Frank J. Mazzotti

Severe mammal declines coincide with proliferation of invasive Burmese pythons in Everglades National Park

Invasive species represent a significant threat to global biodiversity and a substantial economic burden. Burmese pythons, giant constricting snakes native to Asia, now are found throughout much of southern Florida, including all of Everglades National Park (ENP). Pythons have increased dramatically in both abundance and geographic range since 2000 and consume a wide variety of mammals and birds. Here we report severe apparent declines in mammal populations that coincide temporally and spatially with the proliferation of pythons in ENP. Before 2000, mammals were encountered frequently during nocturnal road surveys within ENP. In contrast, road surveys totaling 56,971 km from 2003–2011 documented a 99.3% decrease in the frequency of raccoon observations, decreases of 98.9% and 87.5% for opossum and bobcat observations, respectively, and failed to detect rabbits. Road surveys also revealed that these species are more common in areas where pythons have been discovered only recently and are most abundant outside the pythons current introduced range. These findings suggest that predation by pythons has resulted in dramatic declines in mammals within ENP and that introduced apex predators, such as giant constrictors, can exert significant top-down pressure on prey populations. Severe declines in easily observed and/or common mammals, such as raccoons and bobcats, bode poorly for species of conservation concern, which often are more difficult to sample and occur at lower densities.

Population biology of the American crocodile

The population biology of the American crocodile (Crocodylus acutus) was studied in southern Florida during 1977-1982. Crocodiles occur primarily in inland mangrove swamps protected from wave action. Females use the open waters of Florida Bay only for access to nesting sites. Individuals have large (86-262 ha), overlapping activity areas. Nesting occurs in spring and summer, avoiding the cold and the wet seasons, either of which can affect incubation. Clutches averaging 38 eggs were laid both in mounds and in holes in the ground, either singly or communally. Available data cannot support the view that the number of nests has decreased in recent years. Hatching failure occurred as a result of infertility, predation, and embryonic mortality from desiccation and flooding. Hole and creek nests were most susceptible to embryonic mortality. Seventy-eight percent of nests hatched some young. We found no evidence of adults defending nests or young, but nest opening by adults was essential for hatching. Disturbance at nest sites caused females to abandon the site. All expected age classes occurred in the population. Size at maturity was 2.25 m TL for females. Documented mortality of adults and subadults of approximately 2 crocodiles per year was predominantly human-caused. At least 45 crocodiles have been released into southern Florida in 17 years. We estimate the southern Florida population to be about 220 ? 78 adults and subadults. The northernmost population of the American crocodile (Crocodylus acutus) occurs on the extreme southern tip of the Florida peninsula, USA. The range of this tropical species has always been limited, probably by temperature constraints (Kushlan and Mazzotti, 1989). Early reports of crocodiles in Florida are notices and discussions of its occurrence (Wyman, 1870; Hornaday, 1891; Smith, 1896; Barbour, 1923); Moore (1953) provided the first detailed account of its status. Ogden (1978) documented aspects of its nesting biology. More recently, Gaby et al. (1985) reported on the population biology of a small number of crocodiles living in a power plant cooling system. 3 Present Address: Department of Wildlife and Range Sciences, University of Florida, 3245 College Avenue, Davie, Florida 33314, USA. st population of the Ameriil ( ocodylus acutus) oc urs on the e tip of the Florida peninsula, e of this tropical species has alli ite , probably by temperature ( s la and az ot i, 1989). Early iles in Florida are notices and its occurrence ( yman, 1870; ; S ith, 1896; Barbour, 1923); ) ided the first detailed ac ount . e (1978) documented aspects i iolog . ore recently, Gaby et te on the population biology er of crocodiles living in a powli s ste . ess: epart ent of Wildlife and s, iversity of Florida, 3245 College Many crocodilian populations are endang red because their effective population sizes h ve been reduced by human-related factors such as as hunting and habitat loss. Small poplations, particularly when isolated, risk extirpation because of the action of natural or artificial forces that erode their numbers. The Florida population of the American crocodile is thought to be of limited size (Ogden, 1978), and therefore susceptible to chance and human inerference. As a result it has long been considred to be endangered (Barbour, 1923; Hines et al., 1984). Unfortunately, limited information has inhibited understanding of its population biology, and therefore of its true status or appropriate conservation needs. In this paper we discuss the population biology of the the American crocodile based on a five-year study of the population in the core of its northernmost range, northeastern Florida crocodilian populations are endanbecause their e fective opulation sizes een reduced by human-related factors as as hunting and habi at loss. Small popi s, particularly when isolated, risk extiri because of the action of natural o arti7 This content downloaded from 157.55.39.162 on Thu, 11 Aug 2016 05:12:47 UTC All use subject to http://about.jstor.org/terms J. A. KUSHLAN AND F. J. MAZZOTTI Bay. Where appropriate, we compare information from studies conducted simultaneously and cooperatively with our own on other segments of the population (Gaby et al., 1975; P. Moler, pers. comm.). Other aspects of our overall study program are discussed elsewhere (Behler, 1978; Hall et al., 1979; Dunson, 1982; Lutz and Dunbar-Cooper, 1984; Stoneburner and Kushlan, 1984; Kushlan, 1988; Mazzotti et al., 1988; Kushlan and Mazzotti, 1989).

