[user249]

We here in the US have apex predators in the Rocky Mountains (grizzlies, wolves) so I don't see how this is a blocking issue

[giarc]

Because those ecosystems have existed with those predators for thousands/millions of years and have balanced themselves.

[grapeskin]

Imagine bringing cold-resistant crocodiles to the Rockies and how much that’d shake up the ecosystem.

An apex predator is only an apex predator in absence of a bigger, more dangerous predator.