The living world is not currently in a state of equilibrium. The populations of humans and the animals and plants we keep are growing extremely quickly; almost every other population of living thing is shrinking quickly.
“Rich” refers to the variety and abundance of living things. The total biomass of living things has declined (there’s less overall) and what is left is concentrated into far fewer species than previously.
Just taken as a store of information, the loss is staggering. But from a practical perspective we are less rich; it’s far harder today to catch fish than it used to be, for example.
He means something that would be interesting for a biologist like him to hang out in and observe.
I know some biologists, and they often wish that the world should be "rich" from a biologist's perspective even if the biologist isn't there to observe it.
It's a curious perspective to me. If you don't exist, why would it matter whether the world is (a) green and buzzing with insects, or (b) reduced to grey goo or strange matter? Why would your aesthetic preferences be relevant in a world that you don't exist in? It's not like you're going to get to watch this universe through a viewport.
Aesthetic preferences exist in our minds independent of our direct experience. That’s why people can have aesthetic opinions of things the instant they experience them.
An aesthetic preference is essentially an ideal against which we compare our experience. Whether we expect to meet that ideal does not diminish its power. In fact if it was easily met, it would not be much of an ideal.
A modern day equivalent of this rich state of equilibrium can be observed in Chernobyl. The place was decimated by nuclear radiation a few decades ago. Initially it was believed that the place never support any large life forms for many centuries. However wild life is currently thriving in Chernobyl including large mammals and birds [0].
The notion of equilibrium is something I always disagree with when I read environmentalists. The ecosystem is never in equilibrium. If it were it would be dead. If it reached a meta-stable state evolutionary change would halt.
An icy comet circling way out beyond Neptune is an example of a system somewhat near equilibrium.
