NATIONAL PARK SERVICE Research in the Parks NPS Symposium Series No. 1

At a meeting of the crocodilian specialist group of the International Union for the Conservation of Nature (IUCN) conducted in New York City in March 1971, it was concluded that of the 27 living species and subspecies of crocodilian in the world, all are declining in numbers except for the American Alligator (Alligator mississipiensis). It was recognized that the alligator's favorable status is only a recent development, and is due to several pieces of legislation passed during the last 2 years which served to limit severely the North American market for alligator skins. Of the remaining 26 crocodilian forms, the IUCN group considered 22 to be severely endangered (Anonymous 1971).

Two crodocilian species are native in the United States, the previously mentioned American Alligator, a fresh-water and estuarine inhabitant endemic to portions of the southeastern coastal plain, and the American Crocodile (Crocodylus acutus), which occurs primarily in coastal zones from Ecuador and Venezuela northward to Mexico and southeastern Florida. The range of these two species barely overlaps in mangrove-lined creek and pond estuaries of southeastern Florida, primarily within Everglades National Park. In addition to containing this zone of overlapping ranges where both species are relatively uncommon, Everglades National Park also contains larger areas used exclusively by one species or the other: the broad, interior, fresh-water marshes and swamps, and shallow, saline, Florida Bay. The former habitats apparently support the largest concentration of alligators remaining in Florida, while eastern Florida Bay supports a majority of the remaining Florida population of crocodiles. Studies are now under way in Everglades National Park for purposes of quantifying natural and human-related factors which regulate distribution, nesting success, and population size of the two species, and to establish alligators and crocodiles as indicators of park water requirements, general health of ecosystems, and as a measure for visitor impact on wildlife. More ideally, the studies also explore the desirability and reality of managing portions of alligator and crocodile habitats to reestablish numbers of these two species to population levels that more nearly approximate the historical numbers in the respective habitats. Greatly increased numbers of alligators would be especially desirable for their contribution toward reestablishment of a properly functioning Everglades ecosystem, a relationship to be discussed in more detail later in this paper. The present studies in Everglades National Park are primarily ecological in design, and are intended to build upon a base of previous observations and studies, primarily by Craighead (1968), Hines et al. (1968), and Kushlan (in prep.) in Florida, and Chabreck (1971) and Joanen (1969) in Louisiana.

I will discuss first the American Alligator, restricting this report to habits and ecological relationships of alligators in the Everglades region of extreme southern Florida, south of the Tamiami Trail. Names and descriptions of plant communities used here closely follow terminology and descriptions recently suggested by Craighead (1971).

The subjective quality of most 18th and 19th century first-hand accounts of alligators in southern Florida mean that these reports are of little use for calculating the total numbers that once occurred. However, these early reports leave no doubt that, at least in certain areas and seasons, alligators massed into concentrations much greater than have been seen in recent decades. One recent effort at estimating the former alligator population for the area of Florida south of the Tamiami Trail was made by Craighead (1968). He calculated approximately 1 million alligators, based on the approximate number of limestone solution holes and creeks that exist in extreme southern Florida which appear capable of supporting alligators. Human disturbance to this alligator population, largely through killing for commercially valuable skins, and more recently through habitat disturbance caused by water management efforts, eliminated concentrations of alligators from all areas not vigorously protected or far removed from drainage or water impoundment sites. By the 1960s, the overall reduction of alligator numbers south of the Tamiami Trail probably exceeded 90% of the pre-white man alligator population.

