patterns of species diversity of snake community assemblages, with data on two everglades snake...

Patterns of Species Diversity of Snake Community Assemblages, with Data on Two EvergladesSnake AssemblagesAuthor(s): George H. Dalrymple, Frank S. Bernardino, Jr., Todd M. Steiner and Robert J.NodellSource: Copeia, Vol. 1991, No. 2 (May 16, 1991), pp. 517-521Published by: American Society of Ichthyologists and Herpetologists (ASIH)Stable URL: http://www.jstor.org/stable/1446600 .

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SHORTER CONTRIBUTIONS: HERPETOLOGY

Copeia, 1991(2), pp. 517-5i21 ? 1991 by the American Society of

Ichthyologists and Herpetologists

PATTERNS OF SPECIES DIVERSITY OF SNAKE COMMUNITY ASSEMBLAGES, WITH DATA ON TWO EVERGLADES SNAKE ASSEMBLAGES.-Vitt (1987) evalu- ated the relationship between snake species diversity and latitude and concluded that the relationship was "not impressive." We have re-

analyzed this relationship and found a strong relationship, previously obscured by the com-

bining of estimates of diversity using different

logarithmic treatments by Vitt (1987) vs Brown and Parker (1982). Snakes were collected from two areas within Everglades National Park: an upland area, Long Pine Key, and a wetland area, the Pahayokee region. In this report we com- pare the characteristics of these snake assem-

blages to each other and to other reports on snake diversity, to determine whether the Ev-

erglades assemblages are relatively depauperate due to geographic isolation and/or reduction in habitat diversity at the extreme southern end of the Florida peninsula (Duellman and Schwartz, 1958; Busack and Hedges, 1984; Means and Simberloff, 1987).

Study area and methods.-Long Pine Key (LPK) is an upland region of 8000 ha dominated by pinelands intersected by numerous short hy- droperiod wetlands or transverse glades, with scattered hardwood hammocks throughout and former agricultural lands to the south (Dalrym- ple, 1988). Pahayokee is an area of wetlands, with scattered dwarf cypress forest and marsh- land, that is part of the vast expanse of season- ally flooded wetlands adjoining the Shark River Slough.

We collected snakes by road cruising in an automobile. We did 50 person-h/mo of field work for 3 yr in LPK (1984-86). We cruised a single road 2-3 d each week. We shifted the collecting time from primarily day time in the cooler, dry season to night time in the hot, wet season in LPK. In the Pahayokee region, we collected snakes 1 d/wk by road cruising during 1987 and 1988, for a total of 20 person-h/mo. We surveyed 9.6 km of paved road in LPK, and 11.2 km of paved road in the Pahayokee area.

In LPK, in order to avoid counting the same snake more than once, we marked all live snakes

by the method of Brown and Parker (1976), and we checked snakes found dead on the road for a previous mark. We did not mark snakes in

Pahayokee, because the snakes were mostly caught during short term movements in re-

sponse to annual drying, and a much larger pro- portion of the snakes were dead on the road.

The formula for species diversity used in this

study is the Shannon diversity index:

H = -; pilogpi,

where,

p = ni/N.

For the calculations in this study the numbers were transformed to natural logarithms (follow- ing Brown and Parker, 1982). The formula for Evenness used here is

E = H/Hmax

where H is the Shannon index and

Hmax= logns

where s is the total number of species (see Brown and Parker, 1982:62, and Brower and Zar, 1984 for discussions of these indices).

Results and discussion. -Everglades.-The snake

assemblages of the two areas have very different patterns of activity: in LPK many species were active throughout the year, with peaks in total numbers in the early Summer and Fall. In the Pahayokee area, snake activity was dominated by a peak of activity during the annual drying of the wetlands in the Winter. Differences in snake movement patterns resulted in many more snakes per hour in Pahayokee vs LPK. The general ecological relationships of these species and the habitats of the regions were reviewed by Duellman and Schwartz (1958) and Dalrymple (1988).

