Purpose - the reason for which something exists or is done, made, used, etc. or an intended or desired result;end;aim;goal
Role - proper or customary function: the teacher's role in society.
Worth - usefulness or importance, as to the world, to a person, or for a purpose: Your worth to the world is inestimable or value, as in money.
Humans have this need to assign these values to things, and we often do so unthinkingly. Far too often, these three words are used interchangeably, when in fact they all have subtly different meanings and implications. Whether we talk about an organism, another person, a job, or an item, we make decisions about them based on their purpose, role, and worth.
Take, for example, a person. In most cases, we allow a person's purpose to be self-ascribed because their purpose is their reason for existence. Only an individual knows or has a feeling about why they exist, although we may joke that a certain person's purpose is to annoy or something similar. There's a certain thought process and evaluation behind a purpose. A person's role (or roles) are defined by themselves and society. A person can be a parent, child, sibling, worker, boss, volunteer, whatever. It's what they *do* in their life, and sometimes gives insight to their purpose. A cleric has the purpose of living for their deity, and expresses that by joining the clergy. Another might have the purpose of helping others, and so becomes a doctor or social worker. A person's worth is a tricky matter, possibly the trickiest of all. There is their inherent worth as a human, for which many philosophers have tried to ascribe a dollar value, since (especially in the US) we measure worth with monetary units. There is the added value they bring to society, in the form of earnings, consumption, savings, and investment. All of those are pretty straight forward. And then there is one's worth in relationships- as a friend, mentor, parent, partner, etc. Again, difficult to quantify.
Inanimate objects are easy to evaluate using these criteria. We humans ascribe the purpose for which they were designed, the role they will play, and their worth. There is a creator, or re-creator in the case of re-purposed goods, that assigns the values of these various traits.
But what about ecosystems and non-human organisms? An ecosystem can't think as it's not a single organism, so it can't self-ascribe a purpose, leaving the other option to let a creator ascribe the ecosystem's purpose. But who or what is the creator of an ecosystem? Ecologically speaking, the "creator" of an ecosystem is a function of the environment and the communities within that ecosystem. Here I feel I should point out that I am speaking from a scientific-philosophical approach, not a theological-philosophical approach because proof of an ultimate Creator is scientifically impossible- there is no test to prove or disprove one's existence- and thus it's a matter of faith, which varies from person to person and is not objective. The role of an ecosystem can be ascribed be humans, as roles can be defined by not just the item to which the role refers, but others whom share a relationship with said thing, and humans most definitely share many deep relationships with ecosystems. Worth is still difficult to ascribe, as it is partly subjective, and difficult to quantify the many areas in which an ecosystem has worth (ecological services, recreation, aesthetics, etc.).
On to other non-human organisms. They are living, and may have the sentience to ascribe their own purpose, but would have no way with which to communicate said purpose to us. Not to mention the fact that we, as not-their-creators (except in the case of GMO's) and as not the organism, don't have the right to ascribe a purpose to another organism. The argument that an organism's "purpose" is their place in the food web is a fallacy of logic, as an organism evolves in the direction of maximum fitness. This means that organisms evolve away from predation in most cases, and would be evolving away from their "purpose" if food chain position were a true purpose. An organism's place in the food chain, can however be considered a role, as roles are not just self-ascribing but also circumscribed by others, and those roles may or may not be something that we desire (consider the worker in a fast food service trying to pay their way through college to become a nurse, e.g.). An organism's worth, as with all worth, is difficult to quantify and hard to assess.
Thus, when we consider endangered species protections, to which organisms are they granted, the protection of ecosystems and the environment, and other ecological quandaries, "purpose" should rarely, if ever, enter the equation, although role and worth are definitely things to consider.
So what's the purpose to this post? I, as the creator of said post, have ascribed it the purpose of "productive waste of time." Cheerio!