Terrestrial ecosystems of south Florida have undergone numerous human disturbances, ranging from alteration of hydroperiod, fire history, and drainage patterns through implementation of the canal system to expansion of agricultural activity to the introduction of exotic species such as Melaleuca, Australian pine, and the Pepper Tree. Over historical time, dramatic changes in the ecosystem have been documented, and these changes have been attributed to various human activities. However, cause-and- effect relationships between specific biotic and environmental changes have not been established scientifically. This project is designed to document historical changes in the terrestrial ecosystem quantitatively, to date any changes and determine whether they resulted from documented human activities, and to establish the baseline level of variability in the south Florida ecosystem to estimate whether the observed changes are greater than would occur naturally.
This project is designed to document the terrestrial ecosystem history of south Florida and is collaborating with other projects at the USGS and other agencies on Florida Bay, Biscayne Bay, and the Buttonwood Embankment. The specific goals of the project are 1) document the patterns of floral and faunal change at sites throughout southern Florida over the last 150 years; 2) determine whether changes occurred throughout the entire region or whether they were localized; 3) examine the floral and faunal history of the region over the last few millennia; 4) determine the baseline level of variability in the communities prior to significant human activity in the region; and 5) determine whether the fire frequency, extent, and influence can be quantified, and if so, document the fire history for sites in the region. Data generated from this project will be integrated with data from other projects to provide biotic reconstructions for the area at selected time slices and will be useful in testing ecological models designed to predict floral and faunal response to changes in environmental parameters.
Method of Investigation
The ecosystem history of the Everglades over both the last 150 years and the last few millennia is being studied by analyzing floral and faunal components and charcoal from a series of short cores and surface samples collected in the Taylor Slough region of Everglades National Park, Big Cypress National Preserve, and the Water Conservation Areas. Dating of cores is provided by Lead-210 and Carbon-14. Cores are sampled at closely spaced intervals (1-2cm) for dating and examination of the microflora and fauna; these sample intervals may provide resolution in a decadal scale. Pollen and plant microfossils will be examined from each sample, and where present, mollusks, ostracodes, diatoms, foraminifers, and dinoflagellate cysts also will be studied. Quantitative data from each group will be compared to establish the timing and, ultimately, the causes of biotic change across the region. The concentration of wind-blown charcoal, which is the measure of local to regional fire history, also will be determined from each core to establish the accumulation rate of charcoal and assess temporal and regional trends in burning history. Relict plant morphology, which is retained in charcoal particles, will help identify the types of plants that burned. For each floral and faunal component, the historic record will be sampled intensively to detect biotic changes in response to human activity, and the prehistoric record will be analyzed from selected cores to determine the baseline levels of variability in the ecosystem prior to human interference.
Evidence of Past Changes
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
|Photos (clockwise): everglades, tree island fringe, and mangroves. [larger image]
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 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.
|Figure showing a section of a core from the Buttonwood Embankment. [larger image]
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.
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).
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