A swim in a tropical stream, Corcovado Natl Park.
Conserving Biodiversity in South Florida
South Florida and the Florida Keys are home to a diverse group of animals and plants, with representative species of temperate climates from the north and tropical climates from the south. The diverse ecosystems found in the Everglades and the Keys provide breeding sites, food resources and habitat for a wide variety of organisms for a fairly small geographical area. Over the last hundred years, the booming human population of South Florida and subsequent strain on natural resources has had an impact on the native species of the area. This paper will examine the factors affecting biodiversity in these ecosystems, some representative species that are now threatened or endangered and issues that impact conservation efforts in South Florida and the Florida Keys.
In visiting an area like South Florida and the Keys, it impossible not to be struck by the incredible variety of plants and wildlife that can be found there. This area is native to species that may be familiar to northern visitors, such as those found in oak and pine forests, as well as seemingly exotic tropical species such as mahogany and geckos. Conservation biologists have been using the term biodiversity to describe this wide variety of living things for many years and the word has made its way into common use in high school biology classes and newspapers in the last decade, but the term is often misused. One misconception is that biodiversity of an area is equivalent to the number of different species found there – the more species in an area the more biodiversity (Noss and Cooperwriter, p4). This is only partly true. Biodiversity is defined more broadly to include not only species richness (lots of species in an area), but also species evenness (the relative numbers of species in an area) and the presence of native species (native biodiversity) and exotic or invasive species.
Biodiversity for a region exists really at four levels: genetic diversity, species diversity, ecosystem diversity, and regional or bioregional diversity. (Noss and Cooper writer, p5). Most fundamentally each individual within each species has their own unique DNA sequence or set of genes. Whether formed by mutation or natural variation, collectively these represent a number of different solutions to survival of the species. This forms the basis for evolution and for the long-term survival of the species – by maintaining a large variety of genotypes, the species may have a better chance of survival in the face of natural disaster, changing climate or disease. Species diversity is what most people think of as biodiversity (Noss and Cooperwriter, p6), but species diversity must include not only how many species (richness), but also how the number of species is distributed within an area (evenness) and the presence or absence of native species. South Florida is home to great variety of terrestrial and marine ecosystems such as wetlands, pine forests, coral reefs, mangroves, tropical hardwood forests and many more. Each ecosystem has its own unique set of physical and chemical characteristics (abiotic characteristics) and unique communities of organisms (biotic characteristics) that contribute to the overall diversity. Thus the organisms of the mangrove swamp may be quite different from those of the freshwater wetlands and those of the dry scrub. Many conservation efforts are now aimed at preserving entire ecosystems s as to include all of the abiotic and biotic factors affecting a population. Finally, South Florida is at the border of two large global bioregions, the northern temperate region and the southern tropical region. Bioregional diversity refers to large areas (millions of acres) that have a distinct pattern of landforms, climate and wildlife (Noss and Cooperwriter, p9). The overlap of these two regions in South Florida means that there are organisms at the far southern end of their distribution, such as the white tail deer, as well as many tropical species that reaches their northern most limits of their distribution, like the white-crowned pigeon, and so are found nowhere else in the United States (Wilcove, p172).
With the great diversity of life (at all four levels) found in South Florida and the Florida Keys, what could cause there to be so many threatened and endangered species identified in this region (about 111 according the Federal Fish and Wildlife Service)? The most basic factor affecting survival is tolerance to change. Biologists identify five factors that affect biodiversity of a region: over killing, habitat destruction, introduction of exotic species, introduced diseases and pollution of the air and water (Wilcove, p8). All of these have been at work in South Florida since the Europeans started exploring in Florida in the sixteenth century. Some native organisms have survived despite this human influence like the American Alligator; others have been nearly extirpated from South Florida for good like the Key West Quail Dove (Wilcove, p181). Their survival seems in large part to relate to their tolerance to human intervention of one form or another.
The Everglades and the Florida Keys provide a good example of these threats to biodiversity in South Florida. Historically the Everglades were as rich in wildlife as any other area in United States, the shallow waters, mudflats and wetlands provided a wide diversity of ecosystems including a huge diversity of wadding birds (Wilcove, p173). In later part of the nineteenth century the fashion in women’s hats was for showy plumes of feathers, many coming from the birds of the Everglades and coastal areas of South Florida. Thousands of birds we shot and like alligators and many other species nearly over killed to extinction. When the Everglades were established as National Park in 1947 the population had rebounded somewhat as the Victorian hat fashion had long ended. Since the establishment of the park, the wading bird population has dropped from a high in the forties of a quarter million to present levels of an estimated 20,000 to 30,000 (Wilcove, p174).
How did the population decline while supposedly being protected in a National Park? The answer may lie in one of the other major threats to biodiversity, habitat destruction. When Florida first was developed in the nineteenth century, the Everglades were considered a nuisance – uninhabitable land. The federal government moved in to drain large areas of the Everglades and convert the land to farming and urban development. This process was compounded by a series of severe hurricanes in the forties that prompted the government to create a series of drainage canals and other flood controls. The Everglades was one of the first national parks created to protect an entire ecosystem (Noss and Cooperwriter, p133). The problem has been that the park itself is too small. The Everglades ecosystem actually is made up of most of South Florida (Noss and Cooperwriter, p134). It begins in the lakes south of Orlando, including Lake Okeechobee, and slowly drains southward 200 miles in a “river of grass”, a slow moving shallow water area with an abundance of emergent grasses. The area was historically susceptible to seasonal flooding and drought, but today is marked by extensive loss of native habitat and water loss for agriculture and drinking water for the large neighboring urban areas of Miami and Orlando. Only the southern most area is protected. The Everglades National Park and the neighboring Big Cypress National Preserves are becoming more isolated as urban areas to the north and east take over more habitats. Outside the boundaries of the park, the combination of habitat loss and modern water control methods, alternately drain and flood large areas of the marsh regardless of natural seasonal cycles, have destroyed much of what once the Everglade ecosystem. This now directly effects the ability to protect the areas with in the park boundaries (Noss and Cooperwriter, p134). Human influence outside the park makes it difficult to preserve the Everglades because the ecosystem that makes up the Everglades extends well beyond the borders of the park.
Another human influence on the Everglades has been the introduction of exotic or non-native species. One example is the Australian punk tree (Melaleuca quinquenervia). Since being introduced into Florida fifty years ago as a landscape plant, the Melaleura has transformed some places in the Everglades, the mostly treeless "river of grass", to become the “river of trees”, a completely alien habitat to the plants and animals that have evolved to live in the Everglades. The Melaleuca has taken over and dominated native species, like many other exotics, in part because of its ability to disperse huge quantities of seeds, its dense growth which drives out native species and a lack of natural predators. In the process, wildlife, which depends on the open grassy wetlands of the Everglades, has been reduced in numbers.
In terms of conservation of the biodiversity that currently exists within South Florida, habitat destruction may ultimately be the biggest problem to overcome. In 1967, biologists E. O. Wilson and Robert Macarthur proposed a theory of “island biogeography”. This relates the concept of island species diversity to the conditions created by habitat destruction and fragmentation in areas such as South Florida. The idea that larger islands have more area and so support a greater number of species than smaller islands has been around for centuries. Island biogeography proposes that the number of species on an island is related to equilibrium between the rate of immigration of new species from other islands or the mainland and the rate of extinction of existing species on the island. The closer an island is to the mainland or to other islands the more likely there will be immigration of new species and so the overall species diversity should be greater (Noss and Cooperwriter, p34). This relates to conservation efforts in that human intervention tends to fragment natural habitats into “habitat islands” (Blair and Ballard, p68) that instead of being surrounded by water are surrounded by alien habitat such as urban areas, farms or golf courses. Plants and animals can migrate between these habitat islands just as they do between oceanic islands. Two major factors effecting this migration are the “area effect”, the ability of larger areas to support more diverstiy, and the “distance effect”, the tendency of immigration rates to be higher the closer two islands are to each other (Blair and Ballard, p69). As the human population of South Florida and the Keys continues to grow, the existing native habitat becomes more fragmented, creating “islands” of natural habitat. The number of species able to survive in this remaining habitat is related to the effects of both the area effect and distance effect. In creating nature preserves, conservationists need to be aware of the balance between creation of a large preserve verses a number of smaller preserves and the proximity of one preserve to another. As with the Everglades, the Keys represent a complex series of ecosystems that cannot be conserved in isolation.
Another factor in conservation efforts in South Florida is habitat edge or the “edge effect”. The boundary between two different habitats, such as a forest and a field, is known as an edge and generally has greater species diversity than the interior areas of either the forest or the field (Blair and Ballard, p90). Though there is greater diversity along the edge, the introduction of species from one ecosystem into another may lead to greater rates of predation or other factors that effect delicate species. Also, if the preserved area is small, edge effect may take over much of the preserve. In other words, the interior species that the preserve may have been designed to protect would be confined to small area at the center of the preserve well away from the edge, while the rest of the preserve was dominated by a mix of species from the exterior and interior habitats.
