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Global frog populations are declining and this literature review has delved into the problems and causes behind this dilemma. Batrachochytrium dendrobatidis causes chytridiomycosis which leads to mortality in many frogs by attacking the keratin found on tadpoles' external mouthparts and the skin of adult frogs. Chytridiomycosis is affected by global warming; the increase of temperatures towards an optimum for growth of chytridiomycosis has caused an increase in the spread and prevalence of this disease. Habitat fragmentation has been caused through agricultural activity, deforestation and urbanisation of land. This is breaking up habitats and causing problems with inbreeding in populations that have become isolated. Chemicals in the water are having detrimental effects, not just to growth and development rates, but to the immune system as well. Frogs under pressure from pesticides are losing resistance to parasitic infections meaning they are more susceptible to mortality. When non-native fish are introduced to water bodies inhabited by frogs there is a large decrease in frog numbers. Fish predate on juvenile frogs and compete with adult frogs for the same food. Conclusions brought from these issues are that serious guidelines need to be implemented to protect frogs and stop this amphibian order from becoming extinct.
Frog numbers are drastically falling on a global scale. Since 1980, nine species have become extinct and 113 more could possibly be extinct (Stuart S.N. et al. 2004). The specific focus of this literature review is to analyse the factors affecting the decline in frog populations and will elucidate several reasons that contribute to this decline. The fungus chytridiomycosis has spread worldwide (Skerratt L.F. et al. 2007) and this has been labelled as a major cause for frog deaths (Stuart S.N. et al. 2004). Its prevalence and intensity has been linked to global warming (Pounds A.J. et al. 2006) and this will be looked at in further detail. Habitats worldwide are affected by anthropogenic factors e.g. destruction through deforestation, road building etc., (Harper E.B. et al. 2008). Reasons why this is causing a decline in frog population numbers will be discussed. Frogs are highly susceptible to chemicals in water channels due to their permeable skin (Doyle R. 1998, Morrell V. 1999) and their larval stage being dependent on water (Barth B.J. and Wilson R.S. 2009). These chemicals are affecting mortality rates because they suppress the immune system (Christin M.S. et al. 2004), cause limb malformations (Oullet M. et al. 2007) and affect the number of juveniles that are surviving through to adulthood; this is also affected by predatory stress (Relyea R.A. 2003). This review will contain explanations behind these findings. Another factor that is detrimental to frog populations is the introduction of fish to a water body (Brönmark C. and Edenhamn P. 1994). How this affects frogs and their population numbers will be interpreted.
Batrachochytrium dendrobatidis, the causative agent of the fungal infection chytridiomycosis
Batrachochytrium dendrobatidis (Bd) is an amphibian chytrid fungus and is the causative agent of chytridiomycosis. Chytridiomycosis is an emerging disease that infects amphibians (Daszak P. et al. 1999, 2003). It is a disease that affects the skin, where the zoospores attack keratinised tissues (Voordoouw M.J. et al. 2010) and is present on the external mouthparts of tadpoles and on the outer layer of the skin in the adults (Wake D.B. and Vredenburg V.T. 2008). One of the life stages in chytrid development is a zoospore-releasing sporangium which releases zoospores that infect a new host or re-infects the original host. This creates new sporangia and completes the asexual life cycle (Wake D.B. and Vredenburg V.T. 2008). The fungus depends on water flow because the chytrid zoospores have a restricted swimming ability (Voordoouw M.J. et al. 2010), but Bd has still been found on fully terrestrial species of amphibians (Wake D.B. and Vredenburg V.T. 2008). This has been shown via a survey that took place in Maine, USA. They found that the prevalence of the infection in species that hibernate in terrestrial habitats was nearly three times lower than that in species that hibernate in aquatic habitats (Voordoouw M.J. et al. 2010). This shows that although there is higher prevalence in those species that do hibernate in water, Bd is still present in those species that don't.
