The correct answer is - predator species in a community.
In the terrestrial ecosystems, a very good and usually sure predictor of the biodiversity is the existence of a predator species. The more predator species there are, the higher the biodiversity, and vice versa, the less predator species, the lesser the biodiversity.
The reason why the predators are such a good predictor of the biodiversity is that they are the species that are on the top of the food chain. That means that the predators are dependent for their survival on all the other species bellow them in the food chain, which is pretty much all species apart from themselves.
If a predator is present, than there has to be pray. If there's pray, omnivores and herbivores, than there has to be plants. All of the organisms are connected to one another, and the ones on the top of the food chain are the most dependent on the others, thus their existence usually means a healthy ecosystem with higher biodiversity.
