Each species on
our planet occupies a unique geographic
range where the members of its various populations
live, feed, and reproduce. Some species have extensive
geographic ranges that stretch over several continents.
Species with such distributions are known as cosmopolitan
species. Other species can have more restricted
geographic ranges isolated to a small area on a single
continent. This type of distribution is termed endemic.
Geographic ranges of organisms continually shift,
expand, and contract with the passage of time. These
changes are the result of two contrasting processes:
establishment and extinction. The establishment of
a species takes place when individuals colonize new
areas and are able to maintain reproductively viable populations.
New suitable habitats for establishment may open up
because of abiotic and biotic environmental change.
Species are always attempting to expand their spatial
distribution as it betters their chance for long-term
a process that eliminates members of a species from
all or part of its geographic range. Extinction occurs
when large numbers of individuals from a species are
killed by biotic
interactions or abiotic environmental
change. Limited extinctions occurring within small
sub-regions of a species’ range are usually quite
Dispersal and Colonization
Many of the organisms that inhabit the Earth have
the ability to move. This movement can be accomplished
by either passive or active means. Active
movement requires the organism to use
some appendage to initiate walking, running, flying
or swimming. In passive
movement, the organism uses an external
force to cause transit. Many plants use wind passively
to disperse seeds over relatively long distances. Oyster
larvae can travel hundreds of kilometers by using the
power of sea currents.
Plants have developed a number of different mechanisms
for moving their offspring. Some of the common methods
- The use of specialized morphological
structures to aid the transport of an individual
- The use of particular morphological
structures to transport the individual by moving
- The production of fruit encased seeds that other
organisms consume and disperse.
- Adhesion mechanisms that allow seeds to attach
themselves to other actively moving organisms.
- The physical ejection of seeds.
be defined as the movement of individuals away from
others of the same species. One common reason why organisms
disperse is to find new habitats rich in needed resources.
Through dispersion organisms can evade the competitive
influence of their parents, siblings, and other species.
Ideally, a dispersing organism would like to find a
place where resources for survival are in abundant
supply and competition by individuals of the same species
and other species for these resources is minimal.
Dispersal also involves a large element of discovery.
By finding new suitable habitats, individuals increase
the geographical range and spatial dominance of their
species. Species with large ranges are less likely
to go extinct. Most of the causal factors that result
in the death of individuals work at specific spatial
scales. If the species has a distribution that is larger
than this scale, portions of its population will be
unaffected. Also, with a large range comes greater
variation in habitat types and associated species genetics.
Biotic or abiotic mechanisms that might kill off individuals
often act on specific types of habitats. As a result,
being able to occupy a range of habitat types because
of greater genetic variation provides possible safe
havens for the species when times get tough.
Once dispersed, an individual can try to colonize
a new site. To achieve successful colonization the
new site must have all the necessary abiotic and biotic
conditions needed for survival. For many individuals,
the dispersal process ends in death because colonization
does not take place. Successful colonization often
requires the chance event of finding a site devoid
of other organisms. Sites within ecosystems become
free of organisms through the mechanism of disturbance. Disturbance can
be defined as any process that acts to disrupt an ecosystem,
community, or species population by changing resource
availability, biotic interactions, or physical conditions.
Disturbance often causes the premature death of individuals.
Factors like predation, climate variations, earthquakes,
volcanoes, fire, animal burrowing, and even the impact
of a single raindrop can all lead to a disturbance.
The process of dispersal does not end with the colonization
of an individual on a new site. Once colonized, the
individual must secure enough resources to support
future growth and reproductive efforts. For many individuals,
life after colonization is a struggle for continued
existence because of the stresses associated with various
biotic and abiotic influences. These influences can
involve biotic interactions like competition, predation,
and disease, or abiotic factors like severe weather,
flooding, drought, and fire.
Abiotic Factors and Tolerance Limits
Most species appear to be limited in at
least part of their geographic range by abiotic factors,
such as temperature, moisture availability, and soil
nutrients. No species is adapted to survive under all
conditions found on the Earth. All species have specific
limits of tolerance to physical factors that directly
effect their survival or reproductive success. The portion
of the abiotic factor's range of variation which a species
can survive and function in is commonly defined as the tolerance
range. The level within the tolerance range at
which a species or population can function most efficiently
is termed the optimum.
In 1840, Justus Liebig suggested
that organisms are generally limited by only one single
physical factor that is in shortest supply relative
to demand. Liebig’s ideas were strongly influenced
by agricultural studies that identified nitrogen (N)
or phosphorus (P) as the nutrient limiting the production
of crops. At one time researchers accepted Liebig’s
theory so completely that they called it the Law
of the Minimum, and they tried to determine
the single limiting
factor that controls the growth of numerous
species. However, subsequent studies have shown
that Liebig's concept is inadequate to account for
the distributional limits of a large number of species.
In most cases, the spatial limits of distribution are
controlled by complex interactions between several
different physical factors.