3.1 Reasons why Batrachochytrium dendrobatidis has caused severe declines
Skerratt L.F. et al. (2007) discusses two different theories of how chytridiomycosis could cause severe global population declines. The first is called the "novel pathogen hypothesis" (Rachowicz L.J. et al. 2005). This involves the spreading of a highly pathogenic, virulent and transmissible pathogen which leads to high mortality and the failure to breed to produce. This happens in populations of naÃ¯ve, susceptible species. The second hypothesis is called the "emerging endemic hypothesis" (Rachowicz L.J. et al. 2005). This can occur when an already prevalent pathogen becomes highly virulent, pathogenic and transmissible. This can be caused by abnormal environmental change, for example, climate change, pollution and/or increased UV-B radiation. Daszak P. et al. (2003) summarised that anthropogenic (caused or produced by humans) introduction seems to be a major factor in the emergence of chytridiomycosis. Rana pipiens are more susceptible to chytridiomycosis during April to June and October to March as they breed in April to June and hibernate from October through to March. Both these life stages are dependent on water and this is where the chytrid zoospores are found that can infect the frogs (Voordoouw M.J. et al. 2010). Bd has caused serious declines because certain frog species rely on water either once or twice a year, if the water bodies are infected with chytrid zoospores, then it is more than likely the frog will pick up the infection.
3.2 Impacts of global warming on Batrachochytrium dendrobatidis
It has been found that chytridiomycosis has an optimum temperature for growth which is between 17°C and 25°C (Longcore J.E. et al. 1999, Piotrowski J.S. et al. 2004) (Woodhams D.C et al. 2008). It has been suggested in many journals that global warming is creating an optimum climate for the growth of chytridiomycosis (Woodhams D.C. et al. 2008, Laurance W.F. 2008). It was suggested by Pounds A.J. et al. (2006), that increasing cloud coverage could be controlling daily minimum and maximum temperatures that are creating a "chytrid thermal optimum". This occurs in areas of high elevations (Woodhams D.C. et al. 2008). But there is some opposition to this theory, Bustamante H.M. et al (2010) concluded that all studies published in this matter to date were based on correlative data and that no reliable evidence was available to prove a direct or indirect relationship between amphibian declines and climate change. According to Bustamante H.M. et al (2010), Pounds A.J. et al (2006) assumed that Bd is present in a non-pathogenic state and when the temperature reaches the "chytrid thermal optimum" it converts to a pathogenic state. Laurance W.F. (2008) suggests that instead of a warm-year hypothesis, he found stronger evidence that supported an increase in frog declines after three years of unusually warm weather. But this trend was only visible at tropical latitudes, where increases in minimum temperatures were greatest.
Why frogs are vulnerable to habitat fragmentation
Figure 1. Two graphs showing the dispersal distances of juvenile and adult Columbia spotted frogs. The positive values represent upstream movement and the negative values represent downstream movement (Funk W.C et al. 2005)The isolation of populations of frogs by habitat fragmentation reduces rescue effects (Funk W.C. et al. 2005). 'Rescue effects' cause an increase in a population persistence (Brown J.H. and Kodric-Brown A. 1977) because it involves immigrants reproducing in new populations which in turn boosts genetic diversity and therefore decreases negative inbreeding effects (Tallmon D.A. et al. 2004). Habitat fragmentation means that immigrant frogs cannot find new populations to breed with, and this increases extinction rates (Funk W.C. et al. 2005). Funk W.C. at al. (2005) researched how far frogs actually dispersed from breeding sites. The larger the dispersal value, the more affected by habitat fragmentation the frog species would be. They found that Columbia spotted frogs had remarkably high dispersal rates, with juveniles moving significantly further so are therefore highly affected by habitat fragmentation which can be seen on the graphs. Not all frogs are so affected by habitat fragmentation; Hilliers A. et al. (2008) studied the effect of fragmentation on West African leaf-litter frogs. They defined fragmentation using two parameters: changes in size and connectivity of the forest fragment. Using these parameters they found that fragmentation did not directly affect leaf-litter frogs.
