DESIGNING REFUGES FOR THE FLORIDA SAND PINE SCRUB
Earl D. McCoy
Organisms are threatened by fragmentation of their habitats. The island-biogeographic explanation for the species-area relationship (MacArthur and Wilson 1963, 1967) indicates that the species richness of fragments will decline, because of increased extinction rates, and that the severity of the decline will be greater for smaller fragments. Numerous studies have suggested that this conclusion may be too simplistic, however, and that serious errors might be made if emphasis were placed solely upon sizes of fragments (e.g., Simberloff and Abele 1976; Connor and McCoy 1979; Kushlan 1979; McCoy 1983; Simberloff and Gotelli 1984; Usher 1985; Zimmerman and Bierregaard 1986). The conservation planning for a species residing in habitat fragments must take into account its autecology.
We shall address some issues of reserve design for the vertebrates inhabiting fragments of the sand pine scrub habitat of interior peninsular Florida (McCoy and Mushinsky 1994, in preparation). Sand pine scrub is a xerophytic plant association, characterized by sand pine (Pinus clausa), rosemary (Ceratiola ericoides), and/or scrub oaks (Quercus chapmani, Q. geminata, Q. inopina, Q. myrtifolia). Ground cover over the white sand surface typically is sparse and patchy, although lichens (e.g., Cladonia spp.) may be found in great abundance. In the absence of relatively-infrequent but intense fires, the scrub habitat tends to succeed to xerophytic hammock or sandhill (Christman 1988; Fernald 1989; Myers 1990).
Among the approximately 300 species of non-weedy plants and 70 species of vertebrates found in Florida scrubs are many that are essentially restricted to this habitat. Florida scrub occurs, or formerly occurred, along much of the coastline, as well as throughout much of the interior of the peninsula (Davis 1943, 1967). Peroni and Abrahamson (1985a, 1985b) and Christman (1988) estimated that xeric land, mostly scrub, has suffered a loss to agriculture and real estate development of more than 60% of its original extent. More recent estimates (Fitzpatrick pers. comm.) place the loss near 85%. Most remaining fragments of scrub are small, often less than a few hundred hectares in area.
To examine reserve design for scrub vertebrates, we present six analyses. (1) We establish the relationship of the taxonomic richness of the vertebrates of a scrub to its size and other physical and biological attributes. (2) We establish the relationship of the abundance's of the vertebrate taxa to the environmental attributes of scrubs (see Haila and Hanski 1984; Haila et al. 1987, 1993). (3) We determine if the taxonomic compositions of relatively small and taxonomically poor scrubs tend to be subsets of the compositions of relatively large and taxonomically rich scrubs. (4) We determine if a relatively large scrub tends to support more taxa than several small scrubs of equal cumulative area ("SLOSS"). (5) We determine if small scrubs are relatively depauperate. (6) We determine whether or not the conclusions about reserve design reached by focusing on taxonomic richness agree with conclusions reached by focusing on abundances of taxa.
We studied 16 scrubs in south-central Florida, along a 50 Km portion of the Lake Wales Ridge in Highlands and Polk Counties. These scrubs were either "large" (about 200 hectares), "medium" (25-50 hectares), or "small" (10 hectares or less). When possible, we chose scrubs on state- and federally-owned lands, because they provided the best protection possible for our trap arrays (see below). Also when possible, we selected scrubs studied by Christman (1988), to facilitate our location of potential study sites, and to gain background knowledge of the existing vegetation from his descriptions.
We took two measures of vegetation and seven measures of size and isolation at each site. The measures of vegetation were "vegetational structure," which is the physical arrangement of living and dead plant material, and "vegetational composition," which is the relative abundance of several taxa of characteristic scrub plants. We used aerial photographs and visual inspection to determine: (1) size, (2) distance to the nearest scrub, (3) distance to the nearest larger scrub, (4) presence/absence of potential "corridor" habitats, (5) types of habitats between scrubs, (6) distance to the nearest permanent water, and (7) types, and percent coverage, of surrounding habitats. Also, we used a temporal series of aerial photographs to determine the extent of size reduction.
