Answer:
One of them is the variance of the gene pool--in other words, the genetic adaptations plants and animals have help them to survive. That is where the phrase "survival of the fittest" comes from; if one is not genetically fit to survive in their environment, they will die off and their genes will not be added to the gene pool, making it easier for organisms with more fit genes to reproduce and add to the gene pool.
Another is the availability of resources. It is very hard for an organism to survive without food, water, and shelter. If the organisms of a species live in an environment with a lack of resources, they will die off, thus making them unable to reproduce and therefore go extinct.
Also, the presence of predators makes it difficult for a species to survive. Predators preying on prey make prey poulations go down and predator populations go up. However, going back to the gene pool, many organisms have adapted to hiding from predators with means such as camouflage or burrowing.
Finally, the general conditions of their environment. This one is self-explanatory. For example, you would not expect to find a polar bear in the desert or a cactus in the Arctic. The conditions are simply not right for their survival and they will die off. This is also why birds migrate--they need to go somewhere warm in the winter to keep themselves warm!
Hope this helps!
Explanation:
