Roger Downie: Froglife and University of Glasgow
Most Croaking Science readers will be aware that the Global Amphibian Assessment of 2004 (GAA1: Stuart et al., 2004) found that about one third of amphibian species worldwide were in the IUCN’s ‘threatened’ categories (vulnerable, VU; endangered, EN; or critically endangered, CR), compared to only 12% of birds and 23% of mammals, making amphibians the worst off of the terrestrial vertebrates so far assessed (reptiles had not been assessed at that time). How has the situation changed in the nearly two decades since then?
The headline news is that we now have an assessment for reptiles (GRA1) and a second assessment for amphibians (GAA2). GRA1 (Cox et al., 2022) was made by 961 scientists who contributed their knowledge to 80 workshops over about 18 years (Meiri et al., 2023). They assessed 10,196 species, 21.1% coming out as threatened. However, 14.8% (=1507 species) were classed as data deficient, on grounds of too little reliable information being available. The threats are not evenly distributed across reptile orders: 57.9% of turtles are threatened; 50.0% of crocodilians; and 19.6% of snakes and lizards. Complete extinction is hard to be sure of, as we’ll see later for amphibians. For reptiles, 31 species are judged extinct since the year 1500, with 40 more ‘possibly extinct’, having not been observed over several decades. Overall, the 21.1% figure is a little less than the most recent estimate for mammals (26.5%) and more than that for birds (12.9%).
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GAA2 (Luedtke et al., 2023) assessed 8011 species, about 40% more than in 2004. The proportion judged threatened is 40.7% (=2873 species), an increase on both 1980 (37.9%) and 2004 (39.4%). The data deficient category reduced to 11.3% from 22.5% in 2004 . Newts and salamanders are the worst-off order, followed by frogs and toads, with caecilians the least threatened, although that result is problematic because of the difficulty in assessing these elusive tropical burrowing animals. The extinction figure for amphibians is 37, with 185 ‘possibly extinct’. Establishing extinction is not easy, especially in remote parts of the world little visited by scientists. A Conservation International-funded project found populations of several species not seen for 50 years or more: Moore’s book (2014) provides an entertaining account of his attempts to find some of the missing species. As in 2004, the 40.7% of threatened species figure makes the amphibians the worst-off of the tetrapod vertebrate groups.
In 2004, GAA1 noted that a worryingly high proportion of amphibian population declines were occurring in what seemed to be good quality habitat, and termed these declines ‘enigmatic’. The main cause was soon identified as a previously little studied fungal disease, chytridiomycosis, caused by a microorganism, Batrachochytrium dendrobatidis which infects amphibian skin and can cause death by damaging the skin’s water balance functions. Later, a second chytrid species was found, B. salamandrivorans, specific to newts and salamanders. Much research on chytrid disease followed, with many scientists regarding it as the primary cause of the amphibian crisis. However, it emerged that amphibian interactions with the disease are complex, and some species thought to have disappeared after the disease reached their area, turned out to have survived, albeit in small isolated populations. The harlequin frogs (genus Atelopus) of Mesoamerica provide a key example: 37% of the species believed to have become extinct have now been rediscovered (Jaynes et al., 2022). Overall, GAA2 concludes that habitat loss and degradation are the main threats to amphibians, as to most kinds of wildlife, with disease next in importance and climate change rapidly climbing the agenda.
Can we now conclude that the situation for amphibians has worsened over the last 20 years, with the overall percentage of threatened species increasing, despite the major conservation efforts being made? GAA2 concludes that 788 species have deteriorated in their IUCN red list status since 1980, with only 120 species improving. However, the answer is complicated by at least two factors. The first is the expansion in numbers of described amphibian species. GAA1 assessed around 5700 species, while GAA2 included 8011; even that figure is not the whole story. The Amphibian Species of the World database (Frost, 2023) now (October 2023) lists 8685 species. Why has the list increased so much, and how does this affect the conservation assessment? The increase is partly the result of new exploration to remote areas, discovering previously unknown amphibian populations. But it is also partly the result of taxonomic revisions, mainly driven by molecular methods: wherever a species has previously been found to be very widely distributed, molecular analysis has often found that the population should be sub-divided and sub-populations designated as new species. An example is the neotropical treefrog Boana crepitans previously considered to occur in Brazil and northwards to Trinidad and Tobago. A re-analysis sub-divided the northern population as a new species, B. xerophylla, and further analysis sub-divided that northern population into three, with the Trinidad and Tobago population being named B. platanera (Moises et al., 2021). Although GAA1 judged B. crepitans as widespread and common, therefore of Least Concern (LC), the newly identified populations need new assessments, and this is clearly the case for many new species identified in the same way.
Another issue is the assessment methodology. In the years running up to GAA2, with several colleagues I carried out some species assessments in Trinidad and Tobago. We were quite surprised by some of the GAA1 assessments, and discovered that they were made not as a result of recent fieldwork, but by committee decisions based on the views of a few visiting scientists. We carried out extensive field surveys on the species that GAA1 assigned to threatened categories and concluded that some of these should be regarded as LC: for example, we recommended altering the Trinidad stream frog Mannophryne trinitatis from VU to LC; the marsupial treefrog Flectonotus fitzgeraldi from EN to LC; and the litter frog Pristimantis urichi from VU to LC. My experience of feeding our data into the IUCN process was very positive, and the process was clearly as rigorous as possible, but it is still the case that fieldwork data are not always available, and expert judgement has to be made.
It is good to have up to date assessments of the global status of both amphibians and reptiles, but it is clear that much more effort is needed if we are to halt species declines.
References
Cox, N. et al. (2022). A global reptile assessment highlights shared conservation needs of tetrapods. Nature 605, 285-290.
Frost, D. (2023). Amphibian Species of the World, an online reference. Accessed 13/10/23.
Jaynes, K. et al. (2022). Harlequin frog rediscoveries provide insights into species persistence in the face of drastic amphibian declines. Biological Conservation 276, 109784.
Luedtke, J. et al. (2023). Ongoing declines for the world’s amphibians in the face of emerging threats. Nature online 4/10/23.
Meiri, S. et al. (2023). Done but not dusted: reflections on the first global reptile assessment, and priorities for the second. Biological Conservation 278, 109879.
Moises, E.S. et al. (2021). Zootaxa 4981, 401-448.
Moore, R. (2014). In search of lost frogs. Bloomsbury, London.
Stuart, S. et al. (2004). Science 306, 1783.