At Vegan.com, Erik Marcus responds to Cheri Shankar at Huffington Post by arguing that one can be an environmentalist and eat meat.
As a practical matter--how animal agriculture affects "the environment" (contribution to climate change, damage to local environments near factory farms, etc.)--Marcus has a point. An environmentalist could significantly reduce meat consumption, could actively push for more sustainable, environment-friendly agricultural practices, etc. Plastic bags are bad, but I wouldn't claim you're not an environmentalist if you ever accept a plastic bag at a store. "Environmentalist" is not a term to define some inner essence, whereby you either "are" or "aren't"; different people may have different levels or types of concerns for the environment, may express these concerns in different ways, and may take different actions (personal and political) to protect the environment.
From a ideological or philosophical perspective, though, I think an environmentalist eating meat is problematic. It comes down to what we mean by "the environment." Eating meat, generally speaking, means not considering animals a part of that environment. Thus an environmentalism that allows for killing animals for food (something unnecessary, relating to pleasure, not survival) is a human-centered environmentalism. Such environmentalism is concerned with the global environment and local environments, but primarily how environmental damage affects humans. A meat-eating environmentalist may still want to save the environment (or environments), but to do so for humans. In that sense, even saving endangered species isn't about the animals, but about saving species so that human beings can continue to appreciate and enjoy them.
I also think, from an educational perspective, it is good to publicize the connection between current animal agriculture and environmental damage. In recent years I've come across articles and books highlighting this connection fairly regularly. It is good to point out to people that their choice to eat animals has environmental consequences. Even if a writer makes a claim some will take as extreme (i.e., that you cannot be an environmentalist and eat meat), such claims still publicize the connection and require people to think about it. Will exposure to this connection convert many individuals to veganism or vegetarianism? Maybe not. Will exposure to an extreme claim cause individuals to say "Well that's too radical: to hell with environmentalists, and to hell with the earth!" I doubt it. Hopefully, though, people will learn something, think critically about their own choices and lifestyles, and make positive changes.
So yes, "environmentalists" can express concern and take action for the environment in numerous ways; this concern may include abstaining from meat, or it may only include reducing meat consumption, or it may express itself in ways having nothing to do with food. But meat-eating environmentalists may be denying animals' place in and part of "the environment," and insisting that environmentalists refrain from meat can only help, not hurt.