Can someone make the case using logical arguments on why a higher number of species is any better or worse than a lower number? Why is an ecosystem in equilibrium with 10 million species better than one with 8?
On the same token, more species means more genetic diversity and higher levels of resilience to diseases and other threats.
Different species service ecosystems in different ways. Pollination, water purification, soil enrichment, etc. With too few species, key requirements for cross species survival are less likely to be met.
On a less scientific basis, more species just makes the planet richer. The thought of just a few dominant species across the planet is a very sad prospect and would represent a huge loss regarding the beauty that billions of years of evolution have brought us.
Of course, somewhat paradoxically, these species disappearing is an adaptation to shifting ecosystems.
What's happening now is that ecosystems are globalizing. An ecological niche that was filled by 30 local species before modernity, is now being mixed up and whichever species is "best" may fill that niche everywhere.
So if my local frog species disappears, it's probably replaced by more hardworking immigrant frogs. This can be very disruptive and have negative consequences in many ways. But it doesn't mean animal life disappears. It changes.
One big thing is that on land we converted from hunting and gathering the commons to farming private land thousands of years ago.
In the oceans we're still doing it, and modern technology is devastatingly effective at it.
Look at the diversity within canis lupus familiaris, the domesticated dog.
Yes a moth with different wing shape is regarded as a separate species.
A given species doesn't just eat items X, Y, Z in a food web. In a pinch, they might be able to deal with others things. A W, perhaps. If the W-predating species suffers disease or some calamity, this other species can, to some degree, pick up the slack. It's really a key part of adaptive radiation. Maybe I could live here, although I don't usually. Maybe I could mate there, although it is a bit out of my comfort zone.
When you play those simulation games with just a few species, you see a lot of boom-bust cycles and eventually you get a collapse. Similarly, those sealed terrariums have similar problems: they eventually fall into a lower equilibrium of fewer species and then they kind of turn to mush.
I hesitate to say "role" because that feels assigned, but different species do different things in an overlapping fashion, like a cross-trained staff lowering your "bus factor."
Now, I imagine there's some kind of maximum number (so many species that finding someone to breed with is hard), but the minimums lack resiliency for various cycles.
I would be interested in the answer to this as well.
This isn’t a perfect example, but the other day I was researching shrimp. There is a genus called neocaridina, and they’re generally detritivores. However, I discovered that many species in the genus eat quite differently. Some will eat hair algae, some won’t. Some like to feed on hard algae, some only soft. Some have preference for higher protein foods, others less. Of the 26 known species, the manifestation of their presence in an ecosystem would actually be quite different.
This is a single genus which would have substantially varying impacts on ponds, streams, and lakes for example. If you have a diverse set of these species, you will be far more likely to keep a variety of detritus and algae sources in check. Reduce the diversity, and perhaps hair algae will get out of control. Once this happens, you might see certain fish or insects lose their habitat because they depend on other vegetation, but the hair algae is absorbing too much light and nutrients now.
This is applies to all kinds of animals, in all kinds of habitats. Diversity means greater balance, greater stability, and means of recovering faster when things collapse.
Which ecosystems on Earth are "in equilibrium"?