01Human and natural disruptions
Invasive species (zebra mussels, kudzu, cane toads) lack natural predators and outcompete natives. Habitat destruction, pollution, overharvesting and climate change reduce biodiversity.
Eutrophication: fertilizer runoff → algal bloom → algae die → decomposers use up O₂ → fish die (dead zones).
Biomagnification: persistent toxins (DDT, mercury) concentrate at higher trophic levels — top predators are hit hardest (bald eagle eggshell thinning).
02Climate change
Rising CO₂ from fossil fuels traps heat (greenhouse effect), shifts species ranges, causes ocean acidification (harming coral and shellfish), and changes timing of seasonal events. Natural events (volcanoes, El Niño) also disrupt ecosystems.
03Notebook box
Eutrophication chain: nutrients ↑ → algae ↑ → decomposition ↑ → O₂ ↓ → fish die.
Worked example: Why do orcas carry higher mercury than small fish? → Biomagnification up the food chain.