. We also compare our data to that of Hatt et al. (1948), quantitatively analyze the updated distributions to determine if they are in agreement with the basic tenets of island biogeography, and compare the results to those from other island groups in the Great Lakes.

MATERIALS AND METHODS

We searched the scientific literature for any records of reptiles and amphibians in the Lake Michigan Archipelago, beginning with the sources given in Hatt et al. (1948) and proceeding to the present (Bowen and Gillingham 2002a-e; Case and Scharf 1985; Gillingham 1988; Linton et al. 1988; Phillips et al. 1965; Placyk et al. 2002; Scharf 1973; Scharf and Jorae 1980). Also included were observations from our own fieldwork in the Archipelago. We compared the island and mainland herpetofaunas using the Coefficient of Biogeographical Resemblance (CBR), a species-level approach (Duellman 1990). This is calculated as

CBR = 2C/([N.sub.1] + [N.sub.2])

where C is the number of species that two areas have in common, and [N.sub.1] and [N.sub.2] are the number of species occurring in each respective area. To form a mainland species pool we included all those species that have ranges adjacent to the Archipelago in the lower peninsula of Michigan (37 species; Harding 1997). A CBR of zero indicates no common species while a CBR of one indicates identical species composition.

Statistical analysis was done in two ways. The relationship between the number of species and island area can be characterized by the power function

S = c[A.sup.Z]

where S is the number of species on an island, A is the area of the island, and c and z are constants. Transforming this equation logarithmically gives

log(S) = log(c) + zlog(A)

so that when the log of island area is plotted on the x-axis and the log of species number is plotted on the y-axis, a linear regression will produce a best-fit straight line with intercept log(c) and slope z. The constant z can be used to compare the species-area relationship of one region with another (Arrhenius 1921). Wright (1981) used a z-statistic to compare values of z. The z-statistic is calculated by summing the two z values in question and dividing the sum by the square root of their summed variances. If the z-statistic is larger than 1.96, the null hypothesis of equal z values can be rejected. We calculated z-statistics in order to compare our updated distributions with those reported by Hatt et al. (1948).

We then used Spearman's Rho correlation tests to determine the strength of the relationships between island area and number of species, island distance from the mainland and number of species, and island distance from the mainland and island area. To remain consistent we log-transformed all data. In keeping with basic island biogeographic theory, we predicted that the number of amphibian and reptile species on an island would increase with island area and that the number of species would decrease with increasing distance from the mainland. A statistically significant relationship between island area and distance from the mainland would make interpretation of species-area and species-distance relationships difficult. Partial correlation tests were used to determine if island area and distance from the mainland had confounding effects on the number of species present. All correlation tests were done separately for amphibians and reptiles. All tests were interpreted at an alpha of 0.05 and statistical analyses were done using JMP[R](SAS Institute Inc.).

RESULTS

To date, a total of 10 reptile and 11 amphibian species are documented in the Lake Michigan Archipelago. Qualitative inspection of the data suggests that the largest islands have the largest number of species. This is not true in all cases, however, as some relatively large islands may have fewer recorded species than smaller islands (Table 2). The CBR value for the Archipelago and adjacent mainland (21 island species, 37 mainland species) is 0.724.

Performing a linear regression on log-transformed island area and number of species gave a z value of 0.19 for reptiles ([r.sup.2] = 0.32, p = 0.07) and a z value of 0.28 for amphibians ([r.sup.2] = 0.44, p = 0.02). The values calculated from data in Hatt et al. (1948) are z = 0.13 ([r.sup.2] = 0.30, p = 0.13) for reptiles and z = 0.19 ([r.sup.2] = 0.30, p = 0.1) for amphibians. The z-statistic for reptiles was 0.001 and for amphibians was 0.002, suggesting that the z values calculated with updated distributions were not statistically different from those calculated using Hatt et al. (1948). However, the results for amphibians changed qualitatively in that the slope of the regression line was statistically different from zero with the new data included.

