From black blobs to frogs’ legs

Newly-laid frogspawn close up, Image by Rowena Millar

This March, we’re celebrating all the wildlife and incredible insects that our gardens have to offer. Cornwall Wildlife Trust’s (currently non-roving) reporter Rowena Millar is back with another wildlife gardening blog, this time with an update on her garden pond life!

At least eight plops and splashes revealed the presence (or rather disappearance underwater) of at least eight amorous adult frogs in the pond last month. The frogs seem particularly jumpy this year, diving out of sight as soon as they glimpse me approaching in the distance.

Common frogs mating in the pond

Common frogs mating, Image by Rowena Millar

Last year they spawned on 24th January and this year, I was pretty sure that they were spawning at around the same time, but there was none of the usual throaty croaking throughout January and they held off, waiting until a second spate of icy weather was over. Did they sense the cold snap coming? The valley frogs in nearby woods went ahead and spawned, but maybe there was an ominously iciness to the wind in our higher location.

During the hard frost, I saw a tawny-brown female frog, nearly as big as my hand and bloated with spawn, swimming around just under the ice. Using warm water and my hand, I removed some ice in the corner of the, to make sure she and her fellow frogs had enough air. Iced-over aquatic plants such as hornwort ought to provide plenty of oxygen by photosynthesizing under clear ice, but oak and ivy leaves that I could see decaying on the bottom of the pond, having been blown in during gales, could have the opposite effect.

Frog amongst the duckweed in the pond

Frog in duckweed, Image by Rowena Millar

This year, our frogs started croaking to each other in February and decided to spawn on top of a raft of bogbean interlaced with stray grass in the middle of the pond, safely away from any hungry blackbirds. In around two weeks, the tadpoles will have changed from black dots to squiggly lines cushioned within their bed of jelly. As they develop, the infant tadpoles will feed on the jelly and any algae that form on it, then they will squirm down through any remaining jelly, then through thick, twisting bogbean stems and leaves into the water below, where they can explore life among the stems, like fish fry exploring a mangrove swamp. Their main predators in our pond used to be dragonfly nymphs, but there seem to be fewer of these in recent years, and palmate newts are the most numerous predator. You can find out more about palmate newts in my previous 'Newt encounters in the garden' blog. 

On social media, there are arguments about whether or not people should move, swap or share frogspawn. My own policy is to move drying out spawn (in or near puddles) only metres away, to deeper water on the same site, and to keep tadpoles in tanks only temporarily, for educational reasons (who isn’t fascinated to see them develop?) or perhaps if the tadpoles face total annihilation in their current position and there’s nowhere to go nearby. Such tadpoles should be returned to their exact place of origin later, to avoid possible spread of disease to new sites. If kept until they have grown all their legs, the tadpole keeper should be aware that they need enough surface water for adequate oxygen, and that their diet gradually changes from plant eating (e.g. pond algae) to meat eating (e.g. insects) as they become little frogs. Once they have legs, they need somewhere to crawl to, they need to breathe air from the atmosphere like we do, and they will not survive in a tank or bowl of water with no escape.

If you are able to get a close look into your pond, you will notice each of your frog tadpoles changing from a jet-black round blob to a wiggly longer black blob. It then develops a rounded body with external gills and a tail, and then its gills become internal and it starts to develop hindlimbs (this is around two months after hatching). By the time it develops forelimbs it’s eating animal matter (dead or alive), has a slightly froggy shape and is a paler colour. Then the tail starts to shrink. By three months old, it is becoming a froglet, which can seem tiny and thin compared to the fat, speckly brown tadpoles with legs you had seen before. Not all tadpoles develop into frogs at the same rate – maybe it depends upon water temperature and food availability.

The frogspawn starting to change into tadpole shapes

The blobs starting to change into tadpole shapes, Image by Rowena Millar

But where do they go? Once your froglets have four legs and no more than the stubbiest stump of a tail, they go walkabout around the garden. If you had a lot of spawn and most of your tadpoles survived the host of perils that could face them (including drought, algal blooms, dragonfly larvae, water boatmen, newts, grass snakes, birds and hedgehogs), you might need to be careful where you are stepping! (Don’t be tempted to deter or exterminate predators – nature should be allowed to balance itself out, and tadpoles are part of the food chain.)

Toadlets may emerge from a pond en masse, like swarms of delightful little black crawly gremlins – but I will save toads for a future blog.

To make your wildlife garden friendly to frogs, my best advice (apart from having a pond with some shallow and some slightly deeper water, with easy access in and out) is to encourage insect life throughout your pond and garden, by growing a wide range of flowering plants for every season. Avoid keeping fish if you can. If your pond is new, you should make sure that there is access into your garden from other gardens or wild places, so that frogs can find their way in. Provide plenty of low cover such as tall grass and vegetation near the pond. Provide leaf mulch and log piles at ground level, where frogs can hide. A bog garden would be much appreciated by frogs, too. 

Watch the video above if you're interested in finding out more about the incredible wildlife that can be found in your garden pond and visit the website for a step-by-step guide on how to build your own pond. The Environmental Records Centre for Cornwall and the Isles of Scilly (ERCCIS) also have a great 'Amphibians in your garden' factsheet for you to read. Enjoy your tadpole and frog watching!

By Rowena Millar