We have seen that every plant requires many distinct resources to complete its life cycle, and most plants require the same set of resources, although in subtly different proportions. Each of the resources has to be obtained independently of the others, and often by quite different uptake mechanisms - some as ions (potassium), some as molecules (CO2), some in solution, some as gases. Carbon cannot be substituted by nitrogen, nor phosphorus by potassium. Nitrogen can be taken up by most plants as either nitrate or ammonium ions, but there is no substitute for nitrogen itself. In complete contrast, for many carnivores, most prey of about the same size are wholly interchangeable as articles of diet. This contrast between resources that are individually essential for an organism, and those that are substitutable, can be extended into a classification of resources taken in pairs (Figure 3.27).
In this classification, the concentration or quantity of one resource is plotted on the x-axis, and that of the other resource on the yy-axis. We know that different combinations of the two resources will support different growth rates for the organism in question (this can be individual growth or population growth). Thus, we can join together points (i.e. combinations of resources) with the same growth rates, and these are therefore contours or 'isoclines' of equal growth. In Figure 3.27, line B in each case is an isocline of zero net growth: each of the resource combinations on these lines allows the organism just to maintain itself, neither increasing nor decreasing. The A isoclines, then, with less resources than B, join combinations giving the same negative growth rate; whilst the C isoclines, with more resources than B, join combinations giving the same positive growth rate. As we shall see, the shapes of the isoclines vary with the nature of the resources.
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