Wetlands serve as critical habitat for many species of amphibians and reptiles, collectively known as herpetofauna, or “herps". Their reliance on wetlands makes herps especially vulnerable to the loss and degradation of wetlands; indeed, global population declines in reptiles and amphibians may be related to changes in the quality and availability of wetland habitat.
Use of wetlands by reptiles and amphibians
All amphibians rely on wetlands to some extent. Many species lay gelatinous eggs under water, whereas others, like certain salamanders, lay their eggs on moist land. After hatching, many young amphibians enter an aquatic larval stage, which can last from several days to many months (or even several years, as is the case with the Pacific giant salamander (Dicamptodon tenebrosus). On becoming adults, most amphibians adopt a facultatively terrestrial lifestyle, and may use both wetland and upland habitats. However, exceptions to this general life history pattern exist: for example, eastern newts (Notophthalmus viridescens) have an aquatic larval stage, but then transform into a terrestrial juvenile stage - the "red eft" so visible at certain times of the year in the eastern United States - before finally developing into a primarily aquatic adult stage.
As a group, reptiles generally are less dependent upon wetlands than amphibians, with notable exceptions (for example, turtles). Unlike amphibians, most reptiles lay their eggs (or give birth to live young) on land. However, upon hatching, many young reptiles return to water, and in some cases will live out their adult life within a wetland. Examples of reptile in the United States that depend upon wetlands at some point in their life include: alligator (Alligator mississippiensis), common snapping turtle (Chelydra serpentina), spotted turtle (Clemmys guttata), northern water snake (Nerodia sipedon), cottonmouth snake (Agkistrodon piscivorous), and several species of garter snakes (Thamnophis spp.).
Wetland types important to amphibians and reptiles
Amphibians and reptiles depend upon a variety of wetland types. These may include marshes, swamps, bogs, and fens (and their associated subclasses). Some wetlands are only wet a portion of the year and are considered “ephemeral” wetlands. These wetlands provide important habitat and breeding grounds. Vernal pools, one type of ephemeral wetland, are of critical importance to amphibian populations. As small, often isolated wetlands, vernal pools are only wet for a portion of the year. Periodic drying creates a fish-free environment for amphibians, many of which have adapted rapid egg and larval stages as a race against the dry season. The absence of fish predators in vernal pools benefits amphibian populations.
There are often strong ecological connections among wetlands in a landscape, even those that may be physically isolated from one another. Although some may be permanent and others ephemeral, amphibian populations can depend on multiple wetlands within a given area. To ensure the persistence of these species over the long term, the variety and density of suitable habitat sites within the landscape must be preserved, along with terrestrial corridors that connect the wetlands.
Threats to wetland habitat
Destruction of wetlands
Wetlands around the world are threatened by a variety of human activities. For example, at least 220 million acres of wetlands are thought to have existed in the United States (excluding Alaska and Hawaii) prior to 1700; over half of our original wetlands have been drained and converted to other uses. Though the rate of loss has decreased in recent decades, wetlands and other aquatic resources are still threatened by activities such as ditching, draining, dredging, and stream channelization; deposition of fill material for commercial and residential development, dikes, levees, and dams; crop production; logging; and mining.
Fragmentation of wetlands
To successfully carry out their life history, most herps must move among different wetlands and between uplands and wetlands. For example, the red-eft stage of the Eastern newt moves from the wetland habitat occupied as a larvae to upland areas and then back to wetlands for the transformation to adulthood. When the ability of herps to move across the landscape is diminished, population fragmentation can follow, with the attendant increase in the risk of localized extinctions. Although fragmentation of wetland habitat can result from many causes, roads are an especially widespread problem. Roads may results in actual physical barriers to the movement of herps (for example, the concrete median dividers found on many high-traffic roads), or may pose virtual barriers because so few individuals can avoid by killed by motor vehicle traffic.
The introduction of chemical pollutants to wetlands can reduce habitat quality for herps. Many synthetic organic compounds and metals adversely affect amphibians and reptiles. Sublethal effects of chemical pollutants include reduced ability to swim, catch food, and reproduce successfully. Amphibians are particularly sensitive to chemical contaminants owing to their permeable eggs and skin.
Chemical pollutants may be introduced by sources far removed from the affected wetland, for example stormwater runoff from roadways. Chemical pollutants may even be delivered in an airborne fashion: a recent study by the U.S. Geological Survey (USGS) showed that “organophosphorus pesticides from agricultural areas, which are transported to the Sierra Nevada on prevailing summer winds, may be affecting populations of amphibians that breed in mountain ponds and streams.”
The introduction of Endocrine disrupting chemicals (EDC) is also a significant concern. Studies have shown that chemicals like polychlorinated biphenyls (PCBs) build up in turtle eggs and can reduce eggshell thickness and cause reproductive failure. Other studies have shown reduced male organ size among reptiles exposed to EDCs, which can impair reproductive ability. Both amphibians and reptiles are susceptible to the dangers of EDCs.
The indirect effects of excess nutrients can be detrimental to amphibians. Heavy loads of nutrients such as nitrogen and phosphorous can result in an overabundance of algae, which can reduce habitat quality for some amphibians. Excess nutrients can also reduce the amount of oxygen available in the water for amphibian tadpoles and alter the composition and numbers of the invertebrate communities that are food for the juveniles. In Texas, playa wetlands receiving nutrient-laden feedlot effluent were devoid of amphibians found in natural wetlands. In this case, experiments indicated that the nutrient-concentrated effluent had to be reduced to less than 3% of its original strength in order to minimize adverse effects.
Climate change may threaten aquatic and semiaquatic life by reducing wetland acreage due to frequency and severity of storms and sea level rise. Latitudinal shifts in temperature and precipitation patterns also threaten herps.
Invasive plants and animals can alter the structure and function of wetlands, and in doing so may reduce their ability to support populations of native herps.
Image by Tomas Castelazo
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