Scrub is frequently cited as Florida's most distinct ecosystem; physiognomy and composition are quite distinct from surrounding habitats and between 40-60 percent of scrub species are considered to be endemic. Scrub contains a biological treasure house of plants and animals adapted to life on scattered ridges of sand; the ancient origins of these sand dune communities date back to the Pliocene savannas, and provide a relic example of an extremely old and formerly extensive ecosystem (Deyrup and Eisner 1993).
Scrub vegetation represents an exceedingly xeric plant community in an otherwise lush, subtropical belt. Scrub is often defined botanically as a xeromorphic shrub community dominated by a layer of evergreen or nearly evergreen oaks (Quercus geminata, Q. myrtifolia, Q. inopina, Q. chapmanii) or Florida rosemary (Ceratiola ericoides) or both, in the presence or absence of an overstory of sand pine (Pinus clausa) occupying well-drained, infertile, sandy soils (Myers 1990). Three units of scrub can be defined: inland peninsula, coastal peninsula, and coastal panhandle (Myers 1990). The scrub is home to about 100 plant species, about one-third endemic to this ecoregion (Fergus 1993). An even higher number of invertebrates (45+) are endemic to the harsh desert-like conditions. Perhaps the flagship animal species of this part of the ecoregion is the scrub jay (Aphelocoma coerulescens coerulescens), a distinct race recently elevated to species rank, that has evolved an interesting social system in the scrub islands. High genetic variability among scrub species is likely, as many are poor dispersers.
The largest blocks of scrubs are inland scrubs and the best studied is the Lake Wales Ridge, home to the Archbold Biological Station. Many of the vertebrate and invertebrate species endemic to the scrub have been studied here. Other islands of scrub are less well studied and under great threat from development. Scrub is dependent upon fire to maintain its unique assemblage of species. Left alone, scrub patches will burn at 5 to 40 year intervals. Scrub fires are mediated by the surrounding landscape, which can either be more fire-prone than scrub (e.g. high pine) or nonflammable (e.g., swamps). Fires can be spectacular; the 1935 fire in the Ocala National Forests burned 140 km2 in only four hours. Scrub seems to be remarkably resilient and persistent in the face of these natural disturbances. The absence of the driving disturbance event, fire, may ultimately spell the greatest threat to the persistence of scrub.
Habitat Loss and Degradation
Today only about 10-15 percent of the scrub habitat remains, the rest replaced by citrus groves and housing developments. There has been a relatively uniform loss of scrub habitat across the region, but the most severe loss is in the south (Lake Wales Ridge), which also has the highest biodiversity value. Along with increased urbanization and the planting of citrus groves, fire suppression is another important factor that has led to the conversion of scrub. Proximity of housing developments and other facilities near scrub preclude allowing fires to burn naturally or at all. Specifically, conversion has most affected sand pine scrub, sand pine islands, and scrubby flatwoods. A review of the current conservation status of remaining scrub can be found in Myers (1990).
Remaining Blocks of Intact Habitat
The largest block of scrub habitat is the Ocala National Forest and the Archbold Biological Station, both part of the Central Ridge section of this eocregion. In decreasing order of size, other blocks (all blocks are found in southern Florida) include:
•Jonathan Dickinson State Park (Coastal scrub section)
•Avon Park Air Force Base (Central Ridge section)
•Arbuckle State Forest (Central Ridge section)
•Cedar Key Scrub State Preserve (Coastal scrub section)
•Merritt Island Cape Canaveral (Coastal scrub section)
A more complete list of remaining blocks is listed in Myers and Ewel (1990, p. 142-3).
Degree of Fragmentation
The Lake Wales Ridge section is not naturally highly fragmented, but residual sections have become fragmented by citrus and residential development. Corridors are not as relevant for scrub as for other ecoregions because scrub occurs in naturally disjunct blocks of habitat, although some species require relatively undisturbed intervening habitat for their effective dispersal. Fragmentation and urbanization reduces the ability to maintain fire susceptibility. Existing corridors and linkage zones would be restorable if intense buring is permitted.
Degree of Protection
The most promising development is that relatively large sums of money have been made available for conservation of scrub by the state of Florida through the KARL program and other avenues. Although scrub occurs in national forests, these areas are not being managed in a manner that promotes maintaining scrub over the long term. A number of important areas on private land have been proposed for purchase under habitat conservation programs. Overall, the variation in scrub habitat is sampled adequately? by many small reserves.
Types and Severity of Threats
The major conversion threats in this ecoregion are citrus production and residential development. Fire suppression remains the most pervasive degradation threat. Unlike other Florida ecoregions, there are no serious exotic species problems.
Suite of Priority Activities to Enhance Biodiversity Conservation
•The following activities should be given top priority:
•Important remnants of scrub habitat have been identified. Now direct action must be taken to properly conserve them, especially on the Lake Wales ridge.
•The USFWS refuge currently being discussed should be established and assured of adequate management.
•Remaining scrub habitats should be thoroughly inventoried.
•Greater awareness of the biological value of scrub should be promoted nationally.
•Archbold Biological Station
•Florida Natural Areas Inventory
•The Nature Conservancy
•The Nature Conservancy of Florida
•US Fish and Wildlife Service, (have planned sand pine refuge)
Relationship to other classification schemes
This ecoregion is not delineated by Bailey or Omernik. Boundaries were taken from Küchler (1964) and Myers and Ewel (1990).
Prepared by: E. Dinerstein, A. Weakley, R. Noss, R. Snodgrass, K. Wolfe.