In LPK, 20 species of snakes were found, and the number of snakes per year is summarized in Table 1. The seasonal activity patterns of the more common species are reviewed by Dalrym- ple (1988), and Dalrymple et al. (1991). In Pahayokee, 16 taxa (Nerodia fasciata was represented by both N. fasciata pictiventris and N. f compressicauda) were found (Table 1). The inter- annual variation in numbers in Pahayokee is a reflection of the annual variation in patterns of drying, and the more abrupt, almost explosive,



518 COPEIA, 1991, NO. 2

TABLE 1. NUMBER OF SPECIES (SUBSPECIES FOR Nerodia fasciata) AND INDIVIDUAL SNAKES OBSERVED AND/OR COLLECTED EACH YEAR IN THE LPK AND PAHAYOKEE REGIONS. Twenty-two taxa (21 species, one with two

subspecies) are represented.

Number per species

Long Pine Key Pahayokee

Species 1984 1985 1986 Total 1987 1988 Total

Agkistrodon piscivorus 27 24 23 77 45 83 128 Cemophora coccinea 1 4 1 6 1 1 2 Coluber constrictor 77 73 58 208 5 1 6 Crotalus adamanteus 10 8 13 31 0 0 0 Diadophis punctatus 24 36 59 119 0 1 1 Drymarchon corais 10 2 5 17 0 0 0 Elaphe guttata 17 30 36 83 0 0 0 Elaphe obsoleta 39 47 47 136 0 0 0 Farancia abacura 0 2 0 2 0 1 1 Lampropeltis getulus 1 3 4 8 1 1 2 Lampropeltis triangulum 0 1 0 1 1 0 1 Micrurusfulvius 4 13 5 22 0 0 0 Nerodia fasciata pictiventris 2 5 6 13 43 190 233 Nerodia fasciata compressicauda 0 0 0 0 1 7 8 Nerodia cyclopion 0 0 0 0 2 16 18 Nerodia taxispilota 0 1 1 2 0 0 0 Opheodrys aestivus 50 82 28 160 1 0 1 Regina alleni 0 1 0 1 0 6 6 Sistrurus miliarius 70 102 99 271 10 15 25 Storeria dekayi 22 57 56 135 9 21 30 Thamnophis sauritus 98 69 43 211 60 239 299 Thamnophis sirtalis 100 102 77 279 50 208 258 Total snakes 552 666 561 1782 229 790 1019 Number of taxa 16 20 17 20 13 14 16 Diversity index, H 2.30 2.42 2.42 2.42 1.82 1.66 1.72 Evenness index, E 0.83 0.81 0.85 0.81 0.71 0.63 0.62

seasonal movement pattern of the snakes of the area.

Comparison ofEverglades snake assemblages to those ofother regions.-Brown and Parker (1982) compared the number of species (i.e., species richness), Shannon Indices, Simpson's Indices and Evenness of 10 snake communities. That ap- proach is extended herein with the addition of our data and the calculation of those community measures for thirteen other studies (Table 2). The data in Table 2 indicate that the LPK region is neither high nor low in terms of diversity or evenness for its latitude. The Pahay- okee data indicate a lower diversity and evenness than LPK.

The analysis reveals species diversity values (H) ranging from 0.592-3.660, whereas Vitt's (1987) analysis of Brown and Parker's (1982) values and an additional 20 snake communities

included no species diversity values greater than 2.0, although it included communities with nearly 100 species (Vitt, 1987). Whereas Brown and Parker (1982) used natural logs (logj) in their calculations, Vitt used log to the base 10 (logi0). Vitt (1987) combined the natural log values (Brown and Parker, 1982) with values calculated with log base 10 in his analysis, and this resulted in the incorrect interpretation. (This is not a question of one logarithmic transformation being better than the other, but sim- ply that only one logarithmic transformation should have been used on all of the data sets.) For the relationship of species diversity and species richness in Vitt's review r2 = 0.185, (Vitt, 1987), whereas r2 = 0.956 for the values in Ta- ble 2. Vitt (1987) stated that only 11.1% of the variance in species diversity is explained by latitude, but our analysis reveals that 76% is explained by latitude (r = -0.87, P < .01; Fig. 1).



