Solving the Mystery of Amphibian Decline

by: Laura Girardeau

Introduction

Morphological, physiological, and Behavioral Considerations

Possible Causes for Population Decllines

Solutions

Conclusions

References

Introduction: The Extent of the Problem

     Frogs and toads belong to the most abundant order of amphibians, Anura, containing about 3,960 species. They have endured at least two of the major global mass extinctions in which at least half of all animal species became extinct, including the dinosaurs. A few decades ago, they were so abundant that ìscientists and backpackers recall having to take care not to step on them when hiking in the high country (Phillips, p. 6), and even nights in the suburbs rang with their song. Why, then, do scientists now report that some previously common frogs have not been seen in years, even withnoobvious changes in habitat?
 
In the mountains of Oregon, Colorado, and California, several kinds of amphibians, including Yosemite toads, Cascade frogs, leopard frogs, and western toads, dropped in number in the early 1980s and have not recovered. (Phillips, p. 6)
 
     The story is the same all over the globe. The familiar song of the frogs has become strangely silent. Over half of Californiaís native amphibian species are now in decline (Phillips, 1994), and nearly one-third of frogs and toads throughout the United States may be threatened with extinction (Luoma, 1997). Stories abound on sharp declines and extinctions occurring over a few years or even months. For example, the spectacular golden toad and harlequin frog of Costa Rica, bright jewels of the rainforest which stirred excitement in the herpetological community, seemed to vanish as quickly as they were discovered. In 1987, scientists counted some 1,500 breeding golden toads. Over the next two years, only one appeared , and the next year, none at all (Luoma, 1997). And in 1979, dead mountain yellow-legged frogs (Rana muscosa) were found littering a lakeshore in Kings Canyon National Park, and crashed from a population of about 800 to zero over a period of a few months (Griffiths and Beebee, 1992).
 
     Because amphibians often breed in ponds which dry out during warm years, a certain amount of population fluctuation is natural from year to year. Population estimation can therefore be difficult for researchers, and this problem is exacerbated by the fact that there is almost no historical data on many amphibian species (Hecht, 1993). J. Whitfield Gibbons of the University of Georgia surveyed amphibians in South Carolina ponds for nearly twenty years and found that the biggest constant was change. Our most dramatic finding was how variable the community is naturally Based on any three-year survey, you could come up with a completely erroneous conclusion about the trend of increase or decline,(Luoma, p. 69). Similarly, James Petranka, amphibian biologist at the University of South Carolina, notes that the 50% decline in populations of wood frogs in Great Smoky Mountains National Park could be part of the amphibiansí natural cycles, and it will take 10 to 15 years to assess whether or not they are under stress (Pocson, p. 39). But for some species, including our own, this may be too long to wait. As tropical biologist John Terborgh states, when things start going into decline, itís a pretty late warning, not an early warning,(Phillips, p. 211).Pocson warns:
 
 

Although it is too soon to sound the alarm over amphibian declines in the Great Smokies, overall population reductions could provide a warning to humans. Because amphibians mate and reproduce in ponds and wetlands and spend their adult lives on land, frogs, toads and salamanders are an integral part of both aquatic and terrestrial ecosystems. Sometimes predators and sometimes prey amphibians are a critical link in food chains (p. 39)

     Gary Fellers, ecologist with the Biological Resources Division of the U.S. Geological survey puts the point bluntly: The naysayers can always say you won't know if you're seeing a trend until you survey these animals for a hundred years. But if you go to Yosemite and visit every pond, lake and stream that is suitable for frogs and find major declines or absolutely nothing where there once was abundance, that is well beyond the scope of a natural fluctuation," (Luoma,p.69).

     Some scientists suggest that frogs and other amphibians are canaries in a coal mine, indicating imbalances of a global scale with their decline. Their moist, permeable skin makes them particularly sensitive to environmental pollutants. If a frog or toad population is hurting the changes are good that some other living thing or natural system is also askew (Pocson, p. 39. ) However, the exact nature of this imbalance is unclear. Although in some cases the culprit is obvious--a wetland filled, a river dammed, a forest cleared--in others it is not (Pocson, p. 39). In this paper, I will explore the myriad explanations for the escalating decline in frog and toad populations on a global scale, with special emphasis on Oregon species.

