Introduction and Prime Factors Balancing Over Reproduction

[Bernard NebelKeymaster]

Emphasize to students that no ecosystem should be seen as a grouping of static individuals. Over a period of years, as a result of ongoing life cycles, every plant and animal in the system will be of a subsequent generation of those before. The central question to pose is this: With every species reproducing as it will, how are the populations of each species kept in balance.

Have students consider how every species has the capacity to and invariably does “over-reproduce”, i.e., produce more offspring, seeds, or spores than is mathematically necessary to replace the parents as they die off. That is, given ideal conditions, any species will rapidly expand in numbers over successive generations. (Consider math/graphing exercises to show this.)

Stress: No natural population has a means to control its reproductive habits or to control its birthrate. Therefore, the conclusion is, unpleasant as it may be, most offspring, seeds, or spores are eliminated before reproduction. How does such elimination occur? (See following section)

Prime factors balancing over reproduction

Predation–predator-prey balance.

For a classic example, type into your browser:

lynx snowshoe hare graph images

Have students note the fluctuating balance between the two populations. See the text for the explanation as to how the fluctuating balance between predator and prey works. Note the adaptations on the part of both predator and prey that are required.

Another example: this between populations of moose and wolves:

http://www.phschool.com/atschool/science_activity_library/interacting_populations/isle_royale_ans.html

Also see videos: https://www.youtube.com/watch?v=PdwnfPurXcs

Famine

If all conditions are favorable for a given species, its population will increase rapidly. (Have kids do an exercise such as: Starting with 10 rabbits and assuming each female produces 10 offspring per generation what will be the population of rabbits after 3, 4, 5, 6 generations. Graph the results.) Have them discuss: For how long can such growth continue? What will result?

In nature we find many examples of “overgrazing” and population crash due to famine. See: http://dieoff.com/page80.htm

Such famine often occurs in combination with, or is intensified by one or more years of inclement weather (too much rain, droughts, abnormal temperatures)

Disease

A large dense population becomes more susceptible to the outbreak and spread of a fatal disease, an epidemic, which leads to a population crash.

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