Monday, 3 April 2017

How Evolution Works - Squirrels and Pine Martens

Europen red squirrel, Sciurus vulgaris
Population crash in an invasive species following the recovery of a native predator: the case of the American grey squirrel and the European pine marten in Ireland | SpringerLink

Sometimes, evolution can produce intriguing and unexpected results. At first glance it may be hard to see why introducing a predator can result in an increase in the population of a species. This is especially pleasing when the species benefiting is a charming and endangered species, the European red squirrel and the predator is an almost equally beautiful member of the stoat family, the pine marten.

So, having been away from blogging for a couple of weeks with other distractions, it gives me considerable pleasure to write about this open access paper, albeit from 2014, which reports just that from Ireland.

In Ireland, the UK and Italy, the invasive North American grey squirrel, Sciurus carolinensis, threatens the survival of the Eurasian red squirrel, Sciurus vulgaris, as the effects of competition and disease almost inevitably lead to total replacement of red squirrel populations. However the results of a recent national squirrel survey suggested that the normally invasive grey squirrel had gone into decline in the Irish midlands, which was anecdotally attributed to an increase in European pine marten, Martes martes, range and numbers. This study aimed to quantify changes in squirrel distribution in Ireland and to investigate the role, if any, of the pine marten in red and grey squirrel population dynamics. A distribution survey of the midlands was carried out which confirmed the grey squirrel population has crashed in approximately 9,000 km2 of its former range and the red squirrel is common after an absence of up to 30 years. At landscape level, pine marten and red squirrel abundance were positively correlated, whereas a strong negative correlation between pine marten and grey squirrel presence at woodland level was found to exist. Squirrel demographics were determined by means of live trapping programs which confirmed that the red squirrel in the midlands is now in competitive release and the grey squirrel is present at unusually low density. This study provides the first evidence of a regional grey squirrel population crash and suggests that European pine marten abundance may be a critical factor in the American grey squirrel’s success or failure as an invasive species.

North American Grey Squirrel, Sciurus carolinensis
For those unfamiliar with the red squirrel/grey squirrel story in Britain and Ireland, I have an article written a few years ago to illustrate how the replacement of one species by a related one can give a false impression of sudden evolutionary change in the geological column. This also explains the ecological background to the story.

What seems to be happening in Ireland is that the re-introduction of the predatory pine marten has created an environment in which competitive pressure on the reds from the greys is being reduced by differential selection of greys, possibly because they are less agile and so less able to escape.

The European red squirrel and European pine marten of course co-evolved over several hundred thousand years in post-glacial Europe and maybe longer in ice-free refugia to the south, so the reds have had a long time to hone their escape techniques and the necessary agility. Meanwhile, the American martens do not appear to feature a significant number of grey squirrels in their diet simply because there is little overlap in their respective ranges. There has therefore been little pressure on greys to evolve to escape from them.

European pine marten, Martes martes
Now however, in Ireland at least, it seems the greys have come up against a predator they never evolved in the presence of but reds have retained their evolved abilities from the days when pine martens were much commoner.

This illustrates how change in populations dynamics can occur due to subtle changes in the environment and how abilities evolved earlier and until recently mostly redundant, can reemerge as beneficial adaptations when the environment changes appropriately. Of course, those reds best able to escape the pine martins are more likely to be those from which this population growth is coming. Meanwhile, there is little pressure on the pine martin to adapt to match the red's agility because they have a ready supply of the more common greys.

This study also suggests that it was decline in the pine martin population in Britain and Ireland, due to persecution, which may have contributed to the success of the invasive American grey squirrel in the first place as, faced with little or no predation pressure, it was easily able to out-compete the reds being larger and more aggressive as well as being a carrier of the squirrel parapox virus which is harmless to greys but fatal to reds. It may be no coincidence that the last few remaining mainland refuges for the red squirrel in Britain are also refuges for the pine marten.

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