[2026] PTE Academic Reading - Multiple Choice, Multiple Answers

In ecology, a keystone species is a species on which other species in an ecosystem largely depend, such that if it were removed the ecosystem would change drastically.The concept was introduced in 1969 by zoologist Robert T. Paine.He based his theory on observations and experiments in the rocky intertidal zone of Washington state, specifically focusing on the starfish Pisaster ochraceus.

Paine found that removing the starfish from the ecosystem led to a population explosion of mussels, which rapidly outcompeted other benthic organisms for space.This dominance by a single species significantly reduced the overall biodiversity of the rocky shore.Thus, the starfish acted as a keystone by keeping the mussel population in check and maintaining ecological balance.

Apex predators are often considered classic examples of keystone species, but the title is not limited to carnivores.Certain herbivores, such as elephants in the African savanna, also play this critical role.By consuming small trees and shrubs, elephants prevent the savanna from turning into a woodland, thereby preserving the habitat for numerous grass-eating species.