Ecosystems and the biosphere
Energy flows through ecosystems and matter cycles within them. Human activity is now changing both on a global scale.
Producers and consumers
Producers (autotrophs), mainly plants, algae, and some bacteria, make their own food from sunlight or chemicals. Consumers (heterotrophs) eat other organisms: herbivores eat plants, carnivores eat animals, omnivores eat both. Decomposers, such as fungi and bacteria, break down dead material and recycle nutrients.
Food chains and webs
A food chain shows one path of energy: grass -> grasshopper -> frog -> snake -> hawk. Each step is a trophic level. Real ecosystems are food webs, many interconnected chains, so removing one species can ripple through the whole web.
The 10% rule
Only about 10% of the energy at one trophic level is passed to the next; the rest is used for life processes or lost as heat. So 10,000 units of energy in plants supports about 1000 in herbivores and 100 in carnivores. This limits food chains to a few levels and explains why top predators are rare.
Biomagnification
Some toxins, such as mercury and the pesticide DDT, are not broken down or excreted, so they accumulate in organisms. Concentrations increase up the food chain. DDT thinned the eggshells of eagles and pelicans until it was banned in many countries in the 1970s.
The carbon cycle
Carbon moves between the atmosphere, organisms, oceans, and rocks. Photosynthesis removes carbon dioxide from the air; respiration and decomposition return it. Burning fossil fuels releases carbon stored for millions of years, raising atmospheric carbon dioxide from about 280 ppm before industry to over 420 ppm today.
The nitrogen and water cycles
Nitrogen gas makes up 78% of air but most organisms cannot use it. Nitrogen-fixing bacteria, some living in the roots of legumes, convert it to usable forms. The water cycle moves water through evaporation, condensation, precipitation, and transpiration from plants.
Biomes
Biomes are large regions with similar climate and communities: tropical rainforest, desert, grassland, temperate forest, taiga, tundra, and aquatic biomes such as coral reefs and the open ocean. Temperature and rainfall largely decide which biome forms where.
Human impact
Major threats to ecosystems include habitat loss, climate change, pollution, overexploitation, and invasive species. Conservation responses include protected areas, restoring habitats, reintroducing species, and reducing emissions. The International Union for Conservation of Nature lists tens of thousands of species as threatened.
Key takeaways
Producers capture energy; consumers and decomposers pass it on and recycle matter. Only about 10% of energy passes between trophic levels. Persistent toxins biomagnify. Carbon, nitrogen, and water cycle through living and nonliving parts. Biomes reflect climate. Habitat loss, climate change, and invasive species threaten biodiversity.
Flashcard
Makes its own food
Check your understanding
If producers in an ecosystem store 50,000 units of energy, about how much reaches the secondary consumers?