Posted: 13 May 2005 at 1:59pm | IP Logged

Purely anecdotal, but I've received a number of reports from the public in Kent claiming that the arrival of marsh frogs has led to the decline of common frogs and native newts. One lady has even reported a crash in the grass snake population at a pond now dominated by marsh frogs but previously used by smooth newts and common frogs. I feel it is an area that needs to be looked into. (Why they may have caused the local grass snake population to crash I have no idea at all, you would have thought the local grass snakes would have welcomed their arrival).

If anyone wants to take a closer look at these populations I do have contact details. To be honest it is something I've found difficult to advise about and worrying at the same time. In particular, there is a consistency in the reports that they often state that it only takes a few years after the marsh frogs arrive, for native species to appear to become totally extinct.

Posted: 13 May 2005 at 4:46pm | IP Logged

Thanks Gemma

I'm pretty sure that marsh frog is one of the main reasons we have a strong grass snake population, so I too am puzzled by your contacts report of their decline.

Its interesting that you mention newts as well.  These seem to do fine in all marsh frog populated ponds that I know of...

Posted: 15 May 2005 at 7:35pm | IP Logged

Right

I would say that it not just marsh frogs that impact on common frogs - take  for instance the humble smooth newt or even the great crested newt - both of these species seem to impact greatly on common frog tadpoles -

I have had two successful spawnings over 2004 and 2005 I had a good hatch of tadpoles but all were gone within a few weeks as the smooth newts move into my small garden pond.

We have a thriving marsh frog population in Essex - Hadleigh Castle Country Park - They are really occupying an empty niche - yet we also have a very low Common frog population - which may always have been low - yet a few hundred yards up the hill in back gardens we have a thriving common frog population!

I have seen massive declines of Common frogs over the Essex countryside without the presence of Marsh Frogs - in fact many countryside ponds are not actively managed - there are fewer water meadows providing the temporary ponds/waterbodies which Common Frogs need to breed - rather than blame the marsh frog maybe we need to look at the changes in habitats - which seem to benefit Marsh Frogs and not our native common frog. If we can balance the needs of both As you say they coexist in Europe -

In gardens of course there tends to be an increase in Common frog populations - I have noticed that my pond was colonised by common frogs two years after it was built - in 2004 - 

I can also report that grass snakes in Hadleigh are doing rather well feeding on the Marsh Frogs, while the herons, little egrets are also feeding on them - not to mention the amount of food available to aquatic inverts - feeding on the tadpoles (MF) water beetles and their larva plus many other species possibly feed on marsh frog tadpoles -

I am getting fed up with these 'scapegoats' which we find to cover our misuse of our natural resources etc etc. I do find the issue interesting and I enjoy monitoring the occurence of marsh frogs in Essex - they are still spreading - with the help of humans - we have removed a small population from Davy Down Country Park while they have also manged to get into Rainham Marshes - The RSPB are worried about the effects on crested newts - yet they do not seem to have any effect on crested newts - apart from provide yet another food source (possibly)

The rangers at Hadleigh were concerned that the marsh frogs were eating all the aquatic inverts in their pond dipping ditch/platform - they must have eaten them all - when you look at other factors - introduction of fish (sticklebacks) the 20 times a year pond dipping sessions you can see other factors that have caused a decline.

The Marsh Frog has been reported as benefiting several species - grass snake, medicinal leech, grey heron, little egret and of course the Bittern.

It would be good to look at predator prey relationships within our studies - I fail to see why grass snakes would decline due to marsh frogs - perhaps they were cane toads?

It is often the case that grass snakes eat all the fish in a garden pond - yet it was most likely to be the neighbours cat or the Grey Heron -

The paper on canterbury grass snakes suggests that the artificially high numbers of snakes - by feeding on marsh frogs may also harm our native amphibians????? errrr

Regards

Jon

Posted: 16 May 2005 at 10:36am | IP Logged

Thanks for this input Jon

I appreciate that there are many factors that influence common frog populations, I was just highlighting numerous reports and observations that common frog seems to have declined in areas where marsh frog has become more abundant.  Perhaps I didn't make the two arguments entirely clear - I certainly don’t regard Marsh frogs as  completely unwelcome visitors as I'm sure (at least at Heathrow) there is in fact a net gain in terms of supporting grass snakes, aquatic inverts and birds.  Other amphibians also seem to do well.  The study I mentioned predicted no significant impact of marsh frog upon aquatic invert populations due to the wide variety of prey taken and the fact that despite the frogs aquatic nature the items were primarily terrestrial.  The tadpoles no doubt help the Dytiscus, dragonfly and other predatory taxa.  I saw a heron attempting to swallow a fully grown marsh frog which it struggled with for some time, eventually getting it down.  No bittern, as yet though!

To conclude - I was simply interested if anyone else has observed marsh frog and common frogs coexisting or situations where one has (apparently) resulted in the absence of the other.  It seems that they should coexist and that predation is probably not an issue but it is hard to ignore the decline or absence of common frog in my area - despite the presence of good habitat.  Like I said this could be due to other factors that are also affecting UK populations as a whole.

Regards

Posted: 16 May 2005 at 1:09pm | IP Logged

Jon, I agree with you sentiments and also agree we shouldn’t blame the marsh frog without solid evidence - many other factors could be to blame for crashes in common frog populations. I do feel though we should look to see if there is an impact on common frogs due to marsh frog competion in a scientific rather than anecdotal sense before it is 'too late'.

We had colonisation by Rt in the first season in our Essex garden pond, no breeding this year but the population increases daily with more little heads visible all the time, very encouraging, but as you know Essex is very underrated for herps, we find them practically everywhere we look!

At the other end of the spectrum consider the number of sickly looking or colour mutated common frogs one comes across these days. Though it seems more occur in suburban areas, they are found increasingly in the wider countryside, if indeed you can find any common frogs at all in the wider countryside.

My concern is that if we are loosing common frogs to disease, lack of genetic variability and habitat loss, will the marsh frogs ability to take up alternative niches and also potentially out-compete Rt in classically good habitat, lead to the demise of Rt as the UK's 'common' frog, replaced by a European invader.. think grey squirrel.

Posted: 16 May 2005 at 4:50pm | IP Logged

Rana temporaria to me has colonised the many urban and suburban ponds while the marsh frog in my experience lives in completely different habitats - well they can survive in habitat which the common frog cannot for instance fishing ponds.......ah theres another factor the stocking of fish into ponds.......does wonders in turfing out frogs

I would say that the interaction of the marsh frog and the common toad would be interesting - dont both have tadpoles that are distasteful to fish and other predators that chew they prey.

I feel we need to carry out a study of these animals - the feeding study at the Barns Wetland Centre forced the contents of the frogs stomach by using saline water or distilled - you can obtain a DEFRA license to enable release back into the site which the animal came from - or you could try and kill them but I am sure we would have plenty of discussion of how best to dispatch a marsh frog - I am sure we want to avoid this considering the furore from other sections of society -

I am leading a survey of the marsh frogs at Hadleigh Castle Country Park this coming saturday 21st May - we are meeting at Benfleet station at 9am - could you make that Gemma?

Regards

JC