This argument also implies that “dominionism” is wrong, i.e. all life has a right to not be killed or abused. Yet human life is impossible without killing and consuming, other living organisms, be it plants, animals og fungi. Thus it is unethical to continue living.

This argument is bad, because for human life to be possible, you must draw the line between life that you consider ethical to kill and life that you consider unethical to kill.

It’s not about “all life” but about “all sentient life”. Only beings that are able have pleasant and unpleasant experience should be considered. If something (living or not) cannot experience suffering then you can’t harm it, by definition.

Sentience is studied scientifically. It cannot be stated with absolute certainty but scientists have good sets of criteria and experiences that helps identify it. With the current knowledge it’s almost certain that all mammals are sentient, like us. Fishes and birds are also very likely to be sentient. Some species of insects are probably sentient while others may not be. And plants are likely not sentient.

But even if all living things are sentient, it doesn’t change very much. Speciesism means treating beings differently only because they belong to some specific species. There are good reasons to treat different beings differently but they should be based on the beings’ interests, not their species (and studying sentience helps identifying these interests). It’s very likely that we do less harm by growing plants than by breeding animals. And even if it was the same amount of suffering we would still do less harm by avoiding eating animals because breeding them to eat them actually requires more plants than just eating plants. We should seek to minimise suffering and avoiding eating animal is a good way to do that.

you can’t prove plants aren’t sentient.

And you can’t prove something is sentient. But scientists have criteria that help determine whether a species is sentient. See this review for example.

even if you could, why should sentience matter?

I already answered. If something can’t be harmed there no need to prevent harming it.

what ethical system even accounts for sentience as a factor of right behavior?

About all animal welfare:

Respect for animal welfare is often based on the belief that nonhuman animals are sentient and that consideration should be given to their well-being or suffering, especially when they are under the care of humans.[4]

If something can’t be harmed there no need to prevent harming it.

i don’t really like your use of harm here to exclude everything but sentient beings, but as a term of art, for the purposes of this discussion, i will indulge you.

why does it matter if something CAN be harmed? what creates a duty to NOT HARM something?

what creates a duty to NOT HARM something?

About all ethics is about reducing harm. If you don’t know that harming is bad I don’t think we can have a discussion.

deontological ethics are explicitly not about that.

I guess it depends on the philosopher, but at least one includes “doing no harm” in the obligations[1]:

Ross [20] modified Kant’s deontology, allowing a plurality of duty-based ethical principles, such as doing no harm, promise keeping, etc.

can you name an ethical system that does concern itself with that?

Probably all consequentialism and at least utilitarianism (harm decreases the global well being). Negative consequentialism is more specifically focused on reducing suffering/harm.

I’m not a consequentialist at all, and Ross is not using harm in the same sense as we are. even if he were, his is not a very common strain of ethics.

your ethical theory seems to be on dubious footing to me.

So in your ethical theory, harm doesn’t matter at all?

You seem to follow some kind of deontology. There’s no obligation in your system to not cause unnecessary harm? I guess you have some obligation not to hurt your dog even if you like doing that. Isn’t that obligation related to the fact the dog would be harmed if you did?

Maybe it’s just a difference between consequentialism and deontologism, but I was convinced deontologists generally had some rules that prevent unnecessary harm. They don’t?

There’s at least Tom Regan who was a deontologist (at least in his book The Case for Animal Rights) and talks about harm:

In Regan’s view, not to be used as a means entails the right to be treated with respect, which includes the right not to be harmed.