John C. Ogden

The use of conceptual ecological models to guide ecosystem restoration in South Florida

Conceptual ecological models, as used in the Everglades restoration program, are non-quantitative planning tools that identify the major anthropogenic drivers and stressors on natural systems, the ecological effects of these stressors, and the best biological attributes or indicators of these ecological responses. Conceptual ecological models can be used with any ecological restoration and conservation program and can become the primary communication, planning, and assessment link among scientists and policy-makers. A set of conceptual ecological models has been developed for South Florida restoration as a framework for supporting integration of science and policy and are key components of an Adaptive Management Program being developed for the Comprehensive Everglades Restoration Plan. Other large-scale restoration programs also use conceptual ecological models. This special edition of Wetlands presents 11 South Florida regional models, one total system model for South Florida, and one international regional model. This paper provides an overview of these models and defines conceptual ecological model components. It also provides a brief history of South Florida’s natural systems and summarizes components common to many of the regional models.

EVERGLADES RIDGE AND SLOUGH CONCEPTUAL ECOLOGICAL MODEL

A conceptual ecological model of the effects of the major anthropogenic stressors on the Everglades ridge and slough system was developed as a planning tool for designing and assessing the Everglades restoration program. The pre-drainage Everglades ridge and slough system was an expansive, hydrologically integrated, long-hydroperiod, low-nutrient freshwater marsh, characterized by low-velocity sheet-flow, long-term water storage capacity, moderate-to-deep organic soils, and alternating sawgrass ridges and more open-water slough communities. Depth, distribution, and duration of surface flooding in this environment largely determined vegetation patterns, as well as distribution, abundance, seasonal movements, and reproductive dynamics of all aquatic and many terrestrial animals. Drivers on the system are urban and agricultural expansion, industrial and agricultural practices, water management practices, and human influences on species composition. These drivers lead to five major ecosystem stressors: reduced spatial extent, degraded water quality, reduced water storage capacity, compartmentalization, and exotic species. Attributes that are affected by these stressors and can be used as indicators of restoration success include periphyton, marsh plant communities, tree islands, alligators, wading birds, and marsh fishes, invertebrates, and herpetofauna.

Ecosystem management to achieve ecological sustainability: The case of South Florida

The ecosystems of South Florida are unique in the world. The defining features of the natural Everglades (large spatial scale, temporal patterns of water storage and sheetflow, and low nutrient levels) historically allowed a mosaic of habitats with characteristic animals. Massive hydrological alterations have halved the Everglades, and ecological sustainability requires fundamental changes in management.The US Man and the Biosphere Human-Dominated Systems Directorate is conducting a case study of South Florida using ecosystem management as a framework for exploring options for mutually dependent sustainability of society and the environment. A new methodology was developed to specify sustainability goals, characterize human factors affecting the ecosystem, and conduct scenario/consequence analyses to examine ecological and societal implications. South Florida has sufficient water for urban, agricultural, and ecological needs, but most water drains to the sea through the system of canals; thus, the issue is not competition for resources but storage and management of water. The goal is to reestablish the natural system for water quantity, timing, and distribution over a sufficient area to restore the essence of the Everglades.The societal sustainability in the Everglades Agricultural Area (EAA) is at risk because of soil degradation, vulnerability of sugar price supports, policies affecting Cuban sugar imports, and political/economic forces aligned against sugar production. One scenario suggested using the EAA for water storage while under private sugar production, thereby linking sustainability of the ecological system with societal sustainability. Further analyses are needed, but the US MAB project suggests achieving ecological sustainability consistent with societal sustainability may be feasible.

PULSED BREEDING OF LONG-LEGGED WADING BIRDS AND THE IMPORTANCE OF INFREQUENT SEVERE DROUGHT CONDITIONS IN THE FLORIDA EVERGLADES

