The Influence of Temperature and Salinity Conditions on Chytridiomycosis in Anurans

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The influence of naturally occurring salinity on disease occurrence and severity has been researched through observational studies. Heard et al. (2014) examined how wetland conditions impact chytridiomycosis contraception and survival rate in growling grass frogs (Litoria raniformis). A higher occurrence and severity of infection was found in fresh water bodies with lower salt concentrations. Green and golden bell frogs (Litoria aurea) are found within several areas of the coastline and Stockwell et al. (2014) proposed this is a result of the higher salt levels which naturally minimise the effects of chytridiomycosis and allow populations to survive. A different experiment that aimed to investigate tadpole tolerance to saline surroundings found that this species had 85% survival rates in water treatments with high salt content compared to the fresh water with only 35% survival rate (Kearney et al. 2012). The cause of this adaptation to saline habitats is not proven to be in response to chytridiomycosis. It is important to note that the distribution of the species in saltier water could be due to other influencing factors, such as competing with fewer species for food in less populated ponds.

A number of studies have experimentally investigated the efficacy of salt as an antifungal agent for chytridiomycosis when added to water bodies. Stockwell et al. (2012) and Stockwell et al. (2014) conducted studies on the effects of different salinity levels in aquatic conditions. The initial research used artificial tanks with concentrations between 0 ppt and 5 ppt and the second utilised natural ponds by adding salt to create salinity levels of 2 ppt and 4 ppt with a control at 0 ppt. The aforementioned study focused on juvenile Peron’s tree frogs (Litoria peronii) and the latter observed green and golden bell frog tadpoles. The longest survival and lowest infection rates were observed in the 3 ppt treatment and the 4 ppt treatment, respectively. These results are consistent with the antifungal properties of sodium chloride and its ability to reduce symptoms of chytridiomycosis. An important difference between these in vitro and in situ studies is the additional data that can be obtained from secondary observations in real habitats and ecosystems. Although it produced tadpoles with the highest tolerance to chytrid infections, the in situ 4 ppt treated pond resulted in a significant deduction in dragonflies and larvae. This highlights possible impacts on non-target organisms and the importance of understanding how changing original salt concentrations will influence all biota. The bodies of the tadpoles in the 4 ppt pond were also substantially smaller, suggested to be a consequence of greater energy expenditure on osmoregulation or less available food.

Maintaining ideal temperature and salinity conditions to provide refuge areas

A refuge is an area of the environment where the conditions reduce a pathogen’s ability to damage other species (Stockwell et al. 2015). Development and spread of disease may be lowered by environmental controls by either reducing the pathogen intensity and transmission or enabling the host to respond more effectively (Woodhams et al. 2011). As discussed, the chytrid fungus generally cannot tolerate high temperatures above 25-28˚C and sodium chloride concentrations of 3 ppt or more. Habitats that naturally have low Bactrachochytrium dendrobatidis suitability and decrease the impacts of chytridiomycosis on anurans have been researched by previously noted studies (Murray et al. 2013; Heard et al. 2014; Scheele et al. 2015).

Environments can also be manipulated to become refuges by increasing temperatures or salt concentrations to protect chytridiomycosis-threatened populations. Becker et al. (2012) sampled common green frogs (Lithobates clamitans) from ponds in a New York Park to determine how man-made habitat changes influence disease prevalence and intensity. Evidence showed that cleared canopy cover and vegetation at the water line had the greatest impact on the host-pathogen interactions. As trees and plants cast shade over the water bodies, they create lower temperatures by decreasing direct sunlight and heat radiation. Results suggested that reduced canopy density increased the air temperatures at basking sites where frogs rest outside of the water. Shallow areas warmer more rapidly to higher temperature and it is suggested that this provides hosts a greater ability to reduce and clear infections. Scheele et al. (2014) and Puschendorf et al. (2011) also found that removing canopy cover or maintaining naturally low levels of shade increased survival rates in the wild. Introducing artificial heat sources to habitats is another strategy to increase infected frogs and toads exposure temperatures above the chytrid fungus tolerance and raise their chance of ridding infection (Scheele et al. 2014).

Increasing habitat salinity is another suggested method to create refuges from chytridiomycosis because of salts antifungal properties (Stockwell et al. 2012). The strategy can be implemented easily, is cost-effective and requires minimal monitoring and management (Stockwell et al. 2014). However, the manipulation of water chemistry can have adverse effects on target and non-target species. Australian research on tadpoles tolerance to elevated salt concentrations (16% seawater) decreased the survival rate of spotted marsh frogs (Limnodynastes tasmaniensis) and painted burrowing frog (Neobatrachus sudelli) to 0% and 35% respectively (Kearney et al. 2012). Increasing salt levels in a habitat could be damaging to surrounding vegetation, organisms and the entire ecosystem. (Stockwell et al. 2014). During droughts animals and plants require fresh water sources and increasing saline aquatic habitats may have adverse effects in future dry periods (Clemann et al. 2013). Stockwell et al. (2014) suggested a ‘mosaic’ strategy to create a mixture of salt refuges and normal ponds in order to mitigate disease and minimise any adverse effects.

Conclusion

A deepened understanding of temperature and salinity as environmental influences on the anuran disease chytridiomycosis is paramount to reducing species decline and possible ecosystem collapse. Through knowledge of the pathogen’s physiological limitations, areas that the fungus cannot tolerate but are habitable for target species can act as refuges. Naturally occurring refuges should be identified and protected. There is promising evidence that small habitat manipulation such as vegetation reduction can help mitigate the effects of disease. However, further research on the consequences of changing salinity is required and the precautionary principle should be applied.