Ecology and the environment are fairly broad subjects with plenty of material to dig into. Some of these sub-topics could include:

Energy Production - How do each of these affect the environment, compare with one another, and/or what are the real details as to how they work...?

• hydro-fracture drilling for natural gas (a somewhat controversial method of extraction)
• oil drilling in the ocean (pros and cons)
• nuclear power (the serious dangers posed by energy companies that are not following safety regulations. The Japanese disaster is a great example there...)
• wind energy (the effects of necessary new infrastructure to use wind energy on a large scale; the potential benefits of wind energy vs. the natural limitations of this method)

Also, you could look into innovative recycling programs, recycling facts and figures (how much energy is required to recycle certain goods and is it worth it?, etc.). Hybrid automobiles, electric and and hydrogen powered cars are another subject that may be interesting and pose interesting ideas for speeches.   

You really should choose something that interests you and your audience.  I have found that the best speech topics are related to homework, security cameras in schools, uniforms, school lunches, and other student-centered topics because that is what you know about and what your classmates are interested in.

Here are some potential topics for your list:

What is a man's favorite hair color in a woman?

Clothes influence the way people behave.

Who is the ultimate couch potato?

Cats versus dogs

Type of hot peppers

Feng shui

Some women are from Mars.

Young adults need to serve at least one year in the military.

Passing high school should be a requirement for getting a driver's license.

 America is a melting pot of people.

Until the economy is a better footing, immigration should be stopped.

What makes the best spouse?