Natural Selection Image Map


Within any given species, populations can be characterized in the following three major ways:

·        Density

·        Age Distribution

·        Sex Ratio


Of these three characteristics, probably the most familiar to you is the term "density."  By this, we refer to the number of individuals divided by the amount of area of suitable environment.  This is the density of one population or members of the same species in a given area.  For example, if only two cows can live on 100 acres in west Texas, then the density is 2 cows/ 100 acres.  Do not confuse this with "species richness," which describes the number of different species found within an ecosystem. 
An example of species richness.  Comparison of number of species on Caribbean islands. Note that the larger the island, the more species it supports.

A high density will tax the ecosystem’s supply of available resources and increase the chances for disease and parasites.  On the other hand, a low density will decrease the occurrence of possible mating partners.  How would this be bad for the population?  Also, a low density means that normal levels of disease and predation could drive the species toward extinction. 


What about age distribution?  Look around your classroom.  This would be an example of a very narrow age distribution because most members of this population (your class) have about the same age.  What are some of the drawbacks of this situation for a species? 
Have you ever heard the adage, “An elephant never forgets,” and wondered, "What does an elephant have to remember anyway?"  Some elephant  herds migrate over vast distances to waterholes.  Only the older elephants remember where those holes are. Well, as it turns out, the oldest elephants in a herd use their accumulated knowledge of their environment to make decisions for the entire herd.  When poachers kill these older elephants, their memory is lost as well.  Without this, the surviving elephants cannot interact with the environment as effectively since they don't know where the resources can be found. 

Generally, ecologists use the age distribution to determine the growth of a population.  If most members are of young age, the population is said to be growing. 

One practical application of age distribution deals with the body's response to infection. Bacterial infections stimulate white blood cells to proliferate. Older white cells look different from young ones. Medical technicians are sometimes asked to count the percentage of young white cells and the percentage of old ones. If you have many young cells, it means that your body is responding well to the infection. But if the population is dominated by old white cells, that is a bad sign.

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