Like so many people worldwide, I have a great fondness of, and feel a great connection to, frogs.
Caption: our first pond (Pond 1) was installed in 2009, and was immediately discovered by the one or two Common frogs occurring in the immediate area (here’s a male). Common frogs can live for over 7 years (12 is possible in captivity), but they might only have a single breeding event across that time. This boldly pattern individual was only seen in 2009 and 2010. Image: Darren Naish.
Right from early childhood, frogs (and their larvae, tadpoles) have been among the most familiar of animals to me, either as adult animals encountered in gardens and forests, or as tadpoles observed in local ponds and kept at school for educational reasons. This closeness to frogs makes them invaluable as ambassadors for nature, as educational tools about biology and biodiversity, and as good barometers for the health – or otherwise – of the natural world. For the record, there’s only a single frog species where I live, namely the Common frog Rana temporaria. I’m in the UK, where the herpetofauna is so woeful that it’s easy for us to learn, and get to know, every single one of our very low number of species.
But as I’ve become older, it’s obvious that frogs have locally declined. Ponds which – 20 or 30 years ago – had over 100 frogs in the early months of the year (where they gather to breed) today have none, and gardens and woods which were once familiar froggy haunts are no longer home to any of these animals, or at best to just one or two. Where have the frogs gone? Most of you will be familiar with the Global Amphibian Crisis (GAC), a worrisome event involving the spread of amphibian-killing fungi and pathogens and the impact of habitat loss and pollution. Frogs also suffer as roadkill and as the prey of free-roaming pet cats. For all these reasons and more, frogs have, during my lifetime, disappeared from the places I associate with them, and are absent from the gardens surrounding our current house.
Caption: Pond 1 and Pond 2 during the early stages of their construction, in 2009 and 2018, respectively. Both have been, and are, successful wildlife ponds, but both could have been better. Images: Darren Naish.
Or were seemingly absent, until I created a pond. Yes, back in 2009 I dug a giant hole, and installed a pond (Pond 1). And in 2018 I installed another one, Pond 2. Neither pond is perfect: my attempts at pond creation so far are always limited by the materials and tools I have and the locations themselves, but the point is that something is better than nothing. I do, incidentally, plan at some point to do an article on ‘how to create a garden pond’, and I strongly encourage you to do this… if, that is, you have available land. I’m very much aware of what a privilege it is to have access to your own garden area.
Caption: the only adult frog observed in Pond 2 for 2019 and part of 2020 was this single male. At left, here he is in April 2020, long after the breeding season. At right, a night-time image (note the white eyeshine); he’s calling for females, a fertilised clutch already to his left. Images: Darren Naish.
Thanks to these ponds, frogs have reappeared. Initially, I was aware of just one or two adults which appeared in Pond 1 during the breeding season (January and February… and now December as well due to our warmer winters). Common frogs are easily identifiable as individuals thanks to their specific markings – no two specimens are alike. The fact that clumps of spawn have been appearing since 2010 shows that we have at least two adult frogs in the area, but until this year (2020) there was precisely zero evidence that the tadpoles were making it to frog stage. Mortality in tadpoles and newly metamorphosed froglets can be hideously high and even be 100% (Beebee & Griffiths 2000), and the fact that froglets were nowhere to be seen led me to think that literally none of the tadpoles were making it to froglet stage.
Caption: 2020 saw the appearance of two spawn masses in Pond 2 (and none in Pond 1), and here are the tadpoles as of March 2020. Here, they’re in their sessile phase. They don’t do much more than lie on the top of the jelly and aim to absorb sunlight, so direct light is very important. Image: Darren Naish.
But 2020 has been very different, and in fact it’s the first year here in which it’s obvious that things might be on the up. For starters, a great many of 2019’s tadpoles which overwintered in Pond 2 successfully metamorphosed into froglets from late April 2020 onwards, the consequence being an abundance of exquisite micro-frogs – around 10 mm long at their smallest – at the pond edges and in the undergrowth.
