Both are true and true, both are good, both are true and true.
Island species are indeed very diverse, and many more are found on islands than on mainland. Island ecosystems are more complex and have a lot more species than mainland ecosystems. And islands have some of the highest per capita productivity of any terrestrial ecosystem.
How is that possible? All sorts of things contribute to this. Islands are cooler and wetter than the mainland. Species on islands are more numerous and diverse. Islands are more exposed than the mainland. Islands are more isolated than the mainland. Islands are more dynamic because they are so very small. Islands are more diverse because they are so very diverse. Islands are more diverse because they are so incredibly diverse.
One of the most common ways to compare biodiversity is to count total species. Island species are just as abundant on the mainland as they are on islands. The only difference is that the island populations aren’t as diverse as the mainland populations. Island species have much lower diversities than mainland species. Of course, by this metric, the island ecosystems are more diverse than the mainland ecosystems, so that’s a little misleading to put on a biodiversity comparison. But this is just the best we can do today.
Of course, there are many other ways to compare biodiversity, but counting species is a pretty standard way to do it.
Actually, there are a lot of ways of comparing biodiversity. I’m not sure which is better: The two best ways, or comparing biodiversity, to look at the top 20% of biodiversity on the planet (which I did, but that’s a whole other post).
And the most important thing to think about with biodiversity is that it is a measure of all of the life forms on a planet. And the only way to look at the top 20 is to look at the biggest biodiversity hotspot.
The top 20 biodiversity hotspots from the top 20 are all species of plankton, which are the most complex organisms on Earth. We often think of the top 20 as the smallest biodiversity hotspot, the most complex of all. But in reality it’s the most complex of all. That’s why we have to find out which species are truly worth looking at.
I think that there are two different ways to look at the top 10 of biodiversity hotspots, and only two ways to look at the top 20. The first is to look at the biggest hotspot and say that its the most diverse, the second is to look at the least diverse and say that its the most diverse. The top 20 is a bit of a weird set.