Around the world, habitat for snakes is eroding. Sometimes habitat is destroyed outright to make way for human dwellings or industry. Other times, habitat is contaminated by pesticides or snakes are crowded out by the introduction of exotic animal species. Regardless of the source of habitat loss, the result is always the same: snake populations decline as a result. Biologists were slow to notice the decline in snake numbers, but as early as 1975 biologist Robert H. Mount expressed concern for the survival of some snake species: "Much of the detrimental habitat manipulation is the result of the increasing human population and its demands on the natural environment. Extensive land clearing, whether for housing, industry, mineral exploration or agriculture, takes a heavy toll." In the United States, more than half of the snake species currently listed as endangered have declined as a direct result of habitat destruction.
Snakes that depend on wetlands, streams, or unique coastal habitats cannot live just anyplace but depend on areas with the unique conditions necessary for their survival. Human alteration of such areas to create agricultural fields or housing developments has taken a heavy toll on many snake species around the world.
For instance, some snakes, such as the gulf salt marsh snake and the Atlantic salt marsh snake, have become increasingly threatened with the dwindling of their highly specialized habitat, marshes and swamps located near where rivers empty into the ocean. Unlike snakes that can live in freshwater, these snakes have adapted to life in a primarily saltwater environment. Developers who drain marshes to make room for homesites near the ocean threaten them with extinction.
The copperbelly water snake of the American Midwest also requires very specific and increasingly rare wetland habitat. Though in the winter they hibernate in dry upland areas and spend much of the late summer and fall in woodland areas, copperbelly water snakes spend a few crucial months of the spring and early summer in woodland swamps, which provide them a place to mate and find their prey, which consists of frogs, tadpoles, and fish. Originally, conversion of wetland habitat to agricultural areas reduced the snake's numbers. But since the mid-1980s, the greater danger to the snake is modifications of its habitat such as dredging, coal mining, stream channelization, and road construction, in addition to commercial and residential development. The increased sedimentation associated with residential construction also fills in the snake's remaining wetland habitat, choking out the vegetative cover that the snake hides in and killing off the populations of amphibians and fish that the snake relies on for food.
Copperbelly water snakes represent a kind of indicator species that signals humans about the health of the entire wetland ecosystem. As a predator, the copperbelly lives at the top of the food chain and acts as a gauge of the health of the whole ecosystem it inhabits. If the copperbelly is in trouble, then chances are the entire wetland ecosystem is in trouble as well.
Agriculture and livestock grazing have also taken a heavy toll on snakes. Around the world, large tracts of land have been cleared, burned, plowed, and mowed, leaving many snakes with radically altered habitats. In many countries, fires are set to promote growth of grazing vegetation for vast herds of domestic animals. The fires have either killed snakes outright or left them without adequate prey or vegetative cover. For example, on the Indian Ocean island of Madagascar, the Madagascar ground boa annually loses more acres of its already limited habitat to such controlled fires. The species has been listed as endangered since 1977.
Though sometimes snake species can adapt to such altered habitats, rarely are they able to thrive there. Massasauga rattlesnakes are one good example of the problem. They are barely holding on in the southwestern United States because their grassland habitat has been converted to agricultural use. In some areas, overgrazing has caused mesquite thickets to grow unchecked, choking out native grasses needed by the massasaugas as cover. Because their vegetative habitat has been largely destroyed, massasaugas have had to adapt to living in the dens of banner-tailed kangaroo rats, themselves an endangered species. The dens represent a microhabitat that changes little in temperature from season to season and affords them plentiful pocket mice, lizards, and giant centipedes that enter the holes. Conservationists hope that if the kangaroo rats and their habitat can be protected, the massasauga will benefit as a result.
Forest destruction, whether to promote agriculture or for logging, can cause a number of problems for snake species, the most serious one being loss of populations with adequate genetic diversity. Forest destruction results in the long-term problem of what herpetologist Kenneth Dodd calls "the fragmentation of remaining population size and the resulting potential loss of genetic diversity." As habitats are fragmented by logging, potential breeding snakes are unable able to reach each other. As the available number of breeding snakes grows smaller and more isolated, inbreeding can occur, resulting in offspring with deformities, lower resistance to disease, and myriad other genetic problems. One suggested solution to this problem of habitat fragmentation has been to preserve habitat corridors connecting the otherwise isolated patches of forest, theoretically allowing for movement from one protected area to another. However, herpetologists such as Dodd argue that snakes "will not likely abandon preferred habitats to move between isolated habitat patches."
Other snakes are dependent on the trees themselves. For example, the Round Island keel-scaled boa, an arboreal snake, relies on the trees for both resting and hunting. Also, the trees protect it from attack by predatory mammals. These snakes must come down to the ground to mate, and without the cover of surrounding trees and dead and decaying trees on the forest floor, they are vulnerable to predation by other animals as well as attack or capture by humans.
