The earth isn't split up into separate "ecosystems" split by continent and land mass you realise? It's one big system that isn't even sealed off from outer space. And in terms of a truly barren planet, the Sahara is teeming with life
The Sahara is teeming with life compared to Mars, but we also know it once hosted significantly more life before it was a desert. That old system collapsed.
As I've pointed out, these are arbitrary, they make it easier for humans to reason about the problem, but they don't exist. Purely in your head.
And even your Sahara example is bunk, there are like 10 states within it's borders somehow existing in a barren ecology
Then answer me this... What would be an appropriate level of diversity?
additionally, humans are currently contributing to desertification in a way most other species can not
if your point is that "deserts would have existed anyway" that's fine, but it's still the collapse of a localized system due to external factors (in this case, a system collapse caused by earth's orbit/axial changes)
> 3 in the Shannon index