Morphological, Physiological and Behavioral Considerations

     The physiology and behavior of frogs and toads exposes them to environmental pollutants by default. Since their eggs are anamniotic and must be laid in moist environments, they are vulnerable to all the chemicals in the water in which they are laid. They are also vulnerable to the potentially harmful rays of the sun and the constant threat of dessication due to weather change. Even later in life, tadpoles and adult frogs and toads are still vulnerable, since their permeable skin, made to absorb and release respiratory gases and water, is just as permeable to environmental threats from outside their bodies.

      Behaviors which require water, such as breeding, are also vulnerable to environmental factors. For example, if a breeding pond dries up during the mating season, the mating cycle can be disrupted as appropriate breeding sites disappear. The ability of frogs and toads to hide from predators can also be impaired if plant cover dies due to drought, erosion orcompaction of the soil by livestock.

      Although they have fared well throughout all the changes that evolution brings, frogs and toads have not adapted to the pace of modern environmental change from a myriad of sources, including habitat destruction, global warming, ozone depletion, acid rain, and introduced species, among other threats. This makes them vulnerable to attack on many fronts. It is likely that there is no unicausal explanation for their decline, and that instead, a combination of stressors from the modern world is causing the mysterious silence of the frogs.

Possible Causes

Ozone Depletion

     Andrew Blaustein is a prominent researcher of the effect of ozone depletion on populations of Rana cascadae (Cascade frogs) and Bufo boreas (western toads) in Oregon's Cascade mountains. He accidently found the connection between declining frog populations and increased ultraviolet rays entering the atmosphere through a thinning ozone layer while studying the kinship behavior of tadpoles. Over several years of collecting the frogs from their high mountain meadow and pond habitat and returning them later, Blaustein found that they were becoming more and more difficult to locate for collection. Of the thirty sites he originally surveyed, 40% of the populations of both species no longer existed a few years later, regardless of whether or not the areas had been logged and habitat had been destroyed. Water quality and pH were in the normal range for these areas. Clearly, this indicated that something out of the ordinary for these species, and Blaustein set out to solve the mystery (Phillips, p. 124). He was awarded a National Science Foundation grant to discover whether increased UV-B reaching the earth through a depleted ozone layer were damaging the eggs and tadpoles of Cascade frogs and western toads, and contributing to their population declines. The Pacific tree frog, Hyla regilla, a species which had not exhibited a significant decline in population, was included in the study as a comparison species. Blaustein shielded natural breeding pools from UV-B rays with a series of filters and compared these pools with those which were left unshielded. He found that over 40% of the Bufo boreas and Rana cascadae eggs exposed to UV-B rays died, compared with 10 to 20% of shielded eggs. Nearly all of the Hyla regilla eggs survived, however, regardless of whether or not they were exposed to ultraviolet (Blaustein, 1994).

      Although this study does suggest that UV-B is a contributing factor in the mortality of Rana cascadae and Bufo boreas, it also raises a new set of questions. The fact that ìin some ponds the failed eggs were unable to fight off infection from a pathogenic fungus" (Luoma, p. 62) leads to the question of whether UV-B rays are directly responsible for the decline of these species, or whether the rays simply make them susceptible to other stresses, such as fungal infections and increased water acidity. Blaustein hypothesizes that UV-B is compromising the frogs immune systems, since he found that the anurans most susceptible to UV-B damage, including the Cascades frog and the western toad, produce low levels of a DNA repair enzyme, photolyase. (Luoma, p. 62). Frog species with high levels of this enzyme, including Hyla regilla, are found to be less vulnerable to ultraviolet. Blaustein believes that this could indicate an adaptation to egg-laying in open, shallow ponds. Although the other two species also lay their eggs in these habitats, perhaps they simply haven't evolved this trait and thus lay eggs more vulnerable to UV-B.
     