A new parameterization for estimating co‐occurrence of interacting species

Models currently used to estimate patterns of species co-occurrence while accounting for errors in detection of species can be difficult to fit when the effects of covariates on species occurrence probabilities are included. The source of the estimation problems is the particular parameterization used to specify species co-occurrence probability. We develop a new parameterization for estimating patterns of co-occurrence of interacting species that allows the effects of covariates to be specified quite naturally without estimation problems. In our model, the occurrence of one species is assumed to depend on the occurrence of another, but the occurrence of the second species is not assumed to depend on the presence of the first species. This pattern of co-occurrence, wherein one species is dominant and the other is subordinate, can be produced by several types of ecological interactions (predator-prey, parasitism, and so on). A simulation study demonstrated that estimates of species occurrence probabilities were unbiased in samples of 50-100 locations and three surveys per location, provided species are easily detected (probability of detection > or = 0.5). Higher sample sizes (>200 locations) are needed to achieve unbiasedness when species are more difficult to detect. An analysis of data from treefrog surveys in southern Florida indicated that the occurrence of Cuban treefrogs, an invasive predator species, was highest near the point of its introduction and declined with distance from that location. Sites occupied by Cuban treefrogs were 9.0 times less likely to contain green treefrogs and 15.7 times less likely to contain squirrel treefrogs compared to sites without Cuban treefrogs. The detection probabilities of native treefrog species did not depend on the presence of Cuban treefrogs, suggesting that the native treefrog species are naive to the introduced species.

BIRDS CONSUMED BY THE INVASIVE BURMESE PYTHON (PYTHON MOLURUS BIVITTATUS) IN EVERGLADES NATIONAL PARK, FLORIDA, USA

Abstract We identified 25 species of birds representing nine avian Orders from remains in digestive tracts of 85 Burmese pythons (Python molurus bivittatus) collected in Everglades National Park, Florida, USA, from 2003 to 2008. Four species of birds identified in this study are of special concern in Florida and a fifth, the Wood Stork (Mycteria americana), is listed as federally endangered. This represents the first detailed analysis of the avian component of the diet of the introduced Burmese python, now established in Everglades National Park, Florida and highlights the potential for considerable negative impact of this invasive species on native bird populations.

STRUCTURE OF EVERGLADES ALLIGATOR HOLES

Alligators create disturbance patches within the Everglades, but little is known about spatial and temporal variation in alligator holes throughout this ecosystem. Forty-eight alligator holes in Water Conservation Area 3 of the central Everglades were sampled to assess variation in morphology and vegetation and to identify landscape- and habitat-level features that correlate to alligator-hole variation. Field observations and plant community ordination suggested three distinct categories of alligator holes: holes containing shrub/tree species (shrub holes), holes surrounded by marsh (marsh holes), and holes dominated by cattails (cattail holes). Shrub holes were the most species-rich and diverse of the three types. All alligator holes except for cattail holes contained a greater richness of vascular plants than the surrounding marsh. Cattail holes were deeper than shrub or marsh holes, and all holes were deeper than surrounding marsh. Major landscape features that may influence structure of alligator holes seem to be distance from canals and surrounding marsh vegetation matrix. Alligator holes increase spatial heterogeneity, influence plant community composition and structure, and increase biological diversity. Restoration and management plans for the Everglades ecosystem should ensure that this landscape feature is perpetuated.

Factors affecting individual foraging specialization and temporal diet stability across the range of a large “generalist” apex predator

Individual niche specialization (INS) is increasingly recognized as an important component of ecological and evolutionary dynamics. However, most studies that have investigated INS have focused on the effects of niche width and inter- and intraspecific competition on INS in small-bodied species for short time periods, with less attention paid to INS in large-bodied reptilian predators and the effects of available prey types on INS. We investigated the prevalence, causes, and consequences of INS in foraging behaviors across different populations of American alligators (Alligator mississippiensis), the dominant aquatic apex predator across the southeast US, using stomach contents and stable isotopes. Gut contents revealed that, over the short term, although alligator populations occupied wide ranges of the INS spectrum, general patterns were apparent. Alligator populations inhabiting lakes exhibited lower INS than coastal populations, likely driven by variation in habitat type and available prey types. Stable isotopes revealed that over longer time spans alligators exhibited remarkably consistent use of variable mixtures of carbon pools (e.g., marine and freshwater food webs). We conclude that INS in large-bodied reptilian predator populations is likely affected by variation in available prey types and habitat heterogeneity, and that INS should be incorporated into management strategies to efficiently meet intended goals. Also, ecological models, which typically do not consider behavioral variability, should include INS to increase model realism and applicability.

