1. Extinction and endangerment. Extinction, like speciation, is a fundamental, natural process in even the most well-balanced of ecosystems. Interactions among species at various trophic levels, combined with differences in their capacity to adapt to the ceaselessly changing physical environment, guarantee that plants will continue to become locally and globally extinct. Moreover, in the young flora at the southern end of the long Florida peninsula, ecological theory suggests that species turnover will occur more rapidly than it would within the older plant communities to the north, as taxa better suited to local conditions continue to arrive, disperse, and evolve to fill southern niches more effectively than earlier immigrants (MacArthur and Wilson 1967).
2. Anthropogenic factors affecting viability. A recent catalog of plant populations of special concern (Center for Plant Conservation 1995) suggests that the extinction rate of Florida plants may be poised to accelerate rapidly over background levels, if indeed this is not already the case. The CPC findings are based on subjective assessments by leading Florida conservation biologists and ecologists. According to these experts, there is a strong likelihood that 37 Florida plant taxa may not survive another five years in the wild without immediate conservation action, while another 34 may not survive ten years (Appendix A). The imperiled condition of many of these populations is directly related to human activities. However, in most instances the cause of population decline is not one-dimensional, i.e., it is most likely a complex combination of anthropogenic change, environmental stochasticity, and the ecological idiosyncrasies of the species in question. Human activities adversely affect Florida plants in many ways, and have especially contributed to: 1) Habitat loss, 2) Habitat fragmentation, 3) Altered disturbance regimes, 4) Altered hydrology, 5) Introductionof non-indigenous plants and animals, 6) Overexploitation, and 7) Global climate change. These are addressed below in turn.
Habitat loss. Outright conversion of land to residential, agricultural, and commercial uses (including developed park and recreational facilities) has been the leading mechanism by which human activities have destabilized plant populations in Florida. Habitat loss on a percentage basis has been greatest for plants associated with coastal strand, sandhill, and pine rockland ecosystems, but in nearly all Florida ecosystems native plants currently have less than half the habitat originally available to complete their life cycles (Table l). Furthermore, such figures greatly overestimate the appropriate habitat remaining for individual taxa, since the species of greatest concern are often restricted to specific sites, some of which are very limited indeed. The inseparability of species and habitat is one of the fundamental principles of ecology and natural resource management. If what appears to be an imminent wave of extinctions and extirpations within the state is to be averted, protection of the habitats of rare Florida plants must become a key consideration in the planning process for all future development at local and state levels.
Altered disturbance regime Periodic disturbances such as fire, flood, extreme drought, or catastrophic windstorm have also had formative influences on the evolutionary development of Florida plants. While all of these can cause significant mortality in existing plant communities, such disturbances may also accelerate nutrient cycling, open plant canopies, stimulate flowering, deposit or expose fresh seedbeds and aid in seed dispersal. They are also capable of creating patch structure in otherwise homogeneous landscapes. Indeed, the long term survival of a species may be as dependent on the maintenance of an appropriate disturbance regime as on the preservation of its critical habitat characteristics. While it is rarely possible for resource managers in the current landscape mosaic to precisely replicate historical disturbance regimes (which include severe events that might threaten life and property), there is an emerging recognition that the disciplined use of fire and water as management tools can accomplish ecological purposes and reduce the likelihood of catastrophic events. Moreover, the elaborate protective systems that have been developed to control wildfires and floods have not succeeded in eliminating large-scale cataclysms, even while they have contributed to many less desirable human and ecological side effects. These include the eradication of riparian ecosystems from many of the state's waterways, and the gradual replacement of pine forest acreage by late-successional broad-leaved forest. Thus, as they apply to natural disturbances, measures adopted to protect public safety and property are not always compatible with ecosystem health and, thus, perhaps not even with the longer term well being of humans. On the other hand, use of fire and water as management tools increasingly appears to be more beneficial than was originally believed.
