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American alligator distribution, thermoregulation, and biotic potential relative to hydroperiod in the Everglades

Project Proposal for 2001

Continuation Research Plan [Year 5 of 7 Years]

PROJECT TITLE: American alligator distribution, thermoregulation, and biotic potential relative to hydroperiod in the Everglades

Principal Investigator: Dr. H. Franklin Percival, Leader
Florida Cooperative Fish and Wildlife Research Unit
USGS- Biological Resources Division
Email: percivalf@usgs.gov
Mail address:
USGS
University of Florida
117 Newins-Ziegler Hall
Gainesville, FL 32611-0450
Phone: (352) 392-1861

Co- Principal Investigator: Dr. Kenneth G. Rice
USGS- Biological Resources Division
Florida Caribbean Science Center
Email: ken_g_rice@usgs.gov
Mail address:
USGS
Everglades National Park Field Station
4001 SR 9336
Homestead, FL 33034
Phone: (305) 242-7832

BACKGROUND:

Over the last one hundred years the hydrology of the Everglades has been greatly altered by mankind. Efforts to repair the functioning of the ecosystem are using a multicomponent model, the Across Trophic Level System Simulation (ATLSS), to predict the response of native flora and fauna to alterative water delivery scenarios. This study was designed to provide information on the natural history and population functioning of the American alligator in the Everglades for construction of an ATLSS American alligator population model and to investigate restoration needs and status of the alligator in the Everglades ecosystem.

We initiated a five year study on the home range, daily movement, habitat use, thermoregulation, and body temperature patterns of alligators in both Shark Slough, Everglades National Park, and in Water Conservation area 3A North. A total of 66 alligators were captured and surgically implanted with radio-transmitters. A subset of 29 of these also were implanted with temperature recording data loggers. Data loggers recorded core body temperature simultaneously at 72 minute intervals for 1 year.

Weekly aerial telemetry locations were collected beginning 1 January 1997 to estimate home range size. Weeklong intensive sampling efforts conducted from 7 November 1997 to 31 July 1998 were used to estimate daily movement and habitat use.

Objectives:

  • Determine daily and seasonal movements of varying age/size and habitat (canal, interior marsh) classes of Everglades alligators.
  • Determine the proportion of female alligators in the population that might be expected to nest in a given year and examine reproductive parameters associated with nesting.
  • Elaborate on existing hypotheses of thermoregulation in Everglades alligators.
  • Investigate the relative importance of canal habitats to alligator populations.
  • Relate the above objectives to the dynamic hydroperiod of the Everglades.
  • Determine the relative contribution of fertilization failure, losses during in ovo/embryonic development, to decreased reproductive success in alligators.

INFORMATION NEEDS AND USES

  • This study encompasses 3 of the critical projects for restoration of crocodilian populations determined by a meeting of over 40 biologists, managers, and administrators held in Homestead in December, 1998.
  • Alligators are a key indicator component and are used as ecological attributes and measures in the Everglades Ridge & Slough and Marl Prairie/Rocky Glades Conceptual Ecosystem Models.
  • This study directly addresses the critical ecological pathways outlined in the Everglades Ridge & Slough Conceptual Ecosystem Model.
  • This study examines the effects of compartmentalization on wildlife populations discussed in several of the Conceptual Ecosystem Models, the Federal Objectives for The South Florida Restoration by The Science Sub-Group of The South Florida Management and Coordination Working Group.
  • In the Everglades Ridge & Slough Conceptual Ecosystem Model, a link between water management practices and reduced production and survival of alligators is proposed. This study addresses hypotheses concerning this proposed linkage.
  • Information developed in this study will enhance the prescription of minimum area discussed in the Federal Objectives for The South Florida Restoration by The Science Sub-Group of The South Florida Management and Coordination Working Group.
  • This study addresses critical information needs such as restore ecosystem structure and function, recover populations of selected indicator species, and identification of ecological assessment indicators identified in South Florida Ecosystem Restoration: Scientific Information Needs by the Science Subgroup of the South Florida Ecosystem Restoration Task Force.
  • We investigate the decline in numbers and shift in distribution of the American alligator identified as an Everglades National Park Major Issue and information need in South Florida Ecosystem Restoration: Scientific Information Needs by the Science Subgroup of the South Florida Ecosystem Restoration Task Force.
  • Specific proposed performance measures relate to the alligator such as reduce frequency of water dry-outs during courtship period and duration of below ground water depths to increase alligator nesting and re-establish hydrological predictability for relationship between peak early wet season water levels and late wet season levels to reduce alligator nest flooding.
  • Data from this study is being used to develop an ATLSS alligator population model for comparison of restoration alternatives during implementation as outlined in South Florida Ecosystem Restoration: Scientific Information Needs by the Science Subgroup of the South Florida Ecosystem Restoration Task Force.

