Evolution__3rd_Edition

496 PART 4 / Evolution and Diversity
Species occupy particular ecological
niches ...
. . . but history can also be
influential
17.2 Ecological characteristics of a species limit its
geographic distribution
The distributional limits of a species are set by its ecological attributes. One way of
understanding how ecological factors limit a species’ distribution is in terms of a distinction,
first made by Hutchinson and MacArthur in the 1950s, between the fundamental
niche and the realized niche of a species. A species will be able to tolerate a certain
range of physical factors a temperature, humidity, and so on a and could in theory live
anywhere these tolerance limits were satisfied. This is its fundamental niche. However,
competing species will often occupy part of this range and the competition may be too
strong to permit both species to exist. Each species’ realized niche will then be smaller
than its physiology would make possible: each will occupy a smaller range than it could
in the absence of competition. Much ecological research has been carried out to discover
the factors a whether physical or biological a that act to limit particular species’
distributions.
In some cases, a species’ distribution is limited ecologically, for example the species
could not live outside its existing range because a competing species is present elsewhere.
In other cases, a historic rather than ecological explanation is needed. The
species may be ecologically capable of living at a place, but it is absent because it has
never arrived a that is, it has never migrated and established itself.
In what sense are ecological and historic factors alternatives? If we consider the
particular distributional limit of a species, we can ask whether it lies at the limit of
the species’ ecological tolerance, or whether the species could ecologically survive
on the other side of the border but for some historic reason is not. It can therefore
be meaningful to test between ecological and historic explanations. In most real cases,
however, a complete account of a species’ distribution needs both ecological and
historic knowledge. A species cannot live outside its ecological tolerance range; its
biogeography therefore cannot contradict its ecology. However, within its ecological
tolerances, historic factors may have determined where it is living and where it is
not. The two factors will then not be opposed, and the sensible method of analysis
is to work out how ecology and history have combined to produce the species’
distribution.
17.3 Geographic distributions are influenced by dispersal
A species’ range will be changed if members of the species move in space, a process
called dispersal. Individual animals and plants move, actively and passively, through
space both in order to seek out unoccupied areas and in response to environmental
change. (Plants move passively, at the seed stage.) When the climate cools, the ranges of
species in the northern hemisphere move southwards, and tropical forests fragment
into smaller forest patches. It would also be possible for the range of a species to
change, when the climate changed, without the movement of individuals. Those in the
colder regions (for example) might die off, and the range would shrink and move on
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