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| © John Wilkinson |
Have you heard this call?
If you have please log your record at the Record
Pool
Marsh frogs stay in or around water all year and tend to sunbathe on the sunny banks of
ponds or on submerged vegetation [1]. The photos above and below are classic marsh frog poses. This
behaviour differs from native frogs and toads that tend to leave ponds
after spawning in February and March [*]. Therefore, if you see a frog behaving in this way
between April and September it is very likely to be a marsh frog or a
close relative (see below in More about marsh frogs).
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| © John Wilkinson |
Call
Male frogs have a very loud and distinctive call. It
often consists of repeat bursts of five rapid quack-like sounds that have been
compared to a laugh or cackle [1]. Marsh
frogs call both day and night from late spring sporadically throughout the
summer.
Size
Marsh frogs can grow to 13 cm or more [1], much larger than native frogs and toads.
Skin
Their skin is warty, with folds of skin running
from their eyes down either side of their back. This distinguishes marsh frogs
from another non-native the North American Bullfrog (Lithobates
catesbeianus).
Colour
This can vary between green, brown or grey, but
marsh frogs are often more vivid green than native frogs and toads [2].
Vocal Sacs
Male marsh frogs have grey vocal sacs either side
of their head that inflate when calling[2].
Please Remember
Unfortunately, section 14(1) of the Wildlife and
Countryside Act 1981 makes it an offence for anyone to release marsh frogs back
into the wild once they have been captured; so please only observe and listen
to them.
[*] A native toad called the
natterjack toad (Epidalea calamita) can spawn in ponds until July.
However, natterjack toads are fairly rare, much smaller than marsh frogs and
prefer shallow sandy bottomed ponds so they are unlikely to be mistaken for
marsh frogs.
More about my research
Why is this research
important?
The common frog (Rana
temporaria) has suffered declines in the countryside and only remains
widespread because of its ability to thrive in urban and suburban garden ponds
[3]. The introduction of the marsh frog (Pelophylax
ridibundus) in Kent and Sussex may have accelerated the common frog decline
in this region. Although the decline of common frogs in Kent and Sussex has
coincided with the introduction and proliferation of the marsh frog, factors
other than marsh frogs may be the cause.
Possible mechanisms of impact
This may be as simple as…
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| A marsh frog in Romania eating a common spadefoot toad (Pelobates fucus). © Vlad Cioflec |
Marsh frogs are known to prey on other amphibians [4].
Despite different habitat preferences marsh frogs are found in habitat suitable
for common frogs so predation may occur. However, it is not known whether this
is likely to significantly impact common frogs.
Or other species may also be involved…
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A grass
snake (Natrix natrix) in Kent eating a marsh frog
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Landscape scale analysis
Species distribution models (SDMs) estimate suitable
conditions for species by relating species presence to environmental variables.
They provide a means to predict the distribution of marsh frogs and common
frogs and to identify if there is a relationship between the two species. Data
on elevation, pond density, watercourses, and land cover were used to model the
distributions of marsh frogs and common frogs in Kent.
The results show that there is a separation between the
distribution of marsh frogs and common frogs. The majority of this pattern is
likely to be due to differences in habitat preference between the species.
Marsh frogs are very aquatic frogs and stay close to water. Lots of the areas
of Kent which have a high density of watercourses are classified as arable and
horticultural land (e.g. the red circle).
Common frogs are more terrestrial so they may be less
tolerant of arable and horticultural land. The area indicated by the red
ellipse of the common frog SDM is not dominated by arable and horticultural
land and has a high pond density making it suitable habitat for amphibians. So
why is this area predicted as being less suitable for common frogs? Are marsh
frogs spreading into high pond density areas and reducing common frog numbers?
Local scale analysis
At a local scale repeat amphibian surveys were conducted in
areas where marsh frogs were present (Kent) and where they were absent
(Sussex). Survey ponds were chosen to be in areas with habitat suitable for
both species as predicted by the SDMs and ground truthing.
Results
Less than half the ponds in areas with marsh frogs had
common frog spawn present.
