Lower Keys Marsh Rabbit

Sylvilagus palustris hefneri

Description

Lower Keys marsh rabbits, Sylvilagus palustris hefneri, are small to medium-sized rabbits, with short, dark brown fur and a grayish-white belly. Their feet are small and their tails are dark brown and inconspicuous. Male and female marsh rabbits do not appear to differ measurably in size or color. Lower Keys marsh rabbits are about 12.5-15 in (32-38 cm) in length with a weight of 35-50 oz (1,000-1,400 g). Their hindfeet measure 2.6-3.1 in (6.5-8 cm) and their ears are 1.8-2.4 in (4.5-6.2 cm) in length.

This marsh rabbit differs from mainland (S. p. palustris ) and Upper Keys marsh rabbits (S. p. paludicola ) in several cranial characteristics. The Lower Keys marsh rabbit has a shorter molariform tooth row, higher and more convex frontonasal profile, broader cranium, and elongated dentary symphysis. They are also different in the extent and ornateness of the dorsal skull sculpture. The Lower Keys marsh rabbit is also the smallest of the three marsh rabbit subspecies and is distinguished from other marsh rabbits by its dark fur.

A separate species of marsh rabbit (S. palustris ) in the Lower Florida Keys was first noted in 1952 with additional sightings and scat records. The Lower Keys marsh rabbit (S. p. hefneri ) was recognized as a distinct subspecies in 1984 based on an examination of specimens collected from Lower Sugarloaf Key, Monroe County, Florida. S. p. hefneri is the most recently described of the three sub-species of marsh rabbit. The new subspecies was named in honor of Hugh M. Hefner in recognition of the financial support received by his corporation.

Behavior

Adult Lower Keys marsh rabbits of the same sex do not have overlapping home ranges, and may display territorial behavior if another adult enters their home ranges. The home ranges of these marsh rabbits average 0.7 acres (0.3 hectares). Adult marsh rabbits have permanent home ranges, while male subadults tend to disperse. Adults of both sexes have similar home range sizes, although the size varies widely among individuals. This individual variability may be due to differences in habitat quality, population density, or the status of an individual in a social hierarchy. Juvenile Lower Keys marsh rabbits appear to use a home range near their nest site.

Lower Keys marsh rabbits usually travel through a variety of habitats between their natal and permanent home ranges including areas with dense ground cover, through mangroves, upland hard-wood hammocks, and in vegetation between the road shoulder and the water. Marsh rabbits are good swimmers and will swim when pursued. Dispersing marsh rabbits suffer high mortalities, particularly when there is a lack of habitat between populations or when there are roads to cross. Dispersing Lower Keys marsh rabbits travel up to 1.2 mi (2 km) from their nests, expanding their home ranges with time. This species appears to be chiefly nocturnal, although they can be active on cloudy days and when they are protected by dense cover.

The Lower Keys marsh rabbit, with its small body size, short life span, high reproductive output, and high habitat specificity, exhibits classic metapopulation community dynamics. There are 40 subpopulations of rabbits that occur in small disjunct patches of habitat on four keys. Rabbits living in these habitat patches are socially isolated from other patches but interact through dispersal. Distance among habitats is important because the ability of rabbits to recolonize vacant habitat patches depends upon the presence of habitat corridors. These habitat patches occur in a highly fragmented mosaic of native and disturbed habitat, with few contiguous areas of native habitat greater than 12.5 acres (5 hectares). Random population fluctuation is evident in marsh rabbit populations; several populations were so small and contained so few individuals of the same sex that they eventually became extirpated. The marsh rabbit population is estimated to contain approximately 100-300 individuals. More than two-thirds of the habitat identified in the Lower Keys is currently below carrying capacity.

Both sexes of marsh rabbits begin to sexually mature at about nine months of age. During this time, the majority of the males disperse. Sexually maturing females do not appear to disperse. Similar to other subspecies of marsh rabbits, the Lower Keys marsh rabbit is polygamous and breeds year round. Initial results from a study of 24 rabbits from five populations indicates that all females breed and only a portion of the males breed.

