Biodiversity: the diversity (number of different types) of organisms present in an ecosystem.  Healthy ecosystems usually have higher biodiversity.

Ecology:  The scientific study of the interrelationships going on in the environment.  Ecologists sometimes recommend what ought to be done to achieve a particular goal, but their concern is with what is happening, not with what human beings choose to do about it.  They don't set the goals.

Ecosystem: All the organisms in a particular region and the environment in which they live. The elements of an ecosystem interact with each other in some way, and so depend on each other either directly or indirectly. (source)  This is a term used by ecologists.

Environment: both the natural land, plants, animals, and climate and what humans have done.  Scientists use this term to describe the conditions in a particular place, but historians use it more broadly to mean the ecosystem plus the land and climate plus how it has been modified by humans.

Environmentalism: A political movement concerned with protecting and improving the environment.  Environmentalists push for particular ideas about what the goals should be.

Environmental policy: What the government decides to do to modify human impacts on the environment.  This is a political process by which goals are set, but it is influenced by public opinion.

Indigenous:  originating or occurring naturally in that place.  For people it is used for the people who were there when Europeans arrived, even though they had earlier spread from somewhere else.

Nature: different people use this word with different meanings.

Wilderness: The Wilderness Act of 1964 defined wilderness as an area of undeveloped land that appears “to have been affected primarily by the forces of nature, with the imprints of mans’ work substantially unnoticeable” (source)