Case Study: Amphibians

INTRODUCTION

There is little doubt that human activity is causing loss of species through considerable damage to natural environments, and amphibians have suffered the most. Indeed Sodhi et al (2008) observe that due to their major declines in population, susceptibility to disease, and morphological deformities, amphibians epitomise the modern biodiversity crisis.

Amphibians are cold-blooded vertebrate animals, which metamorphose from water-breathing juveniles to air-breathing adults (Stuart et al. 2004).The current extent of amphibian extinction is perplexing, because during their 350 million year period of evolution, they have managed to overcome mass extinctions similar to that of the present. The exact characteristics enabling these creatures to survive extreme conditions have been the subject of much research (Wake and Vredenberg 2008).

Deep-sea sediment cores suggest that over the past 65 million years, global climate change has been almost continuous, but the recent rapid warming is of particular concern (Carey and Alexander 2003). By comparing the current amphibian extinction rate with background fossil rate, we can improve our understanding of the magnitude of the current biodiversity crisis and the extent to which humans are responsible (McCallum 2007).

Figure 1. Relative percentages of species loss from different altitudes. Source: Pounds et al. (2006).

STATISTICS

Recent research has shown that the current extinction rate exceeds that of both the 1500 and 1980 level, rates of amphibian declines are catastrophic, and projected losses for the future are even more intense.

The 1989 Congress Of Herpetology marked the start of scientific concerns about amphibian population declines, and the IUCN GAA (Global Amphibian Assessment) was conducted in order to gauge the severity of these declines. Results from this indicate that 43.2% of amphibian species are declining, 122 are possibly extinct, and up to a third are at risk of extinction (Stuart et al.2004, Pounds et al. 2006, Wake and Vredenberg 2008, McCallum 2007).

CAUSES

Colins and Storfer (2003) note that there are six underlying hypotheses explaining amphibian declines. Three of these; the invasion of alien species, overexploitation and excessive harvesting of amphibian populations, and land use change, are relatively well understood. By removing, introducing, or changing constants, it is likely that amphibians will suffer. The remaining hypotheses involve complex interactions between global change, infectious diseases and trends amongst amphibian populations.

·      Global Warming

Humans influence on the earth’s climate, large scale warming, and aerosol formation intensify the hydrological cycle and shift the balance between ecological conditions, in turn leading to threats to species survival (Walther et al. 2002, Pounds et al. 2006).

In the Cascade area of Western North America, shallow lakes and ponds provide an ideal location for Western Toads, Bufo boreas, to lay their eggs. Recently, though, the embryos of these toads have begun to die before becoming properly developed. Climate change induced reductions in pond level are thought to be to blame. Shallower water overlying eggs means that protection against exposure to UV-B is reduced, for instance, where water depth is less than 20cm, 80% of toad embryos are killed by Saprolegnia, whereas this figure is only 12% in water depth of 50cm (Pounds2001).

The indirect effects of global climate change include changes to the phenology of breeding, in the case of amphibians, the seasonal variation in timings of egg laying. Evidence suggests that breeding seasons mirror climatic conditions; this puts those hatching early at a disadvantageous risk of mortality due to cold temperatures.

Figure 2. Relationship between climate change, amphibian declines, and extinctions. Source: Pounds (2001).

·      Characteristics of amphibians

Amphibians are directly influenced by temperature and moisture. Their cellular and physiological processes are controlled by heat exchange with air, water, and solar heat. Hence, severe daily temperature fluctuations are dangerous for bodily function.

Most amphibians are found in specific tropical geographic ranges, and aren’t capable of adapting well to different environmental conditions. It is to be expected, therefore, that further habitat changes will accelerate species loss, particularly of those dwelling in ecologically pristine areas. The disappearance of the Monteverde toad and Harlequin frog during unusually warm years illustrate this (Daszak et al. 1999, Wake and Vredenberg 2008, Pounds et al. 2006).

In reproductive terms, water is vital for amphibian existence. Eggs and larvae are deposited in standing water, and so annual variation in rainfall will influence both the number of eggs laid and hatched (Carey and Alexander 2003).

