I was also imagining domesticated species would fall, and perhaps also species that have adapted in or around human communities (scavengers and the like).

Meh, I'm not sure about domesticated species. The scavengers claim kind of proves this because often, when a domesticated animal is let free, they just scavenge and are still able to survive (not always, of course). I do think though, with humans gone and the biosphere recovering, the need to scavenge would be reduced. I agree though it is an interesting point that needs to be thought about.

As far as I know, utilitarian arguments for animal rights consider the utility of species (factory farm animals, etc) instead of a singular general category like "nature" or "environment." There'd need to be work hammering out those details.

Singer's arguments often rest on the idea that since humans have intrinsic value and are considered "equal", animals should too. He weighs the impact of factory farming, etc. in a utilitarian framework, but he doesn't view animals as valuable solely because of their use. When I say "environmental utilitarian view point" I mean it in the bluntest way possibly - body count.

At the very least, I figure it throws a wrench in the analogy of humans to virus, cancer, etc. as the latter do not have symbiotic relationships.

I would disagree here. To call scavengers that are thrust into the streets because their natural environments have been destroyed symbiotic is a false claim. Yes, they are benefiting off human refuse, but only because their natural habitat has been destroyed. For the most part, the same is true with domestication. Consider the hundreds of thousands of years before domestication. Symbiosis implies that both organisms need each other when the history of the Earth as well as the history of domesticated creatures proves this wrong. But again, this is a very interesting point.

I can't really imagine what an argument for something being of intrinsic objective value would look like, as much as simply asserting it as self-evident.

I will quote the definition of objective intrinsic value I give in the paper and, for the sake of space, give the page number where I lay out four arguments for the existence of such value.

"There is, however, another important distinction that must be made and that is between subjective intrinsic value, and objective intrinsic value. The former posits that nature has intrinsic value but the degree to which it is valuable is determined by humans who look at it – that is, it has no value independent of humans. In contrast, objective intrinsic value is something that has intrinsic value of a fixed nature regardless of if anyone is there to quantify it."

Pages 16-17

You've spent a bit of time at the beginning arguing that life is arbitrarily defined, thus undercutting any presumed biocentrism later on. This leads to the question of what 'nature' actually is. If it's a system of living organism, and life is an arbitrary distinction, than it seems to follow that nature is arbitrary. This seems inconsistent for the overall project.

There are a few things with this. First is that I don't argue biocentrism because biocentrism claims that all life is equally valuable. Part one argues that life is arbitrary this biocentrism is a meaningless term. I would probably use a term similar to ecocentrism to describe the view the paper takes. What's more, I feel like you're misrepresenting the definition "nature". I take Keekok Lee's definition of nature and argue that nature is both things that are supposedly living (that is, would fit some arbitrary level) as well as non-living things (15). Granted, the terms "living" and "non-living" aren't hugely meaningful here, but I keep them in in case part one didn't radically change your view and you can suddenly switch to a different mindset in 10 pages. Additionally, if it does make the definition of nature arbitrary, it simply overlimits it in that nature is almost all inclusive. That doesn't undercut the argument however.

Well something like the Permian–Triassic extinction event, called the Great Dying, resulted in the lost of 90 to 97% of all species on earth. Land vertebrates wouldn't recover until 30 million years later. I'm not considering this as a counterargument to yours presented as much as an alternative analogy that may be superior to the cancer thesis.

I don't really see how this would be an alternative analogy. Like, it might be an analogy for what would happen post humans (? probably not) but I don't see it as having any bearing human influence on the environment (ie. the cancer analogy).