The present distribution of alligators south of the Tamiami Trail reflects to a great degree the past human effects on this alligator population. Alligators are most numerous down the center of the principal Everglades drainage, an area known as the Shark River Slough, and along mangrove-lined headwater creeks at the lower end of the Shark Slough. Historically, the Shark Slough may have been a comparatively less suitable habitat for alligators than adjacent cypress swamps, fresh water mangrove swamps, or higher rockland Everglades. However, these adjacent habitats were either more accessible to hunting or, being on higher ground, have suffered greater loss of alligator habitat as south Florida ground water levels have lowered. Therefore, these habitats have experienced the higher percentage of alligator losses. Accentuating the contrast between historical and present alligator distribution is the probability that the Shark Slough is more suitable to alligators at present than prior to white man's influence in south Florida. Some 19th-century journalists and soldiers who traveled in the central Everglades produced reports (e.g., Dimock 1915) which indicate that in some years the Shark Slough was too deeply flooded to allow for alligator nesting, and that because of deep water, large vetebrates were scarce in the central everglades.

The Shark River Slough, where present alligator studies are centered, is a broad, fresh-water marsh, seasonally reflooded during and following an annual rainy season, June through October. Vegetatively, the slough is dominated by expansive marsh communities of sawgrass and spike rush; these marshes are interrupted by numerous islands of woody vegetation on slightly elevated sites. Where limestone bedrock mesas extend 1-3 ft above general ground level, the woody vegetation is predominately hardwoods species of West Indian origin; elsewhere on elevated peat deposits, woody vegetation is primarily swamp hardwoods including abundant willow. Also generally distributed through the slough are solution hole depressions in the porous limestone bedrock, usually bordered by willow or other swamp hardwoods. Narrow, headwater creeks which penetrate the sawgrass marshes in the lower slough are bordered by thickets of swamp hardwoods and red mangrove.

Basic life history of alligators within this environment is as follows. Sexually mature alligators, 5-6 ft in length or longer, occur throughout the Shark Slough and center their activities around water-filled limestone depressions (known as solution holes, survival holes, or alligator holes). Each hole usually supports either one mature male or one mature female, and may also have several juvenile alligators 1-2 years old. Presumably, the juveniles found with big females are her offspring. Also present with either adult male or female alligator may be several 3- to 5-year-old nonbreeding subadults. The solution hole provides a place of refuge from predators for the juvenile alligators, a den for adults, and a source of food. This latter function is of particular importance during dry seasons if surface water disappears from the surrounding marsh. Mating most often occurs in mid- or late spring, during a period of weeks when adult males wander fairly great distances from their holes to locate females. Nests are constructed and eggs are laid primarily in June. Nests are most often located within a few hundred feet of solution holes and are situated either in dense strands of 6 to 10-ft-high sawgrass or in thickets of tall sawgrass mixed with scattered swamp hardwoods at the edge of tree islands. Nest sites and solution holes are connected by multiple trail systems through the marsh. Nest mounds are composed of local vegetation, most often sawgrass leaves and stems, and measure 1.5-2.5 ft high in the center, and 5-7 ft across at the base. Most nests are built in water less than 10 inches deep. Between 20 and 55 eggs are laid in a depression in the nest center, then covered over by vegetation. Female alligators usually remain near nests during the 60 to 65-day incubation period, often digging temporary holes or dens in soil or under willow root systems located near the nests. The nests serve to maintain eggs at rather constant temperatures, approximately 80-85°F, and some 10-20°F below maximum outside air temperatures. Female alligators are most attentive at nests early and late in incubation periods, and may defend nests from intruders, including humans. There is good evidence, but not certain proof, that females assist newly hatched juveniles to escape from nests by tearing away a portion of nest material shortly after hatching occurs. The stimulus for the nest-opening behavior of females presumably is the vocalizations by newly hatched juveniles in the nest. Juveniles often remain in or near the female alligator's solution hole for 1-2 years after hatching, but the exact relationship between adult and juveniles is uncertain. Alligators 3-5 years old are inclined to wander, and often make up the majority of alligators seen in perimeter canals or headwater creeks adjacent to the Shark Slough. Often hundreds of 3-5 ft subadults may be seen in a single creek, suggesting that these age classes may travel several miles to suitable sites. Alligators are opportunistic feeders, with adults feeding primarily on fish, and smaller alligators feeding on a wide variety of aquatic invertebrates and small vertebrates.

