ScienceWeek

ECOLOGY: ON THE DECLINE OF AMPHIBIANS

The following points are made by Nigel Williams (Current Biology 2004 14:R986):

1) Concern about the apparent decline of many amphibian species around the globe over recent decades, often in habitats considered to be of high quality, has prompted fresh worries about the extent of environmental factors that may be playing a part. A new first-ever global study estimates that one third of the world's amphibian species are in danger of extinction. Researchers think that the mysterious collapse in numbers might be a warning that our environment may be in a worse state than we think -- amphibians are known to be the most vulnerable of all animals to subtle changes in their ecosystems.

2) Researchers have known since the 1980s that many of the world's amphibian species are vanishing but the scale of the decline revealed by the new survey has created great concern. More than 520 scientists from more than 60 countries took part in the three-year Global Amphibian Assessment, a study of the world's 5743 known amphibian species. One in three -- a total of 1856 species -- are apparently threatened with extinction. The latest count shows 122 amphibian species have become extinct in the past 20 years. The problem is particularly acute amongst the New World species. This region is home to more than half of the world's known species of amphibians -- its 3046 described species comprise 53 per cent of the world's total.

3) Brazil and Columbia have the greatest diversity with 731 and 698 species respectively. The top five countries for amphibians (including Ecuador, Peru and Mexico) are all in the New World and Venezuela and the US are also in the top ten. At the other end of the diversity scale, a number of Caribbean islands have just one native species each. Nearly 40 per cent of New World amphibians are threatened with extinction, including 337 species that are critically endangered -- on the brink of extinction. Nine species in the region have gone extinct in the past 100 years and another 117 species are "possibly extinct", meaning that researchers are unaware of any existing populations but have not performed the extensive searching required to place these species in the extinct category.

4) Many of these declines are recent. Since approximately 1980, four species have gone extinct, and 109 species have become possibly extinct in this region. Within the New World, Caribbean species are the most threatened -- 84 per cent of the region's 171 species, followed by central America with 52 per cent of its 685 species threatened. The risk facing New World amphibians is considerably higher than for either birds -- 10 per cent -- or mammals -- 16 per cent in the same region. Amphibian species occurring at high elevations with restricted distributions and characterized by terrestrial life cycles, rather than those using a mix of aquatic and terrestrial habitats, are more likely to be threatened than species with other characteristics, the survey reveals.

Current Biology http://www.current-biology.com

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ECOLOGY: AMPHIBIAN DECLINE AND EMERGING DISEASES

The following points are made by J.M. Kiesecker et al (American Scientist 2004 92:138):

1) Across the globe hundreds of species of frogs, toads, salamanders and newts are in dramatic decline. At the same time, new and often serious infectious diseases seem to be sickening people. Might these unhappy developments be connected? Might they indeed share a root cause: the changes in our world brought about by a growing human population? From several lines of inquiry, evidence is accumulating to support such a conclusion -- that environmental degradation wrought by people is contributing to both trends.

2) The human species now numbers 6.3 billion, and collectively we have altered between one-third and one-half of the Earth's land surface. After being stable for millennia, atmospheric carbon dioxide has increased by 30 percent in the last two centuries. Our actions fix more nitrogen than all natural terrestrial sources, and we utilize over half of all accessible surface freshwater. These are dramatic changes, even though we cannot always determine their long-term significance. The immediate consequences seem to be disproportionately borne by frogs, which have suffered massive mortality in recent years.

3) The authors are engaged in studies of this surge in amphibian deaths. While many cases can be linked directly to single, proximate factors such as habitat loss, numerous populations have declined in protected parks and nature reserves, even in remote wilderness areas -- places that are removed from our modern effluvium and that ought to be insulated from human influence. Yet across the globe, many amphibian species have experienced increased disease- and parasite-prevalence, causing massive mortality. Developmental malformations associated with parasitic infection are also frequent: In some groups 90 percent are severely deformed, with extra or missing limbs.

