The abundance, distribution, and apparency of acceptable resources determine their availability to organisms. Resources are most available when distributed evenly at nonlimiting concentrations or densities. Organisms living under such conditions need not move widely to locate new resources and tend to be relatively sedentary. Microorganisms suspended in a concentrated solution of organic molecules (such as in eutrophic aquatic ecosystems or in decomposing detritus) and filter feeders and scale insects that capture resources from flowing solutions of resources may enjoy relatively nonlimiting resources for many generations.
However, necessary resources usually are less concentrated, available at suboptimal ratios with other resources, or are unevenly distributed at the scale of use by most terrestrial organisms. This requires that organisms select habitats where limiting resources are most concentrated or in most efficient balance and seek new sources as current resources become depleted. Although active searching is facilitated by locomotory ability, plant roots are capable of growing in the direction of more concentrated resources. Insects and other animals use various physiological and behavioral mechanisms to detect, orient toward, and move to concentrations of food.
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