AMPHIBIAN DECLINES

Amphibians are checking out all over the planet. Many countries have noticed sudden and massive declines of local frogs, in recent years. Considering that amphibians have been around since before the time of the dinosaurs, we should be concerned that they are starting to disappear now. The following is important reading for anyone who wants to understand why our much loved frogs are disappearing before our very eyes...
 

Introduction

Amphibians are very important components of many ecosystems and they may constitute the highest fraction of vertebrate biomass in some of these ecosystems (Blaustein et al., 1994a). Unfortunately, however, amphibians are starting to vanish before our very eyes.

All over the globe the same trend is being repeated; amphibians are rapidly disappearing from remote areas untouched by humans (Blaustein and Wake, 1990). One of the most dramatic examples of this phenomenon has been documented by Pounds and Crump (1994), who were studying the golden toad (Bufo periglenes) in the Monteverde Cloud Forest Preserve, Costa Rica. In 1987, 1500 toads gathered to mate at a pool, as part of an annual spectacle which had been occurring since at least 1972 when the study was started. In 1988, the following year, only a single toad turned up to breed at the pool. Data revealed that sightings of golden toads and harlequin frogs had decreased by 99% during one year. A similar trend was observed by Californian researchers, who documented a decline in all native amphibians of Californiaâs Central Valley. One species Rana aurora, was found to have disappeared from 24 out of 28 counties (Fisher and Shaffer, 1996). Similarly, in Canada, Hecnar and MâCloskey (1996), documented a significant decline in amphibian species richness in one county. There are numerous other examples documented, and amphibian declines have been recorded on all continents where they occur.

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Declines in Australia

Amphibian declines are well documented in Australia, and Queensland, New South Wales and Victoria, have been especially hard hit by them. In a recent survey of the western division of New South Wales, 8 amphibians were listed as endangered, rare or vulnerable as a result of population declines (Sadlier and Pressey, 1994). Similarly, studies of Litoria spenceri indicated that it had disappeared from 10 of the sites in Victoria that it previously occupied in significant numbers (Gillespie and Hollis, 1996).

Many previously abundant Australian frog species have not been seen for decades. Up to 27 species of Australian frog are currently listed as endangered or vulnerable, and several are presumed extinct (IUCN, 1997). Queensland and NSW, have seen at least 14 species virtually disappear (Laurance et al., 1996). All of these species generally breed in or live near rainforest streams in small pockets of montane rain forest. Declines were first noticed in the 1970âs, with the famous gastric-brooding frog Rheobatrachus silus, which is now feared extinct. Three other species, Mixophyes iteratus, M. fleayi and Litoria pearsoniana, declined by more than 90% and now exist only in small extant populations. The Torrent Frog Litoria nannotis has also seriously declined in stream habitats of north-eastern Queensland.
 

Torrent Frog - Litoria nannotisThe Torrent Frog - A declining species.
The northern gastric-brooding frog, R. vitellinus, disappeared in 1985 and has not been since.
More recently, large scale population declines have been detected, starting in 1989, and resulting in the sharp decline of at least 7 more species, including 4 species of tree frogs and 3 species of the torrent frog Taudactylus sp. (Laurance et al., 1996). Four of these species can no longer be located in the wild, and the other 3 species have disappeared from upland areas, but still persist in lowland areas. It is now feared that Rheobatrachus and Taudactylus species seem to have declined to the point of extinction (McDonald, 1990).
 
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International Concern Grows..

Concern is so great for the phenomenon of amphibian declines, that the International Union for the Conservation of Nature (IUCN) has set up the Declining Amphibian Populations Task Force (DAPTF) to investigate the matter. Many causes have been postulated for the declines being observed. These include acid rain and changing weather patterns (Pounds and Crump, 1994), increased ultraviolet radiation, habitat destruction and most recently, a pathogen (Barinaga, 1990).

There has been a push to quantify declines amid suggestions, that we could be simply observing a natural phenomenon, and the declines may not be ãrealä (Blaustein et al., 1994a). It is possible that observed reductions in populations may be attributable to natural cyclical population events, influenced by weather patterns and other environmental variables. Research has documented long term fluctuations in population dynamics of several frog populations, and tends to suggest that for many species, this is normal (Ingram, 1983; Osborne, 1989; Humphries, 1979).