4) The origins of these catastrophic losses are complex. Several agents can act synergistically to endanger a population. Depending on the specific locale, forces such as climate change, habitat destruction, environmental chemicals, fertilizer runoff and the introduction of exotic species have all been implicated in the threat.

5) So how is the global decline of amphibians related to increased disease prevalence among humans and wild-life? The link is suggestive, not proven, but there are compelling similarities between recent disease outbreaks in many animals. Amphibians have been hit particularly hard because of their life cycle and physiology: Frogs and salamanders are exquisitely sensitive to environmental changes. This property casts them in the role of biological Cassandras, prophesying a pessimistic message of environmental degradation that we don't want to hear. Like Homer's Trojans, we've mostly ignored their warnings.(1-5)

References (abridged):

1. Blaustein, A. R., and J. M. Kiesecker. 2002. Complexity in conservation: Lessons from the global decline of amphibian populations. Ecology Letters 5:597-608

2. Croon, B. 1996. Frog data and observations. Earth Focus, Winter.

3. Daszak, P., A. A. Cunningham and A. D. Hyatt. 2000. Emerging infectious diseases of wildlife: threats to biodiversity and human health. Science 287:443-449.

4. Fauci, A. S. 2001. Infectious diseases: Considerations for the 21st century. Clinical Infectious Diseases 32:675-685

5. Hero, J.-M., and L. Shoo. 2003. Conservation of amphibians in the Old World tropics: Defining unique problems associated with regional fauna. In Amphibian Conservation, ed. R. D. Semlitsch. Washington, D.C.: Smithsonian Institution Press.

American Scientist http://www.americanscientist.org

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ECOLOGY: CLIMATE AND THE DECLINE OF AMPHIBIANS

Notes by ScienceWeek

A growing number of researchers have proposed the possibility that the recent global decline of amphibian populations may be caused by increasing amounts of ultraviolet-B radiation (UVB; UV-B) reaching the Earth's surface. Populations of amphibians in widely scattered locations have been drastically reduced in the last decade, and some of these species (such as the golden toad of Costa Rica) have recently become extinct. While no single cause for these declines has been identified, the fact that the declines have occurred in undisturbed areas and throughout the world has prompted considerations of global phenomena.

The categorizations of ultraviolet radiation (UV) differ in physics and biology. In physics, UV radiation is traditionally divided into 4 regions: near UV (400-300 nanometers); middle UV (300-200 nanometers); far UV (200-100 nanometers); extreme UV (below 100 nanometers). In biology, there are two categorizations of UV: a) The 3 conventional divisions are: near UV (400-315 nanometers), which is absorbed relatively poorly by organisms; actinic UV (315-200 nanometers), which is absorbed most readily by organic matter and which has the greatest effect on biological systems; vacuum UV (less than 200 nanometers, which is absorbed by most substances. b) The second type of categorization of UV in biology is based on the interaction of UV radiation with biological materials: UVA (400-315 nanometers) ("black light"); UVB (315-280) nanometers), responsible for the most marked effects on organisms; UVC (280-100 nanometers), which does not reach the Earth's surface.

Ultraviolet radiation is produced by high-temperature surfaces (e.g., the Sun) in a continuous spectrum, and as a discrete spectrum of wavelengths by atomic excitation in a gaseous discharge tube. Most of the UV radiation in sunlight is absorbed by oxygen in Earth's atmosphere, the oxygen that forms the ozone layer of the lower stratosphere. Of the UV that does reach the surface of the Earth, approximately 99 percent is UVA radiation; when the ozone layer becomes thin, more UVB radiation reaches the Earth's surface and may have hazardous effects on organisms.

In general, ultraviolet radiation, especially UVB radiation, is a potent mutation-producing agent (mutagen), since nucleic acids (both DNA and RNA) absorb light in the UV region, with the pyrimidine bases thymine and cytosine especially reactive. UV radiation causes the hydration of cytosine by the insertion of a water molecule ring at a C=C bond, whereas it disrupts the C=C bonds in thymine and generates thymine dimers.

