Sunday, November 13, 2011

Climate, not space, gives trees more room to range


Everything does not live everywhere; there are no baobab trees in Canada, nor caribou in Florida.  This is not a particularly profound statement.  But, for some of my former Malawian high school students, who had never traveled farther than 20km from their home or watched a nature documentary, it came as a revelation.  “But why, madam?” they would ask me.

Simplistically, the answer has two parts.  One, places are far away from each other, and two, places have different climates; therefore, living things have limited areas where they live because they either can’t get to new places or because when they get there they can’t survive.  Taking this one step further, it follows that species that can survive in a wider range of environments should also have larger ranges.  But, how can we actually determine whether this is true?

One pattern that seems consistent with this idea, is the observation that species that live farther north tend to have larger ranges.  This pattern has been called ‘Rapoport’s Rule’, though it is neither a general rule nor does is belong to Rapoport.  One explanation for why it (sometimes) occurs is that species living farther north have to be able to deal with a wider range of temperatures throughout the year.  This broader tolerance would allow them to live across a wider geographic area.  But again, how would we actually test this?

In the most recent issue of Ecography, Xavier Morin and Martin Lechowicz describe their clever way of figuring out whether the Rapoport effect they found across all 598 of North America’s native tree species actually resulted from tolerance of annual temperature variability, as opposed to just chance.  One alternative hypothesis they had to rule out was the possibility that the trees’ ranges were larger farther north simply because there is more land area in the northern parts of North America than there are in the southern parts.

On a computer, the scientists made an outline of North America and then randomly dropped the tree species onto the continent and let them spread.  When the trees were allowed to spread anywhere, the range sizes that came out of the simulation were much different than the actual ranges sizes for real North American trees and there was no relationship between the size of the range and how far north it was.  However, when the simulation was constrained so that each fake species was given a temperature tolerance and only allowed to spread to places within that temperature range, then the simulation produced range sizes that were larger further north.

This type of null modeling, where a computer makes a simulation that is used to test whether patterns observed in nature could come about just because of chance, is becoming much more popular with all kinds of scientists now that fast computers are cheaper and easier to use. 

You can find this article at:

ResearchBlogging.orgMorin, X., & Lechowicz, M. (2011). Geographical and ecological patterns of range size in North American trees Ecography, 34 (5), 738-750 DOI: 10.1111/j.1600-0587.2010.06854.x

1 comment:

  1. It's interesting to think of north areas having larger ranges, I would have thought the opposite. Is there suppose to be a film clip at the top?

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