Trophic interactions are fundamentally important processes which are manifested at variable scales. Primary producers and lower trophic levels are directly driven by biochemical processes while population dynamics and individual behavior tend to dominate the dynamics of upper trophic levels. Thus the use of a single modeling method is not appropriate. ATLSS is a set of integrated models that simulate the hierarchy of whole-system responses across all trophic levels, and across spatial and temporal scales at which interactions actually occur within an ecosystem.
These are integrated across a spatially explicit (grid-based) landscape model of the Everglades and Big Cypress Swamp. This landscape structure is interfaced with a landscape/vegetation model that simulates within-year and between-year variations in vegetation properties in response to changes in hydrologic conditions and photoperiod, (e.g. rates of vegetation growth and regrowth, caloric and protein content of the vegetation, etc.). Intermediate and long-term vegetation dynamics - regrowth and succession following disturbance and feedbacks between vegetation and geomorphology - will be provided by plant-community succession models currently under development. Spatial resolution at this level is as small as 100m, with the capability to vary this resolution based upon the scale of available input data. ATLSS will be coupled to hydrology models and used to assess the effects of alternative proposed hydrologic restoration scenarios on trophic structure. Simulated or projected changes in species (or taxa) population levels and geographical patterns in response to changes in hydrology and vegetation are able to be statistically analyzed, as well as formatted for display by geographical information systems (GIS).
The ATLSS modeling approach will enhance the South Florida Restoration Initiative Task Force's technical capability to address scientific and management questions affecting these systems. It also provides a framework within which past and ongoing ecological research may be synthesized, further enhancing these predictive and diagnostic models that address ecosystem-level questions concerning the Everglades and Big Cypress wetlands. Future work will include refinement of existing models, development of additional models for several remaining key species and/or functional groups, model performance evaluation, integration of ATLSS model components into a common framework, and refinement of a user-friendly interface.
The Everglades and Big Cypress swamp are characterized by complex patterns of spatial heterogeneity and temporal variability. ATLSS is intended to address fundamental cause-and effect factors associated with these patterns that operate on ecological components of various trophic levels. Such factors affect the population dynamics of key taxa and their vulnerabilities to disturbance. A series of these fundamental factors, arranged in ascending order in the trophic structure of these systems, are stated as questions: (1) How is primary production divided between algal and macrophyte communities? How is production related to hydroperiod? (2) What are the relative flows of carbon from detrital and grazing pathways to the higher trophic levels? (3) Are there differences in fish and macroinvertebrate biomasses between marshes that are, respectively, detritus-based or grazing-based? (4) How do landscape extent, heterogeneity, configuration, and connectivity influence the distribution, abundance and population stability of higher consumer populations?
The work described in this document has benefitted from contacts with a very large number of biologists and modelers. In particular, Dona Neff and Mindy Nelson assisted in data file summarizations for the development of process-oriented and structured-population models for lower and intermediate trophic level organisms. Nancy Urban and Roxanne Conrow shared data for development of the invertebrate models. We specifically appreciate the expertise of Sonny Bass, Jim Schortemeyer, Dave Maehr, and Daryl Land in support of individual-based model development for White-tailed deer and the Florida panther. Modelers who have greatly contributed to the development of ATLSS include Ethel Jane Comiskey, Hang-Kwang Luh, Yiannis Matsinos, Moris Shorrosh, and Yegang Wu.
Animal issues of concern include the overall decline in historical numbers of native and endemic taxa, changes in historical patterns of the spatial and temporal distribution of species, changes in patterns of community structure, and changes in the pattern and magnitude of energy and material flows. Most animal populations in the region are now characterized by reduced densities and biomass, increased mortality, reduced reproductive potential and juvenile survivorship, and changes in persistence and resilience. These characteristics are usually associated with fragmented, natural landscapes in which populations are frequently affected by environmental stochasticity, demographic stochasticity, social dysfunction, or genetic deterioration. The alterations in fundamental landscape characteristics and related trophic disruptions described above may be viewed as chronic stresses to these ecosystems, termed "press disturbances" in the ecological literature. By contrast, major perturbations, such as Hurricane Andrew, constitute "pulse type" disturbances, an important dichotomy. The regional biota are adapted to recover from pulse disturbances, even those as severe as Hurricane Andrew. However, the existence of chronic stresses may render these natural systems more vulnerable to permanent changes from a pulse disturbance. An integrative modeling approach is essential for understanding the coupling between the chronic stresses on these systems and the possible long-term consequences of hurricanes.
