Population Changes



Population Changes

Populations of living beings are constantly changing. The number of humans, animals, insects, or plants living in a given area can vary because of two kinds of factors: biotic and abiotic. Biotic factors are living factors that can influence the size of populations, such as predators or other species competing for food. Abiotic factors are nonliving things in the surrounding environment that can cause population changes, such as weather or sunlight. Biotic and abiotic factors cause continual changes in the number of individuals that make up a population of organisms.

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    <-NARRATOR:-> Now listen to part of a lecture on this topic in a biology class.
    <-MALE PROFESSOR:-> Imagine there's a group of mice living in a large field, and owls living nearby.
    Now, owls eat mice¡ªso the number of mice there are at any given time depends upon the number of owls in the area...
    um, because the more owls there are, the more mice get eaten, right?
    Now, imagine one year,
    there are more owls than usual.
    Since there are more owls in the area to eat the mice,
    what do you think will happen to the number of mice?
    As you can imagine,
    the number would drop... there'd be fewer mice.
    As far as the other factor...
    um, we can use rabbits to help understand this one.
    Imagine a population of rabbits living in an area...
    These rabbits usually start having their young at the end of winter,
    after the cold winter weather is gone,
    and they keep reproducing until the following winter, when they will stop again while the cold winter weather lasts.
    But let's say this year, the winter season is very short...
    and it uh, you know, starts getting warm much earlier than usual.
    Since winter this year is so short,
    the rabbits can start reproducing much earlier.
    That means the rabbits in that area will have at least one extra reproductive cycle,
    so, of course, one extra litter of baby rabbits.
    So the number of rabbits in that area would increase a lot.


Using the examples of mice and rabbits from the lecture, describe the two different types of factors that can cause population changes.


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