Very simple, animals and plants do not live in a vacuum. They have to be able to survive in their environment. If their environment changes drastically and within a short period of time, then they may be ill suited to the changes and become extinct. Take for example, West Texas. It was much wetter just 13,000 years ago, and the landscape was quite different, because it was in the middle of an ice age. Since the end of the last ice age, however, West Texas has turned into a desert. Further, it did not take 13,000 years for the change to complete. It was much closer to a couple thousand years. Any plants or animals that are not suited to desert conditions will simply become extinct.

Another example is the end of the Jurassic extinction of many dinosaurs. Dinosaurs are ectotherms, meaning they do not generate body heat to stay warm. During warm climates, dinosaurs can become larger, because a larger animal has less body surface and they can conserve body heat better than a smaller animal. At night, when it got cooler, the less body surface means less heat will be lost to the environment, so the dinosaur can be active early the following day, without needing to bask in the sun for a long time to elevate its body temperature. However, if the climate is cool, as it was at the end of the Jurassic, then large dinosaurs would be at a disadvantage, because the small amount of body surface prevents it from gaining heat by basking quickly enough to allow it to be active. Since the ambient temperature is cool, the dinosaur will keep losing heat to the environment throughout the day. At night, it gets worse. As a result large dinosaurs can never reach a high enough body temperature in a cool climate to function. Accordingly, the cooling at the end of the Jurassic resulted in a large number of large dinosaurs and large pterosaurs to become extinct. Such familiar species as Stegosaurus and Allosaurus and many groups of sauropods also became extinct. Stegosaurus is famous for those large plates on its back. It is now evident that Stegosaurus was probably trying to use those plates to collect additional sunlight to stay warm. But even that did not help because Stegosaurus became extinct at the end of the Jurassic.

Some animals headed for extinction could be saved by environmental change . An example could be a dam is built on a seasonal creek which causes the regularity of water flow to stabilize saving some fish facing extinction because of decreased rain .

I study local environment here in Crivitz Wisconsin (50 miles North of Green Bay. This study is on a very small scale but makes you think.

A stream has dried up over the past 13 years. The plant life and animale life have changed from aquatic types to dry land types. There were 3 inch fish in the stream, now the biggest is maybe an inch long. There were frogs years ago, now you seldom hear or see one. The area used to have a lot of moss, now there is very little.