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Publication #SL320

South Florida Conceptual Model1

H. Carl Fitz2

Introduction

The South Florida Conceptual Model explores the fundamental linkages among the Everglades landscape and the human dimension of the South Florida region. After describing those overall linkages, the General Ecosystem Conceptual Model for the Everglades is then described, summarizing the ecological interactions among the primary physical, chemical, and biological processes that drive the ecosystem(s). That General Ecosystem Model is the basis for our ecological landscape modeling framework. A hyperlinked (clickable) version of the South Florida Conceptual Model is available at http://ecolandmod.ifas.ufl.edu/background/sfcm.html

South Florida Conceptual Model

Figure 1. 

The ecology of the Everglades should be considered in the broader context of the South Florida landscape. A simple conceptual model of the relationships among the natural system and the different components of South Florida is briefly summarized.


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Water managers in South Florida are responsible for balancing the various demands placed on our public water resources in order to achieve a sustainable and productive environment for humans and for the natural system on which we all depend. Field/lab research and modeling can aid in understanding the dynamics of the Everglades system in response to current and future water management practices. The interactions among the four South Florida Conceptual Model components (Figure 1) drives the ecological and economic system of South Florida. Water management attempts to integrate our societal values with the resource demands of urban, agricultural, and natural components of the regional landscape.

Societal Valuation

Figure 2. 

Depending on societal preferences, water managers attempt to balance the various demands placed on our water resources in order to achieve a sustainable and productive environment for humans and for the natural system on which we depend.


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Partially depicted in Figure 2, the economy of South Florida depends not only on tourism; agriculture also contributes significantly to its productivity. The water resource needs of this sector are a significant consideration in water management planning. Water supply for residential demands is another important component of the regional water budget, while flood control for land used for agriculture and housing poses a different type of demand on water management. With human populations increasing dramatically since the mid-20th century in South Florida, water management has disrupted the natural timing and distribution of water in the Everglades, with concomitant deterioration in water quality. These changes have led to significant deterioration of this internationally recognized wetland. Demands for restoration of this unique landscape have come from the national and local levels, with citizens demanding that the natural system have a much greater consideration than in the past. Thus, a variety of publicly funded projects, including the ca. $9 billion Comprehensive Everglades Restoration Plan (CERP), have been initiated to restore this valued natural system. In this process, management alternatives are being tested to optimize the balance between the natural and human demands on water resources—with the primary objective involving the restoration of the Everglades.

Urban and Agricultural Development

Figure 3. 

As canals and levees were built during the 19th and 20th centuries, agricultural and urban land uses dramatically increased, significantly reducing the spatial extent of the "natural" Everglades system by the mid-1970s.


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Starting in the late 1800s and the early 1900s, long stretches of canals were dug in attempts to drain the relatively pristine Everglades for agriculture (Figure 3). Problems such as devastating floods led to federal authorization (1948) of the Central and South Florida (C&SF) Project, creating an elaborate network of canals, levees, and water control structures to improve regional flood control and water supply. It was ultimately very effective in managing water for those purposes and accelerated the development of urban and agricultural sectors of the region. Agricultural and urban development has generally continued through the present day, particularly along the corridors east and north of the Everglades. The C&SF Project led to a reduction in spatial extent of the Everglades and also fragmented the once-continuous Everglades wetlands into a series of large impoundments.

In the current-day Everglades, the existing management infrastructure bisects the area into a series of impoundments, or Water Conservation Areas (WCA). Everglades National Park is south of these WCAs, while Big Cypress National Preserve is to the west. Agricultural land uses dominate the area just north of the Everglades, while extensive (primarily) urban land uses predominate along the eastern boundary of the Everglades. Lake Okeechobee, historically bounding the northern Everglades marshes, is now connected to those marshes via canals.

Water Management

Figure 4. 

The managed flows of water into, and within, the Everglades are being evaluated by scientists and engineers in attempts to optimize the management network for the needs of this dynamic landscape.