KEY FINDINGS

Based on a preliminary analysis of data (findings subject to change with increased sample size and hydrological conditions), the following key results have been obtained:

  • Spring is metabolically the most important season for alligator populations in the Everglades due to breeding cycles and feeding on concentrated food supplies during the dry season.
  • Due to consistently high temperatures and decreasing food supplies, summer is metabolically expensive.
  • In fall alligator body temperatures decline with decreasing ambient temperatures, with occasional elevations to activity temperature.
  • Winter reduces body temperature to levels that inhibit activity, but do not allow efficient hibernation.
  • Canals provided a thermal refuge for alligators.
  • Home range size for alligators located in WCA 3A North and ENP were not significantly different.
  • Male alligators had home ranges that are on average 2-3 times larger than female home ranges.
  • Female alligators exhibited high site fidelity for a particular area of the natural marsh or section of a canal and exhibited nearly identical home range sizes among seasons.
  • Male alligators moved more than females during a 24-hour period. Male alligators exhibited the greatest movement during the spring (approximately 400 m/24-hr) and females exhibited the least movement during the summer (approximately 25 m/24-hr).
  • Canal alligators had longer, more linear home ranges that were larger in size than marsh animals.
  • Canal alligators exhibited greater daily movement than marsh alligators.
  • Marsh alligators located in WCA 3a North utilized holes approximately 10% of the time and marsh alligators located in ENP utilized holes approximately 40% of the time.
  • Marsh alligators located in WCA 3a North preferred mixed marsh, cattail, and water lily "potholes" and used sawgrass less than its availability would suggest.
  • Marsh alligators located in ENP preferred shrub islands, spikerush sloughs, and holes and used sawgrass less than its availability would suggest.
  • Canal alligators utilized canals approximately 80% of the time and preferred levee breaks and canal edges in the Miami Canal and preferred willows and canal edges in the L-67 Canal extension.

PROJECT DESCRIPTION

Purpose and Goals

The American alligator (Alligator mississippiensis) is not only a top consumer and a keystone species in the Everglades, but also physically influences the system through construction and maintenance of gator holes and trails(Mazzotti and Brandt 1994). The existence of this species is important to the faunal and floral character of the Everglades as it has evolved. Despite its prominence biologically and publicly in the system, many important questions about basic behavioral and population parameters of alligators remain unanswered. Although many assumptions can be made, we are not certain of movements or survival of varying size classes of alligators under either stable or fluctuating water levels. The reproductive contribution of an individual animal or different size/age classes in any given year has been a principal stumbling block for attempts at modeling any crocodilian population. For effective modeling of alligators, more definitive answers to those latter two questions are essential. Further, Everglades restoration requires an understanding of the ecological impact of various restoration alternatives. This study not only provides keys to this understanding through contribution to ecological modeling but is designed to answer questions related to the effects of decompartmentalization on alligator populations, fertilization failure of eggs, and hydrological effects on thermoregulatory function.

