The Problem of Amphibian Population Declines With an Emphasis on Two Species Found to Be Declining in Oregon

Shawn Knoedler


A general overview of amphibian declines and some possible causes

It has recently become evident that amphibians in many parts of the world have suffered serious population declines. Interest in the possibility that there is an on-going global pattern to these declines began in 1989 at the First World Congress of Herpetology in Canterbury, England (Alford and Richards, 1999). Subsequent studies have confirmed that dramatic losses are occurring at alarming rates in widely separated areas of the world. Unprecedented losses of populations and even entire species have been reported from parts of Asia, Europe, North America, South America, Australia and Africa (Petranka, 1998). By 1993, more than 500 populations of salamanders and frogs were listed as declining or of special conservation concern (Alford and Richards, 1999).

So what is causing this disturbing ecological trend? That humans have played a significant role in the decline of amphibians' populations is certainly not a surprise.

The best documented declines have occurred in Europe and North America and are usually associated with habitat modifications resulting from timber harvesting, wetland drainage, urbanization, agriculture, stream pollution and siltation, and the introduction of exotic predators (Alford and Richards, 1999; Blaustein and Wake 1995; Corn and Fogleman, 1984; Fellers & Drost, 1993; Petranka, 1998). Clearly, declines from direct habitat degradation are well researched and documented, but there is growing evidence that many amphibian populations are also declining in areas that have not been overtly altered by human activity. The decline of amphibians in relatively pristine areas may be due to other anthropogenic factors, most notably habitat acidification (Beattie and Jones, 1992; Dunson et al, 1992) and increased UV-B radiation (Alford and Richards, 1999; Blaustein et al, 1994; Blaustein and Wake, 1995; Fite et al, 1998; Pounds, 2001). Subsequent research has revealed that complex interactions between several synergistic factors commonly result in increased rates of amphibian mortality. For instance, Kiesecker et al (2001) found that fatal pathogen outbreaks in many amphibian populations are directly linked to climate-induced changes in UV-B exposure.

Declining amphibian populations in Oregon

The Pacific Northwest has not escaped the worldwide amphibian crisis. A significant proportion of the amphibians of the Pacific Northwest have extremely limited distributions and possess relatively specific ecological requirements (Walls et al., 1992). These characteristics greatly increase their risk of local extinction. As of 1992, five species of Pacific Northwest amphibians were candidates for the United States endangered species list (Walls et al., 1992). Species that inhabit old growth forests and high altitude habitats seem to be most at risk of decimation from direct and indirect anthropogenic factors (Kiesecker et al, 2001; Walls et al, 1992). The old growth forests of the Pacific Northwest are among the most biologically productive ecosystems in the world, but harvesting has had a significant impact on all of the plants and animals within them (Walls, 1992). This paper will concentrate on two species whose Oregon populations have drastically declined in recent years: the Cascades frog (Rana cascadae) and the western toad (Bufo boreas).

As recently as 1983, R. cascadae was described in Oregon as “extremely abundant in their appropriate habitats” (Nussbaum et al, 1983). Currently, however, at least 80% of its historical population has disappeared in Oregon and it has become extinct in most of the southern parts of its range (Fellers and Drost, 1993). Populations of B. boreas were apparently at a healthy level until steep declines were noticed between 1974 and 1982 (Blaustein and Wake, 1995). In 1979, Andrew Blaustein and his students from Oregon State University began a study of these two species. Their experiments were usually conducted above 4,000 feet in remote and undisturbed habitats in the Cascade Mountains. Their research lead to the discovery of extremely high mortality rates of fertilized eggs of both species. They also noted that the numbers of adults of both species were dropping and attributed this decline to the low numbers of eggs that were surviving (Blaustein and Wake, 1995). But why were so many eggs perishing? Laboratory experiments ruled out the possibility that the chemistry of the water at egg deposition sites was to blame. They began to look for other answers and soon formed the hypothesis that increased exposure to UV-B radiation was causing harm to these amphibians.