However, another common frog predator
may be involved…
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Great crested newt (Triturus cristatus)
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There are almost five times as many ponds with great crested
newts in areas with marsh frogs. The great crested newt SDM (below) shows an
increased suitability in the area of high pond density (red ellipse). Great
crested newts are positively correlated to high pond density. Large numbers of
ponds were created in this area to support the Wealden iron industry which may
be the cause of an unusually high number of great crested newt ponds. This
could be causing the absence of common frogs rather than the presence of marsh
frogs.
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| Predicted distributions of great crested newts in Kent. Warmer colours indicate a higher probability of suitable conditions. |
More about
marsh frogs
Relatives
of the marsh frog
The marsh
frog is part of a group of frogs called ‘green frogs’ or ‘water frogs’ and
other members of this group are present in the UK e.g. pool frog (Pelophylax
lessonae) and the edible frog (Pelophylax kl. esculentus).
There are a few differences between the species. Pool frogs are smaller, have
white vocal sacs and a pale dorsal stripe (marsh frogs do occasionally have a
dorsal stripe) [3]. Edible frogs are a hybrid cross
between marsh frogs and pool frogs so tend to have features that are a mix of
the two parent species. For example they are between the two in size and have
light grey vocal sacs [1]. Unfortunately you can only
reliably distinguish the different species by their call unless they are
examined in the hand [3].
Edible frogs
are rarely found without one of the parent species therefore it is only in
areas where pool frogs exist that the presence of marsh frogs could be
misidentified. The pool frog was recently found to be native to East
Anglia but became extinct in the 1990s and is being reintroduced to Norfolk [1]. There have been unmanaged introductions of pool
frogs around the country and in Surrey you are more likely to find pool frogs
or hybrid edible frogs than marsh frogs.
History
of the introduction
In 1935, 12
marsh frogs, thought originally to be from Hungary, were introduced into
gardens near Walland and Romney Marshes, Kent. Since then, the species has
become abundant in the low-lying wetlands of the south eastern counties.
However, most of the southern counties have small population and there are also
records of sightings in the Midlands, as well as a water frog population in
Yorkshire which may contain marsh frogs.
Useful
Information
Sussex
Amphibian and Reptile Group – http://www.sussexarg.org.uk
Amphibian
and Reptile Groups of the UK – http://www.arguk.org/
References
[1] Inns, H. (2009). Britain's reptiles and
amphibians: a guide to the reptiles and amphibians of Great Britain, Ireland
and the Channel Islands. Old Basing, Hants.: WILDGuides ltd.
[2] Amphibian and Reptile Conservation. Alien
Encounters Marsh frog and relatives [Online]. Available from http://alienencounters.narrs.org.uk/marshfrog.html [Accessed 19 Marsh 2013.
[3] Beebee, T. J. and Griffiths, R. A. (2000). Amphibians
and Reptiles: A Natural History of the British Herpetofauna. London:
HarperCollins.
[4]
Cicek, K. and Mermer, A. (2007). Food composition of the marsh frog, Rana
ridibunda Pallas, 1771, in Thrace. Turkish Journal of Zoology,
31(1), 83-90.
[5] Gregory, P. and Isaac, L. A. (2004). Food
Habits of the Grass Snake in Southeastern England: Is Natrix natrix a
Generalist Predator? Journal of Herpetology, 38, 88-95.
[6] ShareGeo
Open (2010). GB SRTM Digital Elevation Model (DEM) 90m.
Originating from NASA/NGA/DLR/ASI. ShareGeo Open [Online]. Last
updated: 09/06/2010. Available from: http://hdl.handle.net/10672/5 [Accessed 05/10/2013].
[7] ShareGeo
Open (2011). MasterMap Linear Water Features. ShareGeo Open [Online].
Last updated: 15 February 2011. Available from: http://digimap.edina.ac.uk/sharegeo/handle/10389/204 [Accessed 12 January 2014].
[8] The Centre
for Ecology and Hydrology (n.d.). Land Cover Map 2007. The Centre for
Ecology and Hydrology [Online]. Available from: http://www.ceh.ac.uk/landcovermap2007.html [Accessed 12 January 2014].
[9] Digimap
(n.d.). Digimap Ordnance Survey. Digimap [Online]. Available from: http://digimap.edina.ac.uk/digimap/home [Accessed 12 January 2014].
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