Such research has emphasised caution when dealing with amphibian declines, and stresses the need for objective, long term monitoring programs, and the necessity for further research into possible causes of declines. Roberts (1993) also makes the interesting point that it is important to distinguish between non-attendance at breeding sites and death. The reduction in numbers of breeding frogs at a site, may not mean the population is low or that frogs are dead. Individuals may simply be surviving in aestivation (such as in response to drought conditions) and will still survive to breed the next season. Thus they have not actually succumbed to mortality.

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Climate Disturbance

Evidence for the role of climatic disturbance on frogs, has come from Pounds and Crump (1994). They found that there was a marked shift in the population distribution of harlequin frogs in Costa Rica, in response to the 1986-1987 El Nino. Elevated temperatures associated with the El Nino, also lead to desiccation of breeding ponds, and subsequent mortality of tadpoles and eggs. It is also likely that increased temperatures could result in higher adult mortality, due to the increased rates of water loss associated with these dry conditions. Many amphibian declines have occurred in montane areas, where climate is especially vulnerable to atmospheric disturbances. This is especially pertinent given the large concentration of amphibian species in high altitude rainforest in Australia, South America and Africa. The decline of several Queensland and Costa Rican frog species in montane habitats, has given some support to this hypothesis.

Another climate theory postulates that abnormally dry and warm conditions (associated with El Nino), could lead to atmospheric contaminants being scavenged by mist and cloud water in montane areas, reaching critical concentrations which could kill frogs (Pounds and Crump, 1994). This is known as the climate-linked contaminant pulse hypothesis.

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Is a virus responsible?

The pathogen-linked amphibian decline hypothesis has been investigated by Laurance et al. (1996), who isolated a viral agent in one of the declining frog species. They also discovered that frogs were more likely to die in an experimental stream set-up, than in a standing pool, suggesting a stream-borne viral agent. The vector for this virus has been suggested as migratory fish, cane toads, aquatic birds, aquatic insects or the stream water itself. Several factors of the declines, including the wave-like front of declines, the rapid and precipitous decline of populations, higher intensity declines at elevated locations, selective decline of certain species and large scale mortality of adult and juvenile frogs, but not tadpoles, all seem to strongly suggest a pathogen is responsible (Laurance et al., 1996). Just recently, strong support for this hypothesis has come from Slocombe et al. (1995). A Perkinsus-like (protozoan) organism has been found in the skin of dead frogs from around Australia including frogs that died during mass mortalities in Queensland. At least one of these mortality events has been associated with a significant population decline. The organism implicated has similarities to the Perkinsus genus which is a virulent pathogen of oysters and abalone. In tests, this organism has been clearly associated with frog deaths, abscesses and ill health, in the laboratory, and has been isolated in all dead frogs.

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The role of habitat fragmentation

Even if the pathogen theory is proven correct, habitat destruction is undoubtedly a major co-factor in amphibian losses, and even if it is not directly causing mass mortalities, it almost certainly influences longer term population numbers and demographics. Habitat fragmentation has been insinuated as an important factor in amphibian declines, by several researchers (Blaustein et al. 1993; Fisher and Shaffer 1996; Hecnar and MâCloskey 1996; Gillespie and Hollis, 1996). Gillespie and Hollis (1996), suggest that timber harvesting, habitat decline and anthropogenic disturbance may be negatively influencing populations of  L. spenceri in Victoria. Hecnar and MâCloskey suggest habitat loss as a key factor in the demise of Canadian amphibian populations, and in studying the decline of Californian amphibians, Fisher and Shaffer (1996), suggest that habitat modification has paved the way for introduced species to extirpate native frogs. Laan and Verboom (1990) found a decline in amphibians in fragmented, unconnected woodland habitat, in comparison to homogenous habitat connected by corridors to nearby woodland. Similalry, Wind (1996) found that forest fragmentation reduced the number of wood frogs (Rana sylvatica) in Canada.