It is widely known that UV-B radiation can cause serious damage to DNA by breaking apart the hydrogen bonds between nitrogenous bases and forming new structures. These changes can disrupt the functions of a cell and even kill it. However, many organisms can activate an enzyme, photolyase, which removes the damaging structures (Blaustein et al, 1994). The researchers determined that both R. cascadae and B. boreas produced relatively low levels of photolyase, and therefore had little protection from the damaging effects of ultraviolet radiation. Further research has lead to the discovery that UV-B radiation has also contributed to amphibian sicknesses caused by outbreaks of the fungus Saprolegnia, which is found naturally in Oregon lakes and ponds (Kiesecker et al, 2001). Because UV rays have been shown to reduce immune function in some animals, the researchers believe that some amount of amphibian egg damage is caused by an ultraviolet-induced breakdown in the ability of amphibian embryos to resist infection by the fungus. Blaustein (1995) also speculates that since high levels of UV radiation has been shown to kill aquatic insect larvae as well as aquatic algae; the corresponding reduction in food supply may also be contributing to amphibian declines in Oregon. In addition, studies have shown that high levels of UV radiation have caused serious retinal damage to several species of high-altitude dwelling amphibians, including R. cascadae (Fite et al, 1998). This would obviously have a negative effect on these species’ predatory success.

Fellers and Drost (1993) determined that the introductions of several species of non-native fish (eastern brook trout- Salvelinus fontinalis, rainbow trout- Salmo gairdneri) have lead to declines in R. cascadae populations. Non-native fish limit dispersal of amphibians. Populations in tributaries are cut off from other populations by fish in the main stream. The introduction of another non-native species, the bullfrog (Rana catesbeiana) has also lead to declines in Oregon amphibian populations. The bullfrog is much larger than native species, and it has out competed many of the native amphibians. While mostly an inhabitant of warm water ponds, there is growing evidence that it has invaded some higher elevation ponds that were previously thought to be too warm for it (Corkran and Thoms, 1996).

The significance of amphibian declines and measures being taken to change the trend

Amphibians are abundant and integral components of many ecosystems. In some ecosystems, they are the most abundant vertebrates and are integral to the maintenance of a healthy and productive food web. Adult amphibians feed on a diverse array of animals, including mosquitoes, flies, fish, birds, and mammals. Also, larval amphibians commonly serve as a food supply for many of these same creatures. The decimation of amphibians could result in the collapse of many ecological communities (Walls et al, 1992).

A major debate has centered on whether amphibian declines are due to “normal” but drastic fluctuations in population sizes or more insidious, human-related causes (Blaustein, 1994; Pechman et al, 1991; Pechman and Wilbur, 1994). Blaustein feels that while Pechman brings a “healthy skepticism” to certain arguments of the amphibian decline problem, he strongly disagrees with any assertion that the current decline in amphibians is a phenomenon distinct from a general, wide-sweeping biodiversity crisis. He also disagrees with Pechman’s assertion that amphibians are not reliable indicators of environmental stress. Blaustein is among a seeming majority of researchers who feel that amphibians do indeed act as “canaries”. They have permeable skin and eggs that can readily absorb substances from their environment. In addition, they have complex life histories that potentially expose them to both aquatic and terrestrial agents. (Blaustein, 1994). They are clearly an early warning signal that ultimately, other organisms are also at risk.

Today, it is widely accepted that amphibians worldwide are experiencing dramatic population declines and that much of the blame for these declines rests squarely on the shoulders of the human race. Habitat destruction is clearly the major cause of overall declines, but there is clearly a host of human related and interconnected causes to this problem. Until our species either stops growing or experiences a radical paradigm shift in regards to our relationships with other species, this is a situation that seems unlikely to change. Luckily, there are some brilliant and dedicated minds that are currently working on this problem. Aside from the research that is crucially needed to answer lingering questions about these declines, land managers must come up with thoughtful plans that consider the needs of amphibians. Walls et al., (1992) outlined several strategies to maintain viable populations of amphibians in old growth forests that are slated for “harvesting”. The most important strategy is to “minimize direct logging effects”. This should be accomplished by placing a heavier emphasis on cable logging in the winter so as to minimize soil compaction caused by tractor logging. This, they say, will decrease the mortality of hibernating amphibians. In addition the forest canopy should be maximized so as to minimize soil desiccation, alteration of under story vegetation should be kept to a minimum, and there should be ample provisions of suitable microhabitat (leaf litter, loose bark, down logs and other woody matter).

It is equally important that the message of amphibian declines and the overall biodiversity crisis be spread as widely as possible. Many middle and high school teachers have introduced biodiversity into their curriculums and several organizations have been formed to address the amphibian decline problem specifically. One of these is SCOPE, an online collection of materials about population declines in amphibians.

Another organization is The Declining Amphibian Populations Task Force (DAPTF). Their mission is “to determine the nature, extent, and causes of declines of amphibians throughout the world and to promote means by which declines can be altered or reversed”.

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