      However, even this seemingly clear connection warrants more study, because some frog species have not conformed to this hypothesis. For example, one species of red-legged frog which produces large amounts of photolyase is in decline over its entire range, even though it does not seem affected by UV-B rays (Luoma, 1997).
This early UV-B work is a brick in the complex answer to why frogs are declining. It may also be an important warning that ozone thinning is a more serious threat today than anyone could have imagined (Phillips, p. 136). Although many researchers argue that increased ultraviolet radiation may be causing population decline in some species, a combination of factors may be at work.


Acid Rain

     Some scientists suggest that ultraviolet radiation and acid rain may be working synergistically to damage anuran populations. Others pin the blame solely on springtime acid surges which melt from polluted snows and damage eggs. Still others suggest a new combination of factors: University of Arizona biologist Cecil Schwabe suspects that cadmium or arsenic leached from ore deposits by acid rain may be partly responsible for local extinctions of the Tarahumara frog in the Southwest (Luoma, p. 63). The answer is unclear, and could vary by species, since amphibians vary in their vulnerability to acidity. As Hecht (1993) points out, Embryos of some species die at a pH lower than 6.1, but others can survive a pH of 4.6.

      Some studies, such as those conducted by William Dunson at Pennsylvania State University, show that amphibians survival is definitely affected by pH or acidity levels in the temporary spring ponds or wetlands in which they breed" (Pocson, p. 40). Highly acidic waters can kill embryos and slow larval growth and development. Acid precipitation can also cause amphibian decline indirectly by depleting their food sources. By killing zooplankton and algae, acid precipitation robs ponds of nutrients and deprives developing tadpoles of food. And as the insects which frogs and toads prey upon decline due to lack of plant food, so do the frogs and toads.

      However, many frog and toad species are declining despite protection from acid rain, which suggests that other factors are at work. Because the coal-burning power plants which are largely to blame for acid rain are not common in western North America, acid rain does not always explain declines in this area. The spring acid spike from melting snow is less pronounced in the West, and these brief acid surges were found to have no effect on those amphibians that have shown the biggest decline. Stephen Corn of the U.S. Fish and Wildlife Service notes that the rapidly declining leopard frog breeds in highly alkaline pools buffered naturally against acid input, rather than the high altitude habitats most likely to suffer an acid flush from melting snow. Similarly, boreal toads, which have disappeared from over 80% of their original habitats in Wyoming and Colorado, breed in vulnerable pools at higher altitudes but the tadpoles do not hatch until after the acid pulses are over (Hecht, p. 6).

       Some studies even suggest that acid precipitation can increase amphibian populations. Karen Clark, a researcher for the Canadian Ministry of the Environment, found that one of the most acidified lakes she sampled in eastern Canada contained high populations of yellow spotted salamanders, but no fish. In this pond, introduced fish normally ate the salamander's eggs and competed with them for food, and Clark hypothesizes that the acidity of the lake was strong enough to kill the fish, thereby actually benefiting the salamanders (Hecht, p. 6). Similarly, Clive Cummins of the Institute of Terrestrial Ecology at Monks Wood found that although highly acidic water killed frog eggs, slightly acid water benefited some tadpoles: Having been thinned out, those tadpoles that survived apparently could acclimatize to those conditions. Because their low density, they grew faster than tadpoles in non-acid pools (Young, p. 27).

      Even when acid precipitation is seen as the culprit, its causes can be complex and not always preventable. For instance, the unsolved disappearance of frogs in Costa Rica's Monteverde cloudforest has recently been linked to acid deposition, but not the expected kind this time, the culprit may be naturally occurring acidifying gases from a nearby volcano. The mystery of amphibian decline thus becomes more complex as more causes are explored.

Global Climate Change

      Global climate change is another suspected cause of amphibian decline which becomes more complex the more we try to sort out the pieces of the puzzle. Although amphibians have survived million of years of weather fluctuations, including the ice ages, weather fluctuations in recent years caused by the greenhouse effect may have pushed some frog and toad species beyond their limits. Adaptation and evolution may not be able to keep up with the accelerated pace of change that humans are forcing on other species. Some scientists speculate that the amphibian declines and disappearances might be pieces of biological evidence that global climate change is already having serious effects (Phillips, p. 158). However, global warming alone may not be the biggest threat that climate change poses for amphibians. Instead, it may be the indirect effects and synergistic interactions caused by climate change that threaten many species.