At the scale of ecosystems and regions, numbers of nesting long-legged wading birds are often highly variable from year to year, and much of this variation is thought to reflect variation in production or availability of prey animals in wetlands. Based on observations during and following a severe drought in the Florida Everglades (1989–1992), we predicted that large nesting events would be more likely immediately following droughts than at other times. Using a 38-year history of wading bird nesting events in the Florida Everglades, we tested the hypothesis that “supranormal” annual nesting events (numbers of nests >1 standard deviation above the long-term mean) would occur more frequently during the period of up to two years after severe droughts (stages <1 standard deviation below the mean) than after non-drought years. Within this database, we identified 8 supranormal events and 8 severe droughts; 7 of the nesting events occurred immediately after a drought, and 7 of the droughts were followed by a supranormal nesting event. There was a highly significant association between the two types of events. Because many studies suggest that wading bird reproduction is food-limited, this result implies that post-drought conditions somehow result in exceptional productivity and/or availability of small fishes and macroinvertebrates. We propose two biological mechanisms for this pattern and suggest that rare, severe droughts in the Everglades are a forcing function for wading bird population cycles and large-scale movements through the action of pulsed productivity in the aquatic food web.

The natural South Florida system II: Predrainage ecology

The goal of the South Florida restoration process is to reestablish a sustainable ecological system that approximates the predrainage system (i.e., the system that existed before canals and water control structures were built). A critical step in the restoration process is to describe the predrainage system with sufficient detail to capture the essential landscape features (e.g., space, connectivity, patterns) that gave the region its defining character and supported its natural abundance and diversity of plants and wildlife.A previous attempt at reconstruction of predrainage (circa 1870) vegetation patterns covered only the Everglades (Davis et al., 1994) but introduced a concept of landscape mosaics that should be extended to South Florida. We propose 16 landscape units that include freshwater landscapes, upland landscapes, coastal wetlands, and estuaries (particularly Florida and Biscayne Bays), the Florida Keys and Reef Tract, and the Inner Southwest Florida Shelf.The predrainage area, because of its enormous size, supported a landscape heterogeneity that was advantageous to animals with several distinct habitat-related life strategies. Five species have been selected to illustrate different dependencies on landscape patterns in predrainage South Florida: wood stork (Mycteria americana), American alligator (Alligator mississippiensis), snail kite (Rostrhamus sociabilis), Cape Sable seaside sparrow (Ammodramus maritimus mirabilis), and pink shrimp (Penaeus duorarum). As individuals or as populations, these animals operated across several spatial and temporal scales. The predrainage systems large spatial extent and complex hydropatterns allowed wood storks and other animals with large feeding ranges to take advantage of the strongly seasonal rainfall pattern, while at the same time providing a refuge somewhere in the system for survival of all other species. Landscape heterogeneity and large spatial extent promoted the regions natural wildlife abundance and species diversity.

TOTAL SYSTEM CONCEPTUAL ECOLOGICAL MODEL

The total South Florida ecosystem encompasses all natural areas that were once interconnected and embedded within the vast Everglades basin that originally extended from coast to coast and from the upper Kissimmee basin headwaters to Florida Bay, Biscayne Bay, the Gulf of Mexico, and Caloosahatchee and Indian River Lagoon estuaries. Restoration of this system will be successful once defining characteristics of the pre-altered system are recovered. Defining characteristics of the ecosystem are 1) abundant large vertebrates and aquatic prey bases, 2) animals with large spatial requirements, 3) healthy, dynamically sustainable estuaries, 4) oligotrophic freshwater wetlands, and 5) complex landscape mosaics and interactions. These defining characteristics have been altered by three external drivers that create stressors on the system: water management, land-use management and development, and climate change and sea-level rise. Stressors on the South Florida ecosystem include loss of spatial extent; loss of connectivity; altered geomorphology and topography; altered volume, timing, and distribution of regional hydropatterns; input of nutrients; altered fire patterns; and introduction and spread of exotic plants and animals. The Total System Conceptual Ecological Model links stressors to changes in the defining characteristics through major working hypotheses of cause-and-effect relationships. The linkages (ecological effects) relate to hydroperiod and depth patterns, sheet flow, salinity gradients, nutrient status and dynamics, fire patterns, habitat availability, and marsh aquatic fauna prey bases. For each defining characteristic, key ecological indicators are identified to collectively track the decline and restoration of the ecosystem.