Lower Keys marsh rabbits do not display an apparent seasonal breeding pattern. Nevertheless, the highest proportion of females with litters occur in March and September, the lowest in April and December. The average number of litters produced during the wet and dry seasons do not differ significantly. Other species of marsh rabbits breed year round, but seasonal patterns are more evident. In South Florida, other marsh rabbit pregnancy rates are usually lower from September through December and higher from February through June. Higher anestrous or infertile periods are also evident from mid-October through mid-March, although anestrous females are present in every month. A large enough proportion of fertile males are able to breed year round. During a breeding season, marsh rabbit males become ready to breed just prior to females, whose breeding may be induced by male behavior. The number of fertile males decreases one month prior to female pregnancy.

Some female marsh rabbits in south Florida may be continuously pregnant and could potentially produce 10-12 litters per year, although this high rate of productivity is rare. Usually, 75% of female marsh rabbits in south Florida are pregnant during the height of the breeding season. Although no estimate is available for Lower Keys marsh rabbits, the average gestation period of marsh rabbits from mainland Florida is 30-37 days, with an average of 5.5-7 litters per year.

The Lower Keys marsh rabbit may be less fecund than other marsh rabbits. Marsh rabbits in mainland South Florida can produce 14-18 young per female per litter, while only one to three young have been observed per nest for Lower Keys marsh rabbits. An average of 3.7 litters per year has been reported for Lower Keys marsh rabbits; which indicates a much lower fecundity rate than for marsh rabbits in southern Florida. Some marsh rabbits experience total litter resorption that can affect their reproductive output. The loss of these ovulated ova can be related to maternal physiological changes in response to stressful events such as overcrowding. It is not yet known if such stresses cause total litter resorption in Lower Keys marsh rabbits, but with the continual loss of habitat, Lower Keys marsh rabbits may experience similar problems.

A population viability analysis (PVA) conducted for the Lower Keys marsh rabbit predicted that this species would go extinct in 20-30 years under the existing conditions. Although the PVA did not evaluate the effects of any increases in the threats, the U. S. Fish and Wildlife Service (FWS) expects that such increases would only accelerate the extinction of the Lower Keys marsh rabbit. When different management scenarios were included in the model, the persistence of the Lower Keys marsh rabbit was extended to 50 years if all predation by cats was removed. Persistence was not extended appreciably if all road mortality was removed or reintroductions into vacant patches were conducted. The PVA did not assess whether habitat restoration, introductions into occupied habitats, or a combination of management activities would change persistence rates. Considering the desperate condition of the Lower Keys marsh rabbit, the continued degradation of its habitat and predation by cats are likely to push the marsh rabbit towards extinction.

Marsh rabbits and other species of rabbits feed throughout the year on a variety of vegetation. Marsh rabbits do show a preference for particular species, but this is not based on seasonal changes. The climate and vegetation in the Lower Keys are relatively stable coinciding with the marsh rabbits invariant diet.

Marsh rabbits eat vegetation in proportion to its abundance. The most important food species for the Lower Keys marsh rabbit appears to be Borrichia frutescens, which is common in the mid-saltmarsh area. This species spends most of its time feeding in the mid-and high-marsh areas. Rabbits have been seen foraging on a variety of grass, sedge, shrub, and tree species, but have not been seen eating tree leaves or bark. Lower Keys marsh rabbits feed on at least 19 different plant species, representing 14 families. The most abundant species in the rabbit's diet include Sporobolus virginicus, Salicornia virginica, Spartina spartinae, Borrichia frutescens, Rhizophora mangle, and Laguncularia racemosa. Dietary habits are not affected by sex or season.

Based on their distribution, Lower Keys marsh rabbits appear to need only a little fresh water to survive. In a study of several mammals from the Lower Florida Keys, the Lower Keys marsh rabbit was found to have one of the highest capacities to concentrate urine. Although further study is warranted, Lower Keys marsh rabbits may be able to survive solely on dew and brackish water. Lower Keys marsh rabbits probably cannot use seawater to meet their need for water; even black rats, the most salt-tolerant mammal in the study, cannot maintain its body mass on seawater.