      For example, studies of the harlequin frog in Costa Rica (Atelopus varius) found that this species changed its behavior as the climate changed. An unusual trait of this species is its preference for wetting the skin indirectly rather than sitting directly in streams or ponds. Researchers found that harlequins normally dispersed along the river to catch the splashes, but that as shoreline wet zones diminished, they began to clump together in tighter colonies near waterfalls to keep their skin moist. They then became more susceptible to attack by parasitic flies which patrolled the waterfall areas. This combination of factors might not have occurred without the climate change caused by El Nino during that period (Phillips, p. 144).

      Another way that climate change affected the behavior, and ultimately the survival, of the harlequin frog was by altering water levels in rock crevices used as hiding places by this species. Water levels in these hiding places declined so much that females were forced to emerge and collect moisture from waterfall spray. Usually, females emerged en masse only during breeding season. This contributed to crowding of populations along the banks, which in turn stressed the frogs and increased the ability of microparasites to pass among them. This combination of factors is what may have made the frogs more vulnerable to bacterial infection, parasites, viruses, and pollutants that led to population decline (Phillips, p. 151).

      Climate change can also affect breeding conditions. The golden toad (Bufo periglenes) of Costa Rica, which mysteriously disappeared over just a few years, heavy rains may have caused pools to overflow, sweeping eggs and tadpoles onto the forest floor. A subsequent day without rain may have left the tadpoles stranded on dry land. Many frog and toad species have a narrow window of time between dry season and wet seasons when their offspring can avoid both desiccation and being washed from the pools (Phillips, p. 149).

      Climate change can narrow this window even further. For example, scientists in southeastern Brazil hypothesized that unusually heavy frosts in 1977 were to blame for dramatic declines and extinctions of amphibians in the early 1980s. And South Carolina researchers postulated that the temporary population fluctuation of amphibians in that area during the late 1980s was due to local drought which caused ponds to dry up before tadpoles had developed into frogs and toads. Similarly, a seven-year drought may have also contributed to the decline of the Cascade frog and western toad in Oregon during the late 1980s. Deanna Olson, a U.S. Forest Service biologist, found that these frogs use natural corridors to travel between seasonal ponds for breeding. These corridors are usually small streams and springs, which dry by late summer. In 1992, drought caused these corridors to dry by early summer, as well as causing lake edges to recede and some seasonal ponds to dry out as early as April and May. This, in turn, caused frog and toad tadpoles and egg masses to desiccate (Phillips, 1994).
The effects of climate change are exacerbated at the edge of a species' natural range. When a species is already living in marginal habitat, a slight change in climate could significantly alter the ecosystem in that habitat by changing the plant and animal life in the range. This kind of vulnerability is what scientists are seeing in some amphibian declines. Some are occurring fastest and are most noticeable at the edge of ranges (Phillips, p. 164). As more and more frogs and toads are pushed to survive at the edges of their ranges, more are affected by the complex changes that accompany the greenhouse effect.

      Since global warming caused by the burning of fossil fuels is accelerating, these problems will only increase for declining amphibian species. Scientists at the July 15, 1997 climate-change conference in Seattle concluded that even if atmospheric concentrations of greenhouse gases are stabilized, temperature changes will continue beyond the year 2050. For the Pacific Northwest, winters will become warmer and wetter, with more devastating floods, while summers will become warmer and drier. Flooding will increase in the low-lying land west of the Cascades, causing more landslides and inundation of wetlands (Hill, 1997). This could be fatal to frog species, because although amphibians need moist environments, an over-abundance of water could flush sensitive eggs and tadpoles away from their birth ponds, leaving them to desiccate on dry land once flood waters recede. Drier summers mean increased summer drought, which is already a stressor for frogs and toads depending on ponds and wetlands for feeding and breeding. Drought can leave eggs and tadpoles without water during critical stages of development--a condition which proves fatal to amphibians.

