Ecosystems are defined as the interactions between organisms and their environment. Energy from the sun enters ecosystems through photosynthesis by primary producers like plants. This energy is then transferred between trophic levels as organisms consume one another, though typically less than 10% of the energy is transferred between each trophic level. Nutrients also cycle through ecosystems, with processes like decomposition and nitrogen fixation carried out by microorganisms regulating their movement between organic and inorganic forms. Human activities are now impacting global nutrient cycles through activities like agriculture, fossil fuel combustion, and deforestation.