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The South Florida region, and much of the greater Everglades region, is driven by a complex engineering infrastructure (Figure 4) that is operated to distribute water for environmental, water supply, and flood control needs. This hydrologic management—in conjunction with deteriorating water quality—had significant negative impacts on the Everglades' ecology. Water historically flowed from the northern parts of the region into and through the Everglades largely as overland sheet flow. This flow regime changed to point releases at water control structures, often altering the timing and magnitude of water distribution into and within the Everglades. With agricultural and urban runoff, many of these inflows also carried higher loads of nutrients into the historically oligotrophic (low-nutrient) Everglades. The altered distribution and timing of flows in a fragmented watershed, combined with increased nutrient loads, changed the mosaic of Everglades habitats—for the worse.

A variety of projects are underway to restore the Everglades by optimizing management of hydrology and water quality, two fundamental "drivers" of Everglades ecology. Multiple research groups are providing critical scientific insights into the benefits and risks associated with these endeavors, integrating quantitative ecological science into decisions on modifying Everglades water management.

Everglades Dynamics

Figure 5. 

The General Ecosystem Conceptual Model shows the interactions among key parts of an ecosystem. As E. P. Odum (one of the "fathers" of ecology) put it, an ecosystem is more than the sum of its parts. The ecosystem feedbacks, or interactions, among the physical, chemical, and biological components of the Everglades landscape are fundamental to the dynamics of this complex system.


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General Ecosystem Conceptual Model

This General Ecosystem Conceptual Model (Figure 5) summarizes the basic interactions among multiple parts of an Everglades ecosystem. As with any complex system, interactions among its different components are a fundamental aspect of its operation and play an important role in sustaining the Everglades. An analogy is the human body, which is a complex system that is highly dependent on the proper interactions amongst its physics (e.g., skeleton, blood flow), chemistry (e.g., nutrients, oxygen), and biology (e.g., organs, growth). Below we describe some of the basic interactive characteristics of the physics, chemistry, and biology of Everglades dynamic ecosystems.

Hydrology

Hydrology is a critical "driver" of the landscape, in that we need to understand and get the water "right" in order to sustain a healthy Everglades.

Hydrology can change significantly on time scales on the order of hours, but climate change can produce decadal shifts in dynamics of the regional hydrologic cycle. While rainfall in South Florida is seasonal, it is highly variable both within seasons and among years. Intense rainfall events are often unevenly distributed at local scales; tropical disturbances can deluge the entire region. The pattern of water distribution is intensively managed via the operations of the water control structures and associated canals and levees. Changes to water depths and flows can alter the habitat because different macrophyte species and algal/periphyton assemblages have distinct hydrologic adaptations. Likewise, water depths can alter the soils through increased accumulation of dead plant matter when an area is wet for prolonged periods (i.e., long hydroperiods). On the other hand, soils are lost (oxidized) when excessively dry for prolonged periods. Soil nutrients are affected by water exchanges between surface and soil/sediment water storages, while surface water flows are an important transport mechanism for nutrients and suspended organic matter in the landscape. Canals move water and nutrients much more rapidly across long distances. Surface water flows also play a role in suspension and deposition of soils/sediments, potentially altering the physical pattern of creeks and sloughs. While most of the horizontal flows in the Everglades are induced by gentle elevation gradients, wind- and tide-driven circulation is predominant in Florida Bay. These surface flows are highly dependent upon the resistance to flow by macrophytes, and groundwater flows vary significantly across the region depending on the porosity of the below-ground aquifer.

Water Quality

Water quality has been responsible for shifts in primary productivity and species composition of macrophyte and periphyton communities and is another primary "driver" of the landscape at fast (weekly to annual) time scales.

