How can predation affect species diversity?
Predation can increase, decrease, or leave unchanged species diversity.
Species diversity, or the number of species and the abundance of species at a given location, can be increased, decreased, or remain unchanged by predation. It all depends on the circumstances of the predators and the circumstances of the species in the area.
For example, if a new predator is introduced to an area and it is a diet generalist, it will likely consume individuals from multiple species. This would not have a noticeable affect on species diversity. In contrast, if a new predator is introduced and it is a diet specialist or if it highly prefers a particular species, over time, it may reduce the population of its preferred species. This would affect and likely lower species diversity.
Generally speaking though, predation alone won't have a huge effect on the populations of prey if the predator and its prey have existed simultaneously for enough time. Ecosystems trend towards states of equilibrium.
If the predator is an invasive species, there is a greater likelihood of predation having a notable effect on species diversity. The lion fish is a good example and you can read a related Socratic question about it here.