Objectives:

  • Determine daily and seasonal movements of varying age/size and habitat (canal, interior marsh) classes of Everglades alligators.
  • Determine the proportion of female alligators in the population that might be expected to nest in a given year and examine reproductive parameters associated with nesting.
  • Elaborate on existing hypotheses of thermoregulation in Everglades alligators;
  • Investigate the relative importance of canal habitats to alligator populations;
  • Relate the above objectives to the dynamic hydroperiod of the Everglades.
  • Determine the relative contribution of fertilization failure, losses during in ovo/embryonic development, to decreased reproductive success in alligators.

Urgency or Timelines

This study provides information critical the construction of the ATLSS American alligator population model which will be used as a tool for evaluation of restoration alternatives during adaptive implementation of the Comprehensive Ecosystem Restoration Plan. We also provide other timely investigations involving the effects of canals and fertilization failure on alligator populations. The alligator is both a keystone and indicator species in the Everglades ecosystem. Therefore, it is critical to understand the effects of restoration alternatives on this species and to include the alligator in restoration alternative selection, evaluation, and monitoring.

Effectiveness

  • This study provides parameter estimates to an ongoing ATLSS modeling project.
  • We provide blood and tissue samples to other ongoing projects on contaminant concentrations, hormonal levels, and blood chemistry of the alligator.
  • We have provided written and oral comments on restoration alternatives based on the findings of this project to several Restudy groups, NPS, FFWCC, and USGS.
  • In the first 4 years of the study, we have recorded over 6000 telemetry locations, 2000 vegetation, water depth, and soil depth recordings at alligator holes, 150,000 alligator and environmental temperatures, and many morphometric, nesting, habitat use, and behavioral observations.
  • We have produced a number of reports, publications, theses, posters, and oral presentations concerning this project to governmental, environmental (both local and international), educational, and citizen groups.
  • We have used graduate students and university OPS personnel for this study for cost effectiveness and to provide educational opportunities to future researchers and management personnel.
  • We have cooperated throughout this project with the Florida Fish and Wildlife Conservation Commission and the National Park Service to use equipment, personnel, and expertise for alligator capture and data collection especially during peak capture and monitoring periods at no cost to this project.

Synopsis of Research Methods

The Everglades is believed to be a harsh environment for alligators. Everglades alligators weigh less than alligators the same length from other parts of their range (Jacobson and Kushlan 1989, Barr 1997). Further, maximum length is decreased, and sexual maturity is delayed (Kushlan and Jacobsen 1990, Dalrymple 1996). Jacobsen and Kushlan’s (1989) model for growth in the Everglades of Southern Florida predicted alligators reaching a mere 1.26 meters in 10 years and requiring at least 18 years to reach sexual maturity. It is currently suspected that the reason for this poor condition is a combination of low food availability and high temperatures (Jacobson and Kushlan 1989, Dalrymple 1996, Barr 1997).

Two study sites were chosen from within the Everglades ecosystem. Water Conservation Area 3A North (WCA) represented a drier and more dynamic hydropattern while Shark Slough, Everglades National Park (ENP) typified the more stable conditions of the central drainage of the Everglades ecosystem.

After a series of consultations with researchers familiar with radio telemetry and the conditions under which this study was to be performed, an AVM model SB2 transmitter in the 166-170 MHz range was chosen. Three different sized radio-transmitters were utilized. The various sized radio-transmitters allowed alligators of different size to be implanted (K-16-H size with a life expectancy of 1.5 years for small animals, C-cell size with a life expectancy of 3 years for intermediate animals and D-cell size with a life expectancy of 5 years for the larger animals). Radio-transmitters were equipped with a single 30 cm to 40 cm external whip antenna. Transmitters were tested under laboratory and field conditions (both from the ground and air) to determine effective ranges.

The disk shaped data loggers (Tidbit Stowaway Temperature Loggers manufactured by Onset Computer Corporation) had a diameter of 3.0 cm and a thickness of 1.5 cm. Each data logger was capable of recording 7944 temperature readings from a range of -5 to 37° C with an accuracy of +/- 0.2° C. Data loggers were programmed to record temperature every 72 minutes, allowing 396 days of continuous data collection. Data collection times were synchronized using Logbook software (Onset Computer Corporation) so that they recorded temperature simultaneously. Once recovered, data loggers were recalibrated using a water bath.

