Human activities are likely to have aided the spread of a disease, known as Ranavirus, which is threatening common frogs (Rana temporaria) (Figure 1) in the UK (1). There are, however, actions that you can take to benefit amphibian conservation and even help reduce the spread of such diseases.
Amphibians under threat
Why, you ask, should we be concerned about amphibian conservation? Biodiversity, the variation within and between all living things and ecosystems, is currently in global decline (2). Within this, amphibians are affected more severely than other vertebrate groups, with a high proportion of species at risk of extinction (Figure 2) (3). Biodiversity plays an important role in the functioning of ecosystems, upon which all life depends. The implications of a large decline in biodiversity are, therefore, likely to be detrimental to all life on Earth, humans included (2). Thus, amphibians, which make up a disproportionately large fraction of biodiversity loss, represent a priority for conservation efforts.
To conserve biodiversity, we firstly need to understand the causes of decline (3), but, what are the main causes of decline in amphibian populations? There are, in fact, a multitude of threats to amphibians, which each affect different species to varying degrees. These threats include, but are not limited to, habitat destruction and degradation, climate change, legal and illegal hunting and trapping, introduced species and infectious disease. Regional, disease-driven, species extirpations have been more prevalent in amphibians, in comparison with other groups, over the last four decades (1, 4). The reasons for this are not fully understood, making infectious disease in amphibians of particular scientific interest (1).
Infectious disease and amphibian declines
Severe regional declines in amphibian populations first began to be observed in Queensland, Australia, and Central America, during the late 1970s. The cause of these declines was later identified, in the 1990s, as a previously unknown infectious fungal disease, chytridiomycosis, which only affects amphibians. Following the initial observations in Australia and Central America, further amphibian declines, accompanied by symptoms of chytridiomycosis, were observed in other regions around the world (5). As a result, several amphibian species have since become extinct (Figure 3) (4, 5). These events led to questions being raised about the origins of the disease. Evidence, relating to the genetic diversity of Batrachochytrium dendrobatidis (often shortened to Bd), the species of fungus that causes chytridiomycosis, indicates that several strains exist, but the strain most associated with symptoms of disease and amphibian mortality is the same throughout the world (5). This suggests that chytridiomycosis may be a disease that has been spread throughout the world within the last century, rather than an example of a disease that has developed in multiple populations as a result of some trigger, such as environmental change (5). The movement of animals, by humans, for reasons such as the pet trade, has therefore been implicated in the spread of chytridiomycosis (4, 5). It is worth noting, however, that the severity and occurrence of symptoms is also related to environmental conditions and the species of amphibian infected. These relationships are complex and not yet fully understood, but, disease causing strains of Bd do not cause disease and mortality in all amphibian populations in which they are present (5).
Chytridiomycosis, caused by Bd, is one disease that has been a driving force in global amphibian declines, but, concerningly, there are other infectious diseases that pose a threat to amphibian populations. Another species of fungus, related to Bd, which has been named Batrachochytrium salamandrivorans (Bsal hereon), was recently identified as the cause of a catastrophic decline in fire salamander (Salamandra salamandra) (Figure 4) populations within Europe (4). Like Bd, Bsal has the potential to cause mortality in infected individuals, but differs in that it specifically affects caudate amphibians (salamanders and newts), rather than all amphibians (4). Bsal also causes disease at lower environmental temperatures than Bd, which has serious negative implications for amphibian conservation, as populations that are less affected by Bd could become threatened if Bsal were spread to them. It is currently believed that Bsal originates from Asia, where it is present in amphibian populations, but doesn’t appear to cause disease, and that it may have been introduced to Europe through the pet trade (4).
Ranavirus is another infectious disease threatening amphibian populations. Species of Ranavirus are found around the world and infect fishes and reptiles, as well as amphibians (1). Not all Ranavirus species cause disease, but those that do can cause mortality in amphibians and have led to localised extinctions when introduced to previously unexposed populations (1). A recent study into the spread of Ranavirus throughout the UK population of common frogs found that incidences of disease outbreaks were significantly greater in areas with a higher human population density (1). These results suggest that human activities, such as catching and moving spawn, tadpoles and adult amphibians between ponds, may have accelerated the rate of transmission (1). In areas where human population density is lower, rates of transmission were also lower, and more consistent with what would be expected due to the natural dispersal of frogs between ponds, which is part of common frog behaviour (1). This study does, however, have implications in terms of things you can do for amphibian conservation.
What can you do to help conserve amphibians?
The movement of animals, by humans, is a common theme when discussing the spread of infectious disease in amphibians (1, 4, 5). Recommendations for the implementation of more rigorous measures for testing animals for disease within the pet trade have, therefore, been proposed for the benefit of conservation. We can all benefit conservation, however, by raising awareness of the possible negative implications of moving wildlife around, and by not engaging in such activities ourselves (1). Allowing children to observe the development of frogspawn into tadpoles, and then into frogs, is a fantastic way to engage children with wildlife, and those of us who were able to do so as children have fond memories of this, but it is important that we do not move animals between ponds in order to do so. Citizen-science schemes are another way we can all benefit biodiversity conservation. The study into the spread of Ranavirus throughout common frogs in the UK was conducted using data collected by the public as part of such a scheme, known as the Garden Wildlife Health project (1). Another good example of such a scheme, for those in the UK, is the Big Garden Birdwatch. Many other similar schemes do, however, exist in different parts of the world. The benefits you could bring to conservation by engaging in citizen-science are huge (1).
- Price, S.J., Garner, T.W.J., Cunningham, A.A., Langton, T.E.S. and Nichols, R.A. (2016). Reconstructing the emergence of a lethal infectious disease of wildlife supports a key role for spread through translocations by humans. Proceeding of the Royal Society of London B, Vol. 283 (1839), 20160952.
- Dirzo, R., Young, H.S., Galetti, M., Ceballos, G., Isaac, N.J.B. and Collen, B. (2014). Defaunation in the Anthropocene. Science, Vol. 345 (6195), pp. 401-406.
- Stuart, S.N., Chanson, J.S., Cox, N.A., Young, B.E., Rodrigues, A.S.L., Fischman, D.L. and Waller, R.W. (2004). Status and trends of amphibian declines and extinctions worldwide. Science, Vol. 306 (5702), pp. 1783-1786.
- Berger, L., Roberts, A.A., Voyles, J., Longcore, J.E., Murray, K.A. and Skerratt, L.F. (2016). History and recent progress on chytridiomycosis in amphibians. Fungal Ecology, Vol. 19, pp. 89-99.
- Fisher, M.C., Garner, T.W.J. and Walker, S.F. (2009). Global emergence of Batrachochytrium dendrobatidis and amphibian chytridiomycosis in space, time, and host. Annual Review of Microbiology, Vol. 63, pp. 291-310.