The term "El Nino" refers to an aperiodic intermittent (2 to 10 years) flow of unusually warm surface water along the western coast of South America, the flow capable of causing abnormally high rainfall in usually dry areas and severe local ecosystem dislocations -- what is termed an El Nino "event". El Ninos are regional phenomena, but they have global consequences. The name "El Nino" ("The Child") arose because the phenomenon usually occurs around Christmas. In 1986, M.A. Cane and S.A. Zebiak proposed a model for making forecasts of El Nino several seasons ahead by applying Newton's equations of motion and the laws of thermodynamics to the dynamics of the ocean and atmosphere of the tropical Pacific.

The term "Southern Oscillation" (SO) refers to a coherent interannual fluctuation of atmospheric pressure over the tropical Indo-Pacific region. The El Nino/Southern Oscillation phenomenon (called ENSO), the interaction between El Nino and the Southern Oscillation, is the strongest source of natural variability in Earth's climate system. Although ENSO originates in the tropical latitudes of the Pacific Ocean, its climate impact is felt globally. Variations in major rainfall systems that are attributed to ENSO range from droughts in Indonesia and Australia to storms and flooding in Ecuador and the US. The crucial role of the interaction between the ocean and the atmosphere in the tropical Pacific was first postulated in 1969 by Jacob Bjerknes (1897-1975), and the development of quantitative models has progressed during the past 3 decades. The essence of the current Bjerknes hypothesis, as it is called, is that ENSO arises as a coupled cycle in which anomalies in sea surface temperature in the Pacific cause the trade winds to strengthen or slacken and, in turn, drive the changes in ocean circulation that produce anomalous sea surface temperatures. Ocean-atmosphere feedback can amplify perturbations in either the equatorial sea surface temperature or what is called the Walker Circulation -- the thermodynamic circulation of air parallel to the equator. Although the oscillatory aspect of ENSO behavior is now understood reasonably well, the irregularity of the observed cycle is a subject of active research.

The term "oomycete" refers to a group of water molds and related organisms, the oomycetes producing motile cells that include parasites. Of the water molds, the genus Saprolegnia includes parasitic species that cause diseases of fish and fish eggs. Although oomycetes are often called "fungi", the cell walls of oomycetes are chemically quite different than the cell walls of most fungi.

The following points are made by J.M. Kiesecker et al ((Nature 2001 410:681):

1) The authors point out that amphibian populations have exhibited widespread declines and extinctions in recent decades. Although climatic changes, increased exposure to ultraviolet-B radiation, and increased prevalence of disease have all been implicated at particular localities, the importance of global environmental change remains unclear.

2) The authors report that pathogen outbreaks in amphibian populations in the western US are linked to climate-induced changes in UVB exposure. Using long-term observational data and a field experiment, the authors examined patterns among interannual variability in precipitation, UVB exposure, and infection by a pathogenic oomycete, Saprolegnia ferax. The authors suggest their findings indicate that climate-induced reductions in water depth at egg-laying (oviposition) sites have caused high mortality of embryos by increasing the exposure of embryos to UVB radiation and consequently increasing the vulnerability of the embryos to infection.

3) The authors suggest that precipitation, and thus water depth/UVB exposure, is strongly linked to El Nino/Southern Oscillation cycles, underscoring the role of large-scale climatic patterns involving the tropical Pacific. Elevated sea-surface temperatures in this region since the mid-1970s, which have affected the climate over much of the world, could be the precursor for pathogen-mediated amphibian declines in many regions.

In a commentary on this work, J. Alan Pounds (Nature 2001 410:639) states: "During the 1990s, there was controversy over whether the [amphibian population] declines were real, or simply a consequence of natural population fluctuations and direct human disturbances such as habitat destruction. Meanwhile, a separate debate focused on climate change and its relationship to greenhouse-gas emission. Today, there is little doubt that both phenomena -- amphibian declines and global warming -- are real. If there is indeed a link between the two, as the work of Kiesecker et al suggests, there is clearly a need for a rapid transition to cleaner energy sources if we are to avoid staggering losses of biodiversity."

Nature http://www.nature.com/nature

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