We used mainly pitfall and double-ended funnel trap arrays (Campbell and Christman 1982) to capture non-avian vertebrates. One array was installed/ten hectares of scrub. Trap arrays were opened for 7 - 10 days and checked daily during six trapping periods per year. Birds were censused twice yearly in each scrub, once during January - March, to include migratory taxa, and once in April - June, to include breeding taxa. Individuals of most taxa removed from traps were given unique marks. We divided the taxa into four groups for analysis: non-avian taxa trapped at least once, "characteristic" (Christman 1988) non-avian taxa trapped at least once, "characteristic" avian taxa, and avian taxa observed breeding.
We related taxonomic richness, estimated population sizes of individual taxa, and shapes of the relative abundance distributions to the measures of scrub size and isolation and vegetational structure and composition. We used correlation analyses to establish the relationships. Methods for examining nestedness, SLOSS, and whether or not small scrubs are depauperate are explained in McCoy and Mushinsky (1994).
The size of a scrub is the only physical attribute that correlates strongly with taxonomic richness of any of the four groups of vertebrates. The size of the scrub also correlates strongly with abundances of about 60% of the individual taxa. More than half of the remaining taxa are endangered or potentially endangered. Distance to permanent water, distance to nearest scrub, and distance to nearest larger scrub all contributed significantly (p < 0.05) to a multiple correlation with non-avian taxa captured at least once and with characteristic non-avian taxa captured at least once, but not with avian taxa observed breeding or with characteristic avian taxa. Among the various vegetational characteristics, only relative abundance of rosemary was correlated significantly (p < 0.05) with the richness of any of the groups of vertebrates, but these correlations could be spurious, because rosemary tends to be relatively more abundant in our larger scrubs than in our smaller ones. Vegetational characteristics, however, are strongly correlated with abundances of several taxa, and with shapes of relative abundance distributions.
Species compositions of smaller scrubs tended to be nested within those of larger scrubs, for all four groups of vertebrates that we analyzed. A single large scrub did not tend to support more taxa than a equal-sized combinations of smaller scrubs. To the contrary, it appeared that archipelagos of scrubs generally supported more taxa than comparably-sized individual scrubs. We could show no tendency for small scrubs to be depauperate. In fact, for all four groups of vertebrates, both medium and small scrubs tended to support more taxa than expected, and large scrubs supported fewer.
Detailed analysis of taxonomic richness patterns of vertebrates in 16 central Florida scrubs has led us to conclude that no evidence indicated the need for single large scrub reserves. An archipelago of individually smaller reserves could support at least as many taxa. We arrived at this conclusion despite the fact that all four groups of vertebrates (amphibians, reptiles, mammals, birds) displayed a significant taxon-area relationship and the taxonomic compositions of the scrubs were nested. Isolation of a scrub appeared to be more important in reducing the richness of amphibians, reptiles, and mammals than birds.
Consideration of the abundances of individual taxa, on the other hand, appeared to support the need for large reserves. Abundances of most taxa were highest in one of the large scrubs. No single large scrub supported high abundances of all taxa, however, and, in fact, some taxa were more abundant in some of the smaller scrubs. Several endangered or potentially endangered taxa maintained relatively-large populations in some of the small scrubs. Differences in vegetation structure and composition and, perhaps, isolation, of individual scrubs appear to account for at least some of the differences in vertebrate abundance among scrubs of similar size. So, while a particular large scrub reserve may support high abundances of some vertebrate taxa, and, therefore, have the potential to reduce their extinction rates, it will not tend to do so for all taxa. Some taxa, even endangered or potentially endangered ones, may maintain relatively-high populations in smaller fragments that provide suitable vegetational structures and compositions, ready access to water, and other preferred characteristics.
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