Prior to the current effort, no rigorous, integrative approach for testing hypotheses about chronic stresses and their interactions with major pulse disturbances had been attempted. Such research is required to formulate a system-wide restoration plan within the context of regional water and nutrient budgets. Past research and restoration efforts have focused on individual species or habitats, usually within limited spatial or temporal scales. A lack of integrated understanding of the whole system's response to natural or anthropogenic perturbations severely restricts ongoing restoration and management possibilities. Several critical questions concerning the ecosystem's productivity and resilience must be resolved to produce a scientific basis for restoration and management.
Overview of methods: This project employes a multi-disciplinary approach over several years, requiring collaboration of NBS and other scientists through cooperative and interagency agreements. The modeling domain will initially include only the freshwater wetland types of the historical Everglades and Big Cypress landscapes. Pending additional funding, the domain of the ATLSS model will be extended to the mangrove estuaries to simulate trophic dynamics across the entire historical landscape of the Everglades and Big Cypress Swamp.
The questions previously listed will be investigated using the ATLSS approach. Trophic responses to simulated water flows over the historical Everglades and Big Cypress landscapes will be based upon hydrologic outputs from a natural system model and analysed as a scientific baseline. The biotic responses will then be compared to responses to hydrologic simulations of contemporary conditions, using the South Florida Water Management Model. Such comparative analyses will identify critical natural-system characteristics, and the threshold responses of the biota to changes in those characteristics. Identification of thresholds will also allow the evaluation of proposed restoration scenarios to guide the further development of alternative scenarios, if necessary.
Landscape description: The Everglades/Big Cypress landscape is the template upon which all of its complex biological interactions occur. In the ATLSS approach, interactions among organisms, and between organisms and their environment, occur at relatively small spatial scales appropriate to the sizes and activities of the individual organisms. These spatial scales are generally much smaller than the diffuse, large-scale processes that drive the physical dynamics of the system, such as hydrologic sheet flow, rainstorms, hurricanes, and fire. A major challenge for ATLSS is to link these large-scale processes to the small scales at which organisms operate.
Lower trophic-level models: The ATLSS project integrates, in a common modeling framework, ecological elements of varying complexity and scale. The simplest populations of the smallest organisms in the freshwater wetlands of South Florida are found at the lower trophic levels, where populations may be simulated by aggregated variables belonging to a set of first-order, ordinary differential equations. Such equations can be integrated to portray the behavior of these components on a very fine time scale. This output will provide information on the resources available to the higher, more structured populations at the longer intervals over which the structured-population models are run. Furthermore, the consumption of these resources by higher-level consumers can be fed back into the lower trophic models, thereby providing a two-way, dynamical bridge between the lower trophic levels and the rest of the ecosystem.
Higher consumer models - Colonial wading birds: Wading birds are important components in the aquatic food web as top-level carnivores. As highly mobile animals, wading birds can influence the structure and dynamics of both freshwater and estuarine prey communities. They also transport nutrients from interior, freshwater wetlands to downstream coastal estuaries. Wading birds depend on patchy resources. Since water management regulation, however, ponding of overland flows in northern reaches of the Everglades catchment area and severe overdrainage of the southern reaches downstream of these impoundments has occurred. Combined, these have altered the locations or spatial arrangement, and reduced the areal extent and heterogeneity, of seasonal foraging habitats for wading birds. Declines in wading bird populations have occurred in all feeding guilds concurrent with these landscape changes. Decreasing numbers also attempt to nest each year, particularly at traditional colony sites within downstream reaches of the Everglades basin.
The focus of present modeling efforts for wading birds in ATLSS, therefore, is to develop the simulation capability for investigating nesting colony dynamics in relation to landscape changes caused by altered hydrology. The species selected for modeling in ATLSS represent approximately 85% of the regional wading bird population and include both short and long-legged, tactile and visual-feeding species.
