Amphibians and winterkill – a worse than average year?
Several species of UK amphibians overwinter in ponds, including common frogs and smooth newts. These animals tend to spend the winter in the silt and decaying leaves at the bottom of ponds. Normally this does not harm the animals and they resume breeding as usual the following spring. The advantage to spending the winter in the pond is the individuals are ready to breed as soon as the weather becomes warm enough. However, these individuals rely on absorbing oxygen through their skin, especially during periods of cold weather. This can be a problem as sometimes during the winter months oxygen levels within the pond can fall and toxic chemicals build-up. This occurs more frequently when the surface of the pond freezes over for extended periods. Unfortunately this can result in the death of some amphibians and is commonly known as winterkill. This process is natural and every year a number of amphibians, mainly common frogs, die as a result of winterkill and this has no negative impact on populations.
During January and February 2018 we experienced relatively mild winter conditions across the UK, with only light overnight frosts. This encouraged the early emergence of common frogs and other amphibians, especially within the south of the UK. Froglife had sightings of great crested newts on 25 December 2017 and the first frogspawn was reported on 17th January. The relatively mild conditions continued until late February, which triggered the movements of many common frogs towards breeding ponds. At the end of February, the UK experienced unusually cold and prolonged winter weather with hard frosts and snow. This sudden cold period of weather came quickly, when a large proportion of breeding frogs were already at ponds. Although these individuals may have tried to avoid the cold conditions, the combination of low temperatures, already low oxygen levels in ponds, along with an extended period of ice covering ponds caused mortality in many frogs across the country. The Froglife enquiry service received a sudden and high level of reports of large numbers of mainly common frogs being found dead in ponds. Between 5th and 16th March Froglife received 26 enquiries relating to frozen frogs in ponds. In several cases frogs appeared to have frozen within the ice. The worst case was from a school pond where over 120 common frogs were removed dead from the water. This number of enquiries was higher than average as in most years Froglife only receives a handful of cases of winterkill across the whole winter and spring seasons.
In addition to an increase in number of dead adult common frogs, frogspawn appears to have been affected. Spawn laid early in the season is at risk of damage due to later frosts. The cold period of weather in early March and then again on the weekend of 17-18th March occurred after the majority of frog spawn had been laid. Although spawn in the frost-free zone of the pond (usually underwater) will survive, any eggs near to the surface usually die. These often contract a white fungal infection which can later spread to all the spawn. The combination of frost and fungal infection may have resulted in a higher than average mortality of frog spawn in 2018.
It is too early to determine the impact of winterkill on common frog and other amphibian populations. Amphibians are relatively long-lived so the loss of spawn from one breeding season is unlikely to have significant impacts on populations. However, the loss of a large number of breeding adults may have an impact, especially where populations are small and are already under stress from other factors such as loss of breeding and terrestrial habitats, habitat fragmentation, introduced disease and pollutants. Climate change may impact amphibians in several ways and an increase in extreme weather events may have long-term implications on amphibian populations across the country. Long-term population monitoring is required on amphibian populations to determine whether they are negatively affected by repeated extreme weather events at key times during their breeding cycle.