4.1 Consequences of habitat fragmentation
Habitat fragmentation can cause bottlenecks and inbreeding and this has been studied in the European tree frog (Anderson L.W. et al. 2004). Anderson L.W. et al (2004) focused their study on the genetic consequences of habitat fragmentation, which causeed a decline in population sizes due to the loss of suitable ponds. Their results concluded that increased inbreeding (due to a decrease in frog numbers) led to increased larval mortality. In tree frog populations that had an increased number of calling males, the genetic variation was also greater. Bickford D. et al. (2010) researched into the affect forest fragmentation had on frog diversity and abundance in Singapore. They confirmed other scientists' findings that forest fragmentation has negative effects on frog diversity (Findlay C.S. and Houlahan J. 1997, Kolozsvary M.B. and Swihart R.K. 1999, Houlahan J.E. et al. 2000). They found that fragmentation is also associated with other changes to habitats. One example they use is the abiotic conditions of the fragment perimeters and how they change. The perimeters of the habitat become more exposed to the sun, wind and climatic extremes. This causes an elevation in temperature and the humidity of the air and leaf litter is reduced. These conditions become unfavourable for frogs and they must then congregate in the centre of the fragment to survive.
4.2 Causes of habitat fragmentation
Johansson M. et al. (2005) discusses how agriculture has affected the habitats of the common frog, Rana temporaria. Research took place in Sweden, where the effects of agriculture in the north was compared to the effects of agriculture in the south. The effects of agriculture were negative in the south where the farming is more intense, is large scale and more crop orientated. The effects in the north were positive, where there are more hay meadows and no deciduous forests. This shows that how affected frog populations are depends on the type of farming taking place. Crosby M.K. et al. (2009) discussed the anthropogenic causes of habitat fragmentation including road building (including a major highway) and road density. Deforestation is a major cause of habitat fragmentation in tropical frogs. Ninety-five percent of the rainforest in Singapore has been cleared in the past 200 years (Brook B.W. et al. 2003).
Frog declines due to chemicals
Frogs habitats are mainly wetland areas which get a build-up of chemicals including pesticides, herbicides and metals (Cohen M.M Jr. 2001). Frogs are more susceptible to these chemicals because they have porous skins through which they breathe (Doyle R. 1998, Morrell V. 1999). Oullet M. et al. (2007) suggested there was a correlation between a high frequency of hindlimb anomalies and the use of pesticides. This only occurred in newly metamorphosed frogs and not in adults. He carried out his study using frogs from the St. Lawrence River Valley in Quebec. These limb malformations make frogs more vulnerable to predators and therefore lead to an increase in frog deaths. Gillilland C.D. et al. (2001) found that in green frogs (Rana clamitans) 0.3% of the frogs examined had limb deformities. This value was lower than the background level of deformities for this species. From this research they concluded that the levels of chemicals tested in the water were not enough to cause any major concern. Relyea R.A (2009) was interested in how the mixtures of the chemicals would affect frogs and not just single chemicals. He found that the two frogs that were used in the experiment (leopard frogs and gray tree frogs) were very differently affected. Leopard frogs showed a low mortality (24%) with diazinon, but high mortality (84% and 99%) with endosulfan and the mix of all ten pesticides respectively. For the gray tree frogs, there was no effect on mortality with any of the chemicals including the mixture of all ten. This shows that some frog species are more affected than others by chemical contaminants and can therefore out-compete their counterpart which may lead to extinctions in certain species.
5.1 Chemical effects on the immune system
Christin M.S. et al. (2004) studied the exposure of two frog species, Xenopus laevis and Rana pipiens, to a mixture of pesticides. The mixture and concentration of these pesticides represented what was found in the environment of St. Lawrence River, Quebec. They found that the mixture of pesticides altered the cellularity and phagocytic activity in Xenopus laevis and altered the lymphocyte proliferation in Rana pipiens. They concluded that this could contribute to the decline in frog numbers because they are more susceptible to certain infections which could include parasitic infection; Christin M.S. et al. (2003) found that after Rana pipiens had been exposed to a mixture of 6 pesticides for 21 days their lymphocyte proliferation was significantly reduced. They then exposed the frogs to a parasitic nematode, Rhabdius ranae. Frogs exposed to the highest levels of pesticides, had a higher prevalence of lung infection by Rhabdius ranae. It was concluded that pesticides can significantly affect how a frog can deal with parasitic infections and this could cause more deaths.
