Answer:
Global warming, urban sprawl, and deforestation to name but a few.
Global warming, urban expansion and deforestation are all causes of animal extinction. These destroy important animal habitat, for example, due to urban expansion the endangered spotted owl's habitat is being destroyed, the problem is that spotted owls as well as other animals need to live in a specific habitat to survive. The effect is that the "cycle" in unbalanced, in nature everything depends on one another, it is like a chain, if one link goes missing the entire environment can be harmed. For example, if lions were to somehow go extinct, the antelope and elephants will grow in number and as a result there will not be enough food to support them. They will slowly die. But that was just an example, lions are not on the endangered species list. But I hope you get the example.
The above completely disregards the fear of global cooling that was the rage in the 1970's. Earth Day was begun to bring attention to cooling, not warming. Previous periods of prehistoric fluxes in animal and plant populations resulted in the present day. If you really think the times are bad for you please take a walk in a country cemetery and collect data on the ages off the grave markers. You'll soon find without much analysis that living past the age of 10 was an accomplishment. The spotted owl population is greater after clear cutting than before. Most of the commercial environmental groups generate unbalanced hysteria with the endangered animal of the moment. Look at the amount of money these groups control, the land they own and use for their interests without regard those that fund them. Don't read anything written on the subject since 1973. After that year everything is tied to groups that are driven by financial gain.
Please disregard the above statement, somehow somebody has taken the question a little personally. the question asked nothing on opinon of interest groups. If you want FACTS just read the first two statements. Also, Earth Day was created in the 1970s following the oil spill in California.