What is the species introduced into ecosystems, intentionally or unintentionally, and usually by humans? some can take over - perhaps destroying - entire ecosystems. the lionfish is one example?

These are called invasive species (like the Giant African Snails in Galapagos for example.)

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.