      However, despite these findings, the jury is sill out on the global warming theory. Griffiths and Beebee (1992) state that if amphibians decline is the result of global environmental change, two other trends would also emerge: 1) amphibian decline would be similar on different continents, and 2), different types of amphibians would be affected in the same way. Although this is a gross oversimplification, considering the wide variety of frog and toad species and habitats, the general idea makes sense. So far, neither of these scenarios has occurred. The largest declines have occurred mainly in upland areas of North, Central and South America, as well as Australia. Europe, despite no lack of herpetologists, has somehow escaped these declines. And even in areas where some amphibian species are declining, others are not. For example, the Wyoming toad (Bufo hemiophrys baxteri) and northern leopard frog (Rana pipiens) have both gone extinct, while the boreal chorus frog (Pseudacris triseriate) and tiger salamander (Ambystoma tigrinum) are thriving (Griffiths and Beebee, 1992).


Pesticides

     Toxic chemicals used in farming are another possible cause of amphibian decline, since permeable amphibian skin readily absorbs large amounts of water and any toxins it may contain. And since pesticides become more concentrated with each move up the food chain, carnivorous frogs and toads absorb larger amounts of toxic chemicals than they would if they ate only plant material. Soil, too, is a less obvious but equally important avenue for pesticide concentration in amphibians. During dry periods there would be less precipitation to dilute the pesticides, and the chemicals could become concentrated in the soil. Amphibians, such as the golden toad, rely on soil moisture for much of the year. They could be susceptible to absorbing unhealthy levels of the chemicals as their skins absorb the soil moisture (Phillips, p. 156).

      Despite these physiological connections, Griffiths and Beebee note that there have been few, if any, attempts to measure pesticide residues in declining amphibian populations in the wild (p. 28). And although over 200 pesticides and other chemicals have been tested on amphibians in a laboratory setting, results have been inconclusive. Some studies, like those conducted in Costa Rica's Monteverde Cloudforest, reveal high amounts of pesticides in water inhabited by declining frogs (Phillips, p. 154). Others conclude that adult and larval amphibians are no more sensitive to chemicals than other land and aquatic vertebrates, and that of all the compounds tested thus far, only a handful are more damaging to amphibians than to other vertebrates. In fact, one study suggests that anuran tadpoles are much less vulnerable to many pesticides than other vertebrates. However, these studies are inconclusive, since this resistance could simply be an adaptation to naturally occurring compounds, called cholines-terase inhibitors, which are similar to these pesticides (Griffiths and Beebee, 1992).

      It is also important to point out that lab studies do not always translate to life in the wild. Although some frogs may survive exposure to certain pesticides in the lab, the tiniest physical or behavioral abnormality may be enough to tip the scales in favor of a predator (Griffiths and Beebee, p. 28). Unfortunately, pesticide companies are not required to test toxicities of new pesticides on amphibians and reptiles, even though they are required to test them on fish, rats and other vertebrates. However, because many conservationists are emphasizing the large differences between amphibian physiology and the physiology of other test species, the EPA is considering adding amphibians to the list.

      Pesticide use may be linked to the incidence of deformed frogs and toads, which has been steadily increasing over the past few years. This phenomenon has been documented in at least eight states, as well as Canada, and involves at least ten species, including the Northern leopard frog, the American toad, long-toed salamanders, and red-legged frogs (Pocson, p. 40). The current problem became national news in 1995 after students in Le Sueur, MN found a large number of frogs with extra limbs. Although the exact cause of the deformities is unexplained, several theories have been posed: pesticides, ultraviolet radiation working in combination with pesticides, and parasites. Again, a unicausal explanation is insufficient for such a complex problem. There is no evidence, however, that amphibian deformities and overall population declines are linked, since frogs with extra limbs can often still survive and reproduce. Both are probably symptomatic of an unhealthy environment suggests Mike Lannoo, coordinator of the Declining Amphibian Population Task Force (Pocson, p. 40).