Because the predominant "native" Everglades macrophyte and periphyton communities have adapted to low-nutrient waters, increases in the nutrients phosphorus and nitrogen can be detrimental to those communities. Phosphorus is generally the more limiting nutrient in the freshwater Everglades, while nitrogen tends to govern plant growth in the southern Everglades/Florida Bay. Typically, anthropogenic (man-made) inputs of otherwise limiting nutrients cause ecological imbalance, shifting the structure and function of the ecosystem. Management of flows through water control structures and canals has significantly modified the distribution of these nutrients across the landscape. Different macrophyte and periphyton communities can uptake nutrients at varying rates, changing the water quality (and the plants themselves). As water exchanges among surface and soil/sediment pore waters, the associated nutrient exchanges can alter the rates of soil/sediment decomposition, releasing nutrients in other forms that are more available for plant uptake. Along with nutrient availability, salinity gradients in the Southern Everglades/Florida Bay have the potential to modify seagrass plant communities that have adapted to particular environmental conditions.

Algae/Periphyton

Periphyton (assemblages of algae and microbes) are sentinel indicators of the quality of many habitats of the Everglades.

Periphyton are found attached to macrophyte stems, floating as mats in the water column, and as a benthic layer on top of the soil. Long considered an integral part of the animal food web, periphyton respond rapidly to changes in water quality and hydroperiod. Like macrophytes, "native" periphyton are adapted to low-nutrient conditions, while a variety of other periphyton are common in high-nutrient waters. Another important control on periphyton and algae is light availability: when emergent marsh macrophytes grow tall and thick due to increased nutrients, they shade periphyton, reducing the growth of this important resource. There are a variety of different types of periphyton species and communities, depending on the subregion of the Everglades and its local environmental conditions.

Macrophytes

Macrophytes are a primary determinant of the habitat quality in the Everglades landscape, which is largely defined by its heterogeneous mosaic of macrophytic vegetation that is dynamic over both annual and decadal time scales.

There is a high diversity of plants in this region, ranging from emergent marsh plants such as the ubiquitous sawgrass, to hardwood trees of tree islands and mangrove forests. These, and many other common species, have very different nutrient requirements, distinct hydrologic needs, and dynamic effects on the hydrologic cycle itself. Different adaptations by these plants create the habitat mosaic in response to a changing environment. For example, cattail is a "nuisance" species that grows rapidly in response to elevated nutrient availability, can thrive in deep water, and easily colonizes areas that have been disturbed by fire or other events. Sawgrass, on the other hand, is a dominant species in much of the Everglades where there are low-nutrient conditions and "natural" fluctuations of water levels and disturbances. Mortality of plants leads to accumulation of organic matter in the form of peat soils. Tree islands have "died" in recent years due not only to excessive water depths covering tree roots for prolonged periods but also in regions that have been overdrained and made more susceptible to catastrophic fire disturbance. Where regions of the Everglades have undergone successional changes in plant types, animal communities invariably are affected. Many animals are adapted to, and rely upon, high quality habitats that are often characterized by the heterogeneous, alternating distributions of dense and sparse vegetation of different species.

Soils

Soils (and sediments) are in a long-term, ca. decadal, balance between processes of accumulation and oxidation (and sometimes erosion) and are closely integrated with the development of different habitats.

In regions of long hydroperiods where water ponds on the surface for much of the year, peat soils tend to accumulate organic material that comes from plant mortality and leaf fall. Under shorter hydroperiods when those soils are exposed more frequently to the air, oxidation of the organic matter reduces the depth of peat. This process can be accelerated by higher nutrient availability. Disturbances such as droughts and "muck" fires can have significant impacts on peat soils, rapidly burning the organic carbon but leaving behind many of the nutrients to which the ecosystem may respond. Throughout much of the Everglades is a upper-soil layer of fluffy organic material (called "floc") that is partly live periphyton but is principally the organic material from dead periphyton and macrophytes. This "floc" appears to play a critical role in nutrient cycling, and it is transported among habitats. Floc also appears to be an important part of the food web for animals. Thus, soils are closely integrated with water quality and plant or periphyton growth and respond strongly to changes in hydrology. Inorganic content of soils varies in importance through the Everglades system, with calcitic periphyton sequestering calcium and phosphorus into an inorganic component that forms marl soils.

