Gap Analysis for Ant Species
Craig R. Allen1, L. Pearlstine2, and D. P. Wojcik3
1South Carolina Cooperative Fish and Wildlife Research Unit, Clemson University, Clemson, South Carolina
2Florida Cooperative Fish and Wildlife Research Unit, University of Florida, Gainesville
3United States Department of Agriculture, Agricultural Research Service, Medical and Veterinary Entomology Research Laboratory, Gainesville, Florida
Introduction
Clearly, when we speak of biodiversity, it is appropriate to consider plant and invertebrate diversity as well as vertebrate diversity. Vertebrates account for less than 2% of presently described animal species (Gaston 1991). Almost all undescribed species are invertebrates. Few of the estimated 27,000 species going extinct each year are vertebrates (Wilson 1992). Vertebrates utilize relatively large home ranges likely to span several vegetation and habitat types. Most vertebrates, even habitat specialists, are habitat generalists when compared to invertebrates. The scale of perception and environmental exploitation of invertebrates is orders of magnitude smaller than that of vertebrates. Furthermore, the ability of technicians to classify vegetation types exceeds the resolution of habitat utilization by vertebrate species (Maser et al. 1984). For example, the Florida Biological Diversity Project is using a habitat classification scheme that recognizes >100 plant associations. At this level, few, if any, vertebrates are specific to any one association (C.R. Allen, unpublished data), and most species span numerous associations.
Nodes of high biological diversity determined from vertebrate species richness are likely to be in the range of 100s to 1000s of hectares (e.g., Cox et al. 1994). Decisions concerning land use, habitat protection, and conservation are likely to be an order of magnitude smaller. Additionally, small areas unable to support a large variety of terrestrial vertebrates may nonetheless be species-rich (containing a high richness of plant and invertebrate species). Land-use and conservation decisions made using vertebrates as indicators of biodiversity will realistically assess impacts on or protect vertebrates but may have little usefulness in conserving overall biodiversity.
The case for using arthropods for the inventory of biodiversity has been convincingly made (Kremen et al. 1993). Prendergast et al. (1993), in an examination of species richness in Great Britain, compared the diversity hot spots of birds, mammals, butterflies, and liverworts and found that the species-rich areas within each taxon rarely overlapped. Landres et al. (1988) cautioned against the use of vertebrates as an index of biodiversity; a range of well-chosen organisms that will explicitly better represent overall biological diversity is needed in an index of biological diversity. Due to the vast number of described invertebrates, it would be impossible to include them all in initial efforts. Therefore, invertebrate groups should be carefully chosen to maximize their contribution to determining overall patterns of biodiversity.
Butterflies have been suggested as an invertebrate taxonomic group to utilize for biodiversity monitoring (Kremen 1992). However, data from Florida indicate that despite the host-plant specificity of some larval forms, adult butterflies are mostly edge-type species with little overall habitat specificity. Indeed, birds may be more habitat specific than butterflies (Debinski and Brussard 1994). Among the invertebrates, the Formicidae exhibit many traits that make them an excellent choice for inclusion in an index of biodiversity. Ants are relatively easy to identify, and a relatively short period of training and adeptness with a dichotomous key will enable most persons to identify the ants of temperate regions. In Florida there are approximately 190 ant species, similar to the number of herpetofauna and avian species. Ant species are easy and inexpensive to sample, and a variety of established sampling methodologies exist. Additionally, the range and habitat affinities of ants are well known when compared to families such as Scaraebidae (an estimated 250 species in Florida; Woodruff 1973), or Staphylinidae (an estimated 450 species in Florida; Frank 1986).
Ants act as keystone species in many instances (Risch and Carroll 1982). They provide key and irreplaceable ecosystem services such as pollination, nutrient turnover, energy flow, and seed dispersal (Handel et al. 1981). The Formicidae exhibit a wide range of habitat specificities and diversity of lifestyles. Some species utilize very specialized microhabitats (see below), and feeding niches are likely to be saturated (Holldobler and Wilson 1990). Because of niche saturation, the Formicidae are excellent indicators of fine-scale habitat heterogeneity, which in turn is an excellent indicator of biological diversity. Additionally, niche specialization means that general ant sampling may be used to bioassay ecological trends by monitoring trends of species with specific life-history traits of interest. Ants, indicative of terrestrial habitat heterogeneity, and birds, indicative of structural heterogeneity across habitats (e.g., Cyr and Cyr 1979), may make an excellent pair of organisms for indexing biological diversity. Short generation time in ant species translates to rapid response to environmental change. These positive characteristics are good for inventory and monitoring and allow for a finer-scale resolution in biodiversity mapping. The inclusion of the Formicidae in an index of biodiversity yields a broader base and more precise information for cross-scale decision making.
