Question 2: The Decline of Frogs
Introduction
Since the last decade a global decline in frog populations has been detected. The rate in which some frog populations disappear or even go extinct is increasingly worrying. This loss has been described by some as the greatest lost of vertebrae species in recorded history. It is important to halt this great decline, as frogs represent a unique evolutionary branch of organisms that not only have vital ecological niches, but also act as an important environmental stress indicator. To stop these declines from continuing, the causal factor of this problem must be determined. This essay discusses the main factors that decrease frog populations, namely, disease, invasive species, habitat destruction, climate change and pollution. With these problems recognized, a wise and well thought plan must be made and acted upon quickly to save as many frogs species as possible from extinction.
Factors of Decline
Disease
One of the most greatly discussed factors that attribute to amphibian decline is disease, particularly chytridiomycosis; a fungal infection caused by a fungus relatively new to science, Batchochytrium dendrobatidis (Skerratt et al., 2007). Chytrid fungus infection has become a pandemic, infecting frog populations in almost every continent where frogs exist (Holland, 2009). B. dendrobatidis is believed to have originated from Africa, as a preserved African clawed frog (Xenopus laevis) kept in an African museum dating back from 1938 was tested chytrid positive (Skerratt et al., 2007; Holland, 2009). The spread may have been caused by the export of this animal for lab research and as pets. Chytrid fungus infections are lethal to most adult frogs. It attacks keratin, a protein found on the skin of adult frogs only. Hence, tadpoles can be carriers of the fungus, dying after it transforms into an adult frog (Holland, 2009). Chytrid fungus kills by overwhelming a frog’s skin with zoospores. Hyperplasia and hyperkeratosis, or the thickening of epidermal tissue then ensues (Schloegal et al., 2006; Muths et al., 2003). Infected frogs exhibit weird behaviours; they are lethargic, they have a slower reaction to threats, their sitting posture is abnormal and their skin becomes reddish. This increases the probability to being predated upon, another way in which frogs can die if not killed by the disease itself (Schloegal et al., 2006). The vectors of chytrid fungus include introduced frogs, fish, birds and even human boots and vehicles (Laurance et al., 1996; Holland, 2009). Chytridiomycosis often work synergistically with other factors, making it a lethal mix to frogs everywhere (Blaustein & Kiesecker, 2002; Holland, 2009).
Introduced Species
The decline of frog species is also attributed to the introduction of invasive species. An example of this happening is the introduction of trout to the once fishless ponds of Sierra Nevada. Trout was introduced by the US Department Fish and Game in parks for sport fishing (Holland, 2009). Trout fry compete with tadpoles of mountain yellow-legged frogs (Rana mucosa) for food and shelter; both prefer shallower waters as breeding sites and deeper waters as overwinter sites. Adult trout predate upon tadpoles and young frogs (Holland, 2009; Knapp & Matthews, 2000). This directly influences the frog population, nearly pushing R. Mucosa to extinction. Another devastating invasive species is Rana catesbeiana, the bullfrog. Bullfrog tadpoles compete with and even predate upon tadpoles of other frog species, like the native, northen red-legged frog (Rana aurora) (Blaustein & Kiesecker, 2002). Adult bullfrogs are voracious predators that feed on anything that can fit in its gape, including other frogs (Corn & Fogleman, 1984). By competing and predating upon other frogs, the introduction of bullfrogs lowers the populations of native frog species and has even driven some frog species to extinction, hence lowering the diversity of frog species.
Habitat Destruction
An obvious and ongoing cause of frog population decline is habitat destruction and alteration. Forests are continually being cleared, wetlands drained, lakes filled and ponds converted for agricultural use (Blaustein & Kiesecker, 2002; Corn & Fogleman, 1984). Road construction also decimates frog habitats, as rocks, branches and dirt clog streams (Holland, 2009; Corn & Fogleman, 1984). Land clearing also indirectly affect frog populations that are in protected areas, as these areas are fragmented, prohibiting frogs from different populations from mating (Corn & Fogleman, 1984).
