Some time ago I posted a series of blog entries that reviewed available Kent data to investigate the relationship between common frogs and ponds.
In the first post I explained how frog occupancy seems to be lower in areas of high pond density.
graph shows frog presence/absence
But surely frogs need ponds?! So what is going on?
In follow-up post I examined the relationship between common frog and great crested newt.
graph shows frog presence/absence
One clear conclusion was that common frogs are less likely to be encountered in a pond if it is already occupied by great crested newts. After all, newts are voracious predators of common frog spawn and tadpoles, so perhaps this result isn't too surprising.
I then tried to be a bit clever and proposed that common frogs may escape predation from newts by choosing to breed in ponds already occupied by fish. Since fish are a major predator of great crested newt larvae and newts are known to avoid fish ponds, this seems like a reasonable hypothesis. Great crested newts prefer areas with high pond density, so the analysis should be quite revealing. Yet the data doesn't fit the prediction...
Graph shows *only* frog presence, in ponds where fish are absent/present
This proposed interaction only seems to hold true in areas of low and moderate pond density - where great crested newt are perhaps less likely to occur. In areas of high pond density, fish appear to have a negative influence on common frog occupancy. But surely it shouldn't be like this? Surely in areas where common frog are more likely to encounter newts, they should prefer the ponds where newts are least likely to be encountered - fish ponds?!
Sometimes the reality of hard data can ruin what appears to be an excellent hypothesis!
But this now presents an interesting question, why do fish only appear to have a negative influence on common frog pond occupancy in areas of high pond density? Well, the culprit may be an introduced species - marsh frog.
Here's what I concluded in my last blog post on the subject:
The ability of common frog to tolerate the presence of fish should allow them to avoid predation by newts. Available data indicates that this does appear to be the case in areas of low pond density. It may also once have been true in areas of high pond density. However, the colonisation of these areas by marsh frogs may mean that common frogs will eventually disappear from the surrounding wider countryside.
It's a neat hypothesis, but is it true? A problem in analysing the Kent data is the paucity of available information. Clearly it would be interesting to study the interaction of all these species in different geographical areas, where their relative influence can be better understood through careful statistical analysis. Sounds like a PhD project to me...
Enter Aidan Mackay.
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I have recently started a PhD studentship at the University of Kent. I am currently doing research to determine whether marsh frogs (Pelophylax ridibundus) are having an impact on the distribution of common frogs (Rana temporaria) in South East England. Getting a better idea of the distribution of marsh frogs is critical to success of this research.
Aidan has set up a website to provide details about his fascinating research project and to allow folks to submit observations. If you have recorded common frog or marsh frogs anywhere in South Eastern England, please visit his site and submit your observations - thanks!
update (12/7/2013): some captions added to the graphs to make them clearer