Geoscience Reference
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were dissected. Aridity increased in glacials and so there was the expansion of desert
areas by the time of the LGM to about five times their present extent. Also, as men-
tioned, in present-day Europe and North America high-latitude boreal species moved
south ahead of the European (Fennoscandian) and North American (Laurentide) ice
caps as they expanded. There was a considerable dynamic as ecological communities
re-organised in terms of composition, latitude and altitude.
Typically, when a plant species migrates it does so through seed dispersal. The
distance that seeds travel from a source plant is largely determined by probability
with a curved distribution: the majority of seeds travel a short distance and a few
travel longer distances. But largely is not the same as always. Circumstances, such as a
storm or a seed getting stuck in an animal's fur, can see seeds transported considerable
distances beyond the norm. So, if one were to plot a graph of the number of seeds
against distance one would not only see a probability curve but also a small leptokurtic
kink representing a small distribution further afield. (This leptokurtic phenomenon
is the same as those bedevilling concerns over the separation of genetically modified
crops from their non-modified counterparts, and concerns over invading species. For
while one may plant a genetically modified crop so far apart from its non-genetically
modified counterpart as to be assured that 99.9% of pollen from one crop will not
reach the other, there is still a small chance of contamination.)
That a small proportion of seeds will travel exceptionally long distances has pro-
found genetic consequences during climate change events. Plants migrating out of
refugia due to changing conditions will be moving into lands that up to the change
had been occupied by other species, many of which will now be ill-suited to the new
conditions. So those few seeds travelling the farthest first grow up with little com-
petition and subsequent generations can grow with comparative impunity. Northern
hemisphere interglacial populations at the edge of their range would, with glacials,
migrate south into large areas of suitable territory with leaders establishing new colon-
ies long before the rest arrive. Indeed, those behind now have to compete to migrate.
Since only a small number of plants make up this leading wave, as mentioned above,
the genetic diversity of the new colonies covering a large area is reduced compared
with those found in the refugia, which cover smaller areas. Indeed, this also happens
with some animal species, for example, as illustrated by an analysis of 41 species of
North American fish (Bernatchez and Wilson, 1998).
In addition to expanding populations having restricted genetic diversity, so a retreat-
ing population will suffer dissection and extinctions so that the last survivors at the
edge of the population's range will also tend to have restricted genetic diversity. And
it is these individuals that will lead the charge with the next climatic cycle (Hewitt,
1996).
With migration due to climate change, animals and plants will inevitably encounter
some environmental parameters different to those found in their refuge. For example,
changing latitude, so as to broadly maintain a constant thermal regimen, also sees a
change in day length with season. This is not entirely insignificant. The reproduction
of many plants is determined by temperature and day length, as are the migration
dates of some birds, and day length even affects the reproductive cycle of some
insects. So while genetic diversity is restricted, there is some genetic variation, which
is necessary for survival.
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