Ecosystems: Florida Scrub (Part 2 of 2)

By Cathy Vogelsong

Reprinted from The Understory, Dec. 2004. Jan. 2005

(My article is based on the classic Ecosystems of Florida by Myers and Jewel, and a seminar given by Mark Deyrup, Senior Research Biologist at Archbold Biological Station. Dr. Deyrup was kind enough to edit my paper, and many of his comments are included here in bold italics.)

The groundcover layer in scrub is always sparse. Its density appears to be inversely proportional to the density of the pine and shrub layers. Where it exists, the groundcover commonly includes gopher apple, beak rush and milk peas. Lichen may cover the open sand patches. Open sand patches may be maintained, in part, by allelopathy in plants such as Florida rosemary.

There are also many endemic and listed species among the scrub wildlife. The Florida mouse, Florida scrub jay, Florida scrub lizard, sand skink and blue-tailed mole skink are found only in the scrub. Gopher tortoise, which is more a sandhill species, will burrow in scrub. Larger mammals will use scrub as part of their territories. Myers and Jewel state any scrub site of a few acres or more is certain to support several thousand species of arthropods. This includes many host-specific herbivores and host-specific parasites. The type and density of scrub is very important to wildlife. As example, the scrub jay, the only bird restricted to scrub, is found only in a low shrubby scrub lacking sand pine.

Florida scrub is pyrogenic. But unlike other fire-dependent ecosystems in Florida, scrub fires occur infrequently (every 20-100 years).- This is due to the sparse groundcover, very slow accumulation of fuel, and frequent occurrence of water (coast, rivers, etc.) along scrub borders. Most scrub vegetation is not very flammable and not easy to ignite. Fires usually start in neighboring habitats and spread to scrub only when severe burning conditions are present. This produces high-intensity catastrophic fires. The shrubby layer is burned to the ground, but most of these plants are capable of resprouting from their roots. Woody scrub species, with the notable exceptions of rosemary and sand pine, rarely regenerate by seed. Sand pine, which is highly flammable (often retaining branches near the ground), is killed by fire but populations regenerate rapidly from seed. Most sand pines produce closed (clausa) cones, which do not release seed until the heat of a fire breaks up the resin. (Some coastal populations, arguably a subspecies, produce open cones , allowing them to regenerate without fire.) Fires release more -nutrients into the soil, so fire-dependent seed germination may be an adaptation to low nutrients. Many wildlife species are dependent upon the aftermath of fire - like a weevil that lives on burned palmetto stems, and a wasp species that lives on those weevils. Due to fragmentation and fire suppression, all of Florida's fire-dependent ecosystems must now be maintained with prescribed bums, but little is known about handling the catastrophic fire needed by scrub.

The scrub ecosystem is closely associated with the high pine ecosystem (called sandhill in xeric sites) both ecologically and historically, though they differ greatly in appearance. They share xeric, infertile, upland sites, soil characteristics, and dependence upon fire. It was once thought that they represented different stages of the same succession, but fire was found to be the key. The two ecosystems are often contiguous, and where they are, a change in fire frequency causes a shift from one system to the other.

Notes from a seminar presented by Mark Deyrup:

• Scrub was so scrunched down during the Pleistocene age, that the scrub jay is megafauna, perhaps the largest of the animals now restricted to scrub. It may have been once associated with antelopes, giant tortoises and ground sloths and other creatures of the Pleistocene.
• A small area of scrub can maintain a very large number of species. Over a thousand species of moths and butterflies have been identified in Florida scrub, along with 114 species of ants. Many scrub species have few if any close relatives. The ancient scrub is an exceptionally stable habitat because its harsh conditions are unacceptable to most of the worst exotic species that threaten Florida, and because the historical irregularity of the fires in scrub demanded considerable flexibility on the part of scrub inhabitants.
• There are two major decomposers in scrub—termites and fire. (There are conspicuous mushrooms in scrub but most species are not decomposers. They fix nitrogen, transport water, work cooperatively with oaks, etc.)
• Much of what, happens in a scrub, happens underground. (Some harvester ants burrow to a depth of 9 feet!) One tiny organism, the pygmy mole cricket, feeds on a thin layer of algae found 3 inches under the surface of the silica sand (light penetrates silica). A doctoral student identified 34 species of algae, diatoms, etc. in this layer. The algae produce a polysaccharide that holds grains of sand together forming a crust. But this is a very dynamic crust. The whole community of algae is mobile, so they can move up and down in the sand to reform the layer/crust after it has been disturbed.

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