In sufficient numbers, alligators have considerable effect on distribution of plant communities and abundance of certain animal species in the interior swamps and marshes of southern Florida. To understand the effect of alligator activities on plant distribution, one must remember that the Shark River Slough and adjacent habitats are extremely flat regions where only minor changes in land elevation determine the length of time a site is flooded, a factor of considerable importance in determining distribution of plant communities. The interface between woody and marsh communities is often sharp, directly related to the abruptness of land elevation change. The effect alligators have on land elevation and plant distribution is demonstrated in the Shark Slough in the following ways.

NEST LOCATION

All nests I have examined that were located at natural sites in the Shark Slough have been situated in shallow water, with the center of the nest mound elevated 1.5-2 ft above water. Although some nests are reportedly used more than 1 year, most are abandoned after the initial use. These abandoned nests, composed of compact, rotting vegetation, remain as low platforms in the sawgrass marsh and provide a substrate capable of supporting plant species which have less flood tolerance than is inherent in surrounding marsh species. Plant species that establish on old alligator nests are a mixture of woody, swamp hardwoods and understory forbs native to nearby tree islands and other forb species characteristic of early successional stages on disturbed lands outside of the Everglades marshes. Survival of these species on the nest sites is dependent on at least two interrelated factors. Vegetative composition and size of nests affect the rate of decay and length of time each nest survives successive seasonal floodings, and thereby the length of time an elevated site exists for the establishment and support of invading plants. Closely related, the amount of rainfall, depth of flooding, and duration of flooding in successive rainy seasons following establishment of seedling plants on nests will affect their survival; the chance of survival obviously is highest if successive rainy seasons produce lower than average rainfall. I have found that most water-located nest mounds in the Shark Slough decompose at such rates during 1-2 years of normal or above normal flooding that little of these nests remain above water after the 2nd year. Although few quantified data are yet available to show the actual survival rate of plants on the nest mounds, it is true that relatively short periods of 1-2 years of mound survival during series of wetter than normal years is too brief for most species, and that most plant survival to adequate size in order to withstand occasional flooding must occur during back-to-back or longer series of below-average rainfall years.

ALLIGATOR EXCAVATIONS

In the Shark Slough it is characteristic for the numerous solution holes to be rimmed by a dense thicket of willow and other swamp hardwoods. The elevated soil banks which border each solution hole and which support woody growth are in part created and maintained by activities of alligators which live in the holes. In certain seasons, particularly as water levels are dropping, alligators actively push and clear sediment and vegetation to the edges of holes, an activity which serves to maintain a relatively deep pool of open water in the center of the hole. Alligators also dig so-called eaves or dens in the sides of alligator holes, causing portions of the solution hole edge to be pushed upward and thereby further contributing to development of earthen banks. Because of the greater soil content characteristic of solution hole banks than of nest mounds, and because of regular re-charge of organic material to the banks, solution hole banks are more nearly permanent than are nest mounds. They provide more stable sites for establishment of woody vegetation in the Shark Slough marsh. The woody vegetation that surrounds solution holes provides an important part of nesting sites utilized by two marsh birds, the Anhinga and Great Blue Heron.

