WHAT CAN GEOLYCOSA WOLF SPIDERS TELL US
ABOUT FLORIDA BIOGEOGRAPHY?
Samuel D. Marshall
Mark A. Deyrup
Florida underwent a series of inundations by the sea during the interglacial periods. During the height of these interglacial periods parts of the peninsula consisted of a series of islands (MacNeil 1951; Neill 1957; McCrone 1963; Deyrup 1989). These inundations shaped the plant and animal assemblages of modern Florida in two ways: by isolating lineages and by creating or modifying the soil types upon which extant plant and animal communities are based.
Two major interglacial events shaped present-day Florida: the Wicomico and Pamlico periods. During these periods there was some reworking of the much older (Pliocene) dunes of the interior ridges when the Wicomico shoreline was established. Later, during the Pamlico, the lower dunes lying closer to the present-day coasts were laid down.
The xeric communities found on these sandy ridges are scrub and sandhill. Scrub may be defined by the fire adapted plant species found there, particularly sand pine (Pinus clausa), and the several species of shrub oaks (Quercus inopina, Quercus geminata, and Quercus myrtifolia). Sandhill typically has long-leaf pine (Pinus palustris) and/or slash pine (Pinus elliotti), turkey oak (Quercus laevis), and wire grass (Aristida stricta) and a higher fire frequency. There are several variants of these habitats, and the conditions at any one site depend on the time since the last fire. This community variation is superimposed on a complex of habitat islands of xeric uplands that have been isolated for various lengths of time. Thus, even though the amount of topographic relief in Florida might seem inconsequential, it is nonetheless associated with highly complex biogeography that is still poorly understood. Any group of organisms that can give us a key to understanding this situation is especially worthy of study, and it appears that the wolf spider genus Geolycosa is one such group.
This study concerns the distribution patterns of wolf spiders in the genus Geolycosa (Araneae: Lycosidae) and how they relate to the biogeography of Florida. Geolycosa wolf spiders are found throughout North America (Dondale & Redner 1990). These spiders are obligate burrowers: all juveniles and adult females spend their lives in a burrow. Males spend their brief adult lives wandering in search of sexually receptive females. Geolycosa burrows are very distinctive and within given scrubs they can be used to identify the species found there. Wherever they are found, Geolycosa occur in sand or sandy soils. Given this as a genus level habitat requirement, it is perhaps no surprise that Florida has more species than any other state.
There are nine species of Geolycosa recorded from Florida, with six of these restricted to the more xeric upland habitats. These six xeric-adapted species are the focus of our study. For the present study, location data were taken from the literature (Wallace 1942; McCrone 1963), museum collections (Florida State Collection), and from our own collecting trips. Collecting sites were located on the basis of past records, as well as serendipity.
Geolycosa xera McCrone and Geolycosa hubbelli Wallace are found on the ancient dunes of the central ridge. Geolycosa xera is restricted to the southern Mount Dora, Orlando, southern Deland, and Lake Wales ridges. Geolycosa hubbelli is found on the Mount Dora, Orlando, and Lake Wales Ridge. These ridges together represent an ancient dune complex from the Pliocene. The Lake Wales ridge has the highest rate of endemism, possibly because it is the southernmost of the ancient ridges (and therefore less subject to the climatic fluctuations of the Pleistocene). Geolycosa xera is further divided into two subspecies: G. x. xera and G. x. archboldi. Geolycosa x. archboldi is only found in Highlands County, at the southern end of the species' distribution.
Geolycosa patellonigra Wallace and Geolycosa micanopy Wallace are associated with the more recent Pamlico shore lines. McCrone (1963) has proposed that G. patellonigra speciated on the island that was the Trail ridge in the north of Florida, and dispersed down the later Pamlico shorelines.
Geolycosa escambiensis Wallace and the closely related Geolycosa ornatipes (Bryant) are almost identical, with only males being reliably told apart. These two species are restricted to the northern parts of the state and the panhandle. However, the ranges of these species are divided by the Apalachicola River, with G. escambiensis only being found to the west. The Apalachicola is a known boundary for many species, and in this case may represent a division in an older northern coastline. These species could have originated in the ancient fall line sandhills to the north.
Many scrub endemics may have relict distributions, being holdovers from periods when the seas withdrew and conditions were dryer and scrub was a dominant ecosystem in Florida (Deyrup 1989). At the end of the Pliocene many of the species of plants and animals with western affinities probably made their move east (Webb 1990), notably the Florida scrub jay, gopher tortoise, and harvester ant. However, the pattern for the Geolycosa species found on the central ridge, G. xera and G. hubbelli, does not exhibit the relict distribution of a western lineage. These species are descendants of an Atlantic coastal dune species stranded inland by the retreating sea.
In conclusion, what can Geolycosa wolf spiders tell us about Florida biogeography? Across the state, the pattern for Geolycosa matches that described for some other taxa: a distinction can be made between the central ridges and the coastal ridges, with a center of diversity in the north-central parts of the state. Given these general patterns, the distribution of certain species, such as G. patellonigra on low lying ridges near the Lake Wales ridge may indicate that these sites in fact have coastal, rather than central affinities despite their proximity to the Lake Wales ridge. In the panhandle, patterns of endemism may not be associated with particular ridges: G. escambiensis occurs both on recent coastal dunes and as well as the older uplands in from the coast. With more work we might be able to identify old refuges in the panhandle with more endemics from which species such as G. escambiensis dispersed.
The complex biogeography of Florida is foremost of interest just because it explains the distribution patterns that we see around us. Studies of Florida biogeography are also studies of evolution on habitat islands, such as made the Galapagos Islands famous. Moreover, all biogeographic studies today inevitably have a link to applied problems of conservation. The natural communities of Florida's uplands are being rapidly destroyed by development, and our choices of where we must concentrate our conservation efforts depend on recognizing the habitat islands that represent the greatest amount of evolutionary divergence. Geolycosa wolf spiders, with their limited dispersal and narrow habitat requirements, are ideal for this purpose.
We thank G.B. Edwards for access to the Florida State Arthropod Collection and for confirming the identification of specimens, Lou Sorkin for access to the collections at the American Museum of Natural History, Dana Bryan for granting permission to collect in Florida parks and preserves, and Doria Gordon for access to Nature Conservancy preserves. This research was supported in part by Archbold Expeditions.
Deyrup, M. A. 1989. Arthropods endemic to Florida scrub. Florida Scientist 52: 254-270.
Dondale, C. D. and J. H. Redner. 1990. The Wolf Spiders, Nurseryweb Spiders, and Lynx Spiders of Canada and Alaska. The Insects and Arachnids of Canada, Part 17. Research Branch, Agriculture Canada, Pub. 1856. Ottawa, Canada.
MacNeil, F. S. 1951. Pleistocene shore lines in Florida and Georgia. U.S. Geol. Surv. Prof. Paper, No. 221-F.
McCrone, J. D. 1963. Taxonomic status and evolutionary history of the Geolycosa pikei complex in the Southeastern United States (Araneae, Lycosidae). American Midland Naturalist 70: 47-73
Neill, W. T. 1957. Historical biogeography of present-day Florida. Bulletin Florida State Museum of Biological Science 2: 175-220.
Wallace, H. K. 1942. A revision of the burrowing spiders of the genus Geolycosa (Araneae, Lycosidae). American Midland Naturalist 27:1-62.
Webb, S. D. 1990. Historical biogeography. In: R. L. Myers and J. J. Ewel, eds. Ecosystems of Florida. The University of Central Florida Press, Orlando, Florida. p. 70-100.