1) - Overpopulation -- on this the critics are right and the malthusians are dead wrong. I should point out that food-first (which is firmly part of the enviromental movement, Barry Commoner, and in fact the whole anti-capitalist portion of the enviromentalist haas always pointed this out. Are anti-capitalists are a minority within the environmental movement? Absolutely. Anti-capitalists are a minority within the feminist movement, the labor movment, the anti-racist movement ....
Unfortunately anti-capitalists are a minority - period.
2) De-forestation. Is forest acreage increasing? Yes - but OLD GROWTH forest is decreasing. Why is this important? Because old growth forest has much more biomass to prevent climate change; also old growth forest supports a much denser and more diverse eco-system, meaning you have a much greater variety of speciess.
3)This leads to the question of extinction. While you can argue about the number of species we are wiping out, I don't think anyone can argue that we are wiping out species at an accelerating rate. From a human point of view we are wiping out a marvelous genetic library we are only beginning to learn how to read.
4)In terms of enviromenalism helping people:
A) No uncontrolled production without reqard to environmental consequences does not help improve the "human environment". Killer smog is not good for humans. Toxic waste does not have nutritional value. Random climate change is not likely to prove beneficial to the poor, or to the working classs.
B)In terms of environmentalism standing in the way of ending say hunger: We have been producing enough food for quite some time to feed everyone. It is distribution, or to be blunt class oppression by capitalism, and bureaucratic oppression under Lenism, and occassionally imperialist oppression by both that has caused famines and hunger.
C) Last point; anyone who is interested in an anti-elitelist, and factually careful argument for environmetalism should read Barry Commoners "Poverty of Power" and "Closing Circle" . They are very old now; but as far as I can see no one has done a decent update; the current facts replacing the old facts cited in these books simply strengthen his case.