| Zero population growth (ZPG)
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| The number of births at which people are just replacing themselves; also called the replacement level of fertility.
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| Zone of aeration
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| Upper soil layers that hold both air and water.
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| Zone of intolerance
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| Range of environmental conditions that an organism cannot survive in.
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| Zone of leaching
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| The layer of soil just beneath the topsoil where water percolates, removing soluble nutrients that accumulate in the subsoil; may be very different in appearance and composition from the layers above and below it.
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| Zone of physiological stress
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| Upper and lower limits of range of tolerance where organisms have difficulty surviving.
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| Zone of saturation
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| Lower soil layers where all spaces are filled with water.
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| Zones of stress
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| Regions where a species finds conditions tolerable but suboptimal. Where a species survives but under stress.
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| Zooplankton
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| Any of a number of animal groups, including protozoa, crustaceans, worms, mollusks, and cnidarians, which live suspended in the water column and feed on phytoplankton and other zooplankton.
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