A science-based strategy for ecological restoration in South Florida

The Everglades and associated coastal ecosystems of South Florida are unique and highly valued ecosystems. One of the worlds largest water management systems has been developed in South Florida over the past 50 years to provide flood control, urban and agricultural water supply, and drainage of land for development. However, this system has inadvertently caused extensive degradation of the South Florida environment, resulting in the loss of more than half the historical Everglades system and elimination of whole classes of ecosystems. The U.S. Man and the Biosphere Program (US MAB) instituted a project to develop ecosystem management principles and identify requirements for ecological sustainability of South Florida. A strategic process developed by the US MAB Project illustrates how ecosystem management and ecological risk assessment principles apply to South Florida, including the development of societal goals and objectives of desired sustainable ecological condition, translation of these goals/objectives into scientifically meaningful ecological endpoints, creation of a regional plan designed to meet the sustainability goals, and development of a framework for evaluating how well the plan will achieve ecological sustainability of South Florida. An extensive federal, state, and tribal interagency process is underway to develop a restoration plan for restructuring the regional management system, essentially following the elements in the US MAB project process. The Florida Governors Commission was established as an institution to reflect societal values and define regional sustainability goals. The U.S. Army Corps of Engineers is developing a science-based plan for Congressional approval to restructure the water management system to achieve the societal goals. Thus, South Florida may become the prototype example of successful regional-scale ecosystem management.

Environmental management scenarios: Ecological implications

The measure of whether a management scenario is capable of establishing regional-scale ecosystem sustainability is the degree to which it recovers the historical characteristics of the regional landscape mosaic. This study examines the ability of alternate management scenarios to recover the defining ecological features of the Everglades and South Florida landscape. Five conceptual scenarios are evaluated for recovering and sustaining the ecological characteristics of the wetland systems in South Florida. First, the regional-scale physical characteristics are identified that created and supported the major organizing and driving forces in the predrainage Everglades and Big Cypress basins. Eight hypotheses are proposed to explain how human-caused modifications to these defining characteristics have been responsible for the substantial level of ecological deterioration that has been documented in South Florida wetlands during the last century. The restoration scenarios are evaluated on their proposed ability to correct the physical and biological problems identified by the hypotheses. Our assessment of the five scenarios shows that all would improve the problems addressed by the eight hypotheses, as all could more effectively move increased volumes of water across broader expanses of contiguous wetlands than do existing management programs. This would result in longer hydroperiods over larger areas, reflecting historical patterns. Two of the scenarios would be successful in increasing flows into Florida Bay and the Gulf coast estuaries because removing internal structures increases the spatial extent of the upstream areas that could be devoted to natural hydropatterns.The benefits of eastern boundary buffer zones include improved flow into the Taylor Slough basin. Using Lake Okeechobee as a site for increased water storage, followed by the addition of eastern buffer zones and portions of the Everglades Agricultural Area, would produce increased flexibility in providing the storage capacity required to meet sustainability goals. Scenarios with maximum areas of buffer not only are more successful in reducing groundwater seepage losses to the east but also are more likely to reduce the level of nutrients and other contaminants entering the natural wetlands.

NONLINEAR POPULATION DYNAMICS OF SIX SPECIES OF FLORIDA CICONIIFORMES ASSESSED BY CHRISTMAS BIRD COUNTS

We analyzed 30 years of Christmas Bird Count (CBC) data for Florida populations of six species of wading birds: Great, Cattle, and Snowy Egrets; Little Blue and Tricolored Herons; and the Wood Stork, a federally listed endangered species. These species are conspicuous components of wetland ecosystems, and the target of numerous management efforts in the state and region. The nonlinear dynamics of these populations were assessed through time series of birds seen during volunteer-based surveys and the first differences of successive annual counts, autocorrelations, and two new metrics: cor- relation time and a momentum oscillator. CBCs have the advantage of being conducted regularly over a large geographic scale. When properly analyzed, CBCs are among the few available sources of reliable population information for small, dark-plumaged species such as Little Blue and Tricolored Herons. Population trends assessed for all species by CBCs paralleled the known or suspected trends determined by breeding-season aerial and ground surveys, conducted by professional biologists. Cattle Egret populations have declined fol- lowing rapid expansion through the 1960s and 1970s; Snowy Egret and, to a lesser extent, Tricolored Heron populations have declined throughout the study period; Little Blue Heron and Great Egret numbers have been relatively stable. Wood Storks, following a period of decline that lasted into the mid-1970s, slowly recovered through the 1980s. Only Cattle Egrets displayed strongly deterministic population dynamics, indicating that predictions of population trends for the other species will require stochastic models with broad confidence bands.