CHAPTER 9: Introduction to the Biosphere

(e). Abiotic Factors and the Distribution of Species

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 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.


Study Guide


Additional Readings

Internet Weblinks
Citation: Pidwirny, M. (2006). "Abiotic Factors and the Distribution of Species". Fundamentals of Physical Geography, 2nd Edition. Date Viewed.


Created by Dr. Michael Pidwirny & Scott Jones University of British Columbia Okanagan

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05/07/2009 15:23