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essential resources

Two resources are said to be essential when neither can substitute for the other. This is denoted in Figure 3.30a 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. Generally, therefore, growth rate will be determined by the resource in most limited supply. This will be true for nitrogen and potassium as resources in the growth of green plants, and for two host species in the life of a parasite that must alternate between them (see Chapter 12).

perfectly substitutable resources

Two resources are said to be perfectly substitutable when either can wholly replace the other. This will be true for seeds of wheat or barley in the diet of a farmyard chicken, or for zebra and gazelle in the diet of a lion. Note that we do not imply that the two resources are as good as each other. This feature (perfectly substitutable but not necessarily as good as each other) is included in Figure 3.30b by the isoclines having slopes that do not cut both axes at the same distance from the origin. Thus, in Figure 3.30b, in the absence of resource 2, the organism needs relatively little of resource 1, but in the absence of resource 1 it needs a relatively large amount of resource 2.

complementary resources

Substitutable resources are defined as complementary if the isoclines bow inwards towards the origin (Figure 3.30c). This shape means that a species requires less of two resources when taken together than when consumed separately. A good example is human vegetarians combining beans and rice in their diet. The beans are rich in lysine, an essential amino acid poorly represented in rice, whilst rice is rich in sulphur‐containing amino acids that are present only in low abundance in beans.

antagonistic resources

By contrast, a pair of substitutable resources with isoclines that bow away from the origin are defined as antagonistic (Figure 3.30d). The shape indicates that a species requires proportionately more resource to maintain a given rate of increase when two resources are consumed together than when consumed separately. Though probably rare, this could arise, for example, if the resources contain different toxic compounds that act synergistically (more than just additively) on their consumer. For example, d,l‐pipecolic acid and djenkolic acid (two defensive chemicals found in certain seeds) had no significant effect on the growth of the seed‐eating larva of a bruchid beetle if consumed separately, but they had a pronounced effect if taken together (Janzen et al., 1977). Whenever resources are substitutable, whether or not they are perfectly substitutable, the growth rate is determined by their joint availability.

inhibitory resources

Finally, Figure 3.30e illustrates the phenomenon of inhibition at high resource levels for a pair of essential resources: resources that are essential but become damaging when in excess. CO₂, water and mineral nutrients such as iron are all required for photosynthesis, but each is lethal in excess. Similarly, light leads to increased growth rates in plants through a broad range of intensities, but can inhibit growth at very high intensities. In such cases, the isoclines form closed curves because growth decreases with an increase in resources at very high levels.