Many snakes depend on trees for protection from the scorching heat of the sun. In India, for instance, massive cutting of the scrub forest in favor of agriculture has caused the Indian python to decline in huge numbers. Not only has the destruction of the scrub forest reduced the populations of the python's small tree-dwelling mammal prey, it has also left the snake without shade trees for taking refuge during the hottest part of the day.
Sometimes, well-meaning efforts at conservation can be harmful if not carefully planned. The threatened broad-headed snake of Australia depends on gray gum trees in the heavily logged forests adjacent to its rocky habitat on the sandstone plateaus from Newcastle to Nowra. During the hot Australian summers, the snake moves away from its typical habitat of sandstone outcroppings into the woodlands, where it seeks out large gray gum trees with hollows in their trunks. Researchers have collected evidence from people who have seen the snakes migrate from the rocks to the trees and have also tracked the snakes using radio telemetry to confirm that they indeed depend on such tree hollows. The standard forestry practice of removing dead or overly mature trees, the ones likely to have the hollows that the snakes prefer, is diminishing summer habitat for the snakes. The study revealed that forestry practices have indeed impacted the broad-headed snake. Scientists have determined that foresters need to leave many such habitat trees in the forest, especially those nearest the snake's rocky habitat. As biologists Jonathan K. Webb and Richard Shine report, current forest management plans do not call for leaving such trees and that "forestry activities are likely to have a negative impact on broad-headed snake populations either because too few habitat trees are retained . . . or because habitat trees are clumped or inappropriately located."
Human residential developments also destroy habitat needed by snakes such as the broad-headed snake of Australia. In the winter months, the broad-headed snake leaves its roosts and inhabits sandstone outcroppings near Sydney. Researchers concerned by the drop in numbers of the broad-headed snake have determined that removal of bush rocks from the outcroppings for use in decorative gardens has caused the snake's population to decline. Researchers say that,
given that, bush-rock collectors remove the same types of rocks . . . as those used by the snakes and their primary prey, and that snake and lizard densities are apparently controlled by rock availability, the implications are clear. Unless the activities of bush-rock collectors can be curtailed, the considerable range reduction experienced by the broad-headed snake is likely to continue.
Sometimes the problem is not that humans remove resources from snakes' habitats, but that they want to live in the areas that were formerly ideal snake habitats. In many countries around the world, the story is much the same for species living in areas desired for human development. The Jamaican boa represents a good example of the effects residential developments can have on a species. The snake is being hedged into a smaller and smaller territory as its habitat is altered by human residential developments. By the end of the twentieth century, it had been confined to a small fraction of its former territory on the island. Now, even that territory is being destroyed to make way for a rapidly increasing human population drawn to the beautiful island. In order to survive, this large, striking, silver-gray snake has changed its diet from birds and mammals it found in the wild to those creatures that thrive in residential gardens such as rats and mice. But even this food source is not completely reliable, since residents have begun to use rodenticides to control the populations of these rodents.
Similarly, the southern United States, an area once rich with a diversity of snake species, is being developed so quickly that many species are dwindling in number. For example, the rim rock crowned snake of southeast Florida is in danger of extinction as a result of residential and commercial development of its habitat. Conservationists hope that it might stand a chance with, as herpetologist Paul E. Moler says, "the incorporation of environmental considerations into open space design, public parks, and green belts along highways and throughout low-density residential developments." Also, the eastern indigo snake, which lives in southern states such as Florida and Alabama, requires large tracts of unbroken habitat. To find enough prey to eat, a single eastern indigo snake requires as much as a twenty-five-hundred-acre plot of land where it is free to move about. Rapid housing development and road construction have made this impossible for many indigo snakes.
Roads designed to move an increasing number of people to and from their jobs and homes also have a devastating effect on snakes. Such roads cut through habitat, and as snakes move about in their territory, they are often killed by vehicular traffic.
Such traffic is a particularly severe problem for snakes in the United States. For instance, on "one 44-km stretch of roadway in southern Arizona, traffic kills between 500 and 1,000 snakes each year." A study of highways that pass through the Organ Pipe Cactus National Monument in southern Arizona uncovered numerous road-killed snakes. Among the 368 dead snakes counted were representatives of two snake species, the Chionactis palarostris and Lichanura trivirgata trivirgata, species so rare that they cannot be found anywhere else in the United States.
Besides the outright destruction of habitat, the introduction of exotic species can render otherwise suitable habitat useless to some snakes. Fire ants, introduced to the southern United States, are just one example of an exotic species -- that is, a species not native to the region. Accidentally brought to the United States from South America, fire ants have established colonies throughout the South, as far north as Virginia and as far west as California. The fire ants do not attack adult snakes; instead, they wait until the eggs of oviparous snakes begin to hatch and then enter the shell and eat the young snake. Eckerd College professor Elizabeth Forys discovered in a study financed by the Florida Fish and Wildlife Conservation Commission that the spread of fire ants is aided by development. She says, "In the really pristine areas, there were not any fire ants. . . . Our hypothesis is they prefer disturbed areas because there are fewer native ants there -- no competitors."
