Essential resources

Two resources are said to be essential when neither can substitute for the other. Thus, the growth that can be supported on resource 1 is absolutely dependent on the amount available of resource 2 and vice versa. This is denoted in Figure 3.27a by the isoclines running parallel to both axes. They do so because the amount available of one resource defines a maximum possible growth rate, irrespective of the amount of the other resource. This growth rate is achieved unless the amount available of the other resource defines an even lower growth rate. It will be true for nitrogen and potassium as resources in the growth of green plants, and for two obligate hosts in the life of a parasite or pathogen that are required to alternate in its life cycle (see Chapter 12).

aposematism

Batesian and Mullerian mimicry zero net growth isoclines

Figure 3.27 Resource-dependent growth isoclines. Each of the growth isoclines represents the amounts of two resources (Rj and R2) that would have to exist in a habitat for a population to have a given growth rate. Because this rate increases with resource availability, isoclines further from the origin represent higher population growth rates - isocline A has a negative growth rate, isocline B a zero growth rate and isocline C a positive growth rate. (a) Essential resources;

(b) perfectly substitutable;

(c) complementary; (d) antagonistic; and (e) inhibition. (After Tilman, 1982.)

Figure 3.27 Resource-dependent growth isoclines. Each of the growth isoclines represents the amounts of two resources (Rj and R2) that would have to exist in a habitat for a population to have a given growth rate. Because this rate increases with resource availability, isoclines further from the origin represent higher population growth rates - isocline A has a negative growth rate, isocline B a zero growth rate and isocline C a positive growth rate. (a) Essential resources;

(b) perfectly substitutable;

(c) complementary; (d) antagonistic; and (e) inhibition. (After Tilman, 1982.)

Inhibition Essential Resources

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