Spearman's Rho correlation tests revealed that there was a statistically significant positive relationship between the number of reptile species and island area (r = 0.82, p = 0.002, Figure 2A) and between the number of amphibian species and island area (r = 0.76, p = 0.004, Figure 2B). There was no statistically significant relationship between the number of reptile species and island distance from the mainland (r = -0.23, p = 0.37) or between the number of amphibian species and island distance from the mainland (r = -0.38, p = 0.22), although both relationships were negative as expected. However, there was a statistically significant negative relationship between island area and distance from the mainland (r = -0.67, p = 0.02). Partial correlation analysis revealed that removing the effects of island distance from the mainland did not qualitatively change the relationship between number of the reptile/amphibian species and island area, although in both cases the strength of the correlation decreased. However, removing the effects of island area from the relationship between the number of reptile/amphibian species and island distance from the mainland changed these relationships from negative to weakly positive. Therefore, it appears that there is no biologically significant relationship between number of species and distance from the mainland source pool.

[FIGURE 2 OMITTED]

DISCUSSION

In comparison to other archipelagos within the Great Lakes, the Lake Michigan Archipelago has a less diverse herpetofauna. For example, the number of amphibian/reptile species is 16/19 in the Lake Erie Archipelago, 14/14 in the St. Lawrence Archipelago, 14/19 in the Georgian Bay Archipelago, and 15/6 in the Apostle Island Archipelago (summarized in Hecnar et al. 2002). Hecnar et al. (2002) reported CBR values for these island groups and their adjacent mainland source pools of 0.894, 0.921, 0.972, and 0.824 respectively, all of which are higher than the Lake Michigan Archipelago. At this time, it is unclear why the herpetofauna of eastern Lake Michigan would be less diverse both overall and in comparison to the adjacent mainland than elsewhere in the Great Lakes. Possible reasons include fewer islands in the Archipelago (i.e., fewer "targets" for immigration, Lomolino 1990), unrecorded extirpations, and an incomplete census, but these reasons are speculative. Hatt et al. (1948) noted that all of the reptile and amphibian species found in the Archipelago were either aquatic or prone to dispersal via floating debris. While this may be true, it does not explain the absence of the Mudpuppy (Necturus maculosus), Fowler's Toad (Bufo fowleri), Chorus Frog (Pseudacris triseriata), Blanding's Turtle (Emydoidea blandingii), or Five-lined Skink (Eumeces fasciatus). All of these species have mainland ranges adjacent to the Archipelago and are either aquatic or deposit offspring where they might be swept into the Lake (i.e., shoreline pools or debris; Harding 1997).

The z-value for both amphibians and reptiles falls within the "normal" range given by May (1975) of 0.15 to 0.39. The z value for reptiles is smaller than but comparable to that reported by King (1987) for the Lake Erie Archipelago of 0.25. However, it should be noted that neither of these statements would necessarily be true using data from Hatt et al. (1948). Furthermore, the slope of the regression line for amphibians became statistically significant with added distribution records. It is interesting that the relationship between species number and area appears to be stronger for amphibians than for reptiles (higher z value). It is difficult to assign biological significance to this result, however, given that the utility and meaning of z values are currently a matter of debate (Hubbell 2001).

Results for z values and z statistics as well as those for Spearman's Rho Correlation tests and Partial correlations suggest that there is a relationship between island area and the number of species within the Lake Michigan Archipelago, but that there is no relationship between the number of species and island distance from the mainland. This result is in agreement with those found for other archipelagos within the Great Lakes (Hecnar et al. 2002). Hecnar et al. (2002) suggest that this stems from the fact that all of these archipelagos are "land-bridge" in origin and are relatively close to mainland source pools. King (1988) noted the same type of relationship between the number of reptile species and island distance from the mainland in Lake Erie and attempted to interpret the results in light of the equilibrium theory of island biogeography (MacArthur and Wilson 1967). However, he also listed three caveats to this interpretation: (1) that species lists are likely incomplete; (2) that some islands are closer to each other than to the mainland and are more likely to exchange immigrants with each other than with the mainland (i.e., they may be experiencing metapopulation dynamics apart from the mainland source pool); and (3) that some species are better colonizers or more prone to local extinction than others. It is possible that our knowledge of distributions in the Lake Michigan Archipelago is still incomplete, particularly for the Fox group and the Beaver group (excepting Beaver Island itself). The "land-bridge" and metapopulation hypotheses are also viable, but currently there are no data that would allow a test of either.