Global Climate Change and UV-B Exposure
Global warming is dying up wetlands and ponds; the nesting sites of tadpoles. This directly affects the global frog populace. Reduced precipitation in the cloud forests of Costa Rica has also caused a corresponding reduction in frog numbers there (Blaustein & Kiesecker, 2002). CFCs released by anthropogenic sources have caused a long-term ozone depletion which leads to increased UV-B (280-315nm) transmittance (Blaustein & Kiesecker, 2002). Global warming also lowers the depth of lakes, increasing the transmittance of UV-B radiation (Blaustein & Kiesecker, 2002). It has been experimentally proven that frog eggs exposed to UV-B radiation has a lower hatching success rate (Blaustein & Kiesecker, 2002). If effects are not immediately lethal, sublethal dosages of UV-B exposure to embryos will cause mutations, cell death and lower growth rates of hatched tadpoles (Blaustein & Kiesecker, 2002). This lowers the fitness of tadpoles, increasing the probability of being predated.
Pollution and Toxic Contaminants
Pesticides, herbicides, and chemical fertilizers are constantly being applied in agricultural lands. DDT and PCBs are immediately lethal to frogs at high dosages, but unknown to many, even at low dosages, it can cause frogs to be more susceptible to diseases, lower growth rate and affect mobility and predator avoidance (Blaustein & Kiesecker, 2002; Withgot, 2002; Carey & Bryant, 1995). In experiments, a mixture of nitrogenous fertilizers and low pH has been proven to cause tadpoles to swim less vigorously and develop malformations which lower the tadpole’s predator avoidance (Blaustein & Kiesecker, 2002). In a recent study, frog disappearances in protected areas of USA parks have been found to be caused by atrazine, a pesticide applied to crops (Withgot, 2002; Carey & Bryant, 1995). Trace amounts of atrazine, about 1 -10 ppb, levels way lower that EPA’s safe drinking level is discovered to be an endocrine disruptor to frogs (Withgot, 2002). Extreme low levels of atrazine can mimic hormones; causing frogs reduce or increase the production of sex hormones. This causes some frogs to develop extra gonads or even develop both sex gonads (Withgot, 2002). However, these gonads are mostly non-functional and are followed with absence of sexual behaviour, affecting the ability of many frog populations to multiply.
Conclusion
In conclusion, multiple factors have been found to affect frog populations around the world. Some of these causes have been determined long ago (habitat destruction and invasive species) and is an ongoing event that ought to be stopped completely, but some have just recently surfaced (discovery of chytridiomycosis). These factors, working synergistically, have proven to be extremely potent and have caused the rapid decline of frogs. Quick and well thought action must be taken to prevent frogs from going extinct before it’s too late.
(995 words)
References
Blaustein, AR & Kiesecker, JM 2002, ‘Complexity in conservation: lessons from the global decline of amphibian populations’, Blackwell Science, vol. 5, pp. 597-608.
Carey, C & Bryant, CJ 1995, ‘Possible interrelations among environmental toxicants, amphibian development, and decline of amphibian populations’, Environmental Science Perspectives, vol. 103, no. 4, pp. 13-17.
Corn, PS & Fogleman, JC 1984, ‘Extinction of montane populations of the northern leopard frog (Rana pipiens) in Colorado’, Journal of Herpetology, vol. 18, no. 2, pp. 147-152.
Holland, JS 2009, ‘Vanishing Amphibians’, National Geographic, vol. 215, no. 4, pp. 138-153
Knapp, RA & Matthews, KR 2000, ‘Non-native fish introductions and the decline of the mountain yellow-legged frog from within protected areas’, Conservation Biology, vol. 14, no. 2, pp. 428-438.
Laurance, WF, McDonald, KR & Speare, R 1996, ‘Epidemic disease and the catastrophic decline of Australian rain forest frogs’, Conservation Biology, vol. 10, no. 2, pp. 406-413.
Muths, E, Corn, PS, Pessier, AP, & Green, DE 2003, ‘Evidence for disease-related amphibian decline in Colorado’, Biological Conservation, vol. 110, pp. 357-365.
Schloegel, LM, Hero JM, Berger, L, Speare, R, McDonald, K & Daszak, P 2006, ‘The decline of the sharp-snouted day frog (Taudactylus acutirostris): the first documented case of extinction by infection in a free-ranging wildlife species?’, EcoHealth Journal, vol. 3, pp. 35-40.
Skerratt LK, Berger, L, Speare, R, Cashins, S, McDonald, KR, Phillott, AD, Hines, HB & Kenyon, N 2007, ‘Spread of chytridiomycosis has caused the rapid global decline and extinction of frogs’, EcoHealth Journal, vol. 4, no. 2, pp. 125-134.
Withgot, J 2002, ‘Ubiquitous herbicide emasculates frogs’, Science, vol. 296, pp. 447-448.
Mark: 34 / 40
Remarks: V.G. (very good?)
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