Several species present in Florida illustrate the potential of the Formicidae as indicators of biodiversity. The tropical fire ant, Solenopsis geminata, acts as a keystone species (Risch and Carroll 1982) affecting invertebrate community composition where it is not displaced by the exotic S. invicta, which itself acts as a keystone (Wojcik 1994). Several species of ants are endemic to Florida, and others are nearly so. Paratrechina wocjiki is endemic to central Florida where it may be found in a variety of habitats (Deyrup and Trager 1986). Conomyrma flavopectus is restricted to sugar sands in central Florida with early successional stages of sand pine (Trager 1988). Paratrechina phantasma is endemic to Florida scrub and dune habitats (Trager 1984). Four species of ants are endemic to the unique Florida scrub and sandhills habitats; this exceeds the number of endemic scrub species for any of the vertebrate taxa or for butterflies.
Many species are habitat-specific. Xenomyrmex floridanus is restricted to mangrove (Deyrup et al. 1989). Leptothorax allardycei is limited to sawgrass and Crematogaster vermiculata to cypress (Deyrup et al. 1989); no vertebrates are sawgrass or cypress specialists. Leptogenys manni is endemic to Florida and feeds only on isopods (Trager and Johnson 1988); such niche specialization is not unusual, given the general niche saturation found in the Formicidae, and may be useful in ecological monitoring.
The predaceous species Odontomachus clarus illustrates interesting biogeographical patterns (mirrored by some much studied and heralded vertebrates, such as the Florida Scrub Jay). Odontomachus clarus is known only from xeric upland areas of Mexico, the southwestern United States, and subtropical Florida (Deyrup et al. 1985). Florida Formicidae are largely a mix of temperate continental species and species of West Indian origin, a pattern also seen in Florida’s birds and butterflies.
Because vertebrates and invertebrates interact with their environment at different scales, there is a critical need to include some invertebrate taxa in an index of biodiversity, and ants are a desirable and defensible taxon to use. Recently the USGS-BRD has considered mapping the Formicidae at a national level. Here we present two different methodologies for spatial mapping of ant diversity and a comparison of patterns of species richness between ants and mammals in southern Florida.
Methods
Literature-based (Florida)
Geographic distribution of species (i.e., ants and mammals) was determined at the county level. For ants, distribution was determined primarily from published sources (Buren and Whitcomb 1977, Carroll 1975, Cole 1982, Creighton 1950, Deyrup 1991, Deyrup and Trager 1986, Deyrup et al. 1988, 1989, Johnson 1986, Klotz et al. 1995, MacKay 1993, Samways 1983, Schneirla 1944, Smith 1930, 1933, 1944, 1979, Thompson 1989, Thompson and Johnson 1989, Van Pelt 1947, 1950, 1956, 1958, 1966, Watkins 1985, Wheeler 1932, Wilson 1964) and from the unpublished data of D. P. Wojcik and C. R. Allen. The availability of data varied by county. For several counties largely in private ownership with limited access, little data was available, and for some other species distribution is poorly known. We interpolated distributions in counties lacking data based on the presence or absence of species in adjacent counties or known biogeographic affinities of species. These data were then used to produce a county x ant species matrix. All resulting county-level distribution maps were reviewed by recognized experts.
Habitat affinities for ants also were determined primarily from literature review. The Florida bibliography of species habitat use and ecology includes >1300 sources (too many to cite, but the bibliography may be accessed at http://coop.wec.ufl.edu/gap) which have been used to create descriptors of habitat use by species, including ant species. These data were then used to produce an ant species x land cover type matrix. In conjunction the two matrices were then used to produce habitat-specific spatial distributions of all ant species present in Florida, as well as a map of overall ant species richness.
