Rationales for Environmentalism
I'll first divide the possibilities into transitive and intransitive. Transitive morals are "duties to" something -- there is some recipient of moral action for whose sake it's done. Both rights-based and consequentialist morality fall into this category, because both posit some potential victim (a rights-holder or welfare-experiencer) to whom moral action is directed.
Transitive morality can be further divided into direct and indirect forms. Indirect transitive morality says that we should act in one way rather than another in our relations with nature for the sake of someone who isn't part of nature. Nature itself isn't harmed, but someone else may be harmed by what we do to nature. The more common kind of indirect transitive morality is anthropocentrism, which Scorse finds inadequate. Anthropocentrism recognizes the fact that humans -- to whom we have moral duties -- are affected by, and depend on, nature. Therefore what we do to nature may violate the rights or decrease the welfare of one or more humans (including future generations).
The other common type of indirect transitive morality is creation care. Creation care argues that our interactions with nature are limited by our ethical obligations to God. Because nature is God's creation (and hence His property), we violate God's rights or harm Him when we misuse nature.
Direct transitive morality holds that we have moral obligations owed directly to nature -- that nature itself can be harmed in a morally relevant way, regardless of the effects on other humans or God. There is theoretically a whole spectrum of direct transitive moralities based on how much of nature is accorded direct moral status (our relations with the remainder of nature being then constrained indirectly by our obligations to those entities that are directly considerable). But there are three common positions. One -- advocated by Scorse -- is animal rights. Animal rights (which here includes non-rights-based systems like Singer's utilitarianism) holds that we have duties to any entity that is a subject. Subjectivity is variously defined as self-awareness, the ability to form counterfactual preferences, or the ability to feel pain. In any event, animals or a subset thereof are given direct moral status, while our relations with the remainder of nature remains subject to indirect obligations.
Biocentrism extends considerability to all individual organisms. In various ways (usually some hybrid of quasi-Platonism and Darwinism), one posits that non-subject organisms can be said to have rights or welfare that we must respect. Note that, contrary to the tired arguments about vegetarians being hypocrites for killing carrots, biocentrism requires additional, and philosophically controversial, moves to bring non-subjects (e.g. plants) under the direct moral umbrella.
In its pure form, ecocentrism rejects the anthropocentric/animal rights/biocentric concern for individuals. Our transitive moral duties are said to be to populations and ecosystems, not individuals, because those larger collectivities are the true units of life and evolution. We are required to respect the rights or promote the well-being of species and forests and rivers. Note that ecocentrism is almost never found alone, but rather is combined with anthropocentrism (different rules for different domains) or biocentrism (both individuals and ecosystems are self-organizing systems, and hence count morally for the same reasons).
Pantheism is a variant of biocentrism or ecocentrism that stipulates that the reason that we have transitive duties to nature is that nature is God or God is in nature.
One final type of transitive morality straddles, or rather effaces, the direct/indirect distinction. Transpersonal ecology (often identified with Deep Ecology) asserts that one can't draw a moral boundary between humans and non-humans. The whole cosmos is ultimately part of each human self. So doing something to nature is nothing more nor less than doing it to oneself (and to everyone else).
Intransitive morality has no grammatical object -- there is no entity to or for whom moral actions are done, and who is therefore victimized by immorality.
Virtue ethics asserts that it is morally good to behave in a certain way toward nature -- not because nature is harmed in some way, but because acting that way just is the right way to behave, or is the behavior characteristic of a good/moral person. One could postulate the existence of freestanding environmental virtues such as the virtue of leaving a small footprint, or one could extend virtues exercised among humans (e.g. compassion) into relations with nature. A variant of this direct virtue ethics would be indirect virtue ethics, which was very popular in the early conservationist movement. In indirect virtue ethics, one's conduct toward nature is not itself virtuous or vicious, but nature is important as an instrument for cultivating certain virtues (such as self-reliance acquired during hiking or hunting).
One may also act in a certain way toward nature on the basis of a divine command. If God orders you to save the whales, you are obligated to save them, no matter how undeserving of saving they would have been in the absence of such a command. Divine command morality is not a major player in the modern West because what divine commands regarding nature one can find in the Bible are typically seen through the creation care lens.
Finally, one may conclude that certain states of the environment, or processes within it, posess intrinsic value. The evolutionary process and diversity are common candidates for intrinsic value. One promotes intrinsic value not as a duty to the intrinsically valued thing, but as a duty full stop. Analogies to the (alleged) intrinsic value of knowledge or great art can clarify how intrinsic value is intransitive.
Pluralistic or hybrid theories are possible as well.