Zl3203, 2001-05-29

The Evidence and Possible Causes for the Disappearing Frogs in Australia

Abstract.

Of the 205 species of amphibians in Australia eleven have declined or disappeared since the 1985 and 1989. (Richards et al., 1993; Alford & Richards, 1999; McDonald and Alford, 1999) Declines and losses of amphibian populations are a global problem with complex local causes largely unknown. However, the causes may include feral pig predation, ultraviolet radiation, habitat modification or degradation, changes in climate or weather patterns or a combination of these factors (Alford & Richards, 1999).

This paper discusses the general biology and natural sensitivity of frogs and possible causes of decline.

Introduction.

A total of 205 species of amphibians have been recorded from five families in Australia. However, only Anurans (frogs and toads) occur in Australia; there are no natural populations of caudates (salamanders) or gymnophods (cacilians) (Barton, 1999). Over 90% of the Australian amphibian fauna belong to two families, Myobatrachidae and Hylidae (Barton, 1999). At species level over 90% of the frog fauna in Australia is endemic (Barton, 1999).

In recent years there has been a dramatic decline in Australian frog populations and disappearance of several species from significant area of their range. However the cause(s) are unclear as different studies have indicated different causes. Much of the research and monitoring carried out up to date have lacked hypotheses testing or tried to generate them (e.g. Richards et al., 1993; Speare, 1995) (McDonald and Alford, 1999). Some authors (Bradford, 1989; Blaustein et al., 1994; Kiesecker and Blaustein, 1997 and Lips, 1999) believe that frogs because of their sensitivity to changes in the environment including presence of contaminants are indicator species of environmental health, which has raised a general concern among scientists and others(Lips, 1999).

If this hypotheses is proven to be correct we are faced with a world wide problem.

This essay will discuss the biology of frogs with respect to their sensitivity and possible causes of decline.

Frog Biology and Their Sensitivity.

Amphibians, especially those which are terrestrial species, generally inhabit environments that are hostile to their basic physiology. Because they are ectotherms and have a permeable skin, they are more susceptible to changing environments then any other tetrapod (Duellman & Trueb, 1986). Even so, amphibians have adapted by many unique morphological structures and physiological mechanisms so they can sustain a life in nearly all terrestrial habitats, ranging from Arctic tundra through rainforest to some of the driest deserts in the world, and from sea level to elevations greater than 5000m (Duellman & Trueb, 1986).

Frogs (Anura) are the most successful living amphibians. In total there are more than 3800 species of living frogs. They consist of more than 20 families and two or three sub-orders. The uncertainty of number of families and sub-orders is due to the frogs being an ancient group so the lineages are unclear which makes them hard to classify (Zug, 1993).

The skin of amphibians and frogs is highly permeable and a very important organ for frogs more so than for other animals. Frogs use their skin for gas and water exchange, physiological regulation (e.g. osmotic and heat regulation), defence, camouflage, sensation (e.g. chemo- and mechano-reception) and as a buffer to the environment (Duellman & Trueb, 1986; Zug, 1993; Barker et al., 1995). The epidermal layer is compressed and keratinized (hard) and is shed periodically and eaten in one piece (Barker et al., 1995).

Within frog species differences have been found in the skin texture in particular terrestrial and arboreal anurans that have granular skin on their bellies and the ventral surface of their thighs while stream dwelling frogs generally have smooth skin (Duellman & Trueb, 1986). The granular skin provides terrestrial and arboreal anurans with a greater surface area for greater water absorption, and therefore it can be argued that they are better adapted to life in wider environments than the stream dwelling frogs.

Possible Causes of Decline.

Decline in amphibian populations have been reported in numerous places around the world (Carey, 1991; Pechmann et al., 1991; Richards et al., 1993; Pound & Crump, 1994; Carey & Bryant, 1995; Alford & Richards, 1999 and Lips, 1999). Present and historical land clearance, habitat loss, habitat degradation, and pesticide or herbicide use have been identified as some of the threats to the frogs (Switzer, 1991; Carey and Bryant, 1995; Alford, 1998). However, many causes of declines in Australia, Brazil and Central America remain a mystery (Alford, 1998) Causes are assumed, however, to be a combination of causes. These are going to be discussed below.

Feral pig predation

A potential source of possibly direct frog mortality by habitat change is the feral pig. Richards et al. (1993) observed increasing erosion due pig grubbing about over the three year monitoring period at Mt Spec, Queensland. Pigs can greatly alter erosion, turbidity and runoff patterns. Pigs can also act as a direct predator on adult frogs and on the eggs of ground nesting species. Further studies are however, needed to examine the impact of pigs on native frog populations (Richards et al., 1993).

Atmospheric and climatic contamination

It has been proposed that increased UV-radiation at high altitudes could be a possible factor associated with declines of amphibians (Richards et al., 1993). It is also interesting to note that the stream dwelling frogs, that are undoubtedly exposed to higher levels of UV-radiation than non-stream dwelling frogs, are the ones affected by the amphibian decline (Richards et al. 1993). However, it seems unlikely that the increased UV-radiation would have an effect as the frogs are nocturnal and live under almost 100% canopy cover (Richards et al. 1993).