A scientific framework for managing urban natural areas

Abstract Due to the natural population growth and the influx of population from other areas of the county, South Florida is experiencing rapid growth. Meeting the challenge of conserving regional ecological integrity in urban and urbanizing landscapes will depend on the development of ecological reserve systems. In southeastern Florida this means managing fragmented, isolated, and frequently disturbed habitat patches for both the conservation of ecological integrity and human enjoyment. A science-based natural area management plan provides a foundation for making the best decisions possible, and the flexibility of modifying them, and fosters public confidence in the process. The scientific framework employed to develop resource management plans for Environmentally Sensitive Lands in Broward County, FL, includes setting goals and objectives based on public involvement through a scoping process and resource inventories and evaluations. Management plans are developed for each site that identify significant resources, threats, restoration potential, and public use alternatives. Management plans are viewed as hypotheses of ecosystem response, and monitoring programs as experiments designed to test them. Management plans should include provisions for modification over time as more is learned about the actual ecosystem response.

Ecology of a Population of Crocodylus acutus at a Power Plant Site in Florida

This paper presents the status and ecology of a population of Crocodylus acutus associated with the Turkey Point power plant in Florida. Distribution, habitat preference, repro- duction, population size and structure, and recruitment and dispersal are discussed. Nesting occurs on spoil berms in the cooling canal system. The Turkey Point population con- tributes 10% of the annual production of hatchlings in southern Florida. Crocodiles have exploited previously unavailable, man-altered nesting habitat in approximately 4 years. The resident population consists of a minimum of 19 adult, subadult and juvenile crocodiles. This population exhibits differential habitat preferences according to size class, and shows seasonal changes in distribution. Salinity regime is a factor in these trends. Ecology and population structure are similar to those of the population residing in the more pristine habitat of Everglades National Park. Long distance dispersal and recruitment into the Turkey Point population are documented. The long-term outlook for the population in this man-altered habitat is for growth and expansion.

American Crocodile (Crocodylus Acutus) in Florida: Recommendations for Endangered Species Recovery and Ecosystem Restoration

Abstract When the American Crocodile (Crocodylus acutus) was declared endangered in 1975, scant data were available for making management decisions. Results of intensive studies conducted during the late 1970s and early 1980s by the National Park Service, Florida Game and Fresh Water Fish Commission, and Florida Power and Light Company resulted in an optimistic outlook for crocodiles. However, new issues face crocodiles today. Florida and Biscayne bays have undergone changes that have caused concern for the health of these ecosystems. The purpose of this paper is to review results of monitoring programs for C. acutus that have been used as a basis for consideration of reclassification of this endangered species and for restoration of its endangered ecosystem. More crocodiles and nests occur in more places today than in 1975. The maximum number of nesting females in Florida has increased from 20 in 1975 to 85 in 2004, and the number of concentrations of nesting effort from two to four. This evidence supports the proposed reclassification of the American Crocodile from endangered to threatened. However, crocodiles are still threatened by modification of habitat because of development adjacent to crocodile habitat and will benefit from restored freshwater flow into estuaries. As crocodiles continue to increase in number and expand into new areas, interactions with humans will occur more frequently. The challenge of integrating a recovering population of the American Crocodile with an ever-increasing use of coastal areas by humans will be the final challenge in successful recovery of this once critically endangered species.

Modeling the Effect of Toe Clipping on Treefrog Survival: Beyond the Return Rate

Abstract Some studies have described a negative effect of toe clipping on return rates of marked anurans, but the return rate is limited in that it does not account for heterogeneity of capture probabilities. We used open population mark-recapture models to estimate both apparent survival (φ) and the recapture probability (p) of two treefrog species individually marked by clipping 2–4 toes. We used information-theoretic model selection to examine the effect of toe clipping on survival while accounting for variation in capture probability. The model selection results indicate strong support for an effect of toe clipping on survival of Green Treefrogs (Hyla cinerea) and only limited support for an effect of toe clipping on capture probability. We estimate there was a mean absolute decrease in survival of 5.02% and 11.16% for Green Treefrogs with three and four toes removed, respectively, compared to individuals with just two toes removed. Results for Squirrel Treefrogs (Hyla squirella) indicate little support for an effect of toe clipping on survival but may indicate some support for a negative effect on capture probability. We believe that the return rate alone should not be used to examine survival of marked animals because constant capture probability must be assumed, and our examples demonstrate how capture probability may vary over time and among groups. Mark-recapture models provide a method for estimating the effect of toe clipping on anuran survival in situations where unique marks are applied.