Sample-based (South Carolina)
In South Carolina the most recent (and only) comprehensive documentation of ant distribution appeared in 1916 (Smith 1916, Smith and Morrison 1916). This general lack of data in South Carolina necessitated that South Carolina take a sample-based approach to mapping ant diversity, which is currently under way. Ants are sampled throughout the state of South Carolina. Sampling is stratified by physiographic region (sandhills, coastal plain, piedmont, mountains) and by generalized South Carolina Gap Analysis land cover types (n = 28). Ten replicates (randomized within the constraints of access to some properties) in each land cover type in each region will be sampled for a total of approximately 1120 sampled habitat patches across the state (not all land cover types are represented in each strata and some may be of minor importance). Each habitat patch will be sampled by establishing a linear transect consisting of multiple sample points. Sample points will consist of bait samples and pitfall traps. Together, these two sampling methods will capture a majority of ant species present in the state. Pitfall sampling is the better method of sampling overall ant diversity, as aggressive species (e.g., red imported fire ants) will preclude other species from baits. We will also conduct limited sampling with other methods, such as arboreal (C.R. Allen, unpublished manuscript) and subterranean sampling. At each sample point, data on habitat also will be collected. Results of these sampling efforts will be used to simultaneously determine both the county-level distribution and habitat affinity of each species.
Ant sampling is relatively easy and fast. However, identification of species can be problematic. To successfully produce a sample-based data set of ant distributions for a GAP layer, we have had to establish a highly cooperative effort. In this case, cooperators include Clemson University’s Department of Aquaculture, Fisheries and Wildlife and Department of Entomology, the South Carolina Cooperative Fish and Wildlife Research Unit, the South Carolina Gap Analysis Program, the National Gap Analysis Program, the USDA Agricultural Research Service, and the South Carolina Department of Natural Resources.
Spatial correspondence between ants and mammals in south Florida
One example of how the data are used is in comparing ant distributions with distributions of mammals. Land cover for the lower peninsula of Florida was mapped at 30-m resolution from classification of 1993 and 1994 Landsat Thematic Mapper satellite imagery. Bands 2,3,4, and 5 of the imagery and a tasseled cap transformation were used in an iterative unsupervised clustering algorithm. Labeling of the spectral clusters with vegetation associations followed the National Vegetation Classification (Grossman et al. 1998, FGDC 1997) to the alliance level (Weakley 1997). Labeling was assisted by auxiliary information such as land use/land cover maps from the South Florida Water Management District, National Wetlands Inventory maps, soils maps, and vegetation surveys and photointerpreted points from low altitude aerial videography in Everglades National Park and Big Cypress National Preserve (Figure 1). Note: Figures can be viewed at http://coop.wec.ufl.edu/GAP/antmammal_spatial_corr.htm.
Ants and mammals were modeled in similar ways, following Gap Analysis procedures (Scott et al. 1993) and as outlined for ants above. We produced species richness maps of both taxa (Figures 2 and 3). Richness of both taxa was normalized such that the highest richness for each taxon was equal to one and the lowest richness equal to zero, so that the two coverages were comparable. A coverage of spatial correspondence was then produced by subtracting the normalized mammal species richness map from the normalized ant richness map.
Results
In the coverage of spatial correspondence (Figure 4), values near 0 (green) reveal that richness between mammals and ants are equivalent. High positive values (red to orange) identify areas with higher mammal richness relative to ant richness, and high negative values (blue to magenta) identify higher ant species richness relative to mammal richness.
Comparisons of mammal and ant species richness reveal interesting patterns of correspondence and disharmony between the two taxa. The large areas of green on the correspondence coverage indicates that richness between mammals and ants was similar over much of the Florida Everglades (but see below). However, two interesting deviations occur. In the Big Cypress area of southwest Florida, there is a lack of correspondence between mammals and ants, primarily in cypress-dominated habitats. This is not necessarily because mammal species richness is especially high in these areas, but because ant richness is low. Further north, the opposite situation exists; normalized ant species richness is higher than normalized mammal species richness in several pine-dominated habitats. In most terrestrial habitats (excluding the saturated everglades habitats which constitute a large area of south Florida), spatial correspondence between ants and mammals is low.
Discussion
Invertebrates contribute far more to overall species richness than do vertebrates, and nodes of high richness among different taxa are likely not to correspond. This mandates the inclusion of invertebrates in an index of biodiversity. Among the Arthropoda, the Formicidae are a good family of choice for mapping because data are available or relatively easy to obtain, they utilize a wide variety and large number of niches, and some ant species are very habitat- and condition-specific. Utilizing the Formicidae in biodiversity mapping efforts offers the chance to increase the thematic resolution when representing geographic nodes of high species richness. Future land-use decisions will likely be at a scale an order of magnitude smaller than decisions made in the past. The inclusion of the Formicidae in programs investigating biodiversity assures that land-use decisions will be made utilizing species information applicable across a range of scales.
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