5.0 Summary Organisms need free energy to survive. Natural ecosystems form trophic pyramids with heterotrophs feeding on autotrophs and other heterotrophs. Population size and density can be studied quantitatively using several techniques, such as sampling and mark-recapture schemes. Natural populations follow the J-shape curve when reproducing under ideal conditions. Any growing population must eventually reach a limit beyond which either space, resources, or predators become limiting, yielding a carrying capacity that represent a maximum sustainable population size.
Populations can fluctuate wildly depending on the behavior of controlling factors.
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