Wilfried F. Wolff

Landscape Modeling for Everglades Ecosystem Restoration

ABSTRACT A major environmental restoration effort is under way that will affect the Everglades and its neighboring ecosystems in southern Florida. Ecosystem and population-level modeling is being used to help in the planning and evaluation of this restoration. The specific objective of one of these modeling approaches, the Across Trophic Level System Simulation (ATLSS), is to predict the responses of a suite of higher trophic level species to several proposed alterations in Everglades hydrology. These include several species of wading birds, the snail kite, Cape Sable seaside sparrow, Florida panther, white-tailed deer, American alligator, and American crocodile. ATLSS is an ecosystem landscape-modeling approach and uses Geographic Information System (GIS) vegetation data and existing hydrology models for South Florida to provide the basic landscape for these species. A method of pseudotopography provides estimates of water depths through time at 28 × 28-m resolution across the landscape of southern Florida. Hydrologic model output drives models of habitat and prey availability for the higher trophic level species. Spatially explicit, individual-based computer models simulate these species. ATLSS simulations can compare the landscape dynamic spatial pattern of the species resulting from different proposed water management strategies. Here we compare the predicted effects of one possible change in water management in South Florida with the base case of no change. Preliminary model results predict substantial differences between these alternatives in some biotic spatial patterns.

Ecosystem flow networks: Loaded dice?

An information-theoretic comparison of the topologies of observed ecosystem transfers and randomly constructed networks reveals that it is not easy to separate the members of the two sets. The distribution of ecosystem flow magnitudes, however, is seen to differ markedly from ordinary probability functions and to resemble the Cauchy or Pareto distributions. The agencies that impart such structure to ecological flow networks are not obvious, but one strong possibility is that autocatalysis, or indirect mutualism, promotes certain pathways at the expense of others, thereby enlarging the tail of the distribution of flow magnitudes.

Importance of landscape heterogeneity to wood storks in Florida Everglades

Declines in populations of and reproductive success of wood storks and other wading birds have occurred in the Florida Everglades over the past several decades. These declines have been concurrent with major changes in the Everglades’ landscape characteristics. Among the plausible hypotheses that relate to landscape change are the following: (1) general loss of habitat; (2) heavy loss of specific habitat, namely, short-hydroperiod wetlands that provide high prey availability early in the breeding season; and (3) an increase in frequency of major drying out of the central slough areas, which can affect prey availability late in the breeding season.These three hypotheses were compared using an individual-based model of wood stork (Mycteria americana) reproduction. This model simulated the behavior and energetics of each individual wood stork in a breeding colony on 15-min time intervals. Changes in water depth and prey availability occurred on daily time steps. Simulation results showed a threshold response in reproductive success to reduction of wetland heterogeneity. Model comparisons in which (1) only short-hydroperiod wetlands were removed and (2) wetlands of both long and short hydroperiods were removed showed that, for the same loss of total area, the specific habitat removal caused a much greater reduction in wood stork reproduction, indicating hypothesis 2 may be a more likely explanation than hypothesis 1. Reduction of initial prey availability in the central slough areas (simulating frequent drying; hypothesis 3) reduced fledging success by an average of more than 90% in the model.

Virtual biologists observe virtual grasshoppers: an assessment of different mobility parameters for the analysis of movement patterns

An individual-based model was developed to investigate the accuracy of three mobility indices commonly used in field studies: mean daily movement, maximal distance and mean activity radius. Although the model is based on a field study of the mobility of the grasshopper Oedipoda germanica (Saltatoria: Acrididae) in Thuringia (Germany), the results are valid more generally. We show that the observational interval is the key factor influencing the accuracy of all three indices. Moreover, there is an optimal observational interval that minimises the absolute error. The influence of individual mobility, recapture probability, accurate registration of the capture and recapture points and the effect of the observer on the movement of the animals, are also considered.

