The objectives of this project are to:
1. Build an age and spatially structured crocodile population model suitable for comparison of CERP restoration alternatives. 2. Couple the local age-structured models into a spatial dispersal model incorporating crocodile movement behavior. 3. Use spatial parameter maps from the Florida Everglades as driving functions on the spatially structured model and to construct crocodile finite rate of increase maps under different management regimes. 4. Perform sensitivity analysis on the model parameters, and release model concept and code for peer-review.
Daniel H. Slone Kenneth G. Rice; Frank J. Mazzotti, Unpublished Material, Spatial and Age-Structured Population Model of the American Crocodile for Comparisons of CERP Restoration Alternatives.Online Links:
U.S. Department of Agriculture - Natural Resources Conservation Service (NRCS) Department of the Interior - U.S. Geological Survey Department of Commerce - National Oceanic and Atmospheric Administration (NOAA) Environmental Protection Agency (EPA) Smithsonian Institution - National Museum of Natural History (NMNH)
Project personnel includes Timothy W. Green
352 264-3556 (voice)
352 374-8080 (FAX)
tgreen@usgs.gov
The American crocodile (Crocodylus acutus) is a federally endangered top consumer in south Florida, imperiled primarily by habitat loss due to expansion of a rapidly growing human population along coastal areas of Palm Beach, Broward, Dade, and Monroe counties. This loss of habitat has principally affected the nesting range of crocodiles, restricting nesting to a small area of northeastern Florida Bay and northern Key Largo by the early 1970's. When crocodiles were listed as endangered in 1975, scant data were available for making informed management decisions. Field and laboratory data suggested that low nest success, combined with high hatchling mortality, provided a dim prognosis for survival. Because of their small size, hatchling crocodiles are vulnerable to biotic and abiotic stressors, such as high levels of salinity. To grow and survive, hatchling crocodiles need to find food and benign environmental conditions (or at least avoid harsh conditions) and avoid predators. Diminished growth rates and higher mortaity or dispersal rates have been associated with areas that pose greater risk to hatchling crocodiles.Crocodiles now occur in most of the habitat that remains for them in southern Florida. Most of the remaining habitat is currently protected in public ownership or engaged in energy production. In these areas, further loss of habitat is not an issue. However, questions of potential habitat modification through continued alteration of freshwater flow due to upstream development and potential curtailment of the range of crocodiles need to be addressed. Patterns of nesting, relative abundance and distribution, growth, and survival of crocodiles can provide insight into restoration of coastal ecosystems in Southeast Florida. For both Florida Bay and Biscayne Bay, restoring a more natural pattern of freshwater flow would provide the most benefit. Characteristics of flow patterns into Florida and Biscayne Bays that are beneficial for crocodiles include sheet flow through the fringing mangrove swamp that extends well into the dry season. Mid-to late dry season discharges of freshwater that cause a reversal of water levels in the receiving body are hypothesized to cause a dispersal of prey items, making them less available to crocodiles. If so, such discharges should be avoided. Shifting water delivery from Biscayne Bay to Florida Bay would degrade the quality of habitat in Biscayne Bay for crocodiles.
Restoration efforts in the Comprehensive Everglades Restoration Plan (CERP) will likely cause changes to salinity levels throughout the habitat of the American crocodile. Changes in salinity were seen after previous hydrologic changes in the area, when reduced freshwater flow was associated with increased salinity levels in Florida Bay estuaries, including one of the core nesting areas of the American crocodile. Large crocodiles are not significantly affected by high salinity, but previous work has shown that high salinity levels may reduce survival of hatchlings and juveniles. The detrimental effects of high salinity on hatchling and juvenile crocodiles has led to the hypothesis that increased freshwater flow to the estuaries will increase the animals' distribution, abundance, and growth.
We will estimate population parameters of the crocodile across hydrological and habitat gradients from available databases (Mazzotti and Cherkiss 2003). Through the development of population simulation models based on these empirical data, we will evaluate restoration alternatives and assess restoration performance measures. By applying crocodile models to restoration alternatives and predicting population responses, we can choose alternatives that result in biotic characteristics that best approach historical conditions and identify future research needs.
The crocodile model will be based on the ATLSS Alligator Population Model, which has been completed and has undergone expert review, calibration, and some validation. An Ecologist was hired late in FY-2007 to work on this task. He has begun training in Matlab programming, and is reviewing the alligator model. Meetings with crocodilian biologists are planned for the near future to outline changes that need to be made from the alligator model, and to begin parameterization of the crocodile model.
In FY2008 we will:
1. Build an age and spatially structured crocodile population model suitable for comparison of CERP restoration alternatives. 2. Couple the local age-structured models into a spatial dispersal model incorporating crocodile movement behavior.
In objective 1, we will represent the age-structured crocodile population as a 3-dimensional array indexing the number in an age group at a spatial location with space assumed to be 2-dimensional, subject to the effects on crocodile parameters relative to the hydrology effects brought on by elevation changes. The net effect will be that we can still use a 2-d spatial representation of the population. We will use a size-class structured version with a Lefkovitch projection matrix having partial class development in each time step. We note that since we can construct the projection matrix at each location, then we can obtain its largest eigenvalue, which will indicate whether the population will increase (l > 1) or decrease (l < 1) at each location. This will enable us to make maps of crocodile population growth and decline over time.
In objective 2, we will include movement and dispersal of certain age groups in the spatial map. We will introduce spatial structure into ecological models by using a discrete spatial convolution model
Person who carried out this activity:
352 264-3556 (voice)
352 374-8080 (FAX)
tgreen@usgs.gov
Slone, D. H. Rice, K. G.; Allen, J. C., 2003, Model evaluates influence of Everglades restoration plan on alligator populations (Florida): Ecological Restoration V. 21, n. 2, University of Wisconsin Press, Madison, WI.
Mazzotti, F. J. Cherkiss, M. S., 2003, Status and Conservation of the American Crocodile in Florida: Recovering an Endangered Species While Restoring an Endangered Ecosystem: Technical Report Final report V. 1, University of Florida, Ft. Lauderdale Research and Education Center, Ft. Lauderdale, FL.Online Links:
accessed as of 6/29/2010
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727 803-8747 ext 3028 (voice)
727 803-2030 (FAX)
sofia-metadata@usgs.gov
U.S. Department of the Interior, U.S. Geological Survey
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