In Monteverde Cloud Forest Preserve, Costa Rica, Pounds & Crump (1994) suggested that the severely warm and dry conditions associated with the El Niño in 1987 could be a direct cause of high adult mortality due to high rates of water loss. However, even though the slope towards the Caribbean of the Monteverde Cloud Forest Preserve remained wetter than the slope towards the Pacific, the frogs disappeared from both slopes (Pounds & Crump, 1994).

In the Wet Tropics World Heritage Area, Australia, the endemism of rainforest frogs is very high with 21 of 23 species (91%) endemic to the rainforest (Richards et al., 1993). Some of the frog species are widespread both latitudinally and altitudinally, while some species are restricted to small pockets of rainforest especially on isolated mountain tops (Richards et al., 1993). Pounds and Crump (1994) suggested that narrowly adopted montane populations, especially small genetically homogeneous ones restricted to mountain tops could be particular lacking in physiological plasticity catering for climate and temperature change e.g. El Niño events.

It has been observed that some frogs tend to shift to a highly clumped dispersion patterns with habitat patchiness die to warm and dry conditions as result of ENSO (El Niño / Southern Oscillation) events (Pounds & Crump, 1994). It has also been found that parasite loads increased with crowding e.g. the microsporidean protozoan Plistophora myotrophica and the bacterium Pseudomonas aeruginosa that both cause adult mortality especially during warm summers (Pounds & Crump, 1994).

The punctuating rain after the dry season could bring contaminants from the atmosphere and deposit them with minimal dilution in montane areas (Pounds & Crump, 1994). Fog is much more efficient than rainwater at concentrating pesticides; for unknown reasons, droplets may contain residue levels up to several thousand times greater than predicted by Henry’s Law which describes the dissolution of airborne compounds in an ideal liquid (Glotfelty et al., 1987). When the dehydrated anurans rehydrate, especially when toxic compounds are prevalent, they might have relatively low volumes of body fluids to dilute these compounds. They therefore may absorb a volume of water equal to 60-70% of their body water content (Pounds & Crump, 1994). However, Richards et al. (1993) were unable to detect any heavy metals or pesticides in their Wet Tropics study.

Disease

Many amphibian population declines have been shown to be due to habitat degradation, but in the last twenty years there have been mysterious population crashes in protected high altitude areas where no habitat problems have been detected, e.g. in the montane rainforests of Queensland and Central and South America (Laurance et al., 1996, 1997; Lips, 1998). A new species of chytrid fungus has been found infecting the skin of dead frogs during mass mortality events in forests in Queensland and Panama (Berger et al., 1998). Some species are free living in soil and water where they degrade organic matter such as chitin or keratin, and others are parasites of algae, plants, nematodes or insects (Berger & Speare, 1998). Before the discovery of the amphibian chytrid, none had been found to cause disease in vertebrates. The epidemiological data supports the hypothesis that this fungus has been introduced to these rainforest areas and is the cause of the population crashes (Berger & Speare, 1998).

In Berger & Speare’s (1998) study all captive raised frogs died with chytridiomycosis in a matter of weeks after metamorphosis. In healthy examined tadpoles fungus was only found in the mouth parts, which is the only keratinised area of tadpoles. Berger & Speare, (1998) argued that the chytrid spreads from the mouth parts to the skin on the body when it becomes keratinised after metamorphosis which could explain why tadpoles survive while adults die by the keratinophilic fungus.

Natural Fluctuation gone bad?

It is clear that populations of many amphibian species have declined in recent years, both at and above the level of regional meta-populations. It is also universally agreed that frog populations fluctuate considerably in size, due to highly variable recruitment (Alford & Richards, 1999) and add to this that adults can skip breeding years (Pechmann et al., 1991). Alford & Richards (1999) found that local populations of amphibians tend to be more likely to decrease than increase in size. However, large populations may be more likely to be noticed and included in surveys by researchers and the data can therefore be biased toward observing peak populations that eventually will decline, rather than the reverse (Pechmann et al., 1991).

The persistence of a population after frequent natural recruitment failures does not necessarily imply that the population would be able to tolerate human induced mortality.

Natural fluctuations and human effects together could result in local extinction more easily than either alone (Pechmann et al., 1991).

Conclusion

Many amphibian populations occur as meta-populations, so local populations will not give rigorous data for the meta-population (Alford & Richards, 1999). An understanding of the factors involved affecting the status and dynamics of the meta population should therefore be the ultimate goal for research aiming to reverse the decline (Alford & Richards, 1999).The problem to distinguish between real declines and natural fluctuations was clearly demonstrated by Pechmann (1991) in South Carolina, U.S.A. Despite fluctuations of several orders of magnitude in the short term, long term observations indicated no declining trend. Richards et al. (1993) stated that it is indeed possible that population declines are manifestations of natural fluctuations. However, even so, those periods when the population numbers are really low it may be critical for the survival of the population (Richards et al, 1993).

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