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Amphibian Declines in Costa Rica
The well publicized phenomenon of amphibian declines in recent years has raised awareness about the source of such declines worldwide as well as what species are rapidly declining. The status and trends of amphibian declines and extinctions worldwide has been pushed to the forefront of environmental concern. Amphibians appear to be far more threatened than either birds or mammals, with 1856 species (32.5%) being globally threatened (BirdLife International, 2004; The World Conservation Union, 2003). At a specific site in Costa Rica alone, 40% of the amphibian fauna disappeared over a limited period of time in the 1980s, which is around the time scientists first became aware of the threats to amphibian populations (Pounds et al, 1997). A wide variety of the declines are due to habitat loss, global climate change, enigmatic declines (declining even where suitable habitat remains), and the recently identified Chytrid fungus (Batrachochytrium dendrobatidis). These reasons for declines have been most noted in species such as Bufo periglenes (Golden Toad), Atelopus sp. (Harlequin frogs) and Dendrobates (Poison Dart Frog sp.). With 427 species (7.4%) listed as Critically Endangered and on the brink of extinction, conservation and protection efforts of amphibians have been pushed to the top of some conservation organizations agendas (Stuart et al, 2004). More specifically, organizations like the Costa Rican Amphibian Research Center (C.R.A.R.C.) and the Nature Conservancy, which owns and protects land in Costa Rica are focusing their efforts on amphibians.
Scientists have documented four major “hot spots” for amphibian declines: western North America, Central America, northeast Australia, and Puerto Rico (United States Geological Survey, 2006). In Costa Rica, which covers only approximately 0.01% of the Earth’s landmass, it is believed that the country hosts approximately 5% of all of the Earth’s biodiversity (Nature Conservancy, 2006). The country is home to representatives of all three orders of amphibians: caecilians, salamanders, and frogs and toads; as well as a variety of diverse habitats found no where else on Earth (Baker, 2006; Leenders, 2001). One such habitat is the Osa Peninsula, where the jungle meets the sea to form a lowland tropical rain forest that lines white sanded beaches. Since the peninsula was once an island that later connected to the mainland, it has an extraordinary rate of endemism—the number of species found no where else on Earth (Nature Conservancy, 2006). This type of habitat is just one of the diverse habitats in Costa Rica that are home to the rarest and most valuable species in the world.
Approximately 174 species of amphibians have been identified in Costa Rica (Leenders, 2001). The class Amphibia forms an intermediate group of vertebrates between the completely aquatic fish and land-dwelling reptiles, birds, and mammals. The most well known of this class is the order Anura, which is compiled of frogs and toads. Although all amphibians are facing declines, frogs and toads such as the Golden Toad, Harlequin frogs and Poison Dart Frogs are in the spotlight of species being affected in Costa Rica. Costa Rica has 133 species of frogs and toads in 8 families (Leenders, 2001).
Several characteristics of amphibians make them more susceptible to anthropocentric and natural threats and give them a unique role as bioindicators. A bio-indicator is any species whose health can provide early signs of environmental stress (Davis and Menze 2002, Blaustein and Wake 1990, Blaustein et al. 2003b). Two characteristics of amphibians make them excellent bio-indicators (Davis and Menze 2002). The first is their permeable skin through which environmental contaminants are readily absorbed and the second is the fact that most amphibians live both in water and on land for at least part of their life (Davis and Menze 2002). Having these characteristics means that anurans are exposed to the degeneration of both aquatic and terrestrial life (Davis and Menze 2002). Not only do most amphibians live dual lives in both aquatic and terrestrial environments, but their permeable skin makes them extremely vulnerable to the dehydrating effects of life on dry land, pollution, and disease. Even though the majority of adult amphibians have lungs, a good portion of them absorb nearly 50% of their oxygen through their skin and the lining of their mouth (Leenders, 2001). Since amphibians inhabit freshwater, terrestrial and arboreal habitats and exhibit a variety of other unique life history characteristics, their sensitivity to the environment and subsequent extinction should serve as a warning of ecosystem health for humans.
Perhaps the most famous amphibian species to recently be considered extinct is the Golden toad, which is one of four mysterious species of highland toads that inhabit locations within the central mountain ranges of Costa Rica. This species has a tiny geographic range less than 10 km2 (2,500 acres) in elevations between 1,480 and 1,600 meters within the Monteverde Cloud Forest Preserve (Leenders, 2001). Considered explosive breeders, Golden toads are rarely seen except for when they emerge in masses during the first early rains of March or April to breed. This species spends a good portion of its time burrowed under ground so the abrupt decline of the species between 1987 and 1988 was somewhat of a surprise. When the Golden toad first began to decline, many scientists and observers assumed they were witnessing a natural fluctuation in the population rather than a mass extinction. Until 1987 the status of the Golden toad seemed secure; but when the breeding season of 1988 began and only a single toad emerged at one of the most important known breeding ponds in comparison to the more than 1,500 seen at the same site the previous year, the species seemed to disappear (Leenders, 2001). In 1989 only a few toads were observed in the Monteverde Cloud Forest and none have been witnessed since. As a result, the International Union for the Conservation of Nature (IUCN) lists the Golden toad as endangered, but many believe it to be extinct because it has not been seen in over a decade despite extensive surveys (Leenders, 2001).