There is much anecdotal evidence to suggest that habitat fragmentation is a significant variable influencing amphibian populations, but no more than a handful of studies have been conducted in this area, and the few that the author is aware of, have focused on Amazonion or American, and not Australian frogs.

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Conclusion

Due to their sensitive skin and aquatic habit, amphibians are extremely useful as indicators of the state of our environment. There have been dramatic increases in sightings of deformed amphibians all over the world, and as we choke our rivers and streams with lead, mercury, selenium, fertilisers and waste, frogs are being poisoned and deformed on a massive scale.

In addition to this, it is now well established that amphibians (salamanders as well as frogs) are checking out all over the planet. We have probably already lost a hundred or more species forever. Modern amphibians have been around for 100 million years, and survived the catastrophe that killed off the dinosaurs. Considering this fact, perhaps we should start getting worried about the fact that they are starting to check out now...

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References

Barinaga, M. (1990). Where have all the froggies gone? Science, 247: 1033-4.

Blaustein, A.R. and Wake, D.B. (1990) Declining amphibians: a global phenomenon? Trends in Ecology and Evolution, 5: 203-4.

Blaustein, A.R., Wake, D.B. and Sousa, W.P. (1994a) Amphibian declines: judging stability, persistence, and susceptibility of populations to local and global extinctions. Conservation Biology, 8(1): 60-71.

Fisher, R. And Shaffer, H. (1996) The decline of amphibians in Californiaâs Great Central Valley. Conservation Biology, 10(5): 1387-1397.

Gillespie, G.R. and Hollis, G.J. (1996) Distribution and habitat of the spotted tree frog, Litoria spenceri Dubois (Anura:Hylidae), and an assessment of potential causes of population declines. Wildlife Research, 23: 49-75.

Hecnar, S.J. and MâCloskey, R.T. (1996) Regional dynamics and the status of amphibians. Ecology, 77(7): 2091-2097.

Humphries, R.B. (1979) Dynamics of a breeding frog community. PhD thesis, Australian National University, Canberra.

Laan, R. And Verboom, B. (1990) Effects of pool size and isolation on amphibians communities. Biological Conservation, 54: 251-262.

Laurance, W.F., McDonald, K.R. and Speare, R. (1996) Epidemic disease and the catastrophic decline of Australian rain forest frogs. Conservation Biology, 10(2): 406-413.

McDonald, K.R. (1990) Rheobatrachus Liem and Taudactylus Straughan and Lee (Anura: Leptodactylidae) in Eungella National Park, Queensland, : distribution and decline. Transactions of the Royal Society of South Australia, 114: 187-194.

Osborne, W.S. (1989) Distribution, relative abundance and conservation status of corroboree frogs, Pseudophryne corroboree Moore (Anura: Myobatrachidae). Australian Wildlife Research, 16: 537-547.

Pounds, J.A. and Crump, M.L. (1994) Amphibian declines and climate disturbance: The case of the golden toad and the harlequin frog. Conservation Biology, 8(1): 72-85.

Roberts, J.D. (1993). Natural History of the Anura. Pp 28-34 in Glasby, C.J., Ross, G.J.B. and Beesley, P.L. (eds) (1993). Fauna of Australia. Volume 2A Amphibia and Reptilia. Australian Governments Publishing Service : Canberra.

Sadlier, R.A. and Pressey, R.L. (1994) Reptiles and amphibians of particular conservation concern in the western division of New South Wales: a preliminary review. Biological Conservation, 69: 41-51.

Slocombe, R., H. Mc Cracken, R. Booth, J. Slocombe, and C. Birch. 1995. Infectious skin diseases of captive frogs. In Australian Society for Veterinary Pathology - Annual Proceedings, J. Taylor (ed.). Melbourne, Australia, pp. 14.

Wind, E. (1996) Habitat associations of wood frogs (Rana sylvatica), and effects of fragmentation, in boreal mixedwood forests. Unpublished honours thesis. University of British Columbia, Canada.

Links

San Fransisco Chronicle  Amphibian Declines Article

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Created: 10/3/98
Updated: 10/8/03