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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. Mr. Deyrup very generously
edited my paper, and many of his comments are included here in
bold italics.)
Part 2: 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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