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Populations of amphibians worldwide have undergone major declines in the past several decades. The International Union for Conservation of Nature (IUCN) conducted an assessment of available data regarding the 5,743 known species of amphibian. They concluded that 43.2 % of all amphibian species are currently in population decline (Stuart et al., 2004). Houlahan et al. (2004) found in an analysis of 936 species that this decline has been happening since the late 1950s and continues to the present. Amphibians are valuable environmental indicators because they are, at different points in their lives, affected directly by water, air, soil, and sunlight. They also tend to stay in limited areas for their whole lives, which makes them good local indicators. Waddle (2006) explains that indicator species react to changes in their environment as a whole. Their decline may therefore be a sign of trouble for ecosystems worldwide. Shrinking amphibian populations are a risk to ecological communities in which amphibians fill important niches. Adult amphibians feed on a variety of other creatures, including mosquitoes, flies, fish, birds, and sometimes small mammals. Larval amphibians are a food source for aquatic insects, fish, mammals, and birds. Loss of amphibians will disrupt food webs, thus affecting these other populations. According to Blaustein & Wake (1995), amphibians also benefit humans as a source of chemical secretions, which are used for pharmaceutical purposes (e.g. treating pain, burns, heart attacks, etc.). If we lose amphibians, we lose our supply of some medicines and the chance to discover some others . Because amphibians serve as indicator species, a source of medicines and contribute to the stability of food webs, decreases in amphibian populations will send repercussions through ecosystems around the world.
Data for amphibian populations in Virginia and nearby areas of the United States reflect the global declining trend. Vial and Saylor (1993) state that a report from the Declining Amphibian Population Task Force (DAPTF) listed seven Virginian amphibian species as threatened, or endangered, or under special concern. Out of 90 amphibian species in the Northeast U.S., 48% currently fall into those categories (Grant, 2009). Highton (2005) observed salamanders of the genus Plethodon at 1,700 sites in eastern North America and found that 180 out of 205 populations had smaller populations in the 1990s than in earlier years. Declines in Virginia may be the result of land development and logging, which affect habitat (Highton, 2005).
Some possible causes of amphibian decline on a large or small scale include the loss or development of habitat, invasive species, global change, excess UV-B radiation, chemical contamination, and disease. Only the first two of these are relatively well-understood. Changes in habitat such as clear-cutting forests, draining wetlands, conversion of nearby land to farmland, and changes in vegetation impact amphibians. Amphibians are vulnerable to changes in both the aquatic and the terrestrial parts of their homes because of their life cycles. Alien species kill amphibians by preying on them, competing with them, or introducing diseases. A variety of pathogens such as viruses, bacteria, parasites, and fungi are also believed to contribute to the decrease of amphibians. A notable disease is Chytridiomycosis, a fatal thickening of an amphibianââ‚¬â„¢s skin which is caused by a fungus. Global changes like climate change, the addition of chemical contaminants to the atmosphere, and seasonal increases in UV-B radiation where the ozone layer has been depleted have the potential to affect amphibians physically or change their behavior. Pesticides, herbicides, fertilizers, and other chemicals used by humans pollute amphibiansââ‚¬â„¢ environments. This may kill amphibians directly, cause deformities, affect their behavior, reduce their growth, or weaken their immune systems (Collins & Storfer, 2003; Blaustein & Kiesecker, 2002; Alford & Richards, 1999). Blaustein and Kiesecker (2002) theorize that a synergistic combination of all of these factors is responsible for the global decline of amphibians. Kiesecker (2002) also argues that pesticides increase the rate of parasite infections. While many causes are plausible, there is no one reason for decreasing amphibian populations worldwide.
Sexton, Phillips, and Bramble (1990) conducted a study of the migration of Ambystoma maculatum. They looked at how temperature and precipitation in the surroundings affected the spotted salamandersââ‚¬â„¢ breeding migration. Their experiment was conducted at Salamander Pond in western St. Louis County, MO over a ten-year period. In the experiment, they found that the spotted salamander generally began migrating to its breeding site following the first rainfall after the snow had melted and the ground had thawed, though rarely before February. Beyond this, there was no significant correlation between the amount of precipitation and migration. They noted, however, that all migration was associated with some precipitation in whatever amount, namely nocturnal rainfall. These scientists also found that migration was substantially linked to the three-day average soil temperature below the ground. Sexton et al. state that while there was no temperature at which migration seemed to consistently occur, there did appear to be a threshold temperature of 4.0 degrees Celsius at a depth of 30 cm in the soil. This means that in all but one instance, the temperature at this depth during migration was greater than or equal to 4.0 degrees Celsius. This suggests that Ambystoma maculatum hibernates at this depth, though there is no concrete evidence. The researchers also noted that the temperature above the ground when the salamanders migrated was always higher than that of the soil 30 cm below. On average, when the mean three-day average temperature 30 cm below the soil was 4.5 degrees Celsius, the temperature of the air above was 5.5 degrees Celsius. Sexton and his colleagues say that salamanders wait for the air temperature to be higher than the soil temperature in order to avoid exposure to cold or inclement weather. They also found that male salamanders tended to migrate earlier than female salamanders. Their theories explaining this pattern are that males have either a different microhabitat (reside at different depths) or a different migration-triggering temperature than females. The latter of these theories was found to be true for Ambystoma jeffersonianum, a close relative of the spotted salamander.
For three years, R. J. Baldauf (1952) observed spotted salamander breeding ponds near Reading, PA. During this time he tracked air temperature, water temperature, humidity, precipitation, and presence of salamanders and spermatophores. Of the four ponds studied, one was semi-permanent while the rest were vernal pools. Salamanders were observed nightly when possible and otherwise during the day. Temperature data in degrees Fahrenheit were collected, but not regularly. Baldauf determined that the first Ambystoma maculatum migrations occurred from mid-February to mid-March, as evidenced by the appearance of adult salamanders and spermatophores. Temperatures during the migrations ranged from 55 to 62 degrees Fahrenheit (12.8 to 16.8 degrees Celsius). The relative humidity on the dates of migration ranged from 83 to 87 and the amount of precipitation ranged from 0.14 to 0.47 inches. The range of temperature for the 24-hour periods in which the first migrations took place was 41 to 76 degrees Fahrenheit (5 to 21.4 degrees Celsius). Baldauf concludes that humidity and temperature were the most important instigating factors in spring spotted salamander migration. He reasons that a suitable temperature combined with moisture from rainfall, melted snow, or water vapor in the air triggers the migration of Ambystoma maculatum.
The purpose of our experiment is to determine whether or not the temperature of the surrounding environment affects when spotted salamanders (Ambystoma maculatum) begin and end their breeding migration. Our hypothesis is that if the temperature increases above a certain threshold, then the rate of migration of Ambystoma maculatum will increase. We intend to place temperature probes in and around Woodmarsh vernal pool, which is located in Mason Neck National Wildlife Refuge (MNNWR), during the spring of 2011 to measure the temperatures of the environmental elements there. The probes will regularly record the temperatures of the soil, air, and water in the area. We will note how the temperatures relate to the migration of the salamanders and analyze the data for a correlation between changes in these temperatures and changes in the rate of migration.