The ecological and physical characteristics of the Lower Keys provide a unique ecosystem for species to live. Many of the endemic species in the Lower Keys depend upon similar or adjacent habitats. The Lower Keys marsh rabbit occupies habitat that overlaps with that of other listed endemic species, such as the endangered Key deer, Key tree-cactus, and silver rice rat. Marsh rabbits and silver rice rats utilize similar vegetation in salt marshes, transitional areas, and freshwater marshes. Coastal berm areas on Long Beach and Sugarloaf Beach are used by marsh rabbits, as well as by Key deer who use these same areas for bedding and fawning. Sugarloaf Beach is also used as nesting habitat by threatened Atlantic loggerhead and endangered green sea turtles. The state-listed threatened white-crowned pigeon and other bird species also feed along coastal berm areas and within forested marsh areas.

Many endemic species, like those in the Lower Keys, have evolved in an environment with reduced levels of competition, predation, and disease, and are thus more susceptible to extinction. Endemic species are also more vulnerable to extinction due to loss of habitat and must balance a dynamic equilibrium between processes of immigration and extinction in order to survive. This equilibrium is contingent upon the habitat itself and the species present. The survival of endemic species in the Lower Keys, as well as other species, is dependent upon the integrity and health of their habitat.

Habitat

In general, other subspecies of S. palustris are typically found in saltmarsh areas of slightly higher elevation, such as ridges or islands. They are also found along freshwater bordered by hammocks and flatwoods. Normally, marsh rabbits are restricted to relatively undisturbed wetlands.

The Lower Keys marsh rabbit is habitat specific, depending upon a transition zone of grasses and sedges for feeding, shelter, and nesting. This species primarily occurs in the grassy marshes and prairies of the Lower Keys, which are transitional areas that are similar in form and species composition to communities interspersed throughout mangrove forests of mainland Florida. These wetland communities lie in the middle of the salinity gradient in the Lower Keys. Key vegetative species include grasses and shrubs, succulent herbs, sedges, and sparse tree cover. Lower Keys marsh rabbits also use marshes at the freshwater end of this salinity gradient. Freshwater marsh areas are dominated by sedges such as sawgrass, with succulent herbs such as seashore dropseed and grasses such as cordgrass. Freshwater is found in the interior of only a few islands, primarily in the Lower Keys. During the wet season these areas can accumulate standing water.

Marsh rabbits also use coastal beach berm habitat, a relatively rare habitat consisting of a vegetated high ridge of storm-deposited sand and shell. Coastal berms are vegetated with more than 84 plant species including blolly, gumbo limbo, poisonwood, seagrape, and Spanish stopper.

Freshwater marsh and costal berm habitats are relatively rare in the Lower Keys. Both freshwater and saltwater marshes are limited because man-groves occupy coastal areas and interior freshwater habitat is scarce.

Lower Keys marsh rabbits prefer areas with higher amounts of clump grass, ground cover, and Borrichia frutescens present, areas closer to other existing marsh rabbit populations, and areas closer to large bodies of water. These marsh rabbits spend most of their time in the mid-marsh and high-marsh, both of which are used for cover and foraging, while most nesting occurs in the high-marsh area. Lower Keys marsh rabbits occasionally use low shrub marshes and mangrove communities for feeding and as a corridor between patches of transitional habitats. In brackish habitats, the two plant species that are most important to the Lower Keys marsh rabbit for cover and nesting are cordgrass and saltmarsh fimbristylis, both of which are thick, abundant grasses. In freshwater wetlands, the Lower Keys marsh rabbit may use sawgrass for the same purpose. Not much is known about how marsh rabbits use vegetation in coastal berm areas.

