This document discusses key relationships in ecosystems, including populations, competition, limiting factors, carrying capacity, symbiosis, mutualism, commensalism, parasitism, predator-prey relationships, and cooperation. Populations of the same species compete for resources like food and space within an ecosystem. Symbiosis describes close species interactions like mutualism where both benefit, commensalism where one benefits and the other is unaffected, and parasitism where one benefits at the other's expense. Predator-prey relationships involve predators consuming other organisms as prey. Cooperation occurs when organisms work together for common goals.