Terrestrial ecosystems of south Florida have undergone numerous human disturbances, ranging from alteration of hydroperiod, fire history, and drainage patterns by water control structures to expansion of agricultural activity to the introduction of exotic species. Over historical time, striking changes in the ecosystem have been documented, and these changes have been attributed to various human activities. However, the natural variability of the ecosystem is unknown and must be determined to assess the true impact of human activity on the modern ecosystem. To document historical changes and to establish the baseline level of variability in the historic Everglades, the project "Ecosystem History: Terrestrial and Freshwater Ecosystems of southern Florida" is analyzing biotic components from cores and surface samples collected throughout the region. These components include pollen and other organic-walled microfossils, peat composition, molluscs, and foraminifers, and these data are integrated with geochronologic data to form a comprehensive picture of the timing and extent of ecosystem changes in the south Florida region.

Shallow cores and surface samples have been collected at numerous sites in the Water Conservation Areas, Big Cypress National Preserve, and Everglades National Park, with highest sampling density in the Taylor Slough region. To accurately interpret vegetational composition from down-core pollen assemblages, it is critical to correlate modern pollen assemblages with standing vegetation and develop an understanding of how over- or underrepresented a taxon is in the pollen record. Surface samples of peats have been collected at 42 sites selected to maximize areal coverage and diversity of vegetational types in a database of pollen abundance in modern sediments. This database is used to identify the closest modern analogs for down-core assemblages, improving the accuracy of interpretation of past vegetational composition from the pollen record. Initial results show good general correlation between pollen and vegetational composition and provide evidence on which plants are greatly overrepresented in the pollen record (such as amaranths) and which ones are underrepresented relative to their abundance in the standing vegetation (such as cattail).

Seven cores, four from the Taylor Slough/Buttonwood Embankment region, two from Water Conservation Area 2A, and one from Big Cypress National Preserve, have been analyzed to date. All show fluctuation in the abundance of different types of marsh and slough vegetation over the "prehistoric" part of the record, reflecting natural changes in hydroperiod, fire regime, and other environmental parameters. A core collected at site MC1 along Mud Creek (near Joe Bay) consists of alternate layers of marl and peat deposited over the last 2,050 years. Pollen assemblages were dominated by Cladium (sawgrass) pollen from the base of the core to at least 800 BP. Most of these sediments are marls, and the presence of Spirogyra zygospores and fresh- water molluscs prior to 800 BP indicate that they are fresh-water marls. In sediments deposited after 800 BP, foraminifers and molluscs characteristic of brackish water are present, and Spirogyra zygospores are sparse to absent, indicating a salinity increase that is consistent with sea-level rise. Over the last 150 years, pollen of Myrica (wax myrtle), Rhizophora (red mangrove), Avicennia (black mangrove), Quercus (oak), and the Asteraceae (daisy family) are more abundant than at any previous time, but little variation occurs within that time interval, and the site appears to have been relatively insulated from anthropogenic changes.

Cores collected at site TC2 along Taylor Creek also show dominance by sawgrass pollen from about 2,000 years ago until the mid- to late 1800's. Some variation is seen in the relative abundances of sawgrass, cattail, Polygonaceae (knotweed family), and Asteraceae through this time, but, in general, the assemblage varies little in the lower three-quarters of the core. The vegetational response to environmental changes between 1900 and 1960 is greater than at the Mud Creek site; by 1960, the abundance of sawgrass pollen dropped to lower levels than seen elsewhere in the core, and tree pollen increased in abundance. The loss of sawgrass appears to be correlated with hydrologic alterations to the area; such alterations include construction of the Tamiami Trail (1915-1928) with the resulting changes to sheet flow patterns and construction of canal and levee system throughout the region in the 1950's and 1960's. Two other cores, one collected downstream from TC2, and another one collected in the middle of Taylor Slough, show similar patterns to these cores. Of the 35 additional cores collected in this area, several will be selected to form transects along and across Taylor Slough; their flora and fauna will be analyzed for biotic components to establish regional patterns of biotic response to environmental perturbations in the Taylor Slough area.

Cores also have been collected in Big Cypress National Preserve, Everglades National Park, and the Water Conservation Areas to provide a more regional picture of biotic changes over time. In Big Cypress National Preserve, one core from a cypress strand has been analyzed and illustrates a fairly stable vegetational history. Logging in the early part of the century is documented by a great decrease in abundance of cypress pollen; that decrease and the subsequent recovery are the only notable changes in pollen assemblages from the site.

Two cores collected along a nutrient gradient in WCA 2A illustrate the affect of proximity to a canal and a nutrient source on vegetational composition. Both cores date back about 2,000 years and document similar fluctuations in marsh vegetation until the early 1900's. The core in the nutrient-affected site close to the Hillsboro Canal shows a doubling in abundance of pollen of the Chenopodiaceae/Amaranthaceae (pigweed/amaranth families) in the early 1900's, when construction of the Hillsboro Canal began, as well as a tenfold increase in cattail pollen beginning in the 1960's. Neither of these changes occur in the core from the "pristine" site.

Continuing analysis of other cores from the historic Everglades will provide a regional overview of both naturally occurring floral and faunal changes and those resulting from human activity in the region. Integration of this information with similar data from Biscayne Bay and Florida Bay is helping reconstruct the ecosystem history of the entire south Florida system and document the impacts of various natural and anthropogenic changes on the ecosystem.

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