Data loggers were also used to record environmental temperatures. Environmental temperatures were recorded in the marsh near the home ranges of implanted alligators. Environmental temperatures were recorded with both the -5 to 37° C range data logger and data loggers that recorded from -20 to 70° C. The air temperature data logger was shielded from direct solar radiation by a glossy white 14 l plastic bucket with slotted vents for ventilation. The deep water temperature data logger was attached to a concrete block that anchored it at the bottom of the water column. Shallow water temperatures were recorded using data loggers suspended from floats approximately 5 cm from the surface of the water. Black body temperature data loggers were placed in a 600 cm3 copper spheroid painted flat black and positioned to maximize solar input.

Alligators were captured at night from airboats. Suitable habitat was searched using 400,000 candlepower spotlights, and alligators were located by the reflection of the light in their eyes. Alligators were then captured using snares or toggle darts. The location was recorded using GPS along with time of capture and a description of the habitat. Alligators were transported to the University of Florida Fort Lauderdale Research and Education Center in Davie, Florida. The total length, snout-vent length (to the end of the vent), head length, hind foot length, tail girth, mass, and sex were recorded. Each alligator was then permanently marked with two individually numbered Monel tags, a size 6 tag on the first sagittal scute on the tail and a size 3 tag in the webbing of the right hind foot.

Alligators were anesthetized using a combination of medetomindine and isoflurine (T.S. Gross, USGS, unpublished data). Data loggers were cold sterilized and surgically implanted intraperitoneally on the left flank between the last rib and the hind limb. This technique allowed low impact access to the center of the body, which provided accurate core body temperature readings. Two sterilized radio transmitters were then implanted between the peritoneum and the muscle layer on each side of the alligator, one accessed via the incision used to implanted the data logger, and the other through an additional incision on the right flank. After incisions were sutured, the medetomindine was reversed using atipamezole hydrochloride and the alligators were monitored for signs of ill health. The alligators were then released within 24 hours at the exact capture location.

After one year of temperature recording, alligators were recaptured, data loggers were removed using the same surgical techniques, and the animals were released at the recapture site. Animals that were not implanted with temperature data loggers and those implanted but not recaptured were located weekly for the entirety of the study. A sample of the animals were located on a 24 hour basis for an investigation of seasonal movement patterns.

Statistical analyses involved calculation of several variants of home-range estimates (White and Garrott 1990, Seaman and Powell 1996, Staus 1998), time series analysis (SAS Institute 1988), Fourier decomposition (Wilkinson, 1996), cluster analysis (Milligan and Cooper 1985), habitat use (Aebischer et al. 1993), and analysis of variance and other standard statistical tests (SAS Institute 1988).

Key Results

During this study, 79 alligators were captured and surgically implanted with radio transmitters. A total of 66 alligators were subsequently radio tracked for a sufficient period of time for inclusion in home range, movement, and habitat use analyses. These analyses include 31 animals from Shark Slough, Everglades National Park and 35 from Water Conservation Area 3A North. A subset of 29 of these alligators were implanted with temperature recording data loggers. Of the data logger implanted alligators, 18 have been recaptured and 15 functioning data loggers recovered. Finally, 15 of these animals nested at lease once during the course of the study.

  • Home range size and shape and daily movement varied according to gender, temperature, reproductive efforts, habitat and water level.

    Graph showing male/female seasonal home range size
    Figure 1. Mean annual and seasonal home range size (95% adaptive kernel) of radio-tagged male and female alligators located in Water Conservation Area 3A North and Everglades National Park from November 1996 to August 1999.

  • An alligator’s location in the Everglades’ landscape also affected their movements. Canal alligators had larger home range length and area in certain seasons than alligators located in marsh habitats.

    Graph showing seasonal canal/marsh home range size
    Figure 2. Mean annual and seasonal home range size (95% adaptive kernel) of radio-tagged canal and marsh alligators located in Water Conservation Area 3A North and Everglades National Park from November 1996 to August 1999.

