Plants have adapted to the differential occurrence of nutrients in the soil. For example, halophytes and metallophytes can survive in the presence of low concentrations of NaCl and high concentrations of heavy metals, respectively, and calcifuges are restricted to acidic soils and calcicoles are restricted to alkaline soils. These specialists survive in a much narrower range of ecological conditions, whereas most plants live within a less extreme range of conditions in the soil. Plants found in nutrient-rich soils produce more biomass than those living in nutrient-poor soils. The low primary production of plants with narrow soil tolerances conserve the acquired nutrients for long periods of time.
A related aspect of nutrient acquisition is that herbivorous mammals intake large amounts of organic compounds, some of which are either difficult to digest, hinder digestion (e.g., tannins), or are toxic. Toxic compounds protect plants by reducing the consumption of consumers, and in extreme cases these plants may be completely avoided. This system is not only toxic to the animals that consume these substances, but also requires energy expenditures by the plants to synthesize them, often at the expense of growth and reproduction. Small herbivores, because of their low energy requirements, can avoid toxic plants by selecting high-quality food, but large species, because of their high requirements, usually cannot avoid toxic or indigestible plants and must use gut fermentation to aid in their digestion and detoxification.
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