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E7. Within communities different species use different reproductive strategies.

Student Outcome: E7.1

Know that species with high reproductive effort, short life, and many offspring are more common in early succession or disturbed communities (r selected).


With unlimited resources, a population can expand very rapidly. Two rabbits that live in Rabbit Utopia and have five male and five female offspring every four months will produce a population of 12 rabbits after four months and 72 rabbits after eight months. Sounds like nothing, right? After one year, the population will be 432 rabbits. After two years, there will be 93,312 rabbits. And after three years, the population will be more than 20 million rabbits. This rabbit population is following the trend of exponential population growth, in which there is nothing to limit the growth of a population and that population correspondingly grows by exponential factors.


Student Outcome: E7.2

Know that species with low reproductive effort, long life, and few offspring are more common in stable communities (K selected).


Perhaps Rabbit Utopia can grow enough lettuce to support 20 million rabbits, but normal nature cannot. In nature, when a population is small, the resources surrounding it are relatively large and the population will grow at near exponential levels. But as populations grow larger, they need more food and take up more space, and resources become tight. Within the population, competition for food and space grows fierce, predators move in to sample some of the bounty, and disease increases. These factors slow the growth of the population well before it reaches stratospheric levels. Eventually, the rate of population growth approaches zero, and the population comes to rest at a maximum number of individuals that can be maintained within a given environment. This value is the carrying capacity of the population, the point at which birth and death rates are equal.

The carrying capacity of an environment will shift as an environment changes. When there is a drought and less vegetation, the carrying capacity of rabbits in a population will decrease since the environment will not be able to produce enough food. When there is a lot of rain and lush vegetation, the carrying capacity will increase.


Student Outcome: E7.3

Understand that r and K strategies are extremes on a continuum.


Sexually reproducing organisms have two reproductive substrategies.

  • Organisms such as insects have many small offspring that receive very little or no parental care, reach sexual maturity at a young age, and reproduce only one or a few times. In an environment with abundant resources, this life-history strategy allows species to quickly reproduce and exploit opportunities for population growth. The disadvantage of this strategy is that it produces high mortality and great instability when resources dwindle.
  • The alternative strategy is to bear fewer and larger offspring that receive intensive parental attention, mature gradually, and reproduce several times. Humans employ this strategy and are better suited to thrive in a competitive environment, exhibiting lower mortality rates and longer life spans. The disadvantage here is that the concerted investment of time and energy into a few individuals makes it difficult for a population to surmount large decreases in population size due to disasters or disease.


Source: http://www.sparknotes.com/testprep/books/sat2/biology/chapter10section1.rhtml

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