SHORTER CONTRIBUTIONS: HERPETOLOGY 519

5.000

D 4.500L I 4.000- E 3.500? R 0 s 3.000 0 S

2.500 5 0 0 0LPK TL 0U Y 2.000-0

S1.500 PAH 0 1.000- 0.500 0 0.000 I I I I I

0 5 10 15 20 25 30 35 40 45 50 LATITUDE

Fig. 1. Relationship between species diversity (H) and latitude for the snake assemblages in Table 2. (Least squares regression equation is Y = 3.7 - 0.06X). LPK: Long Pine Key; PAH: Pahayokee.

The regression of evenness against latitude is not as strong as that for diversity, although it is significant (P < .001); r = 0.70, P < .01; Fig. 2).

Bias in community sampling.-With the correc- tion to natural logs, much of the unexplained

1.000

0.9000 L

E 0.500 0 00 0 E 0.700 0 NPAH N

0.600- 0 0

E 0 s

0.500 s 0.400

0.300

0.200 I I I I I I 0 5 10 15 20 25 30 35 40 45 50

LATITUDE

Fig. 2. Relationship between species evenness (E) and latitude for the snake assemblages in Table 2. (Least squares regression equation is Y = 0.9 -

0.007X). LPK: Long Pine Key; PAH: Pahayokee.

variation in the analysis is removed from Vitt's (1987) interpretation. Vitt considered that some of the variability in his analysis of snake communities was caused by the fact that many of the studies of snake community assemblages he reviewed have biases, and that some of the tropical studies give estimates of abundance "based

TABLE 2. NUMBER OF SPECIES, DIVERSITY, EVENNESS, AND LATITUDE FOR 24 COMMUNITIES OF SNAKES FROM TEMPERATE AND TROPICAL REGIONS. (The first 10 values are from Parker and Brown, 1982. The data for Ecuador are for the Santa Cecilia location only, and the data for Peru are from the Moropon locality only.)

Locality and source No. species H E Latitude

Illinois (Seibert and Hagen, 1947) 3 .592 .539 42.00 Iowa (Klimstra, 1958) 5 1.132 .704 41.00 Maryland (Dargan and Stickel, 1949) 5 1.084 .674 39.30 Utah (Woodbury, 1951) 7 .992 .510 40.30 Utah (Brown and Parker, 1982) 7 1.051 .540 40.30 California (Fitch, 1949) 9 1.180 .568 38.00 Louisiana (Tinkle, 1957) 10 1.472 .639 30.00 Arizona (Pough, 1966) 12 1.874 .754 32.00 Arizona (Pough, 1966) 12 1.658 .667 33.00 Arizona (Parker, unpubl., in Brown and Parker, 1982) 13 2.151 .839 33.00 Missouri (R. Seigel, pers. comm.) 13 2.095 .818 39.00 Missouri (Clawson and Baskett, 1982) 16 2.140 .772 39.00 Everglades (Pahayokee, Total) 16 1.720 .620 25.30 Virginia (Uhler et al., 1939) 18 2.026 .701 37.00 Everglades (LPK, Total) 20 2.418 .807 25.30 Chihuahua (Reynolds and Scott, 1982) 20 2.424 .809 28.30 Georgia (Wright and Bishop, 1915) 21 2.601 .854 30.30 Belize (Henderson and Hoevers, 1977) 22 2.574 .833 17.00 Venezuela (Silva et al., 1985) 25 2.653 .825 10.35 Guatemala (Duellman, 1963) 29 3.076 .913 16.15 Ghana (Leston and Hughes, 1968) 34 3.110 .882 10.00 Thailand (Inger and Colwell, 1977) 47 3.380 .878 15.00 Ecuador (Duellman, 1978) 50 3.343 .855 -0.03 Peru (Dixon and Soini, 1977) 59 3.660 .898 -4.00



520 COPEIA, 1991, NO. 2

on the relative number of each snake species collected by any means possible and are biased toward species that are terrestrial and easy to collect" (Vitt, 1987). Such a bias could under- estimate aquatic and arboreal species. The LPK

study focuses on the least aquatic area within the Everglades, and so aquatic snakes were ex-

pected to be limited in abundance. Within the LPK samples there are only three highly arboreal species (Elaphe spp. and Opheodrys), and all three were commonly observed in this study. The Pahayokee data focus on semi-aquatic and

aquatic species, and the peculiar activity pattern in response to drying may lead to a bias toward

species more sensitive to standing water levels. When a region's fauna is explicitly broken into wetland and upland components, as in the cur- rent study, it should permit a better understand-

ing of the assemblages, and help avoid under- or over- estimation of diversity caused by sam-

ple bias.

