Keystone species are those species see concept 54.2 (page 1223) keystone species are those species see concept 54.2 (page 1223) that have the most biomass in the community that provide important foods and medicines that live primarily on or under rocks and stones with the largest number of individuals in a community whose absence would cause major disruption in a community

The correct answer is the species whose absence would cause major disruption in a community.

The keystone species are the ones that are ''keeping the ecosystem together''.
They might not be the most abundant species or a species on the top of the food chain, but they are the ones that have the strongest effect on the ecosystem.
The keystone species can have this role because they present an important link in the food chain, their presence changes an aspect of the habitat, they introduce a new resource in the ecosystem etc.