The individual-based wading bird nesting colony model used in ATLSS simulates the activities of potential nesting adults for a period of time immediately preceding the formation of a nesting colony and then the whole of the nesting season. In this model each of the adult nesting birds, as well as each of the offspring of these adults, are modeled as individuals. From this general model template, we have also developed calibrated versions for selected species.
Wood Stork Colony Model: Behavior and energetics sub-models of potential nesting adults. Usually three or four eggs are produced asynchronously by each pair of Wood Storks, and hatching takes place in about three weeks. The individual wood storks usually forage between 1000 h and 1600 h each day, with each bird deciding when to start and whether to follow others or to feed by itself. The location chosen by an individual for foraging is based on its partial information concerning the system. For a tactile forager, such as the Wood Stork, the foraging efficiency should be roughly proportional to the (mean) density of prey within a cell.
Energetics and growth sub-models of nestlings: Depending on their age in days since hatching, there is some maximum that an individual Wood Stork nestling can consume. This maximum increases linearly at first, then levels off and finally declines as the nestling approaches fledgling. From experimental measurements, it is known that a nestling must attain some threshold level of accumulated food (taken here as 15 kg) in order to fledge. If the nestling does not receive this amount of food before the rainy season begins, the model nestling will usually die because the parents will no longer be able to provide food sufficient for its survival.
White Ibis Colony Model: White Ibises are tactile foraging birds that feed mostly on small prey items of approximately 2 cm long. The food of a White Ibis in southern Florida consists mainly of crayfish. The average energy content of the diet is 4.05 kcal/gram-wet weight (16.945 kJ/gram-wet weight) with a ratio of 4:1 wet weight:dry weight. This prey-base sub-model is being coupled to the macroinvertebrate and fish models for inputs on White Ibis prey.
The initiation of nesting is governed by rules similar to the Wood Stork model. In particular, the female must obtain enough food to meet the additional energy requirements of ovary and egg production, i.e. 250 kcal (1046 kJ) during 7 consecutive days. This represents an energetic requirement of more than 25% of its daily energy needs. This rule probably underestimates the actual energy required. In the general wading bird colony model, each bird chooses its foraging site independent of the choices of other birds. White Ibis, however, frequently fly in cohesive flocks between feeding and roosting sites.
The rule for selecting a feeding site is the same rule as the corresponding rule in the wood stork model, i.e. a flock leaving a colony site is treated as a single bird; and, flocks form at a feeding site from local enhancement. The model White Ibises discriminate among feeding sites according to the distance from their colony site. Whenever a White Ibis has to choose a new feeding site, sites within 10 km from the bird's colony location are preferred to more distant sites.
Energetics and growth sub-models of nestlings: Nestlings are modeled in the same manner as the adult birds. Simulating nestlings, however, is much simpler because their main activity consists of eating the food brought back by their parents, and sleeping. Because our main interest is in determining whether or not a nestling receives enough food to fledge successfully, it suffices to keep track only of their food intake. To model the growth of the nestling, their relative sizes are assumed to be solely determined by their individual total cumulative food intakes. Total food intake, therefore, determines whether the nestlings have grown to a size enabling them to leave the nest and forage on their own.
In the following we summarize how the general colony model was modified to simulate visually feeding species, in particular Great Egrets (Casmerodius albus) and Great Blue Herons (Ardea herodias). Visually feeding, long-legged waders exhibit a much broader range of sizes of consumed prey than tactile feeding species. To reflect this difference, the basic unit of prey is taken to be 20 g. In this way the adult bird could simultaneously consume prey of 20, 40, 60 g, representing usual weights of prey consumed by visual foragers in the prey sub-model. In the present versions of the individual wading bird models the prey base is estimated from empirical data. However in the integrated model, prey availability will be supplied by the fish and the aquatic macroinvertebrate models.
The actual mechanisms of visual feeding (locating the prey, stalking, and handling) are more subtle than tacto-locating (groping), as is done by tactile feeders such as the Wood Stork and White Ibis. With visual feeders, a number of environmental factors affect their feeding success, i.e. intensity and direction of sunlight, wind, water depth, water temperature, refraction between the prey's apparent and real position in the water column, etc. Foraging success rates also differ by age of the bird.
