Table of Contents Executive Summary 2
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- Global Population Trends
- Anthropogenic Causes of Amphibian Population Decline
- Global Climate Change
- Fungal Pathogen Outbreaks
- Herbicides
- Species Introduction
Global Amphibian DeclineA literature review of global amphibian declines was used to strengthen the need for reducing amphibian mortality on Vermont roadways. Globally, amphibian populations are in dire conditions due to anthropogenic impacts on the environment. Many of the identified factors behind these declines are global problems that have no attainable short-term solution. The combination of these mechanisms is actually enough to lead scientists to worry about the future of amphibians as a whole. Because of this, it is critically important to reduce sources of amphibian mortality wherever it is feasible. The following section of this report will describe some of the sources of global population decline Global Population TrendsA large-scale study of global amphibian population trends was compiled which strongly suggested population decline is occurring at a global scale (Houlahan et al 2000). The study consisted of 936 data sets from publications, reports, and unpublished data from 37 countries and included 157 species. This study found that a rapid global amphibian decline from 1960-1966 was followed by a less intensive decline from 1966-1997. The decline rates were approximately 15% annually and 2% annually respectively with a total of 61 documented extinctions (Houlahan et al 2000). A majority of the historic declines were observed in Europe, but more recent rapid declines have occurred in Costa Rica, Panama, and Australia. Until recently, most scientists believed these declines were simply cyclical downturns in natural population dynamics. However, new evidence indicates that these declines suggest a more catastrophic cycle (Dalton 2000). Many of the recent population declines are of special concern due to the fact that they occurred in protected areas such as reserves and parks with no clear causative agent (Halliday 1998). Several well-documented localized population declines have been observed in relatively undisturbed sites. These declines are strong evidence for some type of global factor that is likely an indirect anthropogenic effect (Halliday 1998). A majority of the species-specific declines have occurred in isolated high-altitude ecosystems (Houlahan et al 2001). There are countless examples of species-specific declines that serve to illustrate the global connection between local declines. The Golden Toad (Bufo periglenes) was discovered in a Costa Rican Cloud Forest and went from fairly common with an estimated population of 1,000 down to a single toad within a few years. The gastric brooding frog (Rheobatrachus silius), only discovered in 1973, was found in relative abundance until its rapid decline to extinction in 1981 (Phillips 1990). Anthropogenic Causes of Amphibian Population DeclineA single clear causative agent for global amphibian decline has yet to be identified, but numerous anthropogenic influences have been shown to negatively affect amphibian populations. Mechanisms range from direct anthropogenic impacts such as habitat destruction, to more indirect impacts such as pesticide interactions increasing susceptibility to parasites and fungal infections. Global Climate ChangeGlobal climate change has been linked to two major vectors of local population declines. Localized warming has produced niche overlapping and increased interactions between species that historically had been isolated temporally and spatially (Walther et al 2002). Climatic warming in Britain has led to a source of population decline from a previously insignificant interaction between populations of newts and frogs. Both species share breeding habitats in shallow lakes and wetlands, but share different temporal breeding niches. Warmer winters have created variable responses within the two populations. Historically, salamanders have entered the breeding areas well after the frog eggs have already hatched, but due to warming they are entering earlier. The frogs have not significantly changed their breeding timing and are now suffering egg mortality from a new source of predation (Walther et al 2002). Climate change has also created strong warming in the air-water temperature averages in the tropical regions of the Pacific Ocean leading to increased occurrence of large-scale climatic oscillation cycles. These cycles drastically affect local climate in many areas and are responsible for the recent widespread devastation observed in numerous ecosystems from coral reefs to cloud forests (Pounds 2001). Climate change has been implicated as an indirect cause for numerous disease outbreaks responsible for amphibian population declines globally (Pounds 2001). Fungal Pathogen OutbreaksGlobal climate change has also been linked through a very complex system to amphibian mortality through fungal infections. Numerous studies have found that certain amphibian populations are being exposed to increased levels of UV-B radiation, resulting in increased rates of fungal infections (Kiesecher et al 2001 and Pounds 2001). Amphibians that breed in shallow montane lakes have been most impacted by UV-B related mortality. Western Toads (Bufo boreas) are the most studied montane species for this mechanism of population decline. Studies have found that their string of eggs develop normally for a few days and then turn cloudy and white experiencing drastic mortality from a lethal fungus infection (Pounds 2001). The fungal strain Saprolegnia ferax has been documented as the primary cause of these population declines found in the Northwestern United States (Kiesecher et al 2001). Most of these populations have rapidly decreased since the 1980’s and numerous montane populations are now extinct (Kiesecher et al 2001). The