Predation

     Predation by other species is yet another cause of amphibian decline that is contributing to some, but not all, of the problem. The practice of stocking lakes with fish for food and recreation has become common throughout the world. Since these fish are not native to the ecosystems into which they are introduced, they often have no predators or natural population controls, and voraciously prey on eggs and larvae in large numbers. Therefore, as Phillips notes, within a few years, introduced fish can decimate a lake or streams entire amphibian population, (p. 184). Blaustein, who found that ultraviolet rays were damaging frog and toad eggs in Oregon lakes, postulates that hatchery-grown fish stocked in the lakes for recreational fishing could be responsible for the introduction of a fungus which is killing the eggs (Phillips, 1994). Rana catesbeiana, or the common bullfrog, is also notorious for adversely affecting populations of native frogs in the western United States. Although the bullfrog is native east of the Rockies, it was introduced in the West, as well as Hawaii, Mexico, Cuba, Jamaica, Japan and Italy (Stebbins, 1985). Although direct predation of smaller frogs by the larger bullfrogs has contributed to the decline of many frog and toad species, including Rana pretiosa in some regions of Oregon, this is another case in which the issue is more complex than it first appears. For instance, in the case of the declining red-legged frog (Rana aurora).
It would be natural to assume that the bullfrogs simply pushed the red-leggeds out by gobbling them up. However, the places where bullfrogs now dominate are almost all less pristine than they once were. Bullfrogs can withstand conditions that red-leggeds can't. The former have more tolerance than the latter for dirty water and high temperatures, and they don't require the same kind of heavy riparian cover. It is possible that the real culprit in may of the places now dominated by bullfrogs is habitat destruction.. Bullfrogs may simply have been able to survive conditions that red-leggeds have not (Phillips, p. 52-53).

     An abundance of bullfrogs relative to other frog species in an ecosystem, then, does not always indicate that predation is directly causing the decline. It often indicates imbalances within the larger ecosystem.

Collecting

     The practice of collecting, or hunting, frogs and toads is a problem on a global scale, but this is not significantly contributing to the decline of most species in the United States. Scientists probably take more frogs and toads than frog-leg fanciers, and even this number is small compared to the numbers taken by other environmental threats to amphibians.

      Typically, a scientist takes a maximum of two or three dozen frogs from a limited area. A bulldozer takes hundreds and thousands of amphibians from across a wide range with a few scrapes and scoops. It wipes out the habitat as well, ensuring that there will be no future population comeback (Phillips, p. 62).
However, frog and toad hunting is not big business in the U.S., our citizens still eat frog-legs collected from other countries. Frog and toad skin is also used to make purses and wallets, and the pet trade may be the biggest threat from collecting in the U.S.

      After scientists publicly raised the declining-amphibian alarm, human trade in amphibians was not staunched. Instead it increased dramatically. More than four times as many amphibians legally passed through U.S. ports in 1992 as in 1990, according to U.S. Fish and Wildlife Service records, people sell frogs for food, pets, purses and wallets, now more than ever. And consumers continue to buy these products (Phillips, p. 85).
According to the World Wildlife Fund, the U.S. imported over 6.5 million pounds of frog meat each year between 1981 and 1984, entailing the destruction of about 26 million frogs per year. Ninety percent of these frog legs came from India and Bangladesh, where such exports have since been banned (Phillips, p. 89). However, no matter what country the legs come from, there is one constant: The legs once belonged to frogs taken from the wild. Frogs are difficult to raise on farms because they donít respond well to crowding, they tend toward cannibalism, and they are expensive to feed since they will only eat live insects as adults (Phillips, 1994).


Habitat Destruction

     This leads us to what is probably the biggest cause of global decline in frog and toad populations. Throughout the world, habitat is being destroyed at a dizzying pace. Wetlands are notoriously seen by developers as expendable. Bogs, ponds, and swamps often seem more useful to modern city-dwellers when they are filled in and paved over, despite the fact that wetlands rival the rainforest in their productivity and play a crucial role in flood prevention, natural water purification, and climate control. Already, over half the wetlands in the lower 48 states have been drained, paved, or filled (Mitchell, 1992). Britian's loss of 20% of its small water bodies over the last thirty years translates to the loss of about 182,000 ponds, affecting countless frogs and toads (Young, 1990). It is estimated that at least 41% of the year-round streams in the U.S. have been damaged by siltation, erosion and channelization (Phillips, p. 186). Therefore, it is not a coincidence that half of the 75% decline in California red-legged frog has occurred since the mid-1960's, after new towns, housing tracts, water diversions, and cattle grazing replaced or damaged most of the remaining habitat (Phillips, p. 53).

