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mountain hill rock tree plant forest nature sky cloud sun light shadow weather landscape scenery view peak valley cliff gorge canyon river stream water lake pond waterfall ice snow cold fresh clean air wind rain storm thunder lightning earthquake volcano eruption lava ash smoke fire heat hot dry wet humid arid desert tropical temperate polar climate change global warming pollution environment protection conservation sustainability ecology ecosystem biodiversity habitat species population growth decline extinction threatened endangered rare vulnerable protected conservation status wildlife animal plant fungus bacteria virus protist cell microbe organism life biology evolution genetics DNA RNA protein enzyme metabolism photosynthesis respiration circulation digestion excretion reproduction growth development aging death disease infection injury healing regeneration adaptation evolution natural selection survival of the fittest extinction threatened endangered rare vulnerable protected conservation status