One example of this habitat fragmentation is the Key Deer (Odocoileus virgineanus calvium). Related to the northern whitetail deer, the Key Deer is much shorter and stockier. Their range once extended to Key West, but currently they are confined to the Lower Keys of Big Pine Key and Sugarloaf Key (US FWS, p 4-4). The Key deer’s smaller stature may be related to the lack of available food on the Keys and the primary limiting factor for the deer – fresh water. The deer are able to swim from island to island in search of fresh water, which is plentiful in the rainy season but exists only on the larger islands in the dry season. Habitat fragmentation from urban development limits the movement of the deer and forces them to cross roads in areas with heavy traffic, many deer are killed in traffic accidents (US FWS, p 4-9). As more and more people have settled in the Keys, the National Key Deer Refuge, which was created in the fifties to protect the remaining natural habitat of the deer, has become one of its last refuges. The deer population, which may have been as low as 26 individuals in the fifties, now numbers in the hundreds (Wilcove, p184). This rebound in population is severely threatened by habitat fragmentation, which limits their ability to find water and puts the deer at risk of a loss of genetic diversity as existing populations are unable to inter-breed (US FWS, p 4-9). In the Keys, the combination of habitat fragmentation and small population size has driven the Key Deer nearly to extinction.
Another Florida Key species that has relatively large numbers but is threatened by development within the Keys is the white-crowned pigeon (Columba leucocephala). A native of the tropics, where it is common, South Florida represents the northernmost extent of its range. The white-crowned pigeon requires two closely connected ecosystems for survival. The adults breed in the relative safety of the mangrove forests along the coast of the Keys, the young birds then fly from the mangroves to tropical hardwood forests known as hammocks. These hammocks provide a food source and safety for the first few days the birds are away from their breeding grounds, acting as a series of stepping stones to larger hammocks (Stap, p18). As the area of the mangrove and hammock forests decreases or becomes more fragmented, the birds are more susceptible to edge species predators such as raccoons and forced to migrate greater distances to find hammocks. As the coastal areas of the Keys become more developed, it is essential to preserve a series of closely linked areas of both mangrove and hammock in order to protect the white-crowned pigeon (Stap, p18).
Another example of an endangered species feeling the pressure of human influences is the Florida Snail Kite (Rostrhamus sociablilis plumbeus). This member of the hawk family once ranged over much of Florida, but is now restricted to a small area of the Everglades National Park and Lake Okeechobee (Blair, p199). This species favors marshes with wide open horizons and shallow open water containing saw grasses mixed with mudflats and occasional islands of trees. They feed almost exclusively the freshwater apple snail, which they snatch out of the water as they fly just above the surface. Habitat fragmentation and other pressures have reduced the population to less than 100 individuals. The reduction in numbers of the Snail Kite are likely the result of habitat destruction caused by drainage of marshes or flooding of the shallow wetland areas it prefers, its reliance on one species of snail and the invasion of the marshes by invasive trees (like Melaleuca) that alter their preferred habitat (Blair, p199).
All of this has applications for how best to preserve biodiversity in South Florida. When designing a plan for preservation of these species, a number of factors must be considered. As with the biogeography hypothesis, there is some merit to the idea the bigger the preserve the larger the number of species that will be protected, but the reality of this is that there is a limit to the number of species that can be gained solely by increasing the size (Shafor, p35) – as well as the political reality of creating large reserves. The “distance effect” may have less to do with actual distance, since many birds and other species can migrate long distances, and more to do with barriers to migration. As humans develop the land they build roads, fences, canals and other barriers to migration that severely effect species such as the Key Deer. Some species may be hard to protect simply because there may have never been many of them here in the first place (Shafor, p36). The white-crowned pigeon may be an example. This bird is fairly common in South America and some Caribbean Islands, but only exists in small populations within South Florida, which is at the northern most tip of its range. This brings up the question of how much effort should be expended to preserve a species that may have nerve been very common in the United Sates and is quite common elsewhere in the world.
As conservation efforts continue, it is important that biologists take an ecosystem approach to designing preserves. Looking at the “big picture” helps to insure that the inter-connections between various species and the variety of habitats that they use within their life cycle will all be preserved. Without all the pieces of the puzzle, species will continue to be threatened and forced to the vanishing point.
Blair, Robert B, and Ballard, Heidi L. Conservation Biology. Stanford, CA:Kendall/Hunt 1996.
Wilcove, David S. The Condor's Shadow: Loss and Recovery of Wildlife in America. New York, NY: Anchor, 2000.
fws.gov. 14 April 2004. US Fish and Wildlife Service.
Noss, R.F. and A.Y. Cooperrider. 1994. Saving Nature's Legacy: Protecting and Restoring Biodiverstiy. Island Press: Washington, D.C.
Stap, Don. "Hammocks get a rest." Audubon, Mar/Apr 1998: 18-21
US Fish and Wildlife Service Southeast Region. South Florida Multi-Species Recovery Plan. US Fish and Wildlife Service. 1999
Shafor, Craig L. Nature Reserves: Island Theory and Conservation Practice. Smithsonian Institution Press. 1990.
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