GEOGRAPHIC RANGE
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 survival.
Extinction is 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 common.
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 include:
- The use of specialized
morphological structures to aid the transport of an individual by wind.
- The use of particular
morphological structures to transport the individual by moving water.
- 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.
Dispersal can 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 LIMIT
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 affect 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.
CITATION
Pidwirny, M. (2006).
"Abiotic Factors and the Distribution of Species". Fundamental of
Physical Geography, 2nd Edition. 11/1/2012.
http://www.physicalgeography.net/fundamentals/9e.html
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