Distribution

Marsh rabbits are found throughout southeastern North America. S. p. palustris is found from southeastern Virginia south to the Georgia-Florida border. S. p. paludicola is found from the Georgia-Florida border south of the Upper Keys. Lower Keys marsh rabbits were first reported from Key West in 1877. The Lower Keys marsh rabbit's original range extended from Big Pine Key to Key West encompassing a linear distance of about 30 mi (48 km). Historically, Lower Keys marsh rabbits probably occurred on all of the Lower Keys that supported suitable habitat, but did not occur east of Seven-Mile Bridge where the species is replaced by S. p. paludicola.

In 1995 a comprehensive survey for Lower Keys marsh rabbits located 81 areas, comprising a total of 783 acres (317 hectares), that provided suitable habitat. Lower Keys marsh rabbits have been recorded at 50 of these 81 areas. The majority of these areas of suitable habitat are smaller than 7.4 acres (3 hectares) and the total amount of habitat occupied by the Lower Keys marsh rabbit is about 625 acres (253 hectares). Lower Keys marsh rabbits have been found on only a few of the larger Lower Keys and the small islands near these keys. There is a large gap in the distribution of Lower Keys marsh rabbits from Cudjoe Key to the Torch Keys.

Threats

Threats to the Lower Keys marsh rabbit include habitat loss and fragmentation, predation by cats, and road mortality caused by automobiles; critical habitat was not designated. Although once abundant on many of the Lower Keys including Key West, habitat destruction has limited this marsh rabbit to small populations on a few keys. Population estimates range between 100 and 300 rabbits in the Lower Florida Keys. The status of the Lower Keys marsh rabbit is considered to be declining. In 1991, there was a high of 300 individuals and by 1993, the population decreased to only 100 individuals. The marsh rabbit population was higher before 1991. Approximately one-third of the total marsh rabbit habitat is owned by the U. S. Department of Defense, one-third is part of the FWS-managed National Key Deer Refuge and Great White Heron National Wildlife Refuge, and the remaining one-third is privately owned.

The Lower Keys marsh rabbit is vulnerable to habitat alteration, contaminants, vehicular traffic, dumping, poaching, domestic animals, feral hogs, fire ants, and exotic vegetation. These threats have resulted in a decrease in the number of populations, a decline in the individuals in those populations, the isolation of populations, an increase in road mortalities, the increase in feral cat-caused mortality, and the loss of foraging, sheltering, and nesting habitat. All of these threats have disrupted the equilibrium between the Lower Keys marsh rabbit's environment and its survival.

The Lower Keys marsh rabbit occurs in small, disjunct populations whose survival depends on the emigration and dispersal of individuals. In order to persist, the emigration rates of the Lower Keys marsh rabbit have to be equal to or greater than the death rates. This subspecies is thought to be less fecund than others, making it more susceptible to demographic and stochastic events. Since breeding occurs year round, urbanization has affected the Lower Keys marsh rabbit reproductive potential. In addition to natural threats, several of the threats resulting from these developments have reduced reproductive potential, including direct mortality and disruption of dispersal. With the lower potential for interchange between subpopulations, the probability of persistence has been decreased substantially.

This marsh rabbit is habitat specific, depending upon a transition zone of grasses and sedges for feeding, shelter, and nesting. Without these important habitat elements, the survival of the Lower Keys marsh rabbit is drastically reduced. In the late twentieth century, the habitat consisted of a mosaic of small native and disturbed habitat patches. In the two years between the study for the Lower Keys marsh rabbit's listing and the actual listing, four of the 15 original sites used in the listing were destroyed. At the end of the twentieth century, the majority of the sites that remained were isolated from each other by urbanized areas, and population interchange seemed unlikely. Few of the contiguous areas remaining were larger than 12.5 acres (5 hectares). Only 81 patches of Lower Keys marsh rabbit habitat remained; of these, 39% were privately owned and vulnerable to urbanization. Only 50 of these 81 patches had rabbits present.