ALLIGATOR TRAILS

Several mangrove-lined rivers, which flow into the Gulf of Mexico in southwestern Florida, have their extreme headwaters in the fresh-water marshes at the lower end of the Shark River Slough. At these headwaters, located along the approximate inner edge of a broad mangrove forest, numerous, mangrove-lined creeks penetrate sawgrass marshes. Alligators are common in these creeks, where a good food supply exists and ideal nesting sites are available in thick sawgrass strands immediately behind the woody creek edges. Along this mangrove-marsh interface, activities of alligators serve in at least two ways to affect the density and distribution of woody plants. (1) Frequent use of the creeks by large alligators both for feeding and routes of travel serves to control encroachment of red mangroves from the banks into the center of creeks as the animals constantly tear away invading mangrove prop roots. In areas where alligator numbers have been reduced and where water tables have lowered, a combination of events of rather common occurrence in south Florida in recent decades, smaller creeks, particularly less than 10 yards in width, have become completely closed by vegetation. In turn, closure of most small creeks in a local area presumably alters surface water drainage patterns, reduces the capability of creeks to serve as survival sites for aquatic animals, and may reduce species diversity and density of small fish populations. (2) Alligators establish and maintain trails through marsh vegetation which interconnect adjacent mangrove-lined creeks. The trails are repeatedly used, and deepen into narrow channels that are clear of emergent vegetation. Trails which connect parallel creeks usually run perpendicular to the direction of local surface water drainage. During seasons when surface water is deepest in the marshes, floating mangrove seeds disperse from the creeks along the marsh trails, and may become lodged in edge growth and germinate. The resulting narrow strands of mangroves are capable of collecting floating debris, and eventually may form narrow dikes through the marshes. The biological consequences of these dikes has not been studied, and in fact their occurrence in the lower Shark Slough is uncommon. However, it does appear that these narrow, mangrove dikes do form shallow impoundments of surface water in local areas.

SURVIVAL HOLES

There is one important relationship between alligators and other aquatic animals in the Everglades region that has long been recognized, but only recently has been quantified at one site through the studies of Kushlan (in prep.). Solution holes that are occupied by alligators are kept open and deep through the activities of the alligators and serve as dry season survival holes for other aquatic animals. As surface water dries from the surrounding marshes, alligators restrict their activities to the deeper holes and maintain these holes as open pools of water throughout dry seasons in most years. The concentrations of frogs, fish, fresh-water crustacea, and other invertebrates that occur in these solution holes provide breeding stock to repopulate the flooded marshes during the following rainy season, and a most important food supply for upper trophic species during dry months. The failure of Wood Storks, White Ibis, and egrets to nest successfully in Everglades National Park in most recent winters is almost certainly due to an inadequate food supply during the critical bird-nesting season. The recent decline in food species is due to more than one single factor, but an important part of the problem could be the loss of hundreds of solution holes which have become filled with sediment and vegetation as alligator numbers and the Everglades water table both have dropped.

The work of Kushlan clearly shows the survival value of open solution holes in the Everglades region. Table 1 shows the density of 17 species of fresh-water fish and 1 fresh-water shrimp during low-water and high-water sampling dates in a large alligator hole near the Everglades National Park boundary. The important comparison to note is the approximately 953 fish and shrimp per square meter of water in May, compared to 11.9 fish per square meter in August. The dry season concentration is due primarily to animals that moved into the hole from the surrounding marsh, and less to reproduction in the solution hole. Concentrations of birds that feed at these holes during dry seasons are often spectacular.

Thus far I have been summarizing the effects that alligators in sufficient numbers have on certain plant and animal species or communities in the Shark River Slough. The reverse considerations are the effects that various environmental elements have on alligators. We are especially interested in determining how such environmental elements as fire, flood, and drought, which have been altered either in frequency or intensity by human activities, affect the size, density, and nesting success of the Shark Slough alligator population.

Very briefly stated, water management activities in south Florida have created a complex network of drainage canals which have altered fresh water environments by increasing the frequency and duration of drought conditions as ground water levels have generally lowered throughout the southern peninsula. A by-product of overdrainage has been more intensive and frequent wildfires. Conversely, the frequency and duration of flooding or general high-water conditions in the Everglades region have declined. Yet, paradoxically, the water management system through manipulation of storage water is able to produce unnatural and sometimes sudden rises in surface water at almost any season of the year. I will discuss the effects of these changes as they relate to alligator nesting success.

