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Amphibian Health & Disease

Causes of adult amphibian loss: Predators

Adult frogs are part of the diet of a long list of predators. 

Natural predation can be upsetting to us but will do little real damage to the number of frogs in your garden population.

In most cases predation is likely to leave no trace of the frog, but sometimes bodies are left behind. If you come across a dead frog in your garden, in many cases it’s possible to tell what predator may have been involved.

Below is a brief description of tell-tale signs to look out for:

Herons – you will often see a pointed entry wound, the result of being stabbed with the beak.

Rats – rats will attack and kill many toads and frogs as they congregate to breed. Often you will find bite marks around the belly through which they pull out the internal organs.

Cats – if your cat brings in a live frog put it outside in some undergrowth and keep the cat indoors so the frog has a chance to escape. It is also a good idea to keep your cat inside at night so it can’t go after the nocturnal wildlife in your garden.

Otters – often will peel the skin from toads’ legs and eat the muscle underneath. This is because, like many animals, they find toad skin distasteful. Frogs may be eaten whole as they lack the distasteful skin chemicals.

Predation is linked to other factors. For instance, if pollution is a factor, then tadpoles or frogs most affected by pollutants may be easier prey. Similarly, amphibians suffering from disease may also be ‘easy pickings’.

Crossing roads

Every year thousands of toads and other amphibians are killed on roads while migrating to their breeding ponds. The problem can be so bad that populations of toads have become locally extinct, sometimes in only a matter of years. The days following migration the dried bodies of the dead (often squashed) amphibians are seen to litter the road. Toads and newts can also fall into roadside drains, from which escape is normally impossible.

If there is a road you know of where every year numbers of toads are killed and you would like to get involved and help stop this from happening then get involved with Toads on Roads

Winterkill (anoxia)

Some frogs will choose to over winter at the bottom of ponds as they can breathe through their skin and because the water temperature there stays fairly constant. However, in severe winters when a pond is completely frozen over for a prolonged period of time, the decomposition of vegetation within the pond can cause a depletion of oxygen that causes ‘anoxic’ conditions. This can suffocate not only the frogs but many other organisms which are down there.

After the frosts have gone the dead frogs will float to the surface of the pond and will often appear bloated. Death of a few individuals through winterkill is a natural process and not a problem for the population, but some garden pond owners decide to reduce this cause of mortality. The best way to prevent winterkill is to place a ball in a pond before it freezes over and then remove it after the ice layer has formed. The hole that remains should allow oxygen and other gases to enter and exit the pond. If the pond has already frozen over, place a bowl of hot water on top of the ice until a hole has formed. In the longer term, removing excess decaying material from the base of your pond may help (but leave some material behind as it is a great habitat for many pond animals).

Inflated Amphibians

Occasionally frogs, toads or newts are found with an inflated appearance. We do not know the cause of this, though it seems likely that some infection causes problems with the animal’s water balance, causing it to swell up with fluids. In many cases the animal recovers if left alone.

Drowning

Female frogs that arrive early to a breeding pond are immediately grabbed by males – often many at once - which will often not let go until she expels her spawn. Many females become exhausted from all of this male ‘attention’ and can drown. Competition for female toads can be higher than for frogs – females can be grabbed by multiple males which, again, often leads to females drowning, or suffering from exhaustion before, or (more often) after spawning. This is a normal part of amphibian behaviour and it is not advisable to try to separate frogs or toads.

Toad fly

This can be a particular problem to common toads in late August. A species of green-bottle fly lays its eggs on the head of a toad and, once the eggs hatch, the maggots will crawl up the toad’s nostrils and begin eating the soft tissue in the nose and head. This can cause them to make a ‘clucking’ noise as they try to breathe in air. This native parasite will, in most cases, ultimately lead to the toad’s death.

If you are noticing symptoms that are not described above it may be that you have come across adults suffering from an amphibian disease.  If so, please click here for more information.

HELP OUT: During the summer months of 2008 Froglife is appealing for specimens of amphibians which have died for any reason - disease or otherwise.  If you are willing to help out (this will mean storing dead amphibians in your freezer) please get in touch for more information.

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