Gaps in Maps

One question which triggered an early interest in botany and ecology for many of us is: Why does this particular plant grow here but not there? Or, having seen a distribution map for the species, "Why is there a gap in this map?"

Arctic Cottongrass (Eriophorum angustifolium)
growing in a sedge-tundra meadow
at Colesdalen (78˚N) Spitsbergen
Image: R. Crawford

Eminent botanist and author Bob Crawford, Professor Emeritus at the University of St Andrews, addressed this issue in his presentation at the BSBI Mapping Conference in 2012 and the images he used to illustrate his talk were breath-taking, so we were delighted when he agreed to write it up as a paper for New Journal of Botany. This has taken a while, as Bob was busy reading the proofs for his latest book, 'Tundra-Taiga Biology: Human, plant and animal survival in the Arctic'. This has now been published, and you can read more about it here. This should whet your appetite for Bob's paper 'Gaps in Maps' which appears in the August issue of New Journal of Botany, due out in the next few days.

Bob said "Maps of species distribution are normally used for information regarding occurrence. Valuable as such information is, the converse of knowing where species do not occur can also be meaningful. This is particularly the case where there are no obvious reasons for absence. There is a long history of hypotheses in botanical writing as to the causes of gaps in distribution, which vary in their geographical dimensions from limited local absences to more extensive non-occurrences. Examples of this latter situation are usually described as disjunct distributions. Some of the commonest disjunctions are found in the distribution of the species commonly referred to as Arctic-Alpines. The evolutionary relationship between the occurrences of these species in the Arctic as compared with more southern montane habitats has long been a source of speculation. 

Glacial Buttercup (Ranunculus glacialis)
Image: R. Crawford 

"The main questions have been whether or not these disjunct distributions are merely accidents of dispersal or the remnants of former widespread and contiguous distributions that have suffered either from physiological failure or other aspects of evolutionary maladaptation to a changing environment. The advent of molecular methods and their role in the discovery of previously unsuspected migration routes now makes it possible to reconsider the causes of some of these gaps".

If you want to read all about Gaps in Maps, and see some of Bob's fabulous images  illustrating his points, you will have to wait just a few more days for your copy of NJB to arrive. If you're not a BSBI member.... you may never know why that plant grows here but not there! Better join BSBI at once and start receiving New Journal of Botany three times each year :-)