TABLE 1. Comparison of the density of aquatic organisms in the alligator pond during low and high water periods from the studies of Kushlan

Alligators do have the ability to adjust nest height, at least within certain limits, to water levels in the marshes at the time of nest construction. This is shown in Table 2, where the heights of measured nests shortly after construction, and the depth of eggs in these nests, are compared with local water depths, for 3 different years. This comparison shows that the highest nests were built in the year (1970) of deepest water, while the depth of eggs in the nest remained relatively constant. Also shown in Table 2 are measurements from two nests constructed out of water on higher ground. These two nests were the smallest measured, a fact which further suggests that alligators have the ability to adjust nest height to local water depths.

TABLE 2. Relationship between nest measurements and water depth at alligator nests in the Shark Slough, Everglades National Park, 1970-72.

These data in Table 2 show that a rise in water any time during the 60-65 incubation period which is much greater than 6 inches above June levels, probably will destroy the eggs in many nests in the Shark Slough. The average June-August rise in the Slough (P-33) for years 1953 through 1970 was 7 inches, indicating that it is not unusual for lower eggs in nests to be flooded late during incubation, and perhaps most nests contain a few eggs that do not hatch due to flooding. An important water management consideration, therefore, is that excessively heavy deliveries of surface water into the Shark Slough be avoided during the 2 summer months when most alligator nests contain eggs, to avoid loss of most nests through flooding. For this reason, a 6 to 7 inch rise in water probably should be considered the maximum desirable water rise for the period between mid-June and mid-August. In years similar to 1971 when there was an excessively dry spring and late starting rainy season, alligators will not start nesting until late June or early July, and hatching will occur by early September. In these years, an excessive rise in water should be avoided if possible until after the early-September hatching of most nests.

The effects of drought on nesting success are less well known. Drought conditions which existed in the southern Everglades during the spring of 1971 provided our first opportunity to study nesting under such conditions. Summer rains began in early June and were sufficient to reflood the Shark Slough between mid- and late June. Alligators built nests about 3 weeks later than average, mostly during early July, and produced a large hatch of juveniles in early September. The density of nests in the Shark Slough was less than in the previous 2 years, although the number of eggs per nest was the same as the average for earlier years. The large hatch in September may be misleading as a measure of real production, as our observations indicate that juvenile alligators are less well prepared to survive the stress of food shortage such as occurred in the Everglades region after the almost total loss of surface water early in 1971. The total rainfall over the Shark Slough for 1971 through mid-December has been 17 inches below normal, and almost certainly the Slough will become dry again in the spring of 1972, adding further stress to the 1971 hatch of alligators. Such back-to-back droughts may result in higher than usual mortality to juvenile alligators (less than 2 years old), but most subadults and almost all of the adult breeding stock survive.

Aside from the obvious direct effect of fire on alligators, fire in the Everglades marshes also has an important negative effect on acceptability of sites for nests. As I previously mentioned, the usual nest locations in the Shark Slough are the interiors of dense sawgrass, either in pure strands or along the borders of tree islands. The height and density of sawgrass at these nesting locations is a function of soil depth and time since the previous local fire. Following an Everglades fire, recovery of sawgrass heights to the height at acceptable nest sites is rapid, often taking no longer than 6-8 months. However, density of plant leaves and stems and accumulation of dead vegetation per square meter, to a condition similar to sawgrass densities at active nest sites probably require 2-3 years following a fire. Therefore, large Everglades wildfires may eliminate expansive sections of marshland for 1-3 years, forcing a considerable reduction in alligator nesting or relocation of nests (tree islands) where nesting success may be lower. We are presently attempting to better quantify these effects of fire.

The American Crocodile population in Florida has not been accurately surveyed, but probably is between 100 and 300 individuals. The area where a viable population of crocodiles certainly continues to exist is a rather small strip of estuary in extreme southern Florida, from the portion of Biscayne Bay that is south of Miami, south and west into central Florida Bay and along the main Florida Keys, at Key Largo and probably Plantation Key. There is also an isolated, small number of crocodiles in the lower Florida Keys in the area of Big Pine Key. Crocodile distribution within this range is about as one might expect, that is, numbers are very low and distribution is spotty except for a few areas where human development has been limited. Apparently the greatest density of crocodiles remaining in south Florida is along a 15-mile section of shoreline in northeastern Florida Bay, and eastward around the shores of Barnes Sound-Lake Surprise. Most of this shoreline is low and swampy and not readily suited to human use; additionally, Florida Bay is within the boundaries of Everglades National Park. Interestingly, this same northeastern Florida Bay shoreline was described by Dimock (1915) as the center of crocodile abundance, at a time prior to much human influence to the overall Florida Bay-upper Keys area. Therefore, habitats in northeastern Florida Bay may represent some of the most suitable available to crocodiles in Florida.