Feeding aggregations of Great Blue Herons form only when the prey density is high. Birds which forage in such groups gain more food simply because flocks form when food is abundant. When resources are not so abundant, herons often forage solitarily and establish and defend feeding territories. Such solitary visual feeders spend more time searching neighboring sites, walking slowly to search for prey when it is more scarce. The cell size in the landscape sub-model was changed, therefore, to 50m x 50 m to account for the increased time spent by solitary visual feeders searching nearby sites for prey.
Great Blue herons are long-legged, visual feeders foraging both in shallow and deep-water areas. Their foraging mode is to usually stand and wait. The maximum depth for wading is set to be 39 cm. They are diurnal and nocturnal feeders. The prey for Great Blue Herons consists mainly of fish.
Behavior and Energetics Sub-models for potential nesting adults: The adult Great Blue Heron weighs 3.0-3.5 kg, with the male usually about 10% larger than the female. The energy requirements are as follows: The initiation of the nest is governed by rules similar to those in the previous models. A threshold of 600 kcal for a week is the required energy that a female must obtain in order to start a nest. Each nesting pair lays 3 eggs, asynchronously, with a period of two days between eggs. Each egg is incubated for 28 days before hatching. A nestling fed at the maximal rate can grow at the rate of 40 g/day reaching a weight of 400 g within 10 days, and a maximal weight in ~ 3 1/2 weeks, remains at the maximum for the next two weeks, finally decreasing linearly its daily demands to 150 g at the final period in the nest. Each nestling fledges successfully after 60 days from hatching, if it has accumulated a total of 15 kg of food throughout that period, assuming it has not starved.
American Alligator model: The American alligator (Alligator mississippiensis) is a keystone species in the Everglades and Big Cypress Swamp, as defined by its role as a top-level carnivore and architect within these systems and its influence on the structure, distribution and abundance of native plant and animal communities. Although the alligator is a large, mobile carnivore that represents a versatile and selectively opportunistic predator, it depends upon stable resources within its local environment, e.g. presence of surface water and related prey resources. During a long life span of nearly continuous growth, an individual feeds on a variety of prey species and sizes that vary by an alligator's size. As alligator populations consist of overlapping size classes, they are consumers at all trophic levels.
General structure of the model: A model is presently under construction to simulate alligator responses to varying hydrologic regimes in a variety of freshwater, local environments. These local environments include short- and long-hydroperiod wetlands. The model consists of two parts ('modules'). The first module simulates the life stages of individual adult and subadult alligators and, in particular, nesting female alligators and their reproductive performance. The module produces daily data on adult and subadult locations and numbers within these wetland types, and tracks the state of each alligator, which includes: age, sex, weight, and various factors related to reproduction. The alligator data change daily, based upon the behavior and physiological responses of adult and subadult alligators to changes in air and water temperature, water depth, season, food intake, and other factors. The second module simulates the life stages of hatchlings and juveniles (modeled as cohorts) in typical nest areas. The module tracks their daily growth and survival based upon a set of environmental factors and stochastic survival probabilities.
Florida Panther model: As the largest terrestrial carnivore in the Everglades and Big Cypress Swamp, and an animal that depends upon large home range sizes, the Florida Panther (Felis concolor coryi) serves as a key species for assessing the health of these ecosystems. A spatially explicit, individual-based model of the Florida panther population has been constructed to be coupled with the White-tailed deer model previously described. The individual-based model for deer in the Everglades and Big Cypress swamp allows the deer population to be simulated as individuals daily, on a landscape divided into 500 meter x 500 meter spatial cells, with varying vegetation and water depth characteristics. Data from a hydrology model determines changing water depths in each cell on a regular basis (this may be daily, weekly, or monthly).
Model structure: This spatially explicit model of the Everglades and Big Cypress Swamp landscapes has been extended by adding individual panthers. Each panther is assigned a state that includes: age, sex, weight, maximum weight ever attained, location, whether it is at a kill site or not, and number of days since last kill. Additionally, there are sex-specific states. For a female, the state may include an associated set of kittens (linked to the states of these kittens), the number of days pregnant, and a next mating date if she is not pregnant. For males, whether or not a female is within mating distance is a state variable. Each panther is assigned an initial value for all of the above components of its state.
