This bat was impaled on thorns. How did this happen? Poor Navigation? Where was Air Traffic Control? Guanacaste Province, Costa Rica.
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April Maurer
Amphibians have extremely important niches within their habitats. In the tropics, amphibians provide a control system for many species from algae to insects that spread disease and destroy crops. In many cases, amphibians make up the greatest biomass of any vertebrate group in the area. This means that they can be thought of as a holding cell for a huge amount of nutrients and energy that exist in the rain forest.
Despite their importance, amphibians can be considered relatively vulnerable. Since 1980, over 120 species have likely gone extinct on a global scale. Currently, about one third of all amphibian species are considered globally threatened. Tropical areas fall into this horrible situation as well. Sixty seven percent of harlequin frogs in Central and South America have disappeared over the past twenty five years. La Selva Biological Station has produced research that depicts systematic, community-wide decline in terrestrial amphibians. Information from this site over the past thirty five years shows that population density of all species of terrestrial amphibians has declined by 75% since 1970. Thirty five years is practically nothing in the whole scheme of things.
In a study in the Tropical Science CenterÕs Monteverde Cloud Forest Preserve, Alan Pounds and other specialists found that over half of the 70 species they studied had gone extinct. Many species are what are called Òindicator speciesÓ meaning that they
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reflect stress on the entire ecosystem. You can think of them as a canary in a coal mine. One example of stress is environmental change.
The term Òglobal warmingÓ refers to a worldwide increase in temperature. This very real issue has negatively influenced many diverse habitats around the globe. Die-offs and reduction of species are becoming increasingly common disasters. It has taken a devastating toll on amphibians, which are particularly sensitive to change. The direct connection between global warming and amphibian loss has yet to be identified and is widely disputed. Suspected specific causes of these disasters include increase in disease, all-around stress to the animals, and destruction of natural habitat. Here, we will proceed to examine these three culprits in greater detail.
One of the speculated links between global climate change and amphibian loss is disease. More specifically, researchers are suspicious of the chytrid fungus Batrachochytrium dendrobatidis. Pounds et al. hypothesized that climate change could have led to an increase in daytime cloud cover and warmer nights, which would have created the perfect environment for Batrachochytrium dendrobatidis. Normally, temperatures are extreme, keeping the fungus under control. But greater cloud cover means less sunlight during the day and trapped heat at night. Pounds remarks, ÒDisease is the bullet killing the frogs, but climate change is pulling the trigger.Ó Thus, we can think of the fungus as a link, which would show that global warming is at the origin of the great loss in amphibian species.
In opposition, many argue that this is not the case by pointing out the populations which have survived outbreaks and die-offs where no trace of the fungus has been found.
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After studying a significant loss in New South Wales, Australia in 1999, the chytrid was found in only seven of thirty two moribund frogs. Trends identified by Whitfield et al. do not show any correlation between recent amphibian loss and the increase in chytridiomycosis. Furthermore, by attributing the loss of amphibians to a fungus, many argue that we are limiting the solution to only one type of animal. This chytrid fungus affects amphibians because of their sensitive, moist skin. Thus, we cannot link it to the loss of other species including many types of birds and reptiles.
It may be safe to say that the fungus can cause die-offs in certain areas, but not that this fungus can spread like a wave over large areas destroying all amphibians in its path. It seems unlikely, then, that tropical amphibian loss can be linked with global warming entirely by the fungus.
Perhaps then, we can call upon a more general idea that it is simply stress taking its toll on the amphibians in various ways. Stresses like global warming can begin affecting frogs and other amphibians very early in their lives. During development, asymmetry is much more likely to occur when an animal is affected by disease or physical stress.
A study in Queensland, Australia showed that, while asymmetry could not be linked directly to global warming, it occurred at an increasing rate in declining species and coincided with a pattern of regional warming. This local climate change mimics a more global, large scale change, and serves as a model for what may happen in the future. While this is a strong lead and a just argument, there is not enough evidence to say that
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global warming directly causes physical deformities such as asymmetry of limbs. This is just one more hypothesis that requires further investigation.
The most easily supported hypothesis is that amphibians cannot survive simply because their habitat is being destroyed or at least reduced. This is an accepted view with little controversy. One very important habitat for amphibians is leaf litter. Researchers blame global warming for an increase in the rate of decomposition of leaf litter (with warmer and wetter conditions) and for an upset in the fall of leaves. This has been noted specifically in La Selva, Costa Rica.
Global warming can also be blamed for the shift in habitat for many mountain species, including but not limited to amphibians. Rising temperatures means animals that live at a certain elevation may not find this particular location suitable anymore. Studies have found species migrating up mountains as the entire climatic layout of the landmass changes. This leads to an increase in predators for many amphibians. In Monteverde, at 1540 meters, the number of birds has increased at a rate of about nineteen species per decade in the wake of recent climate change. Also, as they move upward with warmer temperatures or away from predators, frogs and other amphibians leave behind the moist layer of air and move into a dryer layer. This can be detrimental for them as they require a wet environment.
Patterns of losing amphibians that have very specific climatic needs are not uncommon. The first species to go extinct because of global climate change was a mountain species in La Selva. What habitat could be more specific than one on a mountain? It has a specific elevation, amount of wind, cloud cover, sunlight, etc. If this
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migration upward continues, we could lose the pristine cloud forests of Costa Rica altogether.
While the reasons behind pointing the finger at global warming vary and are often disputed, there is no doubt that a connection does in fact exist. In 1987, Monteverde, Costa Rica saw a huge species crash and the loss of the golden toad (Bufo periglenes) from the cloud forest following a particularly warm year. Not only will we see these steady losses, but we may also encounter abrupt disturbances. If a Òflip-flopÓ in the trade winds occured, it would be sudden, not gradual, and spell disaster for tropical mountain cloud forests. These forests are affected greatly by cloud formation due to horizontal precipitation, which would be disrupted by a change in wind patterns. This is just one extreme example of what could happen, but many other destructive events have already begun.
About 1,850 species of amphibians have been classified as threatened, and hundreds more are on the verge of extinction. This does not just mean a loss of amphibians, however. As we all know, amphibians are a vital part of an ecological web. Adjust one part of the web, and the entire structure could and likely will fail. Studies have already shown that areas that suffer from significant amphibian loss have also lost birds, reptiles, mammals and fish.
The current response to global warming has been for individuals as well as large industries to make the attempt to help. Individuals know to reduce energy use and recycle as much as possible. Companies are working on alternative fuels and ways to conserve. But there are still people out there who think of this as a Òhippie movementÓ
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and refuse even to accept its existence. Hopefully, this evidence will reach enough people before it is too late.
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