Which statement best describes a keystone species

A keystone species plays a unique and crucial role in the functionality of an ecosystem. Without these species, the ecosystem will be different or can even cease to exist altogether.

Hence, the correct answer is option 4 - it has a strong influence on the ecosystem.

what statement best describes the difference between the sympathetic and parasympathetic nervous systems?

the sympathetic nervous system prepares the body for intense physical activity and is often referred to as the fight-or-flight response. the parasympathetic nervous system has almost the exact opposite effect and relaxes the body and inhibits or slows many high energy functions.