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Will the cold winter check the spread north of global warming species?

Updated Wednesday, 4th March 2009

As global warming species have been gradually spreading north, Mike Dodd asks if they'll now be vulnerable to a cold winter?

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In recent years many species have been spreading north due to the series of mild winters, however the prolonged cold spell late December 2008/early January 2009 - and then the February snow - might set them back.

It would be interesting to compare for example the sightings of kingfishers in 2009 to those in 2008. Kingfishers need access to water to feed and if this is frozen for a prolonged period then they may die.

Kingfishers are not a climate change species in UK, so could be used as a standard to compare the other species against. If their numbers go down then it may be a cold enough winter to cause widespread ecological effects rather than just the normal year-to-year variation in weather.

I would expect several of the insect species that have been rapidly spreading in Britain to have their numbers checked in 2009 and even reduced. Some, though, may be much less affected than the kingfishers if they have an overwintering strategy that can withstand the cold.

Kingfishers have no choice they are here year round, and have to constantly catch food in water to survive. Many invertebrates, however, overwinter as cold resistant eggs or pupae, well insulated in the soil; or perhaps they live in our centrally heated houses. This wintering method is used by creatures such as the spindly spider.

In fact, a number of other species of spiders have taken up residence in our homes in recent years including relatives of the black widow which can have an unpleasant bite.

photo of relative of black widow in flats in Milton Keynes - might have an unpleasant bite so I left it well alone and did not check [image Mike Dodd © copyright Mike Dodd] Copyrighted  image Icon Copyright: by Mike Dodd © copyright Mike Dodd
Photo of relative of black widow in flats in Milton Keynes - might have an unpleasant bite so I left it well alone and did not check.


Another effect, of the snow particularly, was to break branches on evergreen trees. I have recently been looking at a 'lost' arboretum where about 60 species of oaks from around the world were planted.

Most of the species are deciduous and they were unaffected by the snow but the evergreen species from warmer Mediterranean climates such as cork oak had many of the main branches smashed down and split. Different evergreen species from northern forests, such as fir trees, have downward pointing branches and needle-like leaves that easily shed snow; the branches also tend to be very flexible and spring back once the load is gone.

One species that I thought might be checked somewhat is the water fern Azolla filiculoides which is an invasive species from North America. It forms a symbiotic relationship with the blue-green alga Anabaena azollae, which fixes atmospheric nitrogen enabling it to rapidly cover water bodies and cause a considerable nuisance.

It generally turns red and grows poorly in winter so I thought the low temperatures may kill it this year but no during fieldwork last week we saw it still smothering one of our boggy woods. However there is now a 2milimetre-long weevil that seems to be eating the plant and acting as a biological control - so its days of smothering ponds may be numbered.





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