      Cattle grazing is a major cause of riparian destruction and the concomitant decline of the species which call streams and rivers home. As livestock trample stream banks in their efforts to find water and cross over to new grazing land, they erode the soil and increase sedimentation in the water. They also pollute streams with nitrogenous wastes as they urinate and defecate, which in turn changes the character of the plant and algal communities growing there. Soil along the streams becomes compacted as livestock trample surrounding areas, reducing its ability to absorb rainwater. As streamside vegetation dies off, water temperature and bacterial growth increase due to lack of plant cover over the water. Even without formal studies many herpetologists are convinced from their routine field observations that cattle grazing is a serious amphibian killer (Phillips, p. 170). Cattle grazing, therefore, has the potential to adversely affect many frog and toad species in Oregon, especially those inhabiting in the heavily grazed Great Basin region, including Spea intermontana (the Great Basin spadefoot toad), Bufo boreas, Rana pretiosa, and Hyla regilla.

      Logging harms frogs and toads by removing the trees and plants which provide them with shade and hiding cover. Resultant higher water temperatures can stress eggs, juveniles and adults, and higher temperatures on land can reduce moisture and increase the danger of dessication. Forest fragmentation can cut on amphibian population off from another by removing a forest or stream pathway between the populations (Phillips, p. 173). Only about 5% of old-growth forests in the United States are still standing, leaving little habitat for the species which depend on its shaded, multi-layered canopy, moderate, moist climate and clear, naturally-filtered water. The downed woody material common in old-growth forests provides shade, cover and moist microclimates for terrestrial amphibians, and logs which have fallen into stream and ponds attract the insects which feed many frogs and toads. Logging without adequate streamside protection has had an effect on many population of the tailed frog and torrent salamander throughout the Pacific Northwest (Pocson, p. 39).

      Urbanization is another cause of habitat destruction which is often overlooked because it is literally right under our feet. The miles of cement which line our cities and suburbs often replace prime wetland habitat for many species. Much of Oregon's Willamette Valley, for example, was a rich and diverse wetland and floodplain before it was paved and covered with farms, malls, and golf courses.

Solutions

Lifestyle Change

     Although the causes of declines in frog and toad populations are not always clear, several factors seem to be at work, and all connect to increasing environmental destruction throughout the world. Therefore, solutions entail solving or reducing these larger systemic problems. Although ozone depletion, acid rain, the greenhouse effect, pesticide use, and collecting are huge problems of a global scale, each individual can help solve these problems through changes in lifestyle. Reducing the burning of fossil fuels by leaving the car at home or not buying one at all is the first step in reducing the greenhouse effect. Reducing the production of ozone-depleting chemicals such as the CFC's used in air conditioners, refrigerators, styrofoam manufacture, and the aerosols sold outside the United States addresses the problem of increased UV-B rays, and growing and buying organic food reduces the use of pesticides and the contamination of water supplies. Reducing our need for coal-fired electric plants and other industrial factories addresses the problem of acid rain. Refusing to buy products made from frog and toad skin or to keep frogs as pets addresses the problem of collecting, as does refusing to eat frog legs (although many would argue that eating Rana catesbiana, or bullfrog, legs would benefit other frog species).

Conservation

     Reducing habitat destruction is perhaps the most powerful way we can help to preserve frog and toad species. Wetlands and wild places, as well as not-so-wild vacant lots in the suburbs must all be valued for the homes and habitats that they are before we automatically pave them over. As Joni Mitchell sings,

Don't it always seem to go
That you don't know what you've got Till it's gone?
They paved paradise
And put up a parking lot.

Legal Protection

     Although legal protection will not necessarily prevent amphibian declines caused by global environmental problems such as ozone depletion, it is one way to prevent further declines due to habitat destruction.. There are currently only ten amphibians on the Endangered Species List, although many more are probably worthy of listing or some form of protection. Similarly, although hundreds of birds and mammals are listed on the Convention for International Trade in Endangered Species, which protects certain species from collection, less than 100 amphibians are listed (Pocson, 1997). Affording legal protection to declining species is a long and arduous process. Long-term monitoring is necessary in order to establish baselines and prove significant, human-caused declines rather than natural population fluctuations.