The decline of the Golden toad is not an isolated incident and a series of events in 1987 did not only affect the Golden toad, but other amphibian fauna in the Monteverde Cloud Forest. As a result, in the case of Monteverde, three hypotheses have been proposed as possible explanations for the declines of the Golden toad, Variable Harlequin frog (Atelopus varius) and other species. Since 1987, biologists have concluded that 20 species of frogs and toads (40% of the anuran fauna) in Monteverde have disappeared since 1987 (Leenders, 2001). Two of the proposed hypotheses are linked to the 1986-1987 El Nino weather patterns, which resulted in extreme droughts those years.
The first hypothesis suggests that the frogs died from dehydration due to the extreme drought of 1987 which may have desiccated breeding pools, habitat, and underground retreats on an irreversible scale. This hypothesis is based on the observation that just before the Variable Harlequin frogs disappeared they were recorded in extremely high densities around the last remaining wet areas in drying streams that year. If the species did not fall to dehydration then biologists speculate that their dehydrated state may have weakened individuals and caused them to be more susceptible to parasitic, fungal, or bacterial infections (Leenders, 2001).
The second hypothesis suggests that a sudden deposition of high concentrations of pesticides on the reserve is responsible for the 1987 crash in amphibian numbers. It is speculated that such pesticides could be carried to the reserve by the ever-present mist and clouds that the forest is known for. Insecticides, herbicides, and fungicides which are used heavily in the surrounding areas may have evaporated from exposed soils and been carried and deposited onto the reserve. Since rainfall that year was very limited, these chemicals would have been deposited in high concentrations that would be undiluted.
The third hypothesis suggests that sea surface temperatures, which have risen as a result of global climate change, have increased the number of dry cloudy days in the Monteverde Cloud Forest. Three crashes in the size of amphibian populations-in 1987, 1994, and 1998-coincided with an unusually high number of dry days in the preserve (Leenders, 2001). These periods of dryness, which were indicated after analyzing the daily weather patterns in the preserve between 1977 and1998, have not only affected frog populations, but lizards and birds as well.
The two types of Harlequin frogs most commonly found in Costa Rica are the Chiriqui Harlequin frog (Atelopus chiriquiensis) and the Variable Harlequin frog (Atelopus varius). Both of these highland toads live at higher elevations in cloud forests. The Chiriqui Harlequin frog can be found at altitudes from 1,400 to 2,500 m (4,600 to 8,200 ft) where the Variable Harlequin frog lives at elevations between 1,200 and 2,000 m (3,950 and 6,550 ft) (Leenders, 2001). Some populations of the Variable Harlequin frog have been known to inhabit the Pacific lowlands of Costa Rica where they have been found at 16 m (52 ft) above sea level. Harlequin frogs are known to perch on rocks in or near streams where they are exposed to direct sunlight and can absorb moisture from the surface. Despite generally being found near streams and other waterways, these frogs are very poor swimmers.
Both males and females of this species are very territorial and defensive of their home range. Some individuals have been observed occupying the same territory for up to two years (Leenders, 2001). In some cases, individuals will be washed away by rising waters and will return to the same territory although it is not known how they orient themselves (Leenders, 2001). The Variable Harlequin frog is of interest because it has “no eardrums and no middle ear cavity, and it also lacks some of the bones normally required for the transduction of sound waves; yet its ears are fully functional” (Leenders, 2001). A laboratory study conducted with the Chiriqui Harlequin frog found that the hearing capabilities of this species were found to be the same as a similarly sized tree frog that did not lack these ear structures. As a result it is speculated as to whether or not these species hear sounds by perceiving vibrations through their arms. Although the Harlequin frogs are poisonous and produce tetrodotoxin, which is a very strong nerve toxin that is secreted from glands scattered throughout the skin on the back, head and limbs of the frog, at least one species of snake, the Fire-Bellied Snake (Liophis epinephelus) is immune to the toxins (Leenders, 2001).