Disturbances

Disturbances such as fires, hurricanes, and severe drought or flooding can alter the ecological characteristics of the landscape over short and long time scales. There exists an important interaction between response to disturbances and the preexisting structure and function of these dynamic ecosystems.

The Everglades landscape has adapted to natural variability in climate and fire disturbances. While droughts and fire may appear to decimate the landscape, most of the vegetation and animal communities of the region can respond in positive ways: fire occurring in relatively local "patches" at infrequent intervals can enhance the system by opening up new space or clearing away brush species amongst plant communities; hurricanes may flush accumulated organic debris from the shallows of Florida Bay. However, there is potential danger in management regimes that exacerbate the natural response to disturbances. If the frequency of disturbances is significantly altered, areas that remain overly dry can experience severe "muck" fires that burn deeply into the peat and eliminate large amounts of soil. Some macrophyte species, such as nuisance cattail, rapidly colonize and thrive in such a highly disturbed environment. Regions that have accumulated stresses, such as long-term nutrient loading, can be "primed" for dramatic, potentially catastrophic shifts in the ecological balance.

Animals

Animal communities tend to integrate and respond to many of the factors that change the habitat mosaic of the landscape. Different populations of animal species have distinct reproductive and migratory habits that result in complex seasonal, annual, and decadal shifts in their population viability as the landscape evolves.

Although most animals do not appear to significantly affect ecosystem processes or landscape patterns, some modify local habitats at small spatial scales, such as the development of ponds excavated by alligators for nesting or local nutrient enrichment from colonies of birds. Wading birds are one of the conspicuous animals that thrive in the various hydrologic and habitat gradients of the Everglades. They respond to changing water levels and availability of (fish and other) prey and can select for subregions throughout South Florida as conditions change among seasons and years. While fish are capable of migrating within regions of suitable hydrology and habitat, they obviously become limited in range (and potentially more available as prey) as a region dries out. Many Everglades fish are omnivorous, feeding on a variety of detrital and invertebrate food sources. The nature of the interactions among animal populations, and among animals and their habitats, is one (very dynamic) indicator of the "health" of the landscape.

Integrated Landscape

An integrated landscape perspective allows us to synthesize the principal aspects of this dynamic system. The interactions among the ecological processes modify the landscape pattern, while there is a critical effect of this pattern on the nature of these ecosystem processes themselves.

Many research projects are conducted at relatively small scales in the laboratory or in the "field." By synthesizing these data, modeling and other analyses facilitate our understanding of the interactions of this complex system. As part of this procedure, regional maps of the vegetation and soils give insight into the landscape pattern. To understand temporal interactions, many research projects provide insights into the mechanisms underlying the changes in soils, habitats, animals, and landscape drivers such as disturbances, hydrology, and water quality.

Further Reading

Grunwald, Michael. 2007. The Swamp: The Everglades, Florida, and the Politics of Paradise. Simon & Schuster.

Lodge, Thomas E. 2010. The Everglades Handbook: Understanding the Ecosystem, Third Edition. CRC Press.

Websites:

Comprehensive Everglades Restoration Plan: http://evergladesplan.org. The official site describing the plans to restore America's Everglades.

Ecological Landscape Modeling: http://ecolandmod.ifas.ufl.edu/background. More details on the Everglades modeling referred to in the text, including scientific manuscripts and references describing Everglades ecology.

Footnotes

1.

This document is SL320, one of a series of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date June 2010. Reviewed March 2013. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

H. Carl Fitz, assistant professor, Soil and Water Science Department, Fort Lauderdale Research and Education Center; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

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