Undistributed middle

Biological Parallel

'All cheetahs are fast. Some predators are fast. Therefore, some predators are cheetahs.' The middle term (fast) connects premises but isn't distributed—doesn't cover all instances. In ecology: 'All keystone species affect ecosystems. Sea otters affect ecosystems. Therefore, sea otters are keystone.' Invalid—many species affect ecosystems. The fallacy mistakes overlap for identity. Shared properties don't imply shared category membership.