Figure 1. Hierarchy of body condition of alligators in several study sites in the Everglades of south Florida from 1999-2002 using a Fulton's K analysis (n=395). Brackets indicate condition does not differ significantly within that group. [larger image]

The American alligator (Alligator mississippiensis) is an integral part of the Everglades ecosystem. They effect and are affected by the landscape and changes in hydrology, which makes them an excellent organism to use in evaluating current Everglades restoration efforts. Information on alligators (census data, capture or morphometric data, blood chemistry, and reproduction statistics) has been collected in the Everglades since the 1950's. Available historical information includes a suite of life history characteristics and population parameters (health and condition, nesting effort, growth rate and survival, and density) that are useful for evaluating restoration. However, some life history traits (e.g. absolute density and survival) are difficult to accurately measure and can require decades of data to detect trends. In contrast, body condition can be measured using indices and requires less data to begin an analysis. If used properly, condition can be a useful monitoring tool to assess the health of a population and thus the health of its ecosystem. This project evaluated morphometric measurements taken for the American alligator, available condition indices and, using a stepwise process, recommends an appropriate index for use in ecological applications such as Everglades restoration.

Condition indices have been used to analyze the fitness of animal populations for the last 50 years. However, the indices are complex and can be used inappropriately if one is unfamiliar with the constraints. For example, condition for crocodilians has been calculated numerous times using Relative K (length/volume ratio), developed for fisheries in 1951 (LeCren 1951). Relative K is useful for comparing the condition of a population over time, but not appropriate among populations.

Alligators were captured during March, April, and October from 1999 to 2002 by a multi-agency team that consisted of members from U.S. Fish and Wildlife Service, U.S. Geological Survey, University of Florida, and the Florida Fish and Wildlife Conservation Commission. Study areas were A.R.M. Loxahatchee NWR (LNWR, 1 site), Water Conservation Areas 2 (WCA2, 1 site) and 3A (WCA3A, 2 sites), and Everglades National Park (ENP, 2 sites: ENP-SS Shark River Slough, ENP-EST Estuaries). Animals were captured from all study areas in marsh habitats only.

We analyzed morphometric measurements of the captured animals to determine which are measured most accurately and are appropriate for condition analyses. Condition indices are functions of a body length indicator and a volumetric measurement, and are only as accurate as the measurement used. Head length (HL), snout-vent length (SVL) and total length (TL) are suitable for body length indicators and tail girth (TG), neck girth (NG), chest girth (CG), and mass can all be used as volumetric measurements. We then compared four condition indices and two models of volume/length relations for their ability to distinguish between populations with known qualitative condition differences. Condition indices were Fulton's K, Relative K, a simple length/volume ratio, and relative mass. We also modeled volume/length with a residual index and ANCOVA.

It was determined through ANOVA/LSD analysis of the condition indices that the HL/Mass combination of Fulton's K and the SVL/Mass combination of ANCOVA were best able to distinguish differences in condition between areas of the Everglades. ANCOVA is best for determining temporal differences between populations. However, it cannot be used to compare differences across populations, unless strict assumptions can be met. HL/Mass Fulton's K can be used to spatially compare populations of the American alligator, and is suggested by this study as the best condition factor to use for that purpose.

Condition is a very fluid measurement. Water management practices and rainfall can dramatically change condition of animals in a relatively short amount of time, because so many aspects of their life history (feeding, courtship, and nesting) depend on seasonally fluctuating water levels. In this study, we found that ENP-SS alligators had the highest condition (fig. 1), but Dalrymple (1996) and Barr (1997) observed Shark Slough alligators to be in very poor condition only five or six years earlier. Water levels were high during capture periods for these earlier studies, possibly affecting food availability.

Because the alligator's life history is so closely linked to hydropattern, body condition can reflect the impacts of changes in hydrology. The condition indicies recommended here should provide a monitoring tool of alligator population health for the current restoration process.

REFERENCES

Barr, B. 1997. Food habits of the American alligator, Alligator mississippiensis, in the southern Everglades. PhD. University of Miami, Miami, FL.

Dalrymple, G. H. 1996. Growth of American alligators in the Shark Valley region of Everglades National Park. Copeia(1): 212-216.

LeCren, E. D. 1951. The length-weight relationship and seasonal cycle in gonad weight and condition in the perch (Perca fluviatilis). Journal of Animal Ecology 20(2): 201-219.

Contact: Christa L. Zweig, Department of Wildlife Ecology and Conservation, University of Florida, 3205 College Ave., Fort Lauderdale, FL 33314; Phone: 954-577-6304, e-mail: czweig@ufl.edu

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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