sustainability in geography?

I'd use wikipedia if you aren't going to use your book.

I think by natural sustainability they mean the ability to use resources (renewable and nonrenewable) in a responsible way so we do not waste them or cause pollution.  Basically we want to use our resources but not deplete them, future generations will need them as well, and if we use them to fast they will not have enough.  We must preserve the environment.

I'm not 100% sure about human sustainability; what would make sense to me is that we must realize that over population is a problem.  With to many people we will consume far to many resources far to fast.  Large epicenters of populations such as cities are harmful, they create large amounts of waste and consume large amounts of resources.  We need to control our population and consume only what we need, if we reach 9 billion people on Earth we will surely have trouble with pollution/water/food/space/and the climate.  We must also live sustainably and live in energy efficient houses and make use of the green movement.

Economic sustainability is even more confusing.  Basically we want to keep the economy working in an efficient way so that we all have equal standards of living.  If we grow too much the economy will suffer, if we grow to little the economy may need to downsize, which can created problems of its own.  Overall I think we should support local business and try to be more self-sustaining with less connections to outside world.  Don't get me wrong, we will need some connections.