Each simulation run of the panther model takes into account hydrology/vegetation/deer/panther interactions and is initiated with specific numbers, locations, and states for all deer and panther across the landscape. Models are run with hydrology input files from either of the hydrology models for South Florida, and the panther population changes through time due to hydrology effects on vegetation and deer, stochastic demographic factors operating within the panther population, and the current structure of the population. To remove effects of initial conditions, the model is run for an appropriate time period (determined from the simulations). The predicted spatial distribution and abundance of panthers from this initial model run is then used to initialize all further simulations, with a single set used to compare alternative hydrologic scenarios. Since the model contains stochastic components for individual behavior, as well as for mortality and reproductive factors, a number of simulations must be run for each alternative hydrologic scenario.
White-Tailed Deer Model: The White-tailed deer (Odocoileus virginianus seminolus) is the only large (native) herbivore in the Everglades and Big Cypress Swamp. In addition, it is the main prey base for the endangered Florida Panther. Deer are selective feeders and depend upon patchy resources. Consequently, population sizes are closely coupled to both vegetation and water level in the Everglades and adjacent wetlands. Water levels have effects on this consumer species both directly, due to restrictions imposed on deer movements by water level, as well as indirectly through effects on the quantity of quality forage. As the primary large ungulate in South Florida wetlands, deer also have the capability to alter the vegetation through the impact of foraging.
The deer model component of ATLSS simulates the birth, growth, movement, and death of individual deer as they move across the Everglades and Big Cypress landscapes. Each deer is simulated using an energetically based model that tracks energy input through food intake and energy loss through basal metabolism, movement, lactation, and other activities. Weight losses and gains, as well as size at maturity, are consequences of the amount and quality of forage available to each individual deer and the energy it must expend to obtain that food.
Deer herd structure is based on multi-generational groups of females. Fawns travel with their mother until 18 months of age, at which time male fawns disperse. Females remain with their mother, foraging as a group if sufficient forage is available, but scattering if forage becomes scarce. Deer mortality occurs as the result of a number of processess, including starvation, predation, road hazards, catastrophic disturbances, disease, and age-dependent mortality. Food intake is decreased when movement is restricted by water depth. In some parts of the Everglades, deer may become stranded on tree islands during extended periods of inundation. Movement across standing water is restricted as a function of body size, so fawns are affected most. Bucks are assigned a higher probability of moving out of stranding situations. Starvation may also occur due to seasonal regional depletions of high and medium quality forage, or (in fawns) due to poor nutrition of the mother. Mortality due to panther predations is modeled explicitly as a function of the encounter rate between panthers and deer. The bobcat is another important predator for deer in parts of the Everglades. Bobcats are not modeled individually, but are included as a stochastic factor along with other sources of mortality. Spatially-explicit mortality risks due to road hazards and hunting are currently being added to the model.
Each simulation run of the ATLSS landscape model of hydrology/vegetation/deer/panther interactions is initiated with a specific number of deer and panthers. As the model is run over a number of years (with water levels driven by the Natural Systems Model or the Water Management Model), populations increase or decrease as a result of births and deaths until they approach a steady-state that represents the carrying capacity for the system as it is defined by vegetation parameters, deer and panther parameters, and the hydrologic regime. A typical model run may stabilize with approximately 10,000 individual deer and 30 panthers.
Development of additional ATLSS model components: Several new models are planned for development. Process-oriented models for several key lower-trophic-level functional groups of species not included in the present version of ATLSS are planned (e.g., bacteria/fungi, microinvertebrates, other macroinvertebrates). These modeling efforts, however, will only be done pending the funding of empirical studies and the timely availability of study results in support of this effort. The set of age- and size-structured models of intermediate trophic level groups will be expanded to include several invasive, non-indigenous species and selected macroinvertebrates (i.e., Apple Snail). These additional structured-models will also include mercury fate and bioaccumulation components. The set of higher consumer models will be expanded to include individual-based models for the Snail Kite and the Cape Sable Seaside Sparrow. Completion of the Cape Sable Seaside Sparrow model, however, will be dependent on continued funding of an ongoing, empirical study over the next three years. Inclusion of mercury uptake and effects in these additional individual-based models will depend on securing funds above the base-project budget level. In addition, a multi-species, multi-colony simulation capability for wading birds will be developed.