Some exchanges with a philosopher of biology have been really instructive for me. How would you define a species?
There’s an ambiguity between “define species (the concept)” and “define a species (an individual species)”.
Personally, I leave the latter to the biologists. On the former, I’m a conventionalist. That is to say, I see it as a matter of human social convention. Yes, there are all kinds of guidelines as to what we mean by species. But the guidelines are imperfect. In the end, it boils down to a human decision that becomes widely accepted.
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As mentioned above, species is a human construct. It’s an attempt to compartmentalize different living entities into distinct groups when in reality they live in an analog world with blurred indistinct boundaries. As such where we draw the line between the groups is always going to have a measure of arbitrariness. It’s like deciding where red stops and orange starts on the color spectrum.
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This is a good, if somewhat in-the-weeds, discussion of the topic:
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Have you read John Wilkins’ book (based on his Ph. D. thesis)?
The cynical species concept is that a species is what a competent taxonomist thinks is a species.
My thumbnail answer on species reality is that every species is real in its own way except those that aren’t.
Species are abstractions from the observation that biological diversity comes in chunks. It’s a handy idea most of the time, within a restricted scope of time and space. Within that abstraction, different species concepts are handy with different taxa. In my work, I like the so-called “biological species concept”, but even there it needs to be applied loosely. Doesn’t work in rotifers or Daphnia, though.
For some idea of the variety of concepts involved in plant species see Verne Grant’s Plant Speciation.