Snake assemblages and habitat in southern Flori- da.-The diversity and evenness of the LPK

community are predictable based upon the general relationships analyzed by Brown and Par- ker (1982), Busack and Hedges (1984), and Vitt (1987), and do not reflect a depauperate fauna. The higher diversity and evenness of snakes in LPK relative to Pahayokee confirm the "Ev-

erglades effect" of Means and Simberloff(1987), that involves a "huge Everglades hiatus" in the middle of many amphibian and reptile species' southern Florida distributions. Of the 22 taxa of snakes observed in this study, 10 listed as absent or rare in the Everglades wetlands by Means and Simberloff (1987) and also absent or rare in our Pahayokee samples (absent: Dry- marchon, Elaphe guttata, Nerodia taxispilota, Cro- talus, and Micrurus; rare: Cemophora, Diadophis, Lampropeltis triangulum, Opheodrys, and Storeria). The few representatives of those species, and the fact that Thamnophis sirtalis was common (instead of "occasional" as stated by Means and Simberloff, 1987), indicate that the Pahayokee samples were dominated by semi-aquatic species. "Species diminution in the Everglades- Big Cypress region is a direct consequence of the low habitat diversity possessed by this large area by reason of its low elevation, low topo- graphic relief, and extensive continuous wetland nature" (Means and Simberloff, 1987).

The high diversity found in the isolated uplands of LPK illustrates the biogeographic and conservation value of this area. Only isolated remnants of the Atlantic Coastal Ridge system

of uplands remain outside of Everglades Na- tional Park. Approximately 230 natural forest

community sites, encompassing only 1458 ha, remain in Dade County; 99% of the uplands of the county have been destroyed (Dade County, 1984; U.S. Fish and Wildlife Service, 1988). Remnant uplands are still being transformed into housing developments and farmlands, and are in need of rapid and aggressive protection in order to preserve an important part of this

region's biotic diversity.

Acknowledgments.-W. Gibbons, E. Juterbock, B. Means, and W. B. Robertson, Jr. made many suggestions for improvements, and we thank them for their efforts. Thanks to R. A. Seigel for making his unpublished data on the snake

community from Missouri available. We es-

pecially thank L. Vitt for his careful reading of an earlier version of the manuscript and for his thought-provoking comments. This research was sponsored by the U.S. National Park Ser- vice-Florida International University Coop- erative Agreement (CA-5000-3-8005, Supple- mental Agreement No. 2, 1984) and the Florida International University Foundation.

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Copeia, 1991(2), pp. 521-524 ? 1991 by the American Society of

Ichthyologists and Herpetologists

INTERMALE SPACING AND CALLING SITE CHARACTERISTICS IN A SOUTHERN MISSISSIPPI CHORUS OF HYLA CINEREA. -Acoustic communication plays an important role in the reproductive be- havior of green treefrogs (Blair, 1958; Ger- hardt 1974a, 1974b; Oldham and Gerhardt, 1975; Fellers, 1979). In treefrogs, calls may function to maintain spacing among males and to attract mates (Wells, 1977; Narins and Hur- ley, 1982). Information in vocalizations may be greatly reduced or altered by the position of the calling male. Vegetation structure and perch height of the calling male can affect call characteristics and influence mating success (Fellers, 1979; Greer and Wells, 1980; Wells and Schwartz, 1982; Brenowitz et al., 1984).

Herein we characterize the calling sites of male green treefrogs, Hyla cinerea, and describe their spatial distribution. To determine if vegetation type influenced perch height, we compared mean perch heights among vegetation types. We compared nearest neighbor distances (NND) across vegetation types to determine if vegetation type influenced the spatial distribution of males. We tested the hypothesis that males were randomly distributed in relation to each other by comparing the observed distri-



patterns of species diversity of snake community assemblages, with data on two everglades snake...

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