Modeling effects of chemicals on a population: application to a wading bird nesting colony

Abstract An approach for the study of effects of chemical contamination of an avian population is described. The protocol consists of four components: (1) an individual model that is coupled with (2) a chemical exposure model; and (3) a population model together with (4) an effects model. The model-based protocol is generic, delineates types of information required for ecological assessment and lays the framework for the implementation of toxic effects in an individual-oriented model. The protocol is illustrated by modifying an existing individual-oriented model of a wood stork colony to incorporate sublethal and lethal effects of mercury contamination. Because of the paucity of available data on effects of chemicals on wading birds, information from experiments on other avian species is, by necessity, frequently extrapolated to dose-response formulations for wading birds; consequently, the results are qualitative in character and portray relationships determined to exist between certain species of birds and the toxic effects of mercury pollution. Simulation results and applications focus on a population level effect, colony survival of the endangered species Mycteria americana in the Everglades of Florida. Because of the lack of information about model processes and parameter values for mercury effects on wood storks, model sensitivity studies were performed. Given the results for sublethal contamination levels along with the information on levels of mercury found in fish of the Everglades, it would seem that if our assumptions are even close to accurate, this model suggests that wood stork colony losses due to mercury contamination are feasible, possibly in the short term, but definitely in the long term.

An individual-oriented model for ecological risk assessment of wading birds

The contribution of certain contaminants to reproductive failure in many avian species has been an ongoing concern. Appropriate quantitative techniques have focused either on the individual organisms by providing explicit bioaccumulation dynamics or on whole ecosystems by looking at the fate of the contaminant but fail to make the necessary link via population dynamics of interacting individuals. We used the individual-oriented approach in an effort to quantify effects of chronic contaminant exposure on individual birds. This was made possible by the use of an object-oriented model, where individual birds are interacting objects, and their actions are implemented by passing to them appropriate messages. Using this modeling approach a breeding colony of Great Blue Herons (Ardea herodias) is simulated as an assemblage of interacting individuals whose daily actions (foraging, growth, feeding of the young) are simultaneously followed over short time intervals for a nesting season. Spatial distribution of the contaminants in prey resources is used on a cell by cell basis and their effects on certain behavior characteristics of adult birds (e.g. foraging efficiency, effects on flying efficiency, parental care) are taken into account. Results showed that sublethal effects could have a considerable effect on colony success. Appropriate selection of endpoints for risk assessment yields a variety of scenarios for colony success.

Can Individual-Based Models Yield a Better Assessment of Population Variability?

The variability of a population is one of the key attributes influencing its ability to persist over a long period of time. Thus, population variability and the sources of that variability are of great interest to conservation biologists and to population ecologists in general. The central questions biologists want to answer are, can the factors controlling the variability in a population be identified, and, once identified, can they be included in a model of the population that can be used to predict changes in the population in response to those factors? We contrast two modeling approaches to this problem, the state variable approach and the individual-based approach. Population variability is determined by the temporal variabilities of both reproduction and survival in a population. Part of this variability is a result of demographic stochasticity; the fact that, to some extent, births and deaths are influenced by factors that are effectively stochastic. The remainder of the variability is determined by temporal variability in the environment that changes the average survivorship and reproduction through time.

Using an object-oriented model for ecological risk assessment on a great blue heron colony

The contribution of certain contaminants to reproductive failure in many avian species has been an ongoing concern. We used the individual-oriented approach in an effort to quantify effects of chronic contaminant exposure on small groups of interacting individual birds rather than the population as a whole. This was made possible by the use of an object-oriented model, where individual birds are interacting objects, and their actions are implemented by passing to them appropriate messages. Using this modeling approach, a breeding colony of Great Blue Herons (Ardea herodias) is simulated as an assemblage of interacting individuals whose daily actions (foraging, growth, feeding of the young) are simultaneously followed over short time intervals for a nesting season. Spatial distribution of the contaminants in prey resources is used on a cell by cell basis, and their effects on certain behavior characteristics of adult birds (e.g., foraging efficiency, effects on flying efficiency, parental care) are taken into account. Results show that sublethal effects can have a considerable effect on colony success.

The Importance of Landscape in Ecosystem Integrity: The Example of Everglades Restoration Efforts

Ecosystem integrity consists of two parts: functional integrity and structural integrity (Westra 1994). An ecosystem’s functional integrity is the maintenance of characteristic ecosystem processes, such as primary production, decomposition, energy flows and nutrient cycling. Structural integrity encompasses the persistence of specific organisms and biotic communities in the ecosystem. It also includes the spatial extent, heterogeneity, configuration (or spatial arrangement) and connectivity of landscape or terrain patterns representative of the ecosystem, upon which the organisms depend. Functional integrity might be sustained even if an ecosystem affected by human impact has lower species diversity compared with the natural ecosystem, but the affected system would be lacking in structural integrity.