Our primary interest in crocodiles is to preserve a viable population in Everglades National Park, in the face of a slow decline in the numbers remaining in Florida. Field studies presently underway are restricted primarily to northeastern Florida Bay and are designed to determine factors affecting crocodile nesting success, and to better understand habitat requirements and the present pattern of distribution. Northeastern Florida Bay is the upper end of a broad, shallow bay approximately 35 miles long and 7-20 miles wide. The eastern end is largely cut off from tidal influence by the main chain of Florida Keys. This portion of the bay exhibits some marked seasonal fluctuations in salinities and composition of aquatic animal and plant species. Highest salinities, up to 55/1000, are recorded in drying ponds on Florida Bay keys during dry season months, while at the low extreme, salinities near 5/1000 have been detected close to the mouths of creeks which drain the mainland during the summer-fall rainy season. The mainland shoreline of eastern Florida Bay is an irregular series of coves or bights, intersected by several narrow creeks which drain small, inland bays and inland fresh water wetlands. Eastern Florida Bay is composed of several large, deeper pools, called "lakes," 2-4 miles across and 6-8 ft deep, separated by groups or chains of low keys covered mainly by mangroves.

The keys are often interconnected by submerged marl banks which support Thallasia and other marine plants. Vegetation on the mainland shoreline is largely mixed thickets or fringes of red and black mangroves growing on low marl banks. Where there are slightly elevated shorelines, small dense hammocks of West Indian hardwoods occur, and where these hammocks are immediately on the shoreline, small sandy beaches have developed.

Life history and ecological data for crocodiles in southern Florida is less well known than for alligators, but basically the story in Florida Bay is as follows. Adult crocodiles are distributed throughout the eastern bay, but are most numerous near the edges, either along the mainland shoreline or close to Key Largo. Nests are located on relatively high ground within 5-35 ft of water. The two most frequently used types of nest sites are banks of narrow creeks which cut through the marl, mainland shoreline, or inside the edge of shoreline hammocks at the heads of narrow sandy beaches. Nest mounds are most often composed of sand or marl, with little or no vegetation, and are quite variable in size. Largest mounds are 15-20 ft in diameter at ground level, and 2.5 ft high at center, while at the other extreme are those nesting sites where eggs are deposited in an excavation below ground level, and covered over so that the site is flat with the surrounding terrain. There apparently is no correlation between nest composition, size of mound, or susceptibility of a nesting site to occasional flooding. Nest mounds are built during late March or April, and eggs are usually laid by the end of April. The incubation period is approximately 90 days, with hatching occurring between late July and mid-August. The average clutch size in eight nests in 1970 and 1971 was 39 (range: 28-52), and the average number of young hatched from five of these nests was 29 (range: 11-39). There is not a great deal known of the relationship, if it does exist, between newly hatched young and adult females, but hatched young do disperse rather quickly from immediate areas of nests, particularly nests located in areas of relatively high salinities. Most sightings of juvenile crocodiles (those less than 1 year old) have occurred in low salinity ponds and creeks at mainland sites adjacent to Florida Bay. Our hypothesis is that the movement of newly hatched crocodiles to areas of low salinity is food-oriented. Reports of stomach contents in Crocodylus acutus from Central America, by Schmidt (1924), support the hypothesis by showing that juvenile crocodiles there feed opportunistically on juvenile fishes and a wide variety of aquatic invertebrates, primarily insects, crustacea, and mollusks. In Florida Bay these types of prey species are most abundant in low salinity areas, presumably due to low salinity tolerances characteristic of many estuarine invertebrates. We do not yet know the time required for juvenile crocodiles to move from nest sites to nearest creeks or inland ponds, but the distance from some nests to nearest likely feeding areas is as much as 3-4 miles.