In the last two decades of the twentieth century, more than half the area of suitable Lower Keys marsh rabbit habitat was destroyed for construction of residential housing, commercial facilities, utility lines, roads, or other infrastructure in the Lower Keys. Most of the remaining suitable habitat had been degraded by exotic invasive plants, repeated mowing, dumping of trash, and off-road vehicle use. Urbanization fragmented the sites occupied by the marsh rabbits and eliminated many of the corridors that allowed marsh rabbits to move from one site to another. In larger, urbanized areas where the vegetation has been mowed, dispersing marsh rabbits have no cover from predation. In general, residential and commercial activities in the Keys have affected the Lower Keys marsh rabbit by: increasing the number of residences, increasing habitat alteration, increasing species mortality, interfering with reproduction, and decreasing the water quality. These actions have appreciably reduced the likelihood of this species' survival and recovery in the wild.

Because the Lower Keys marsh rabbit exhibits classic metapopulation dynamics, it relies on the re-colonization of vacant habitat patches for survival. Subpopulations in habitat patches are vulnerable to extinction, but vacant habitat patches have the potential to be recolonized by dispersing rabbits. Those sites that are not occupied are just as vulnerable as occupied sites and are important for future dispersal and recovery. The potential for recolonization has been decreased or eliminated because of habitat loss or fragmentation at both occupied and unoccupied sites.

Habitat alteration is the most significant development that prevents this species from returning to its natural state. Continued habitat fragmentation hinders the probability of successful recolonization due to the isolated natured of the habitat, increased road mortality, and cat-caused deaths. Urbanization has isolated subpopulations, and interchange between the majority of the sites is unlikely. Adult territories do not overlap; therefore the Lower Keys marsh rabbit may be forced to have smaller territories if habitat is continually fragmented. If urbanization proceeds, habitat will continue to be fragmented and dispersal and migration will be hindered. The minimum habitat size considered suitable to support the Lower Keys marsh rabbit is based on the minimum home range size of 0.7 acres (0.3 hectares). The destruction and fragmentation of habitat may result in habitat patches that are too small to support subpopulations of the Lower Keys marsh rabbit. For example, five occupied habitat patches located on isolated islands without cat predation were determined not large enough to support viable, long-term populations of this species.

Although, habitat loss is responsible for the original decline of the Lower Keys marsh rabbit, high mortality from cats may be the greatest threat to the persistence of the Lower Keys marsh rabbit. A detailed study of cat diets in the Keys has not been conducted, but rabbits were the largest component of feral cat diets in several studies that have been conducted elsewhere. Even though the exact extent cannot be determined, the number of cats present in the Lower Keys increased during the late twentieth century, along with the increase in the residential population. Cats are responsible for both juvenile and adult mortality. Lower Keys marsh rabbits appear to be equally susceptible to cat predation, regardless of gender or age. In the late 1990s, 14 of 19 occupied patches had domestic and feral cats present.

As urbanization increased during the last two decades of the twentieth century, construction of new roads, or the improvement of existing roads, has been necessary to accommodate more vehicles. The construction of roads results in two main threats to the Lower Keys marsh rabbit: interference with dispersal and increased road mortality. Vehicular traffic interferes with dispersal and may prevent essential interchange between subpopulations. Dispersing males are the most vulnerable to road mortality. Dispersal is responsible for repopulating sites that have been extirpated. Since only a portion of the males breed during the year, the loss of these males may lower the likelihood of mating and hence decrease the reproductive potential. The threat of roads and vehicular traffic has increased in significance because of the magnitude of habitat fragmentation: the size of the remaining habitat fragments forces more adult males to disperse in order to establish territories, putting them at a greater risk of being killed by cars.

A significant portion of the remaining population of Lower Keys marsh rabbits is found on the U. S. Navy's Naval Air Station (NAS), Key West. Four Lower Keys marsh rabbit road kills had been reported on NAS, Key West, between 1992 and 1994. This represents only those animals that have been recovered; it is reasonable to assume that others were never recorded. Most Lower Keys marsh rabbits are killed by vehicles during the rabbit's most active period between dusk and dawn. Off-road vehicular activities also affect the Lower Keys marsh rabbit through habitat degradation and direct mortality. At least one animal has been killed by an off-road vehicle on NAS, Key West. The amount of road mortality has not been determined for other areas in the Keys, but marsh rabbits may experience the same mortality as on NAS, Key West.