Most famous for their bright and contrasting colors, Poison Dart frogs are also found in Costa Rica. This group of frogs belongs to the family Dendronbatidae, which got its common name from Phyllobates terribilis, one species of the genus Phyllobates, used by the Chocó Indians of Columbia to envenom their blowgun darts (Leenders, 2001). The toxins of Costa Rica's Poison Dart frogs are not nearly as lethal as their Colombian cousins, but nonetheless they serve as a defense mechanism for the frogs from predators. The bright reds, blues, greens, and blacks, which are trademark colorations of the frogs, actually serve as an anti-predator mechanism known as aposematic coloration. Poison Dart frogs are small, diurnal frogs that typically have two shield-like flaps on the tip of each finger and toe, which aids in identification of this species (Leenders, 2001). Posion Dart frogs are found in warm parts of Central America, such as lowland rainforests, and prefer areas where the humidity level is very high. They are mostly arboreal and are known to climb far into the rainforest canopy where they have been observed depositing their eggs in bromeliads (Leenders, 2001). Climbing trees is very easy for this species because they have adhering disks at the ends of their fingers.
More recently, the poison of this species has been the subject of important research. Scientists have discovered that batrachotoxin, which is the poison found in the glands of the frog does not immediately block the nervous system of animals that fall victim to it. It appears that this chemical makes the contractions of the heart muscle stronger. Researchers claim that the pumiliotoxin released by Dendrobates auratus might be used as a cardiac stimulant after a heart attack (Leenders, 2001). A variety of other uses for the secretions may be discovered in the future. According to the National Institute of Health, poison dart frogs offer over 300 alkaloid components–chemicals that are similar to cocaine and morphine and can be used for medical purposes (Leenders, 2001).
There is no denying the dramatic global decline of amphibians worldwide, but until recently the speculation that global warming has been the culprit has been a theory. Scientists have speculated for years that rising temperatures and changing weather patterns are the driving force of amphibian declines, but a recent study documents for the first time a direct correlation between global warming and the disappearance of approximately 65 amphibian species in Central and South America (Eilperin, 2006, Butler, 2006). A study, published in the journal Nature, links climate change with frog extinction thanks to an international team led by J. Alan Pounds of the Tropical Science Center’s Monteverde Cloud Forest Preserve in Costa Rica. This study focused on the Harlequin frogs which are classified as critically endangered across their range and faced an enormous decline in numbers between the 1980s and 1990s—with almost two-thirds of the 110 known Harlequin frog species going extinct (Butler, 2006). The link between rising tropical temperatures and the spread of the deadly Chytrid fungus, which is the leading cause of many recent amphibian declines, was proved to be the leading reason for amphibian declines in this study.
This paper helps explain how global warming has improved conditions for the Chytrid fungus, which kills frogs by “growing on their skin and attacking their epidermis and teeth, as well as by releasing a toxin” (Eilperin, 2006). The higher temperatures which have resulted from global warming have allowed for more water vapor to be present in the air. This vapor creates a cloud cover that results in cooler days and warmer nights, both of which the Chytrid fungus favors because it grows and reproduces best at temperatures between 63 to 77 degrees Fahrenheit (Eilperin, 2006, Butler, 2006). Looking at the more than 65 Harlequin frog species that have vanished, researchers found that “80 percent of the time there was a correlation between higher temperatures and the species’ disappearance” (Eilperin, 2006). For instance, after a recorded warm peak in 1987, five species disappeared (Eilperin, 2006). J. Alan Pounds sums the results of his study up best by saying, “disease is the bullet killing frogs, but climate change is pulling the trigger” (Eilperin, 2006, Butler, 2006). Ultimately there seems to be an obvious pattern of disappearances and this study shows that global warming is affecting outbreaks of this disease.
Global warming and the accompanying emergence of infectious diseases are not the only threat to amphibian populations worldwide, although they appear to be the immediate threat. Habitat loss and enigmatic declines still pose a treat to amphibian biodiversity. Enigmatic decline species “present the greatest challenge for conservation because there are currently no known techniques for ensuring their survival in the wild” (Stuart et al, 2004). In Costa Rica, 12.9% of amphibian declines have been classified as enigmatic (Stuart et al, 2004). One trend seems to be that well sampled countries have higher percentages of enigmatic declines, but even so, for species facing enigmatic declines there are not many conservation options available. Especially since the source of these declines is still a mystery. However, rapid amphibian declines exhibit important taxonomic and regional patterns. Three amphibian families are declining at significantly higher rates than other species and enigmatic declines seem to be the driving force of their disappearance. These families are: Rheobatrachidae (gastric-brooding frogs), Leptodactylidae (typical Neotropical frogs), and Bufonidae (true toads) (Stuart et al, 2004).
