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Species density of hylid frogs in North Carolina and Virginia

map of species density of hylid frogs in NC and VA

General patterns of species density:

This map shows the approximate density of distribution of 16 species of hylid frogs of the genera Acris, Pseudacris (excluding P. kalmi), and Hyla in North Carolina and Virginia. Darker color corresponds to higher species density. Click the image to enlarge.

Hylid density is generally centered on two hotspots in the Atlantic Coastal Plain; one in southeastern NC (where two adjacent counties reach a density of 14 species: Cumberland and Sampson) and one in southeastern VA (where two adjacent counties reach a density of 12 species: Isle of Wight and Surry). These hotspots are principally due to the number of hylid species (N = 10; A. gryllus, P. brimleyi, P. ocularis, P. ornata, P. nigrita, H. andersonii, H. cinerea, H. femoralis, H. gratiosa, H. squirella) whose global distribution is mainly limited to the Atlantic and Gulf Coastal Plains. Widely distributed species (N = 5; A. crepitans, P. crucifer, P. feriarum, H. chrysoscelis, H. versicolor) are lesser components of Coastal Plain species density (these species achieve their highest density in the Piedmont of southern VA). The last species (P. brachyphona) is limited to the Appalachian Highlands.

Five species of the above Coastal Plain-limited hylids reach the northern global limit of their range at or near the VA hotspot (A. gryllus, P. brimleyi, P. ocularis, P. nigrita, H. femoralis). One additional species (P. ornata) reaches the northern global limit of its range at the NC hotspot and another (H. andersonii) occurs in the NC hotspot and is absent from the VA hotspot but occurs north of VA. The hotspots are separated by a narrow zone of relatively lower-density counties stretching east to west across northeastern NC. This gap in density aside, Coastal Plain hylid density conforms to the Latitudinal Diversity Gradient. Patterns of hylid density in other regions of NC and VA are less distinct but follow a weak longitudinal gradient.

The density gap in northeastern NC can be attributed in part to a repeating pattern of disjunction in the range of Coastal Plain-limited hylids: three species (P. nigrita, H. gratiosa, H. squirella) are absent from the density gap; four others (A. gryllus, P. brimleyi, P. ocularis, H. femoralis) experience a longitudinal narrowing of their range across the density gap.

Possible factors influencing these patterns:

Physiography and biogeography. The Coastal Plain province is often subdivided into Inner (western; characterized by higher elevations, sandy soils, and pine flatwoods) and Outer (eastern; characterized by barrier islands and low-elevation mainland wetlands) along the Suffolk Scarp, which runs approximately North-South between of the mouth of the James River in VA to mouth of the Neuse River in NC. Much of the Coastal Plain of southeastern VA and northeastern NC falls east of the Suffolk Scarp, in the Outer Coastal Plain. (South of the Neuse River in NC, the Inner Coastal Plain widens while the Outer narrows to a fringe. The highest hylid densities occur here, in the Sandhills of the Inner Coastal Plain, in the Cape Fear and Lumber River basins). Counties east of the Suffolk Scarp on the Albemarle-Pamlico Peninsula in NC (an area dominated by lowland Pocosin wetlands) and between the lower James River in VA and north of the Albemarle Sound in NC (and area dominated by the Great Dismal Swamp wetland) have hylid densities among the lowest in the Coastal Plain.

Climate. A minimum winter temperature isocline (inferred from USDA hardiness zones, separating zones 7b and 8a) runs NE – SW across NC in such a way that the majority of the Inner Coastal Plain south of the Neuse River falls into the warmer zone 8a. This zone narrows considerably towards southeastern VA. Subsequently, much of the Inner Coastal Plain north of the Albemarle Peninsula falls into the cooler zone 7b (a difference of > 5° C in minimum winter temperature). The NC hotspot falls into zone 8a while the lower-density counties north and west of it In VA, a second isocline (separating zones 7b and 7a) approximates the Fall Zone from the Virginia Peninsula south in such a way that the Inner Coastal Plain south of the York River falls into the warmer 7b while the Coastal Plain north of the York River, and essentially all of the VA Piedmont fall into the cooler zone 7a.

Keystone species. Like the five hylid species listed above, the Longleaf Pine (Pinus palustris) also reaches the northern global limit of its range near the hylid density hotspot in southeastern VA. Fire-regulated Longleaf Pine savannas historically dominated much of the Atlantic and Gulf Coastal Plains. This habitat supports high levels of biodiversity, and the historic range of Longleaf Pine correlates closely to endemism-driven hylid density in North America. In northeastern NC, proportionally more of the Inner Coastal Plain is associated with Gum-Cypress and Pocosin wetlands than with pine savannas (which are patchily distributed and uncommon), and the former may represent marginal habitat for hylids more strongly associated with the latter.

Anthropogenic change. Only 5% of historic Longleaf Pine savanna is intact globally. Much of the Coastal Plain of NC and VA has been converted to agricultural use. In some Outer Coastal Plain wetlands, this has involved large-scale and intensive hydrological modification. Some portions of the Coastal Plain have been heavily urbanized (i.e. the Norfolk–Virginia Beach metropolitan area). Protected lands are extensive but are not evenly distributed in the Coastal Plain.

Sampling bias. Northeastern NC is regarded by some as herpetologically under-sampled in comparison with adjacent areas in southeastern VA, the NC Piedmont, or the Sandhills of south-central NC. The difficulty in surveying unfragmented Pocosin and Cypress-Gum wetlands may also contribute to an underestimation of density based on county-by-county distributions.

Multifactorial explanations and model taxa:

Patterns of distribution, including hylid density, are most likely explained by the complex interactions of many factors, including but not limited to those outlined above. It is unlikely that a single factor can adequately account for the distribution of a single species, and this is likely to be compounded when considering family-level densities. Research methods capable of accounting for or reducing complexity are therefore essential. One complexity-reducing method is the model taxon.

The historic ranges of the sister species of Acris form a zone of sympatry in NC and VA which extends in a continuous band through the upper Coastal Plain and includes both hotspots. Although total global range size is similar for both species (A. crepitans sensu stricto, not including the range of A. blanchardi), A. gryllus is primarily limited to the Coastal Plain and reaches its northern global range limit near the southeastern VA hotspot while A. crepitans is distributed across more physiogeographic provinces and a wider latitudinal range. Recent studies of the fine-scale distribution of Acris in sympatry in NC and VA show that A. gryllus appears to be declining while A. crepitans has remained stable or expanded its range further into the Coastal Plain. Specifically, A. gryllus appears to have disappeared from portions of the hylid density gap in northeastern NC and from the vicinity of its global northern range limit in VA.

The distribution of A. gryllus overlaps with the majority (14 out of 16) of hylid species in NC and VA and approximates the principle density patterns of the family in the Coastal Plain. By studying the fine-scale distribution of A. gryllus (i.e., comparing historic and modern distributional data in order to compare distributional change of wider- and narrower-distributed sister species, building niche models and comparing among heterospecific models and other spatial patterns) it may be possible to learn more about the factors which shape the distribution of all hylid species in the Coastal Plain, and in a broader sense develop tools which can aid in the conservation of amphibian biodiversity in this region.