Nesting success by crocodiles at 16 nests during 1970 and 1971 was as follows: nine successfully produced young; four nests destroyed by predators; one nest opened by unknown human; one nest constructed but never received eggs; and one nest contained eggs, apparently undisturbed, which failed to develop. Based on this sample, the most important regulators of nesting success are natural predators. At the four nests destroyed by predators, our identification of the culprits were raccoons, notorious egg-eaters in Everglades National Park. Previous studies in the Everglades, including the work of Holden (1965) with Loggerhead Turtles and Allen (1942) with Roseate Spoonbills, have also reported on the adverse effect raccoons may have on reproductive success in egg-laying species.

One other consideration concerns the effect of exotic Australian pine (Casuarina) infestations on crocodile nesting sites. Casuarina is an aggressive invader of disturbed beaches in south Florida, and its seeds are rapidly dispersed in salt water, particularly during tropical storms. Klukas (1967), in recent studies of Loggerhead Turtle nesting in Everglades National Park, reported on the way dense stands of fast-growing Casuarina eliminate turtle nesting sites at the heads of sandy beaches. The same adverse relationship exists between Casuarina and crocodiles, although infestation of eastern Florida Bay is less advanced than on the Cape Sable beach where Klukas worked. Nine of 11 crocodile nesting sites in Florida Bay that have been closely surveyed for vegetation were found to support Casuarina, and therefore are considered to be in danger of being lost as acceptable nesting sites. A Casuarina control program is called for, to insure that stands do not develop on crocodile nesting beaches.

REFERENCES

ANONYMOUS. 1971. Crocodiles. Proceedings of the first working meeting of crocodile specialists sponsored by the New York Zoological Society and organized by the Survival Service Commission, IUCN, at the Bronx Zoo, New York 15-17 March, 1971. IUCN Publications New Series. Supplementary Paper No. 32. Morges, Switzerland. 190 p.

ALLEN, R. P. 1942. The Roseate Spoonbill. National Audubon Society, New York. 142 p.

CHABRECK, R. H. 1971. The foods and feeding habits of alligators from fresh and saline environments in Louisiana. Proc. 25th Annu. Conf. Southeast. Assoc. Game Fish Commissioners.

CRAIGHEAD, F. C., SR. 1968. The role of the alligator in shaping plant communities and maintaining wildlife in the southern Everglades. Fla. Nat. 41(1):2-7 and 41(2):69-74.

______. 1971. The trees of south Florida. Vol. 1. The natural environments and their succession. Univ. of Miami Press, Coral Gables. 212 p.

HINES, T. C., M. J. FOGARTY, and L. C. CHAPPELL. 1968. Alligator research in Florida: A progress report. Proc. 22nd Annu. Conf. Southeast. Assoc. Game Fish Commissioners.

HOLDEN, M. W. 1965. Further notes on sea turtle nesting on Cape Sable, Everglades National Park. 8 p.

JOANEN, T. 1969. Nesting ecology of alligators in Louisiana. Proc. 23rd Annu. Conf. Southeast. Assoc. Game Fish Commissioners.

KLUKAS, R. W. 1967. Factors affecting nesting success of Loggerhead turtles at Cape Sable, Everglades National Park. Unpubl. data. 58 p.

KUSHLAN, J. A. (In. prep.). An ecological study of an alligator pond in the Big Cypress Swamp of southern Fla. M.S. Thesis. Univ. of Miami, Coral Gables.

SCHMIDT, K. P. 1924. Notes on Central American crocodiles. Field Museum Hist. Pub. 220 Zool. Series Vol. 12(6).