To me it would make sense if it was indeed true. Mainly because on land there is a huge variation in possible ecological niches. The sea, including both the sea

To me it would make sense if it was indeed true. Mainly because on land there is a huge variation in possible ecological niches. The sea, including both the seabed and the surface, look kind of homogeneous in comparison. Another interesting factor is that moving around is much harder on land (both above ground and underground) and so also the density of prey-predators that can coexist in a given territory. This does not invalidate (nor address) any of your point. Just some reflection on the topic.