  • Water level effected home range sizes and daily movements. Alligators moved less and had smaller home ranges in portions of the marsh that experienced near-dry conditions. Alligators located in canals and sloughs, where water was more abundant, were less affected.
  • Everglades alligators in the marsh are dependent upon hole habitats. Alligators were found to have 1 or more preferred gator holes or depression potholes within their home range. Regular movements were observed for alligators within their respective home ranges and daily movements were often from one preferred hole to another.
  • Everglades alligators may live in an adverse environment, but have adapted accordingly. Adult alligators establish a home range that encompasses several habitat types that allow them to access the basic requirements of food, water, shelter, and reproductive opportunities. Of these natural habitats, traditional gator holes and depression potholes are the most critical.
  • Alligators in the Everglades exhibit distinct patterns of Tb within the annual temperature cycle. Alligator Tb is more variable in spring than in any other season. Warm temperatures not only aid sperm and egg production for the coming reproductive season, but they also facilitate prey digestion (Lang 1987). Importantly, the normally limited prey base in the Everglades becomes concentrated in pools during the dry season, which peaks in spring. The increased activity of spring reflects the importance of this season in the ecology of alligators in the Everglades.

    Graph plotting Everglades gator body temp vs. month
    Figure 3. The typical pattern of body temperature of an alligator from the Everglades from 1 August 1997 to 31 July 1998. [larger image]

  • The high temperatures of summer increase the metabolic cost to alligators in the Everglades. When combined with low food availability resulting from the high water levels of the renewed wet season, summer appears to be a time of negative energy balance for Everglades alligators.
  • Fall alligator Tbs declines with the declining ambient temperatures. Alligators avoid metabolically active temperatures in fall since prey are dispersed. This limits the fuel needed to maintain higher metabolic rates.
  • Although Everglades alligators maintain low Tbs during the winter months, Tbs are occasionally raised to activity levels. These heating events are not synchronized as would be expected if environmental factors alone were inducing basking behavior. Since digestion of prey could not be accomplished in such a short time period (Barr, 1997), elevated Tbs in winter may be necessary for excretion of metabolic wastes.

    Graph showing seasonal WCA and ENP alligator body temps.
    Figure 4. Seasonally smoothed body temperatures of 10 alligators from WCA and 4 alligators from ENP from 1 August 1997 to 11 July 1998. [larger image]

  • Everglades alligators are adapted to fluctuating hydropatterns. While low water levels decrease their ability to thermoregulate, they also concentrate prey in an otherwise nutrient poor system. Without a food base to support the resulting high metabolic rate, a high stable Tb would be detrimental.

INFORMATION PRODUCTS

Technical Reports

See www.fcsc.usgs.gov. (see http://cars.er.usgs.gov/)

Percival, H.F., K.G. Rice, C.R. Morea, and S.R. Howarter. 1997-1999. American alligator distribution, thermoregulation, and biotic potential relative to hydroperiod in the Everglades. Interim/Annual Reports. USGS-BRD. Gainesville, Fl.

Percival, H.F., K.G. Rice, and S.R. Howarter. 2000. American alligator distribution, thermoregulation, and biotic potential relative to hydroperiod in the Everglades. Contract Final Report. USGS-BRD. Gainesville, Fl.155 pp.

Data & Models

All data from this project have been stored in a database (MS Access) maintained at both the USGS-BRD, Florida Cooperative Fish and Wildlife Research Unit in Gainesville Florida and at the USGS–BRD, Florida Caribbean Science Center, Restoration Ecology Branch, Everglades National Park Field Station in Homestead Florida. The data also have been reported in the two masters theses. Stanley R. Howarter's "Thermal Ecology of the American Alligator in the Everglades" and Cory R. Morea's "Home range, Movement, and Habitat Use of the American Alligator in the Everglades" are deposited at the University of Florida's science library and available as reprints from the Florida Cooperative Fish and Wildlife Unit.