For species facing enigmatic declines and the threat of the Chytrid fungus, the only conservation option currently available is captive breeding, but even that option is discouraging because many of these species do not survive well in captive situations. One attempt at combating the spread of the Chytrid fungus is the “Noah’s Ark” solution, which is a mission to rescue frogs in the path of the fungus, led by Zoo Atlanta’s Mendelson and Ron Gagliardo of The Atlanta Botanical Garden (Schulder, 2006). The plan involves capturing enough males and females from each species that is threatened and breeding them in captivity before the fungus decimates their entire population. The hope of this study is that establishing survival colonies may buy researchers enough time to figure out how to combat the fungus and reintroduce individuals into their natural habitats.
Other conservation organizations such as the Nature Conservancy and C.R.A.R.C. are doing their part to raise awareness about the rainforests of Costa Rica and conserve biodiversity. The Nature Conservancy helped create Corcovado National Park in Costa Rica in 1975, which protects one-third of the Osa Peninsula today. Their strategies include “consolidating disparate public lands, encouraging conservation management of public and private lands, and strengthening local partnerships” (Nature Conservancy, 2006). Currently, the Nature Conservancy is part of a coalition to create a 10,000 acre conservation corridor between Corcovado and Piedras Blancas National Parks in Costa Rica (Nature Conservancy, 2006).
In an attempt to highlight rainforest conservation publicly, The Nature Conservancy recently hosted an expedition that was featured in the May 2, 2005 episode of Trippin’. The series, which was shown on MTV, brings “Hollywood stars to remote, beautiful, environmentally-sensitive corners of the world to raise awareness about the importance of conservation issues” (Nature Conservancy, 2006). In this particular episode, actors Cameron Diaz and Jessica Alba were among the group of celebrities chosen to meet up with the Nature Conservancy and learn about what can be done to protect such valuable ecosystems.
The C.R.A.R.C. is a biological research center dedicated to conserving amphibians in Costa Rica. This center, which is located in the town of Guayacan is the first of its kind in the country. Since nearly 200 species of amphibians have been recorded within Costa Rica’s 51,000 square kilometers, the center was created to further biological and conservation efforts directed specifically to Costa Rican Amphibian fauna (C.R.A.R.C., 2006). The C.R.A.R.C.’s research and projects involve the conservation goal of restoring parts of the reserve that were destroyed or altered by human activities such as agriculture, logging, and cattle grazing. Habitat restoration includes making “new ponds and seepages or simply replanting areas with foliage more conducive to amphibian needs” (C.R.A.R.C., 2006).
Hopefully the efforts of such organizations and new findings in the future will open up avenues of research that could provide scientists with the means to conserve the amphibians that still survive today. As of last year, the Global Amphibian Assessment (GAA), a survey of the plants amphibian species, published that nearly one-third (32%) of the worlds 5743 amphibian species are threatened and 129 species have gone extinct since 1980 (Butler, 2006). Scientists believe that there may be more like 10,000 amphibian species on Earth, but the current rates of declines threaten ever knowing what really exists. Many amphibian species are on the brink of extinction, with 427 species (7.4%) listed as critically endangered (Stuart et al, 2004). There is a chance that GAA has underestimated the rates of amphibian declines, especially in areas where they have been insufficiently monitored, but one thing is for certain the declines that are recorded appear to be very rapid and have taken place since 1980. The bottom line is that future of amphibians is unknown and their disappearance is troubling on a variety of levels.
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Butler, Rhett A. (2006, January 11). Climate Change is Killing Frogs Finds New Research. mongabay.com. Retrieved May 15, 2006 from World Wide Web:http://news.mongabay.com/2006/0111-frogs.html
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IUCN-The World Conservation Union, 2003 IUCN Red List of Threatened Species. Retrieved 2003 from World Wide Web: http://www.redlist.org
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Leenders, Twan. (2001) A Guide to Amphibians and Reptiles of Costa Rica. Zona Tropical, South America.
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Schhulder, Michael. (2006, March 1). Frog Killer Found After 6-Year Stakeout. CNN.com. Retrieved May 15, 2006 from World Wide Web:http://www.cnn.com/2006/TECH/science/02/21/frog.fungus/index
Stuart, S.N.; Chanson, J.S.; Cox, N.A.; Young, B.E.; Rodrigues, A.S.L.; Fischman, D.L.; Waller, R.W. (2004). Status and Trends of Amphibian Declines and Extinctions Worldwide. Science Vol. 306 pp.1783-1786.
U.S. Geological Survey. (2006, May 15). Where Have All the Frogs Gone? Research May Solve the Puzzle. USGS Website. Retrieved May 15, 2006 from World Wide Web: http://www.usgs.gov/amphibian_faq.html
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