The Lower Keys marsh rabbit may be exposed to pesticides used in marsh habitat. They may also come in contact with poisons used to control black rats. These contaminants can either be ingested while foraging on plants or drinking water. In a 1993 biological opinion, the FWS investigated the effects of vertebrate control agents on endangered and threatened species and determined that several chemicals would jeopardize the continued existence of the Lower Keys marsh rabbit. This conclusion was based on the already endangered status of this species, the lethality of certain chemicals, and the high probability this species would encounter the chemical. Chemicals, such as Pival, a rodenticide used to kill rats, are lethal to rabbits if ingested. Given that the majority of occupied habitat is adjacent to urbanized areas, and that urbanization continues to expand into their habitat, chemicals may severely threaten the remaining small population.

Other human-related effects include contamination, dumping, poaching, feral hogs, and fire ants. Increased nutrients from septic tanks and fertilizers degrade water quality in habitat of the Lower Keys marsh rabbit. Illegal dumping deteriorates habitat and allows the invasion of exotic plants and animals to occur. Poaching has decreased, although it still occurs infrequently. Feral hogs destroy Lower Keys marsh rabbit habitat while foraging, but the extent of the impact has not been analyzed. Fire ants have been increasing in marsh habitat and pose a threat to newborns. These human-induced effects threatened the Lower Keys marsh rabbit in the late twentieth century, but to a lesser degree than habitat loss and feral cat predation.

Since the status of the Lower Keys marsh rabbit is declining, urbanization is predicted to have adverse effects that are likely to drive this species to extinction because of its narrow range and distribution, habitat specificity, classic metapopulation community dynamics that rely on dispersal, and low recovery potential. The Lower Keys marsh rabbit is a very sensitive species that is naturally vulnerable to stochastic and deterministic events and continues to exist in an endangered condition. Increasing human impacts to natural occurring events only reduces its likelihood of survival.

Conservation and Recovery

To alleviate negative effects on the marsh rabbit at NAS, Key West, the FWS consulted with the U. S. Navy in 1993 concerning marsh rabbit road mortalities and mowing activities on the base. Several actions have been initiated by the Navy in an attempt to reduce these effects, including the posting of "no mowing" signs in important rabbit habitat, the fencing of some rabbit habitat to prevent illegal vehicle traffic, the removal of some exotics, and the elimination of the feral cat population on the installation.

In April 1996, the Marsh Rabbit Recovery Team established four main recovery objectives to prevent the extinction of the Lower Keys marsh rabbit. These include the acquisition of suitable habitat with an upland buffer, the control of predation by feral and domestic cats, the monitoring of existing populations, and trial reintroduction of rabbits to unoccupied suitable habitat.

The Lower Keys marsh rabbit's recovery potential is quite low due to the lack of available habitat and increased mortality due to cats and vehicular traffic. Because urbanization affected both occupied and unoccupied sites during the 1980s and 1990s, not only is survival affected, but the opportunity for natural or managed recovery has been precluded in some areas.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

U. S. Fish and Wildlife Service
Jacksonville Ecological Services Field Office
6620 Southpoint Dr. South, Suite 310
Jacksonville, Florida 32216-0958
Telephone: (904) 232-2580
Fax: (904) 232-2404
http://www.fws.gov/r4jafl/

References

Howe, S. E. 1988. "Lower Keys Rabbit Status Survey." U. S. Fish and Wildlife Service, Jacksonville, Florida.

Humphrey, S. R., ed. 1992. Rare and Endangered Biota of Florida: Mammals. 2d ed. University of Florida Press, Gainesville.

Lazell, J. D., Jr. 1984. "A New Marsh Rabbit (Sylvilagus palustris ) from Florida's Lower Keys." Journal of Mammology 65 (1): 26-33.