Altered hydrology. Artificial drainage has been the engine that propelled Florida from the steamy subtropical world pictured in Part I to its current status as a leading tourism- and agriculture-based economy. The environmental costs of this metamorphosis have been enormous: the Everglades has been subdivided and reduced to less than half its original extent, water tables have dropped by five feet or more in much of the state, water shortages are a recurrent problem, wildlife populations are a fraction of what they once were, etc. Declines in certain characteristic wetland plant populations, the result of modified water levels, have paralleled those of animals (Alexander and Crook 1973). Alterations in the timing of water delivery may also have initiated changes in the character of wetland vegetation. Hydrologic modifications may affect low-lying upland habitats as well, especially the extensive area of transitional pine forests throughout the state. However, few acres of drained wetland have become functional upland habitat, as non-indigenous plants have aggressively invaded those areas not immediately developed for residential or agricultural use. Finally, water manipulation in Florida has been accompanied by changes in water quality, which may independently induce changes in vegetation composition and structure. The best-documented example in Florida has been the replacement of native sawgrass marsh by communities dominated by cattails in areas subject to high-nutrient farm runoff south of the Everglades Agricultural Area (Davis 1994). Mitigation plans propose a system in which the polluted water is passed through filtering marshes; costs of acquisition, construction, and operation are currently estimated to exceed $400 million.
Non-indigenous plants. Plants continue today to be an integral part of life for Florida residents and visitors alike, though their amenity value is not always recognized as such. As the native landscapes have receded before the expanding human population centers, their beauty and utility (e.g., shade, protection from wind) have been replaced in everyday life by a wide assortment of strange and exotic plants from all over the world. Many of these introduced plants were supplied by the thriving $1 billion Florida horticultural industry. Others arrived via unintentioned human vectors, e.g., in the holds of ships, attached to clothing, etc. Set in Florida's wet, mild climate, and free from the herbivores and competitors that may have controlled them in their native environments, many of these imports have rapidly become naturalized pests. Ecosystems vary in their susceptibility to invasion, though the reasons for this are not well understood. In general, disturbed habitats are particularly susceptible (Center et al. 1991; Elton 1958; Duever et al. 1986), as are ecotonal areas between upland and wetland ecosystems (Ross n.d.). Many exotic plants have become so well established in certain habitats that native plants have been virtually excluded. This group of aggressive invaders includes trees (e.g., Casuarina equisetifolia , Melaleuca quinquenervia , Schinus terebinthifolius ) , shrubs (e.g., Ardisia solanacea ), vines (e.g., Jasminum spp., Paederia foetida ), graminoids (e.g., Neyraudia reynaudiana, Pennisetum purpureum ), and aquatic plants (e.g., Eichhornia crassipes, Hydrilla verticillata ). Within the U.S., probably only in Hawai'i have non-indigenous plants had a more detrimental effect on native plant communities. Unfortunately, decades may pass between the time of arrival of a plant and the time that its population growth in the wild becomes sufficiently intrusive to prompt its recognition as a problem. By then, the costs of control may be prohibitive.
Overexploitation. The same taste for the unusual that has brought many exotic tropical life forms into Florida has fueled the exploitation or removal of the most novel elements of the local flora, especially palms, epiphytic ferns, orchids, and bromeliads. Collectors of these plants are both amateur horticulturists and commercial enter prises. Their activities have led in some cases to the near elimination of species from habitats in which they were common earlier in this century. Harvesting of native tropical hardwoods for furniture and specialized wood products has also lead to decreases in the range and abundance of species such as lignumvitae (Guaicum sanctum ) and mahogany (Swietenia mahagoni ).
Global climate change. Sea level along the Florida coast has risen about 10 meters over the last 8,000 years. The rate of increase was very rapid at the beginning, slowing gradually to about 0.4 mm per year by about 3,000 B.P. (Lidz and Shinn 1991). Tide records at Key West and other Florida stations indicate that this rate has increased to more than 2 mm per year within the last century (Maul and Martin 1993). It is not yet known whether the latest acceleration has resulted from the recent increase in greenhouse gases associated with the combustion of fossil fuels. Regardless of the cause, a continuation of this trend will result in changes to low lying coastal plant communities over the next century. Replacement of upland species by halophytes has already been documented for isolated pine rocklands in the Florida Keys (Ross et. al. 1994). The spatial and temporal pattern of this replacement during the past century has closely paralleled the upward trend in sea level over the same period. Results consistent with this interpretation have recently been reported for cabbage palm hammocks along the west coast of the state (Williams and MacDonald 1994). Unlike sea level, the temperature records from long-term Florida weather stations are too imprecise and problematic to yet reveal much in the way of a trend. However, given a small increase in minimum winter temperatures, a northward advance of tropical species into temperate communities in central Florida may be expected. The precise distribution of vegetational change may also be affected by other, less predictable facets of global climate change, such as shifts in rainfall pattern and storm intensity.
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