Increased Monitoring

     Since baseline population levels have not even been established for many frog and toad species, increased monitoring of the health and structure of current populations is imperative. More research must be conducted and supported in order to solve the mystery of amphibian decline as well as simply to add to our appreciation of Anuran diversity.

Scientists still do not know the exact magnitude of the amphibian declineEven where they are confident that there is a decline and have some hard numbers as they do for certain species in the western United States, scientists often don't have enough basic information about the species and their relationship to the environment to identify the reasons for the declines (Phillips, p. 208)

      Although amphibians all over the globe should be monitored, national parks are a good place to start, since these areas are often protected from the habitat destruction that affect other areas, such as pesticides and logging. This way, at least some of the variables in the mystery of frog and toad declines can be isolated. In addition, researchers can return to these areas on a long-term basis to monitor populations without significant changes in the nature of the habitat.

Conclusions

     Despite increasing concern and numerous studies on amphibian decline, the once-abundant boreal toads that began declining in the 1970s in the Colorado Rockies can still hardly be found there. And despite a return to normal weather patterns, the golden toad and harlequin frog still haven't returned in Costa Rica's Monteverde cloudforest. All over the world, ìthere have been no signs of significant rebounding by dozens of amphibian species (Phillips, p. 209). These declines show perhaps more than those of other animals just how vast and deep is the impact of even subtle habitat destruction (Phillips, p. 210).

      Research shows that unicausal theories are insufficient when dealing with sensitive frog and toad populations. As Griffiths and Beebee state, ìdescribing the problem as global is premature and misleading, for it implies a common cause (p. 29). There is no smoking gun. In solving the mystery of amphibian decline, we must find all the pieces in the puzzle and learn how they are meant to fit together in the first place. One thing we do know is how important frogs and toads are in the habitats in which they live, as well as the larger global system.
Frogs often dominate the animal and plant communities in which they live. Tadpoles are major consumers of plant life, removing large quantities of algae from streams and ponds. Once they have metamorphosed into adults, they become voracious carnivores that prey on a wide range of insects and also other invertebrates. This dual ecological role of amphibians makes them excellent bioindicator species whose fortunes reflect the general health of the environment (Young, p. 27).

      In addition, frogs and toads have found to contain chemicals in the skin which could prove extremely useful to humans. This includes a painkiller hundreds of times more potent than morphine, a whole new class of powerful and versatile antibiotics, and a growth hormone that is now use to help detect cancer in humans (Miller, p. 282). Despite this promise, less than 5% of the world's frog species have been studied for these compounds.
But regardless of whether frogs and toads are directly useful to humans, they are intrinsically worthy of preservation in their own right, as well as linked to the preservation of all natural systems. Conversely, their decline may be linked to the decline of ecosystems throughout the world. Let's take a moment to listen to the silence of the frogs.

References

Blaustein, Andrew R. (1994). Amphibians in a bad light. Natural History, 103 (10), 32-38.

Hecht, Richard (1993). Acid rain 'not guilty' of killing amphibians. New Scientist, No. 1885, 6.

Griffiths, Richard and Beebee, Trevor (1992). Decline and fall of the amphibians. New Scientist, No. 1827,      25-29.

Hill, Richard L. (1997). Climate-change forecast: Wet, warm, wild. The Oregonian, July 15, A1.

Luoma, Jon R. (1997). Vanishing frogs. Audobon, 99 (3), 60-69.

Miller, G. Tyler, Jr. Living in the Environment. Wadsworth Publishing Co.: Boston.

Mitchell, Joni (1970). Big Yellow Taxi. Capitol Records.

Mitchell, John G. (1992). Our disappearing wetlands. National Geographic, 182 (4), 3-45.

Phillips, Kathryn (1994). Tracking the Vanishing Frogs. St. Martinís Press: New York.

Pocson, Sheila W. (1997). Amphibian assault. National Parks, May/June, 38-41.

Stebbins, Robert C. (1985). Western Reptiles and Amphibians. Houghton Mifflin: Boston.

Young, Stephen (1990). Twilight of the frogs. New Scientist. No. 1713, 27.