In Australia, the Caribbean, Europe, and many other places, too, such exotic species have threatened the survival of some snakes and caused still others to become extinct. Exotic species can cause untold damage to the existing balance of predator and prey already in the area. Feral, or wild, cats, rats, or even goats often take hold in a snake's habitat, either killing the snake for food or breaking down, consuming, or otherwise contaminating its habitat. For example, a nonnative toad introduced into Australia is poisonous, and countless native snakes have died after eating it. Another example is the mongoose, a small, slender mammal native to India and known to prey on snakes, that was introduced by humans to protect them from venomous snakes found on the islands of the Caribbean. The mongoose has thrived, directly causing the decline of several snake species.
One species of island boa, the Round Island boa, has been harmed indirectly by introduction of exotic species. Nonnative sheep, goats, and pigs introduced into this snake's island home in the Indian Ocean eat too much of the vegetative cover necessary for the snake to forage and breed in. This shy snake, which lives in holes in crumbly earth, is also endangered even by rabbits, who eat tree seedlings, which contributes to deforestation. To offset these harmful effects, conservationists are replanting damaged areas of forests and attempting to control the populations of these nonnative species.
Similarly, the primary problems facing the Virgin Islands boa are the huge populations of mongoose, rats, and feral cats roaming over its tiny island habitat. Though the small uninhabited cays and islets that are part of the Virgin Islands ecosystem provide protection from these exotic predators and from humans, the snakes living on these tiny bits of land are vulnerable to severe ocean storms. Conservationists hope that their aggressive rat eradication programs on the larger islands will provide more habitat for this seriously endangered snake.
Though their effects are not as easily identified or controlled as those of exotic species, pesticides have also had detrimental effects on snake species. Aside from killing snakes directly, these chemicals can cause a number of other problems for snakes, including causing snake eggs to be too delicate, causing deformities in young snakes, and killing off a snake's prey animals or contaminating that prey so that the snake is then poisoned by its food. Herpetologist Robert H. Mount describes the subtlety of the pesticide problem:
It is reasonable to assume that the effects vary widely with the pollutant as well as with the species. Some forms not killed outright by a harmful chemical may be affected adversely in subtle ways. Reproductive functions may be impacted, or the chemical may reach toxic levels through bioaccumulation. Some pollutants may not affect a reptile or amphibian directly, but may reduce or eliminate its food supply. It is not unlikely that environmental contaminants are responsible for many of the cases in which certain species are declining when other adverse factors are not apparent.
Though the effects of pesticides on mammal and bird populations have been widely documented, the effects on reptiles are only now being investigated and discussed by scientists. One reason may be that snakes are, due to their secretive nature, not readily observed. Moreover, the effects of pesticides are usually not obvious in snakes.
In a few cases, however, the connection between pesticides and snake deaths are fairly clear-cut. Dieldrin, a pesticide sprayed in Africa to control the tsetse fly that spreads sleeping sickness, has directly killed many snakes. A study revealed that Mirex, a chemical used to control fire ants -- ironically, an exotic species harmful to oviparous snakes -- remains in snake tissue for eighteen months, causes the snake's health to decline, and makes it difficult for it to produce viable eggs and healthy young. One study suggests a link between pesticides and declining snake populations. In two adjacent river valleys in southern Texas, one treated with agricultural pesticides and the other left untreated, researchers noted a clear difference. Herpetologist Harry W. Greene says that the untreated valley "harbors a typical snake fauna, while the second lacks ratsnakes and other egg-laying species; given their proven effects on reproduction in other vertebrates, a reasonable guess is that pesticides eliminated those Texas snakes."
The dangers for snakes go beyond habitat loss, however. Even if endangered snake habitat is preserved and protected from contaminants, snakes still face the danger of people who continue to seek out and kill them out of fear or to sell them as food or leather to domestic and international markets.
Some snake species are adversely impacted by not just one but several types of human activity. This means that even if one threat is addressed, the others may continue to endanger the species. The threatened Kirtland's snake of the American Midwest is just such a snake. It requires a specific and increasingly imperiled wetland habitat made up of wet prairies, meadows, and prairie fens that humans have drained to convert to land to agricultural fields. Though the Kirtland's snake has adapted somewhat to its drier habitat, it must live in crawfish burrows in the vicinity of a pond, lake, or creek area prone to seasonal flooding. Most of the population has been isolated into low grassy areas along creeks, ponds, and ditches, and some have even been isolated in the midst of housing developments where they will inevitably decline due to lack of suitable den sites.
Even if part of the snake's wetland habitat remains intact, it faces other dangers as well. A survey in Illinois revealed that controlled fires, automobile traffic, field and roadside mowing, and herbicides sprayed in railroad rights-of-way have also caused the Kirtland's snake to decline.
Source: Endangered Animals & Habitats: Snakes, Lucent Overview Series. Lucent Books. 2001.