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Causes
of tadpole
loss
In
some cases, ponds may appear to be literally full of tadpoles. If your
pond appears to have a very high density of spawn, it is not necessary to
remove any however.
Tadpoles can survive at very high densities and will be
naturally ‘thinned out’ as the weakest die or are predated upon. This
is a completely natural process; a method that amphibians have evolved to
ensure that only best suited individuals to a habitat will survive.
Reasons
behind 'disappearing tadpoles':
-
Predators:
This
is a very risky time for amphibians, since tadpoles are eaten by almost
every aquatic predator there is. The list includes dragonfly larvae,
diving beetles, water boatmen, newts, ducks, blackbirds and grass snakes. Again,
this is nothing to worry about since in most cases some of the tadpoles
will make it to emerge as froglets.
-
Lack
of available food source:
Tadpoles
feed primarily on the algae that grows on plants and rocks in the pond,
along with decaying matter such as dead pond animals. In some ponds that
are newly created or poor in nutrients there may be little food available.
This can lead to slower growth and later emergence, or even cause the
tadpoles to over-winter at the bottom of the pond. Longer development
times are not normally something to worry about, and adding food is not
recommended as it can lead to excess nutrient levels. Over-wintering
of tadpoles can also occur if there is a mild summer or if your pond is in
a shaded position that means less light is reaching the pond. The algae
that the tadpoles feed on requires light to grow so in some cases it might
be worth cutting back overshadowing branches or foliage near your pond. In
rare instances you may also come across neotenous tadpoles. These are
tadpoles that are unable to metamorphose and hence continue growing,
occasionally to lengths of up to 4 inches! Please let Froglife know if you
notice tadpoles of this size in your pond since it is possible you might
unknowingly have an exotic species of larger amphibian that has taken
residence in your pond.
Please contact
us if you have concerns about non-native species).
Dried
Up Ponds:
if
a pond dries up before the tadpoles have metamorphosed, they will not
survive. While you have tadpoles in your pond it is best to keep an eye on
the pond’s water level and top it up if it’s getting low. If possible
you should use rainwater which you can collect in a water butt - this is
free from chlorine which can sometimes kill tadpoles (although a small
top-up from a hose is not normally harmful). In the autumn after all the
froglets have left your pond you may want to replace the lining to prevent
similar problems the following year. For information on relining garden
ponds see Froglife’s ‘Pond Heaven’ booklet (for details see the 'Frogalogue').
-
Anoxia:
Another
possible reason for loss of tadpoles is anoxia. This can occur when a pond
is too full of algae, or if certain contaminants are in the water.
Further, pollutants in general may cause massive tadpole mortality. A
possible indication of this would be any recent spraying or fertilizing
activity, road building or construction nearby with associated run-off, or
even a sudden algal bloom in the pond.
-
A
note on 'Disappearing
tadpoles':
People
often report that their tadpoles simply ‘disappear’. There are a
number of reasons why this could be so. Sometimes they may be hiding in
the silt at the bottom of the pond or in the vegetation at the edge of the
pond because as they grow larger they tend to become more secretive. In
other cases, it might suggest that predators, such as newts or dragonfly
larvae exist in the pond in large numbers. Tadpole
development can often be dramatically fast in favourably warm periods, so
in many cases ‘disappearing’ tadpoles can suggest that metamorphosis
has taken place and froglets have emerged en-masse and into the
surrounding habitat without you ever being aware.
Click
here
to go back to the 'Amphibian Health and Disease' home page
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