mechanism for the Saprolegnia ferax infection is an example of widespread environmental problems working together synergistically. Previous studies had linked fungal mortality to stratospheric ozone depletion, but new research has shown that climate is the primary factor (Pounds 2001). The climatic oscillations mentioned earlier in this report are directly correlated with snowfall in the Cascades and therefore annual lake levels (Pounds 2001). Fungal infections have been linked to decreased water levels, which increases UV-B exposure in oviposition sites utilized by the Western Toad as shown in Figure 1 (Kiesecher et al 2001). This mechanism has been observed in the Western Toad and other montane s Amphibian populations in Central America and the Australian Highlands are experiencing population declines from fungal infections that appear to be from a different mechanism. Tilarian rain frogs (Eleutherodactylus angelicus) in Costa Rica have experienced fungal infections very similar to Western Toads, but the rain frogs lay their eggs in shade, ruling out UV-B exposure as a causative agent (Pounds 2001). Amphibian declines observed in Australia and Central America were caused by a chytrid fungus, which affects cutaneous respiration. Concurrent declines in reptile populations, which are not susceptible to this type of infection, suggest that a different mechanism must be present (Pounds 2001). This type of chytrid fungus had not been previously documented. The fact that it turned up on opposite sides of the world leads scientists to believe that it is an opportunistic fungus which appeared due to compromised immune system ability, increasing the susceptibility of amphibians to infection (Halliday 1998). Numerous other pathogens have recently increased in abundance suggesting that environmental factors are leading to decreased immune system function (Halliday 1998). HerbicidesA more direct anthropogenic cause of amphibian mortality has been shown through numerous studies relating the popular herbicide Atrazine to endocrine disruption in frogs. Atrazine is the most commonly used herbicide in the United States, with approximately 27,000 tons applied annually. It is also used in over 80 countries, making Atrazine one of the most important herbicides globally (Dalton 2002). Atrazine contains endocrine disruptors that have been linked to disruptions of sexual development in frogs (Dalton 2002). This source of amphibian mortality was first described in the 1980’s when researchers observed increases in frog mortality and deformities (Netting 2000). Studies have found that levels as low as 0.1ppb cause male tadpoles to develop ovaries in addition to testes. Levels greater that 1 ppb lead to the deformation of larynges, organs that are used to call for mates. Levels higher than 25 ppb are shown to cause drastic drops in testosterone levels, which have been observed in wild leopard frogs in numerous sites with high levels of Atrazine. Different from most lab studies of toxicity, these numbers are ecologically relevant because American waters routinely have levels above 50ppb and in some extreme cases, concentrations can be measured in parts per million (Dalton 2002). Species IntroductionSpecies introduction is yet another anthropogenic influence that is leading to amphibian population decline. The introduction of predatory fish to water bodies has led to an indirect source of stress and mortality in tadpoles. When in the presence of predatory fish, tadpoles take cover on the lake bottom and remain relatively motionless. This behavior reduces fish predation, but it greatly increases the risk of parasite infestation. Parasitic infestations rarely result in direct mortality, but the attacks damage kidneys and other organs, leaving a legacy of increased susceptibility to disease as adults. Increased stocking programs have introduced predatory fish to countless aquatic systems, greatly increasing this mechanism of amphibian population decline (Netting 2001). Wetland destruction can also lead to increased parasitism by concentrating tadpoles and snails that host the parasites, in the remaining wetland patches (Netting 2001). Another factor increasing parasitism is the potential for some pesticides to reduce vigor and mobility of tadpoles (Netting 2001). Bull-frog (Rana catesbeiana) and exotic fish introduction has become common practice among permanent wetlands in the United States for aquaculture and sport-fishing purposes, and have recently been shown to reduce populations of native anurans (Adams 1999). Bull-frogs are generalist feeders and predators that negatively impact native amphibians through predation and competition. Many of the exotic fish commonly introduced to aquatic systems prey on tadpoles and adult amphibians. Tadpole mortality from fish predation can be incredibly high in some species that evolved in the absence of predatory fish. This source of predation is very prominent in more developed areas, where most wetlands have bullfrog or fish introduction. Some studies have found that it is very rare to find any native amphibians in systems once bullfrogs have become established (Adams 1999). Download 181.31 Kb. Share with your friends:
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pecies that lay their eggs in shallow water areas. Water column depth is an efficient filter of UV-B radiation, with eggs laid in 50 cm of water receiving half of the UV-B as eggs in 10cm of water (Pounds 2001). The direct mechanism for fungal infection is not yet know but laboratory manipulation has shown in 33% increase in infection rates for Western Toad eggs exposed to UV-B. In natural lakes, mortality rates of 80% are common in shallow sites (20cm) while only 12% in water deeper than 50cm (Pounds 2001). Increased UV-B radiation from either stratospheric ozone depletion, or water-draw down may also harm any other shallow water breeders as well(Pounds 2001).