The amphibian biodiversity crisis: causes and research priorities

Biodiversity crisis: we are all connected

The word ‘crisis’ is among the words that I hear most in the media nowadays, however hardly ever in the context of a biodiversity crisis that we are experiencing. Understandably a lot of attention goes out to the several economical crises, because this is perceivable by the general public as influencing their own lives in the short run. Interestingly though, the climate crisis has received much attention around 5 years ago, while biodiversity has remained an elusive word to many, probably because it hasn’t been popularized as much. Exemplary for the lack of popular understanding are the recent results of a survey among people from several European countries, showing that although the term biodiversity is known, more than half of the people didn’t know what it means.

Luckily, many great efforts have been undertaken to show to the larger public why and how we should be conserving our biodiversity and E.O Wilson’s ‘The diversity of life’ is a great example of this. His words in the concluding chapter still apply to our situation even though he has written them now 20 years ago: “The ethical imperative should therefore be, first of all, prudence. We should judge every scrap of biodiversity as priceless while we learn to use it and come to understand what it means to humanity.” This remark comes after he has shown us how little we still know about biodiversity and the uses of it, but also how, what we do know about it, biodiversity has shown to be invaluable and inextricably linked to our well-being. Something human cultures have always known but we have forgotten of in our strife for economic development and our concurrent disconnection from the natural world around us. Even though we have always been and are still deeply connected to the biological world. More eloquently put: “Humanity coevolved with the rest of life on this particular planet; other worlds are not in our genes.”

The question that pops up a lot: why should we care about the biodiversity crisis? Well, there are many responses to that question, but I won’t go into them here, E.O. Wilson has already made a strong case for that. I just want to add that it is unfortunate that the only answer that suffices for many people is one that will stress the importance for their own self-interest in the here and now. This is typical for our contemporary culture and in contradiction with how other cultures perceive this. Tim Flannery gives some nice examples from his own experience with New-Guinean cultures in his latest book ‘Here on earth’. He intends not to promote the image of the ‘noble sauvage’, but to show that a thorough understanding of the world around us and the insight that our fate is linked to everything around us, is a healthy outlook on life.

So what about amphibians?

Tim Flannery describes in the same book how the most catastrophic decline of species’ numbers is now occurring amongst amphibians, with a third of all species declining in numbers and a large number of species that already have gone extinct. Two of the main reasons he mentions for this decline are climate change and the disease caused by the chytrid fungus. Probably a decent estimate of the real situation, however there are many scientific issues still to be resolved in research on amphibian conservation. Beebee & Griffiths have put together an overview of the agents of decline and the omissions and practical difficulties in amphibian research. And furthermore they propose a future research direction for scientists working on amphibian conservation.

Agents of decline

Firstly, they sum up and discuss shortly each of the main agents of decline and: habitat destruction and use of chemicals are especially important drivers in Europe where intensive agriculture has transformed the landscape over the last century. However, there is no evidence that this can be linked to recent amphibian declines that occur mostly in remote regions away from human influence. These are probably linked to five other drivers of change, including UV-B irradiation: the depletion of ozone on ground level at high latitudes has been linked to amphibian declines. A counterargument to this is that many declining species live in tropical rainforest where they are not exposed to UV-B as intensively as mountainous species.

Evidence from mass mortality in recent years has shown that diseases can play a significant role in the decline of some amphibian species. The type of agent of disease differs and susceptibility to these can be increased by factors as eutrophication and UV-B irradiation. The most well known disease among amphibians is caused by a chytrid fungus, Batrachochritium dendrobatidis, and has been held responsible for amphibian declines throughout the world. It causes death in post-metamorphic amphibians by impairing respiration through the skin and osmoregulation. However, the claim that this fungus is spreading death and terror with all amphibian species throughout the world and is therefore the major cause of recent declines, is not fully supported by the scientificevidence; some species are more susceptible than others and some seem to cope well with an infection. There seems to be an interplay between different factors in giving way to the disease and debate still exists on whether this a primary or secondary cause in the decline of populations.

Two other drivers are the presence of introduced species and overexploitation of amphibians. The latter mainly occurs in Asia, where mostly frogs are illegally collected to serve as food. Introduced species, like crayfish, bull frogs and cane toads can have a negative effect on endemic species because of diseases they bring with them, much like the Europeans wiped out the Indians of the Americas, and competition or predation. The cane toad is a text book example of how successful an introduced species can become and an example I have witnessed myself in Australia where I saw cane toads in the Northern Territory so many miles away from the Queensland cane fields they had once been introduced. (Off-topic but also interesting is the research that is being done on the evolution of these cane toads, showing that natural selection drives these toads to become better adapted to conquering new territory.)