Permits

Permits for alligator capture, surgical implantation of data loggers and radio transmitters, alligator nest/egg investigations, and survey were obtained annually (1996 to present) from the following agencies:
1. Everglades National Park;
2. Florida Fish and Wildlife Conservation Commission;
3. University of Florida Animal Care and Use Committee.

Publications and Presentations

1. Abercrombie, C.L., S. Howarter, C.R. Morea, K.G. Rice, and H.F. Percival. 2000. Thermoregulation of alligators (Alligator mississippiensis) in southern Florida. J. Thermal Biology. SUBMITTED.

2. Barnett, J., K.G. Rice, H.F. Percival, and P.T. Cardeilhac. 1997. A method for the intramuscular implantation of transmitters in alligators. Proc. Inter. Assoc. Aquatic Animal Medicine. 28:45-48.

3. Howarter, S.R., C.R. Morea, H.F. Percival, K.G. Rice, and C.L. Abercrombie. 1999. Thermal ecology of the American alligator in the Everglades. Abstract/Presentation in Managing Biodiversity. Fl. Chap. Wildl. Soc. Orlando, FL.

4. Morea, C.R., S.R. Howarter, K.G. Rice, H.F. Percival, and C. L. Abercrombie. 1999. Habitat preference and movement of the American alligator in the Everglades ecosystem. Abstract/Presentation in Managing Biodiversity, Fl. Chap. Wildl. Soc. Orlando, FL.

5. Rice, K.G., F.J. Mazzotti, and H.F. Percival. 1999. Effects of restoration on alligators and crocodiles in the Greater Everglades Ecosystem. South Florida Restoration Science Forum. Boca Raton, FL. Posters.

6. Howarter, S. R. 1999. Thermoregulation of the American alligator in the Everglades. M.S. Thesis. University of Florida, Gainesville, Florida. 73 pp.

7. Morea, C. R. 1999. Home range, movement, and habitat use of the American alligator in the Everglades. M.S. Thesis, University of Florida, Gainesville, Florida. 88 pp.

8. Morea, C.R., K.G. Rice, H.F. Percival, and S.R. Howarter. 2000. Home range and daily movement of the American alligator in the Everglades. 19 pp. in Crocodiles. Proceedings of the15th Working Meeting of the Crocodile Specialist Group, IUCN, Gland, Switzerland. In Press.

9. Howarter, S.R., K.G. Rice, H.F. Percival, K.M. Portier, and C.R. Morea. 2000. Thermal ecology of the American alligator in the Everglades. 18 pp. in Crocodiles. Proceedings of the15th Working Meeting of the Crocodile Specialist Group, IUCN, Gland, Switzerland. In Press.

10. Morea, C.R., K.G. Rice, H.F. Percival, and S.R. Howarter. 2000. Home range and daily movement of the American alligator in the Everglades. 15th Working Meeting of the Crocodile Specialist Group, IUCN, Varadero, Cuba. Poster.

11. Howarter, S.R., K.G. Rice, H.F. Percival, K.M. Portier, and C.R. Morea. 2000. Thermal ecology of the American alligator in the Everglades. 15th Working Meeting of the Crocodile Specialist Group, IUCN, Varadero, Cuba. Poster.

12. Morea, C.R., K.G. Rice, H.F. Percival, and S.R. Howarter. 2000. Movement of the American alligator in the Everglades. Wildl. Soc. Bull. IN PREP.

13. Howarter, S.R., K.G. Rice, H.F. Percival, and C.R. Morea. 2000. Alligator thermal ecology in the Everglades. J. Herp. SUBMITTED.

