Monday, 21 May 2018

Evolutionary Paradox - Predation Pushed Red Squirrel Numbers Up!

Eurasian red squirrel (Sciurus vulgaris).

The enemy of my enemy is my friend: native pine marten recovery reverses the decline of the red squirrel by suppressing grey squirrel populations | Proceedings of the Royal Society of London B: Biological Sciences

A while ago I wrote a book under my real name about my childhood in rural Oxfordshire and included a brief mention of the loss of the native red squirrels, Sciurus vulgaris, that had been so familiar to us in the 1940s and 50's but which had disappeared almost completely by 1960 to be replaced by the non-native grey squirrel, Sciurus carolinensis.

In it I speculated, based on observations in Ireland, that it might have been the loss of the predatory pine marten, Martes martes, that was actually a contributory cause because it had allowed the grey squirrel to proliferate.

It's rewarding to see that this link has now been given added credence by a study published in Proceedings of the Royal Society of London B: Biological Sciences.

Here is what I wrote in In The Blink of An Eye: Growing Up in Rural Oxfordshire:

The beautiful red squirrel is, to all intents and purposes, extinct in much of the British mainland, though it continued to thrive in off–shore islands and in a few refuges in the Scottish Highlands.

When once they were common in the woods around Fawler, I saw my last red squirrel probably in about 1958, maybe as late as 1960. For a while it had been a bit of a game to identify if it was a red or a grey as a squirrel scampered through the branches above us...

The red squirrel has been the victim of the introduced North American cousin species, the grey squirrel. The reasons for the grey’s success is partly to do with its more robust build and more aggressive nature in that it can easily drive the red out of the niche they both need to occupy to survive. But it is probably mostly due to germ warfare. The grey carries the parapox virus which is fatal to reds but harmless to greys. The greys have failed to displace the reds in parts of Europe where squirrel parapox is not endemic.

Recently, it has been noticed that, in Ireland, where the pine marten has been reintroduced, this has led to a surprising increase in the population of red squirrels. The reason is believed to be due to the pine marten predating more on greys than the more agile red, so allowing the reds to bounce back. It might be that the introduction of greys had such a devastating effect on reds in Britain because it coincided with the decline due to persecution of the pine marten.

The reason greys are less successful against the pine marten is probably because, in North America, where they evolved, their range does not overlap that of the North American martens. The European reds however evolved in the presence of pine martens so they evolved greater agility or some other escape method that the greys lack.

Now the study by a team led by Emma Sheehy, an honorary research fellow at the University of Aberdeen, Scotland, UK, has provided the evidence that this speculation is in fact very probably correct.

Shared enemies may instigate or modify competitive interactions between species. The dis-equilibrium caused by non-native species introductions has revealed that the outcome of such indirect interactions can often be dramatic. However, studies of enemy-mediated competition mostly consider the impact of a single enemy, despite species being embedded in complex networks of interactions. Here, we demonstrate that native red and invasive grey squirrels in Britain, two terrestrial species linked by resource and disease-mediated apparent competition, are also now linked by a second enemy-mediated relationship involving a shared native predator recovering from historical persecution, the European pine marten. Through combining spatial capture–recapture techniques to estimate pine marten density, and squirrel site-occupancy data, we find that the impact of exposure to predation is highly asymmetrical, with non-native grey squirrel occupancy strongly negatively affected by exposure to pine martens. By contrast, exposure to pine marten predation has an indirect positive effect on red squirrel populations. Pine marten predation thus reverses the well-documented outcome of resource and apparent competition between red and grey squirrels.

Pine martens differentially predate on grey squirrels, probably because in their native homeland in North America where they evolved, grey squirrels and the North American equivalent of the pine marten do not overlap, so the grey has not evolved the necessary agility to escape. Given a choice, pine martens appear to go for the larger and less agile grey.

This is a good example of how evolution and ecology are different side of the same coin, biologically speaking. In the presence of the predatory pine marten, natural selection acts against the larger and more robust grey squirrel and favours the smaller, more agile reds. In the absence of the pine marten, those same characteristics give the grey an advantage over the red and natural selection selects in favour of the grey.

The result of this is the paradox that reintroducing a predator can result in an increase in the population of one of the prey species.

I really would like to see a creationist explanation of this paradox together with reasons why this should not be seen as an example of evolution in action, or indeed why evolution would not be inevitable in this situation.

For more detail, see the article on by Twitter contributor @GrrlScientist: Scotland's Imperiled Red Squirrels Have An Unlikely Ally.

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