The permeable skin and hormonally regulated development of amphibians makes them highly vulnerable to endocrine disruption. For instance, the herbicide Atrazine, a common contaminant of ground and surface water where amphibians breed, is highly active at low concentrations. This compound chemically castrates and feminizes male amphibian larvae, retards development and growth, and is the cause of unusual behaviour and immunosuppression (Hayes et al. 2006). Although now banned in the EU, Atrazine is still widely used in the USA.

·      Disease

Chytridiomycosis is a panzootic fungal amphibian disease, caused by Batrachochytrium. It develops and spreads in moist aquatic habitats, particularly during the winter. The disease is persistent at low densities, and attacks the moist skin of amphibians by degrading cellulose, chitin and keratin, and producing zoospores. The discovery of a new form of chytrid fungus, Batrachochytrium dendrobatidis, coincided with the observation that amphibian declines were taking place, and sporangia of this fungus were found within mouthparts of tadpoles, particularly from montane habitats (Daszak et al. 1999, Carey and Alexander 2003, Pounds et al. 2006, Pounds 2001). Similarly, the Saprolegnia ferax fungus has caused mortality of amphibian eggs in the Pacific North West, and loss of the Western toad Bufo boreas. Ranaviruses are another concern, and spread of these has increased due to humans (Collins and Storfer 2003, Carey andAlexander 2003).

In addition to these problems, limited knowledge of the true numbers of creatures no longer existing on earth presents further challenges to ensuring the survival of those remaining (McCallum 2007, Stuart et al. 2004).

References

Carey C. and Alexander M.A. (2003) ‘Climate change and amphibian declines: is there a link?’, Diversity and Distributions, 9, 111-121.
Collins J.P. and Storfer A. (2003) ‘Global amphibian declines: sorting the hypotheses’, Diversity and Distributions, 9, 89-98.

Daszak P., Berger L., Cunningham A.A., Hyatt A.D., Green D.E. and Speare R. (1999) ‘Emerging infectious diseases and amphibian population declines’, Emerging infectious diseases, 5, 6, 735-748.

Hayes T.B., Case P., Chui S., Chung D., Haeffele C., Haston K., Lee M., Mai V.-P., Marjuoa Y., Parker J., and Tsui M. (2006) ‘Pesticide Mixtures, Endocrine Disruption, and Amphibian Declines: Are we underestimating the impact?’, Environmental Health Perspectives, 114, 1, 40-50.

McCallum M.L. (2007) ‘Amphibian Decline or Extinction? Current Declines Dwarf Background Extinction Rate’, Journal of Herpetology, 41, 3, 483-491.

Pounds J.A. (2001) ‘Climate and amphibian declines’, Nature, 410, 369-340.

Pounds J.A., Bustamante M.R., Coloma L.A., Consuegra J.A., Fogden M.P.L, Foster P.N., La Marca E., Masters K.L., Merino-Viteri A., Puschendorf R., Ron S.R., Sanchez-Azofeifa G.S., Still C.J. and Young B.E. (2006) ‘Widespread amphibian extinctions from epidemic disease driven by global warming’, Nature, 439, 161-167.

Sodhi N.J., Bickford D., Diesmos T.M.L., Lian P.K., Brook B.W., Sekercioglu C.H. and Bradshaw C.J.A. (2008) ‘Measuring the meltdown: drivers of global amphibian extinction and decline’, Public Library of Science, 3, 2, 1-8.

Stuart S.N., Chanson J.S., Cox N.A., Young B.E., Rodrigues A.S.L., Fischman D.L. and Waller R.L. (2004) ‘Status and trends of amphibian declines and extinctions worldwide’, ScienceExpress, [www] available from: http://people.nnu.edu/jocossel/Stuart%20et%20al%202004.pdf, [19/12/2011].

Wake D.B. and Vredenberg V.T. (2008) ‘Are we in the midst of the sixth mass extinction? A view from the world of amphibians’, 105, 1, 11466-11476.

Walther G.-R., Post E., Convey P., Menzel A., Parmesan C., Beebee T.J.C., Fromentin J.-M., Hoegh-Guldberg O. and Bairlein F. (2002) ‘Ecological responses to recent climate change’, Nature, 416, 389-395.