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Undercompensating density dependence
Density dependence in which death rate increases, or birth or growth rate decreases, less than initial density increases, so that increases in initial density still lead to (smaller) increases in final density.
Unit leaf rate
The dry weight increment made by a plant over a period of time expressed as a function of the plant's leaf area.
Unitary organisms
Those that proceed by a determinate pathway of development of a tightly canalized adult form, e.g. all arthropods and vertebrates. The contrast is with 'Modular organisms' (see above) in which growth occurs by the indeterminate iteration of repeated units of structure (modules).
Unstable equilibrium
In an ecological context, a level of a population, or populations, or of resources, from which slight displacements lead to larger displacements.

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