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ecology ecosystems middle school science teaching science atmosphere hydrosphere biosphere lithosphere abiotic biotic population community sustainability distance learning ecological pyramids trophic levels food web teachwithfergy food chain scavenger omnivore carnivores herbivores consumers producers niche cellular respiration photosynthesis teachwithfergy teachwithfergy teachwithfergy controlling invasive species native species invasive species keystone species extinct species endangered species extirpated species threatened species vulnerable species biodiversity ph acid precipitation greenhouse effect fossil fuels biomass secondary succession primary succession ecological succession prey predator symbiosis carrying capacity tolerance limiting factors science teaching high school carbon cycle phosphorus cycle nitrogen cycle water cycle