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Open-File Report 2010–1284

Prepared as part of the
U.S. Geological Survey Priority Ecosystems Science Initiative

Spatial and Stage-Structured Population Model of the American Crocodile for Comparison of Comprehensive Everglades Restoration Plan (CERP) Alternatives

By Timothy W. Green, Daniel H. Slone, Eric D. Swain, Michael S. Cherkiss, Melinda Lohmann, Frank J. Mazzotti, and Kenneth G. Rice

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ABSTRACT

As part of the U.S. Geological Survey Priority Ecosystems Science (PES) initiative to provide the ecological science required during Everglades restoration, we have integrated current regional hydrologic models with American crocodile (Crocodylus acutus) research and monitoring data to create a model that assesses the potential impact of Comprehensive Everglades Restoration Plan (CERP) efforts on the American crocodile.

A list of indicators was created by the Restoration Coordination and Verification (RECOVER) component of CERP to help determine the success of interim restoration goals. The American crocodile was established as an indicator of the ecological condition of mangrove estuaries due to its reliance upon estuarine environments characterized by low salinity and adequate freshwater inflow.

To gain a better understanding of the potential impact of CERP restoration efforts on the American crocodile, a spatially explicit crocodile population model has been created that has the ability to simulate the response of crocodiles to various management strategies for the South Florida ecosystem. The crocodile model uses output from the Tides and Inflows in the Mangroves of the Everglades (TIME) model, an application of the Flow and Transport in a Linked Overland/Aquifer Density Dependent System (FTLOADDS) simulator. TIME has the capability to link to the South Florida Water Management Model (SFWMM), which is the primary regional tool used to assess CERP restoration scenarios.

A crocodile habitat suitability index and spatial parameter maps that reflect salinity, water depth, habitat, and nesting locations are used as driving functions to construct crocodile finite rate of increase maps under different management scenarios. Local stage-structured models are integrated with a spatial landscape grid to display crocodile movement behavior in response to changing environmental conditions.

Restoration efforts are expected to affect salinity levels throughout the habitat of the American crocodile. This modeling effort examines how CERP restoration alternatives will affect growth and survival rates of hatchling and juvenile crocodiles, hatchling dispersal to suitable nursery habitat, and relative abundance and distribution in response to changing salinity and water depth for all stage classes of crocodiles. The response of the American crocodile to restoration efforts will provide a quantifiable measure of restoration success. By applying the crocodile model to proposed restoration alternatives and predicting population responses, we can choose alternatives that approximate historical conditions, enhance habitat for multiple species, and identify future research needs.

Future modeling efforts will include climate change scenarios and will cover an expanded area that includes Biscayne Bay and the Ten Thousand Islands.

First posted November 23, 2010

For additional information contact:
Timothy W. Green
U.S. Geological Survey
Southeast Ecological Science Center
7920 NW 71st Street
Gainesville, FL 32653
352-264-3556

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Suggested citation:

Green, T.W., Slone, D.H., Swain, E.D., and others, 2010, Spatial and stage-structured population model of the American crocodile for comparison of comprehensive Everglades Restoration Plan (CERP) alternatives: U.S. Geological Survey Open-File Report 2010–1284, 57 p.



Contents

Abstract

Introduction

Methods

Model Overview

Model Structure

Hydrologic Inputs

Model Environment

Crocodile Interactions with the Environment

Basic Model Cycle

Survival

Effect of Salinity on Survival

Effect of Growth Rate on Survival

Density-Dependent Survival

Effect of Distance to Nursery Habitat on Survival

Growth

Stage Transfer

Dispersal

Rate of Dispersal

Reproduction and Female Dispersal to Nesting Locations

Model Initialization and Nesting Decisions

Sensitivity Analysis

Hatchling Survival Rate Changes

Juvenile Survival Rate Changes

Changes in Influence of Salinity on Hatchling and Juvenile Crocodiles

Results

Validation

Discussion

Rainfall

Conclusions

References Cited

Appendix 1

Appendix 2

Appendix 3


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