5.2 Chemical effects and predatory stress
Relyea R.A. (2003) has found that carbaryl, normally considered sublethel, can have dramatic effects and become increasingly lethal under different conditions, for example competition and differences in temperature. It has been found that predatory stress can make carbaryl 2 to 4 times more lethal. This was found in gray tree frogs (Hyla versicolor), when predatory stress was combined with carbaryl it became up to 46 times more lethal. These results were found using relatively low levels of carbaryl that are considered to be safe, but this experiments show that they are not necessarily safe when combined with other factors. This could be the same for many other chemicals and could be a main contributing factor for why chemicals have such an influence on frog deaths.
6.0 Introduction of fish for sport
As a result of fishing becoming an increasingly popular sport, fish are being introduced into lakes that are inhabited by frogs. Pope K.L. (2008) conducted an experiment to see how the Cascades frogs (Rana cascadae) were affected by the introduction of fish. This was carried out by removing fish and recording the differences in population density and survival. 3 years after the fish had been removed, frog population densities had increased by a factor of 13.6 and the survival of the young adult frogs had increased from 59% to 94%. This concludes that the introduction of fish for sport is severely affecting the survival rate of the newly metamorphosed frogs.
6.1 Introduction of non-native fish
Knapp R.A. and Matthews K.R. (2000) have found that fish introduced into protected areas in the Sierra Nevada is severely affecting the yellow-legged frog (Rana muscosa). The yellow-legged frog was 6 times more abundant in fishless water bodies compared to water bodies that contain fish. Finlay J.C. and Vredenburg V.T. (2007) show that introduced fish and the yellow-legged frog are both competing for the same benthic invertebrates and due to fish introductions this reduces the availability of prey to the yellow-legged frog. They also find that in water bodies that have no fish, post metamorphic frogs are 10 times more abundant. Teplisky C. et al. (2003) discovered that the introduction of fish causes an impact on the tadpole of Rana dalmatina. R. dalmatina tadpoles changed their behaviour in the presence of fish, they displayed lower activity rates and higher refuge use. These lower activity rates were related to lower developmental and growth rates. This stunts the growth of these tadpoles and therefore decreases the fitness of the post metamorphic frog.
From these findings it can be concluded that frog populations are declining for several different reasons of which urgently need addressing. It is not enough just to protect areas because disease (Batrachochytrium dendrobatidis) knows no boundaries. Climate change and global warming is affecting the spread and prevalence of chytridiomycosis (Pounds A.J. et al. 2006). The spread of chytridiomycosis needs to be controlled to stop it becoming more prevalent, the import and export of frogs needs to be strictly controlled. Williams E.S. et al. (2002) suggested that chytridiomycosis should be listed as a notifiable disease that should have testing requirements implemented on all imports and exports. More protected areas are needed and this will protect frogs from habitat fragmentation and degradation but the introduction of non-native fish must be stopped because this is still occurring even in protected areas (Knapp R.A. and Matthews K.R. 2000). Farmers that are causing habitat fragmentation through agriculture could farm in a more environmentally friendly manner (Beaufoy G. et al. 1994); governments could offer incentives for them to leave some land, hedgerows and wetlands alone. The use of pesticides in agriculture needs to decrease; the run off of the chemicals into water bodies is severely affecting frogs (Cohen M.M. Jr. 2001). More organic farming needs to take place where no pesticides are used, and more public education is needed about the benefits of organic food. As the decline of amphibians is a global problem, global action needs to be taken. Each country needs to implement conservation activities to try and preserve a deteriorating amphibian order.