The reasons why species go extinct are many and varied. The most common is that they are not able to adapt to changes in their environment, either because those changes are too sudden or too slow. For example, if a species is adapted to living in a certain habitat and begins to live elsewhere, it will be unable to adapt--unless there are other changes that allow it to do so.
The second most common reason for extinction is habitat destruction or loss. This can include things like deforestation, damming of rivers and streams, overfishing, or pollution of water sources by agricultural runoff or industrial waste.
Another reason for extinction is competition with other species for resources such as food and water. If one species has an advantage over another due to size or other traits (such as color), then this can lead them to outcompete each other until only one remains in existence (this is called "the tragedy of the commons").
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