PLANNED ACTIVITIES - 1999/2000:

  • Continue weekly monitoring of telemetered animals in ENP and WCA IIIA.
  • Continue capture, surgical removal, and analysis of temperature data loggers from animals in ENP and WCA IIIA.
  • Prepare final report on characterization of alligator holes and canals used by telemetered animals. ® Continue nest location and nest/egg characterization of telemetered animals.
  • Conduct seasonal characterizations (e.g., hourly temperature, vegetation, water level) of alligator holes associated with telemetered animals. Alligator holes are recognized as dry season refugia for alligators, fish, wading birds, and many other species. It is critical for restoration of habitats and modeling of restoration alternatives to understand the ecological role of these refugia in relation to varying hydrologic conditions.
  • Continue intensive monitoring of movement patterns of nesting females.
  • Examine the effects of canal habitats on alligator nest success including: viability rates, hatch rates, clutch size, egg size, and female size and condition.
  • Examine the effects of canal habitats on alligator early age class survival.

SCHEDULE OF ACTIVITIES AND DELIVERABLE - 2000/2001:

Reports and Deliverables:
Type of Product* No. of Copies Due Date
1. Brief letter report with the following: [1] date of receipt of executed contract (i.e. start date), [2] this "table" with specific due dates, [3] status of progress made towards providing data/metadata/model-source-code, [4] draft text w/ graphics/slides for 2-pager Fact Sheet. Original + 2 + electronic copy 60 days after award of contract.
2. 1st Trimester Report – a brief report updating progress/problems to date and all data and metadata file, &/or model source code to date. Plus, final of 2-pager Fact Sheet. Original + 2 + electronic copy 120 days after award of contract.
3. 2nd Trimester Report – with updated data/metadata file[s] &/or model source code. Plus, "Request for Continued Funding" for the next year funding. Original + 2 + electronic copy 240 days after award of contract.
4. Annual Report with all data to date with metadata file[s]. Original + 2 + electronic copy 1 Year after award of contract.
* Note: Manuscripts, peer reviewed publications, book chapters, graduate student thesis/dissertation, etc. are both acceptable and desirable as chapters or sections of annual/final reports. At least one 2-page fact sheet is required for each of these major types of publications.

Literature Cited and Related References

Aebischer, N.J., P. A. Robertson, and R. E. Kenward. 1993. Compositional analysis of habitat use from animal radio-tracking data. Ecology. 74(5):1313-1325.

Barr, B. 1997. Food Habits of the American alligator, Alligator mississippiensis, in the southern Everglades. Unpublished Ph.D. Thesis, Univ. Miami, Florida.

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

Jacobsen, T. and J. A. Kushlan. 1989. Growth dynamics in the American alligator (Alligator mississippiensis). J. Zool., Lond. 219(2): 309-328.

Kushlan, J. A. and T. Jacobsen. 1990. Environmental variability and the reproductive success of Everglades alligators. J. Herpetol. 24(2):176-184.

Lang, J. W. 1987. Crocodilian thermal selection. in G. J. W. Webb, S. C. Manolis, and P. J. Whitehead (eds.), Wildlife Management: Crocodiles and Alligators, pp. 301-317. Surrey Beatty and Sons, Ltd., New South Wales, Australia.

Mazzotti, F. J. and L. A. Brandt. 1994. Ecology of the American alligator in a seasonally fluctuating environment. In S. Davis and J. Ogden, (eds.), Everglades: The Ecosystem and its Restoration, pp. 485-505. St. Lucie Press, Delray Beach, Florida.

Milligan, G. W, and M. C. Cooper. 1985. An examination of procedures for Determining the number of clusters in a data set. Psychometrika 50(2):159-179.

SAS Institute Inc. 1988. SAS/STAT user’s guide, release 6.03 edition. SAS Institute Inc., Cary, NC. 1028 pp.

Seaman, D. E. and R. A. Powell. 1996. An evaluation of the accuracy of kernel density estimators for home range analysis. Ecology, 77(7):2075-2085.

Staus, N. L. 1998. Habitat use and home range of West Indian whistling-ducks. J. Wildl. Manag. 62(1):171-178.

White, G. C. and R. A. Garrott. 1990. Analysis of Wildlife Radio-Tracking Data. Academic Press Inc., New York.

Wilkinson, L. 1996. SYSTAT 6.0 for Windows: Statistics. SPSS Inc., Chicago.

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