Simple answer: for me the problem of environmental destruction and amelioration are tied to matters of wealth and income inequality, and efforts to mitigate the problems with human use of the environment are directly tied to efforts to mitigate inequality.
So many politico-environmental problems are problems of class. The logger sees the environmental protester as an urban hippie brat. The logger has to feed his family, and he has a love/hate relationship with the owner of the mill, who is not only of a higher class, but of such a higher class that the logger depends on him. The environmental protester, taken out of the context of class, can only see the logger as a source of ruin for the environment, instead of a dependent on the economic process that brought it about.
This is just the most superficial way at which class and environment intersect. But it in itself is a vital observation. To me the NDPs approach towards class is more viable and consistent with a long-term view of human relations with the environment than any movement that takes the environment apart from this.