Caption: at left, a brand-new metamorph of late April 2020, part of a clutch from 2019. At right, another 2020 metamorph living in preferred habitat (in July 2020), also from a 2019 clutch. Images: Darren Naish.
I’ve always known that Common frog tadpoles can and do overwinter, but I didn’t know that it was especially common. Indeed, technical studies say that overwintering in the Common frog is commonest where temperatures are low and food supplies are too (Beebee & Griffiths 2000, Walsh et al. 2015). My experience at Pond 2 shows that overwintering is very common here in Southampton and may be becoming more so, though I don’t yet understand why (Pond 2 is neither cool, nor especially shaded, nor notably nutrient-poor).
Caption: the fact that many of our tadpoles overwinter means that, by March 2020, Pond 2 was home to both young, tiny tadpoles as well as larger ones from a 2019 egg clutch. And by April 2020, the 2019 tadpoles (shown on the right) already had hindlegs. Images: Darren Naish.
The habitat favoured by the tiny froglets is worth discussing. More by luck than by design, we have a section of garden dominated by low-growing, juvenile brambles and wild strawberry, all of which is just in front of an area dominated by taller grasses and shrubs (including Red hot poker Kniphofia). This area appears to be favoured by the froglets, between 5 and 10 being visible on any given day. And there aren’t just 2020 froglets here, but larger individuals of anything between 20 and 50 mm SVL (snout to vent: obviously, you want body length when talking about frogs, not a length which incorporates the limbs). What this means is that 2020 wasn’t really the first year in which tadpoles successfully transitioned to froghood, but that there are also animals here which metamorphosed in 2019 and 2018, at least. Which is very encouraging.
Caption: the habitat preferred by our froglets. It’s not adjacent to either of the ponds but about 3 m away from Pond 2. It evidently provides the right sort of protection, cover, humidity and food supply. Image: Darren Naish.
Right now (early August), there’s still a large number of well-grown tadpoles in Pond 2 which are months away from metamorphosis (they don’t even have any legs). It’s possible that some of these will become froglets before the end of 2020 but it’s likely that many will overwinter, in which case we’re likely to have new metamorphs leaving the pond in April 2021 onwards.
Caption: more 2020 froglets, encountered in Pond 2 and in the frog patch during July 2020. Again, these animals are from a 2019 clutch, not a 2020 one. Images: Darren Naish.
There are a great many ‘ifs’ in frog survival and metamorphosis, but it’s finally conceivable that at least some of the many froglets currently in the garden here will make it to adulthood and breeding age themselves. In which case we may get to a situation where many adults are spawning, not just one or two pairs. I’ll continue to keep an eye on things.
The resilience of nature is remarkable, and it shouldn’t really be surprising that the frogs here look to be capable of building up their numbers from borderline absence. But none of this would be happening if we hadn’t maintained a wild, green garden and created ponds in which the frogs have been able to breed. Do likewise if you can, and encourage others – again, if you can – to enable green spaces, ponds and wild places, and the animals which live in them, to persist.
Caption: a substantial number of non-metamorphosed tadpoles remain in Pond 2; here’s how they were looking as of just a week ago (July 2020). Some of these will overwinter and complete metamorphosis in early 2021. Image: Darren Naish.
Common frogs are part of the great neobatrachian frog group Ranidae, the members of which have been covered at TetZoo a few times before…
In pursuit of Romanian frogs (part II: WESTERN PALAEARCTIC WATER FROGS!!), July 2011
In pursuit of Romanian frogs (part III: brown frogs), August 2011
The Remarkably Weird Skeletons of Frogs, December 2016
The 2017 Amphibian Conservation Research Symposium, June 2017 (wayback machine version, but with broken formatting)
Refs - -
Beebee, T. & Griffiths, R. 2000. Amphibians and Reptiles. HarperCollins, London.
Walsh, P. T., Downie, J. R. & Monaghan, P. 2015. Factors affecting the overwintering of tadpoles in a temperate climate. Journal of Zoology 298, 183-190.
