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|Frog fallacies and facts, UK|
> Frogs found dead in ponds in Spring must
have caught a disease
Not necessarily - in particularly cold weather, such as experienced in early Spring, frogs may die during hibernation. Whereas females like to hibernate on land, male frogs may choose to lie in a layer of mud at the bottom of ponds, so they can be first on the scene when mating starts. But if the pond freezes, depletion of oxygen under the ice may eventually suffocate them. Female frogs found dead in the breeding season may have been clasped by too many over-amorous males; male frogs may also die due to the exhaustion of mating.
There is no such thing as too much spawn - frogs do lay huge numbers of eggs, but their high mortality rate means that usually fewer than 10 in 1,000 will develop into adult frogs. Tadpoles will survive in a pond at surprisingly high densities; 'thinning them out' is pointless, because numbers quickly build up to replace ones removed. But at tadpole stage many are sure to be eaten by grass snakes, birds, insect larvae, newts, ducks and fish, or lost to frost damage or fungal infection. Transferring spawn is a dangerous business - you might easily import diseases or invasive alien plant species. In some jurisdictions it may also be illegal. Anyway, if there are no frogs in the new pond there may well be a good reason, such as an over-abundance of fish or ducks.
Spawning happens at different times in different regions. Frogs will spawn as early as they can, but, intriguingly, the pattern of starting dates, if you plot them on a map of Britain, has been found to mirror the effect of the Gulf Stream - moving eastwards from Cornwall (where the peak month is January) to the Hebrides, and then the West Coast of Scotland, to the central counties in March and then across to England's East coast. The latest spawning tends to happen at Teeside and Ipswich, where frogs have been known to wait until April. Subtle alterations in the currents of the Gulf Stream will affect spawning times - so frogs are now being used to help monitor and predict global climate change.
This is sadly no longer the case. In parts of Nottinghamshire, the so-called common frog, Rana temporaria, is increasingly scarce. Ironically, it is in Britain's rural areas that frogs are most threatened, due to hedgerow loss and the dredging of ponds on farmed land. However, in suburban areas, frogs are doing rather well, as artificial ponds are created by swelling numbers of amateur gardeners.
If anything it is the other way round - fish eat spawn and so reduce the frog population of a pond. Tadpoles are an asset to fish because they feed on algae, helping to keep pondwater clear.
In fact, the skin of frogs is thin and highly sensitive. frogs breathe through their skin, and are extremely vulnerable to pollutants in water. Because their fortunes reflect the health of their environment, frogs can warn of dangers which may threaten other species. Tests indicate that the weed-killer paraquat, once used in gardens and farms, can deform and kill tadpoles if it becomes concentrated in certain pond weeds.
Our native common frog is often green - but can also be brown, orange or a reddish colour. Genetic mutations provide bright orange, canary-yellow or cream-coloured individuals with red eyes. It has been noticed that some tadpoles are white when they hatch. Increasing reports of these pigment changes are being received mainly in the South, but scientists do not yet know the cause. It is thought that white tadpoles may survive better due to the damaged ozone layer, which has led to warmer summers and higher levels of ultraviolet light.
Camilla Bonn, adapted