We feel that we have succeeded in creating an updated and more useful record of reptile and amphibian distributions within the Lake Michigan Archipelago. Furthermore, our analysis of the distributions has improved knowledge of biogeography in the region and in the Great Lakes as a whole. Not surprisingly, however, our analysis has created more questions than answers. Are the distribution records now complete? Why is the Lake Michigan Archipelago species-poor compared to other archipelagos within the Great Lakes? Why is the z value stronger for amphibians than for reptiles? Why is there no relationship between number of species and island distance to the mainland? All of these questions are suitable for future research.
TABLE 1. Major islands of the Lake Michigan Archipelago with approximate
surface areas and distances to the lower peninsula of Michigan. Surface
areas were converted from data in Hatt et al. (1948) and distances to
the mainland were determined using topographic maps.

Island Surface Area (ha) Distance to Mainland (km)

Beaver 15,136 28
Garden 1990 29
Gull 109 54
High 1495 42
Hog 1024 21
North Fox 362 36
North Manitou 5286 10
South Fox 1373 26
South Manitou 2037 10
Squaw 31 39
Trout 47 45
Whiskey 52 40

TABLE 2. Known distributions of reptile and amphibian species in the
Lake Michigan Archipelago. See the Materials and Methods section for a
list of the sources used to compile data. An asterisk (*) indicates that
this record is based on unpublished field observations.

Species Islands

 Beaver Garden Gull High Hog N.Fox N. Manitou
Notophthalmus
 viridescens X X X
Plethodon cinereus X X X X X
Ambystoma laterale X X X
Ambystoma maculatum X X
Bufo americanus X X X X X X
Hyla versicolor X
Pseudacris crucifer X X* X X X
Rana catesbeiana X X
Rana clamitans X X X X
Rana sylvatica X X
Rana pipiens X X
Chelydra serpentina X X X
Chrysemys picta X X X X
Thamnophis sirtalis X X X X X
Thamnophis sauritus X X
Nerodia sipedon X X X X X
Storeria dekayi X
Storeria
 occipitomaculata X X X
Diadophis punctatus X X X X
Liochlorophis
 vernalis X
Lampropeltis
 triangulum X X X X

Species Islands

 S. Fox S. Manitou Squaw Trout Whiskey
Notophthalmus
 viridescens X
Plethodon cinereus X X X
Ambystoma laterale X
Ambystoma maculatum X
Bufo americanus X X X X X
Hyla versicolor X
Pseudacris crucifer X X
Rana catesbeiana
Rana clamitans
Rana sylvatica
Rana pipiens X
Chelydra serpentina X
Chrysemys picta X
Thamnophis sirtalis X X X X X
Thamnophis sauritus
Nerodia sipedon X X
Storeria dekayi X* X
Storeria
 occipitomaculata X X
Diadophis punctatus X X
Liochlorophis
 vernalis
Lampropeltis
 triangulum X


ACKNOWLEDGEMENTS

We acknowledge J. Rowe, C. Zurenko, and D. Vogel for help in the field. E. Bowen, M. Haussmann, J. A. Holman, and K. Rogers provided helpful comments on earlier versions of this manuscript. Fieldwork was done under MDNR Cultural and Scientific Collector's permit CA 341 issued to J. C. Gillingham. K. D. Bowen acknowledges a Graduate College Fellowship from Iowa State University for financial support.

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_____. 1880. Island life; or, the phenomena and causes of insular faunas and floras, including a revision and attempted solution of the problem of geological climates. London: MacMillan and Company.

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KENNETH D. BOWEN

Iowa State University

AND

JAMES C. GILLINGHAM

Central Michigan University
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Author:Bowen, Kenneth D.; Gillingham, James C.
Publication:Michigan Academician
Geographic Code:100NA
Date:Jun 22, 2004
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