The last driver that is discussed and the one I am most interested in, climate change. Climate change has been pinpointed as an important factor in the presumed extinction of the enigmatic golden toad . In the Monteverde Cloud Forest, where these creatures lived, a delicate chain of events has led from warmer sea surface temperatures in the Pacific to higher cloud formation and the forest becoming drier. This impairs breeding success in amphibians. However, few declines in populations can be directly linked to climate change and seeing that such a climatic change is not unprecedented and hasn’t before led to extinctions, Beebee and Griffiths conclude by saying that “it is therefore uncertain whether recent climate change is a significant cause in amphibian declines”. Because the research on climate change is still very much developing, I will discuss a newer article on this specific subject in an upcoming post.

As we have seen, there is a lot of uncertainty whether these separate drivers are responsible for the large declines in amphibian populations that we have recently seen. But one can also see that they all add up and that weakening of a population by one driver can make it more vulnerable to another. In reality there will also be multiple factors playing a role in the dynamics of a population. Research should therefore also focus on multifactorial approaches.

Practical problems in research

After this short overview on drivers of amphibian declines, it is interesting to see which difficulties are encountered in the study of amphibian declines. Measuring declines in a population or even a species is not as straightforward as one might think. Beebee and Griffiths make a distinction between measuring at the distributional level (species as a whole), population level and measuring the effective population size. For the distributional level it is mainly the workload that makes it practically difficult to monitor decline of a species. As they put it: “Estimates of overall distributional changes are very sensitive to survey effort and require multipleyear resurveys for high confidence”. For a species with a broad distributional range and many different populations, the decline of one or even multiple populations cannot be extrapolated to the species level. This means putting in a lot of effort and time to monitor all different populations.

In measuring populations there are basically three issues that need to be resolved; firstly the non-trivial issue of how a population is defined. Secondly, if the population is defined and a decline is seen in this population, it might be part of the natural fluctuation of this population. It is shown that several amphibian population combine long periods of decline with short bursts of rapid recruitment and hence large population growth. So when measuring a decline this has to be separated from the ‘default’ behavior of the population. And thirdly, a methodological issue, concerning the lack of control over detection probabilities in population studies. This has to do with the fact that probability of detection differs between recorders, between habitats and between time periods. When these problems in measuring populations have been tackled, we still cannot be sure of the viability of a population. Now we have to look at the effective population size  which is more meaningful for conservation planning. This effective population size is even harder to determine than ‘normal’ population size, because molecular genetic methods have to be applied. A small effective population size, which is seen in many amphibian species, makes it more vulnerable to genetic drift and inbreeding, especially with habitats becoming more and more fragmented. For conservationists it is therefore good to be informed on how small or large this effective population size is.

Status of amphibians

With these restrictions on amphibian research in mind, what can we say about the current status of amphibian populations? Interestingly, research that looked at the status of amphibians from around 1600 until 1970/1980 found only 2% of amphibian populations declining and reported only 2 extinctions. However, a review by Stuart et al. (2004) , looking at the ‘current’ status of amphibians, showed a more dramatic picture with 1856 amphibian species (32.5% of the total) under threat and 427 species (7.4%) critically endangered. These numbers indicate that the situation has worsened immensely over the past 25 years.

Research priorities and the future

Beebee and Griffiths conclude by looking into the research priorities for studying the dramatic decline in amphibian numbers. They refer to an essay by Caughley (1994) who makes a distinction between two paradigms in conservation biology: the declining population paradigm and the small population paradigm. The main difference between these two is that the former focuses on empirical studies, trying to identify the agents of decline in real-world populations, while the latter focuses on theoretical and experimental work on the problems that arise within a small population. He argues that these two paradigms should be brought together in conservation programmes. Amphibian research mostly fits within the declining population paradigm: “what is therefore missing from amphibian decline research are explicit applications of the theoretical approaches embraced within the small population paradigm”.

As recommendations for further research it is proposed to use a multidisciplinary approach with experimental and modeling methods. This approach will need to connect studies that target causes of decline in individuals to the landscape-level impacts on populations. In that way it will possible to determine whether these drivers are really having an impact on populations. In addition to this, studying reintroductions of species will also help to demonstrate whether the supposed agents of decline were successfully identified and remedied.

What to me sprang out of the concluding words of the article, is the pessimistic view the authors have of the survival probabilities of amphibians. In a table they enumerate the different agents of decline, as mentioned before here, and give possibilities to ameliorate the situation for this particular agent. They conclude that many of these possibilities in fact seem to be impossible to implement, because they are too expensive, impracticable or will be too late to still give a positive effect: human population pressure will continue to rise, ever increasing habitat destruction, the introduction of exotic species and pollution, while the long generation time and small effective population size in amphibians makes it hard for them to adapt to pathogens and pollutants in a short time. As well, creating corridors for species to promote movement in response to climate change is in many cases impractical. They believe biological research has helped a great deal in understanding the amphibian crisis, but they believe only politics, legislation and socioeconomics can help to reverse these global threats. This means the gap between social sciences and biological sciences has to be closed further. Even though a lot of scientists try to bring hope with concluding their article with a positive outlook on the future, I believe this kind of realism might help to increase the sense of urgency that is needed in this biodiversity crisis.