Croaking Frogs

One of the features of our new garden that surprised me when we first saw the design on paper was the pond.

I don't remember explicitly asking our designer to include one, yet once I saw it on the plan I quite liked the idea. We had a pond here when we moved in, but it was a sad little effort. Hidden under the large acer planted in the border between us and our semi-detached neighbour, it had clearly suffered some damage to its liner as it never filled up beyond half-way and the half that did fill was always choked with weed. Notable mainly for its brief occupation by a drunken party guest on the evening of our housewarming, the pond nevertheless proved to be home to a small population of frogs. When we dug it out and filled in the hole in April 2009 the frogs disappeared, only to reappear again one by one whenever we moved a large rock, wooden sleeper, or pile of twigs anywhere in the garden.

So I had great hopes that once garden construction was complete and the pond had become established it would attract back some of our original colony. As we moved into Spring and the water lost its winter chill this did indeed prove to be the case. We caught the occasional glimpse of a pair of eyes gazing at us from just above the still surface, and while watering the garden early one morning during the recent hot spell I disturbed four adult frogs with the hose as they slept in the damp patches under the leafier shrubs and bushes. It appeared we had succeeded not only in attracting back the originals but adding to the numbers. Until things took a turn for the worse.

We began to find bodies. One or two only at first - a dried-up husk on the path and a pale, mottled corpse lying on its back in the shallows near the pond edge - but having disposed of these they were soon replaced by others. The area is occasionally visited by a heron, so initially we wondered if this had been attacking the frogs. Or maybe one of the many local cats was to blame? But in all cases the frogs appear not to have been injured and in the majority of cases they die in the pond itself, their ghostly pale bodies often floating upright with their back legs resting on the bottom so they appear to be standing up under the water. One awful weekend morning I fetched four bodies out of the water.

These aren't pets, of course, but I must admit to feeling sad at their demise, along with a nagging feeling of guilt in case we're somehow responsible for the deaths. It's nice to have somewhere for the frogs to live, but I didn't expect or want it to become a death trap. I've started mentally referring to it as the Dead Pool.

The sides of the pond are fairly steep, and the exposed liner is slippery when wet. Added to this the recent hot spell meant water levels became quite low, so it occurred to me that the frogs may not be able to get out once they'd jumped in. Is it possible for frogs to drown? Amphibians breathe, at least in part, through their skin, so I guessed that if they were trapped in water that had a low oxygen concentration (which our pond almost certainly has, with no submerged oxygenating plants or other means of replenishing the levels) it could be possible they were effectively asphyxiating. As you will have noticed from the photo, I've added a rudimentary "frog ladder" to the pond to help them get out. They're still dying.

Brief research online suggests the problem may be a slow-release fertiliser in the baskets of the pond plants that is toxic to frogs. I don't remember seeing any dead frogs before the plants were installed, so this is a possible cause, but there again the plants arrived fairly early in the year, so I don't remember seeing any live ones before then either.

My research also pointed up that, since amphibians absorb much more than oxygen through their delicate skins, they are a kind of "environmental indicator" species that will be affected by any chemical pollution. So maybe it's that. Discussions with neighbours who also have a pond have revealed they too are suffering from frogs that croak in the less traditional sense. I don't take much comfort from the knowledge that we're not the only ones this is happening to.