Part III: Why does it matter?
Before I go on to plant labor – complete with quotes from Luke 12:27 and Buenaventura Durruti – there's the question of why any this matters. So you can describe animals as doing labor if you define your terms in a certain way: why should this matter to anyone?
It matters because the classic European leftist imaginary is all about work. It doesn't matter that newer (or older) imaginaries exist that aren't so focused on producerism: the left was dominated by Marxist ideas until the fall of the USSR, and the neoliberal era after that has suppressed any widespread adoption of anything else. For better or for worse, when many people on the left think of what defines the left, the answer has to do with the working class. Since Marxism is basically a 19th century ideology, it has no ecological understanding, and has a false model of value in which all value comes from human work. No socialist state informed by the Marxist tradition has ever done better than capitalist states have in ecological terms.
It's not necessary to argue against Marxist ideas directly: these have more or less died out as living technical ideas for most of the population. What's left is a kind of folk Marxism. Psychologists now say that Freud was wrong about many important things: this doesn't stop generally literate people from thinking about the conscious and unconscious, the superego, ego, and id, the Freudian slip, the father complex and so on, or the more pop culture versions of these ideas like “daddy issues”. In the same way, people who adhere to class forms of the left rather than identity forms will immediately come up with ideas about workers, solidarity, and class interest.
I'm one of these class-form people myself: as an anarchist, I've seen what happens when people claim leftism on the basis of identity but otherwise have a liberal politics. Neoliberalism runs on that kind of thing. So it seems to me to be imperative to re-use or recycle folk ideas about workers, solidarity, and class interest into a form that can address the most important problems that we have as a civilization: our ecological limits. To do that, it becomes necessary to see animals and maybe even plants as workers, to feel solidarity, to understand that there is a common interest between them and ourselves.
This is a long-standing part of the anarchist lineage. In the Eurocentric tradition it goes back to works like Kropotkin's Mutual Aid: A Factor of Evolution, published in 1902. As such it is nothing new, but rather represents a kind of 20th century that should have occurred but did not, either in the capitalist west or the Marxist east. For real 21st century ideas we'd probably do better to turn towards Indigenous anarchists. But I myself am a product of the Euro tradition, and this is all I can write about.