blog

Roger Downie

Froglife and University of Glasgow

A quarter of a century ago, James Hanken (1999) drew attention to the ironical situation that at a time when the serious worldwide declines in amphibian populations were becoming recognised, the number of described amphibian species was increasing at a faster rate than for any other vertebrate class. In 1985, the number of species was 4003; given the current rate of new descriptions, Hanken expected the number to reach 5000 by the turn of the century (1.7% more per year). Now, half-way through 2025, Frost’s database lists 8883 species (actually 3% more per year since 1985), 7828 of them anurans (frogs and toads), with no sign that the rate of new species description is likely to decline soon. But sadly, as these numbers continue to rise, so too do the threats: the Global Amphibian Assessment (GAA) in 2004 concluded that 32.5% of species were threatened with extinction, the highest proportion for any vertebrate class; GAA2 in 2022 revised that figure upwards to 40.7% (although part of the rise results from a decrease in the proportion of species classed as Data Deficient, 22.5 to 11.3%). That figure can only be a tentative estimate, since the conservation status of the newly-described species is rarely known.

If we are to roll back the wave of amphibian extinctions, a key step is to determine how many species exist and where they live (to non-biologists, it is probably a surprise to realise that after centuries of species recording, the world species catalogue is nowhere near complete). However, if we are to protect species into the future, we need to know much more than is contained in a basic species description. Hortal et al. (2015) listed the ‘shortfalls’ (or deficiencies) in our current knowledge about biodiversity under a set of headings named after prominent scientists, for example:

• Linnean: description and cataloguing of species
• Wallacean: distribution of species
• Prestonian: abundance and population dynamics
• Darwinian: relationships and variability

Faria et al.(2020) identified another class of shortfall: Haeckelian, named after the prominent 19^th century German embryologist Ernst Haeckel. This class encompasses knowledge of a species life-history stages. Now, a group of mainly Argentinian biologists has applied this idea to frogs and toads (Vera Candioti et al., 2023; Nori et al., 2025): they note that for more than 50% of species, we have only adult descriptions, with no information on embryonic, larval and juvenile stages, including on their habitats and habits. This clearly matters for species with biphasic life histories where the larval/tadpole stage is fundamentally different from the adult in form, habits and habitat, since measures aimed at conserving adults may be ineffective in assisting earlier stages. The authors note that the IUCN species assessment process does not yet take this information shortfall fully into account, finding that many species where knowledge of larval stages is entirely lacking are currently listed as of Least Concern.

Nori et al.’s (2025) paper describes a strategy for reducing the Haeckelian shortfall for anurans, using geographical and other data to identify the areas where focussed fieldwork should be capable of finding and documenting the missing larval stages, habits and habitats: these are parts of the tropical Andes, eastern Brazil, tropical Africa, India, southeast Asia and New Guinea.

In the UK, with our very limited anuran fauna, including limited larval diversity (all species having pond-dwelling algal browsers), it is hard for us to appreciate the vast diversity of larval forms found elsewhere, and to understand that much diversity remains to be discovered. A few recent examples illustrate this point:

• Dias et al. (2024a) analysed the food processing mouth-parts of several so-called ‘sand-eating’ tadpoles from Madagascar: the ruffled ridges used for separating organic material from sand-grains are described as ‘astonishingly novel’ and not seen previously in any anuran larvae.
• Dias et al. (2024b) described the adhesive oral discs of two Andean toad tadpoles as so different from those of relatives that they justified assignment to a new genus Adhaerobufo despite the adults being quite similar to relatives.
• Romero-Carvajal et al.(2023) describe early development of a ‘plump toad’, one of eleven species found in the Andes, where early stages are previously unknown. Small numbers of large terrestrial eggs are produced, with no free-living tadpole stage; however, the pre-hatching stages include an elongated tail, unlike the findings for other direct-developing species.

When I started research on the amphibians of Trinidad and Tobago in the 1980s, I was lucky that the existing account (Kenny, 1969) included illustrations and natural history notes on most of the tadpoles, and we were able to extend these to cover some new species, including the glassfrog and stream-frog tadpoles of Tobago. But much remains to be done. We made observations on some key variables: for example, the time taken from hatching to metamorphosis, as little as 12 days in some cases; the time taken for the period of metamorphosis itself, and the behaviour of newly metamorphosed froglets. All these are crucial to the success of the anuran life-cycle, but remain hugely under-researched for most species. Let’s hope that the focus provided by Vera Candioti and colleagues provides an impetus for the needed research.

Acknowledgement

Thanks to Robin Bruce for drawing my attention to Nori et al’s paper.

References