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A population is a group of organisms that are all the same species. That's it. You're done. A group of ducks would not be a population if there were mallard and wood ducks combined. Scientists would say that there are two populations existing in the same area.

Population biologists study groups in a specific area. For example, among political regions there are a U.S. population and a Mexico population. Life for these groups is totally different in many ways. We also said a population is all the same species. When you look at the U.S. and Mexico, you might be looking at human populations or dog populations. They all interact, but they are different populations.
You need to remember that populations don't just sit there. Things are always happening. Organisms are born and the population size increases. Other factors that will increase the size of a population are good weather, medicine, geographic isolation and no predators. Organisms will also die of old age and the population will decrease. Populations can also decrease when there are more predators around, after a natural disaster, when competition is too great, or when some of the organisms just leave the area ( migration).
No matter how well a population succeeds in its area, it is still limited even if no outside forces are applied. Three main factors limit population expansion. There are others, but htese are some biggies.

There are physical limits. Sometimes other environments are just bad places to live. They may be too hot, too cold, or too dangerous.

Next are the competition limits. You might have a great place to live but there may be other organisms competing for your food. If there is a smarter bird in the next area, you might not be able to get all of the food you need. A worse situation would be to meet a bird that wants to eat you.

Last are the geographic limits. If you are a mouse living in a valley and there are mountains in every direction, are you going to leave your area? Probably not. You live a good life and might die if you tried to get over the mountains. An ocean might be another geographic limitation.
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