the plant on which it may specialise. Extreme examples can be found in the larvae of various species of oak gall wasps, some of which may specialise on young leaves, some on old leaves, some on vegetative buds, some on male flowers and others on root tissues.
… but the composition of all herbivores is remarkably similar
By contrast, the composition of the bodies of different herbivores is remarkably similar. In terms of the content of protein, carbohydrate, fat, water and minerals per gram, there is very little to choose between a diet of caterpillars, cod or venison. The packages may be differently parcelled (and the taste may be different), but the contents are essentially the same. Carnivores, then, are not faced with problems of digestion (and they vary rather little in their digestive apparatus), but rather with difficulties in finding, catching and handling their prey (see Chapter 9).
Differences in detail aside, herbivores that consume living plant material – and saprotrophs that consume dead plant material – all utilise a food resource that is rich in carbon and poor in protein. Hence, the transition from plant to consumer involves a massive burning off of carbon as the C : N ratio is lowered. The main waste products of organisms that consume plants are carbon‐rich compounds: CO2, fibre, and in the case of aphids, for example, carbon‐rich honeydew dripping from infested trees. By contrast, the greater part of the energy requirements of carnivores is obtained from the protein and fats of their prey, and their main excretory products are in consequence nitrogenous. The crucial role of gut microbes in digestion for all animals, but for herbivores especially, is discussed in Section 13.6.
ecological stoichiometry
These ratios in the composition of organisms, and the changes in these ratios from one trophic level to the next, are the realm of ecological stoichiometry (Hessen et al., 2013), defined as ‘The balance of multiple chemical substances in ecological interactions and processes, or the study of this balance’ (Sterner & Elser, 2002) – see also Sections 11.2.5 and 20.4.3. The approach can be traced back to what is commonly called Liebig’s ‘law of the minimum’ (Liebig, 1840), which states that organismal growth rates are limited by whichever element has the lowest rate of environmental supply relative to the organisms’ demands. A more direct connection still can be traced to the work of Alfred C. Redfield going back to the 1930s, focusing especially on the apparent constancy of the ratio of nitrate to phosphate (around 16 : 1) in the biomass of phytoplankton and in seawater (Gruber & Deutsch, 2013). Indeed, ecological stoichiometry has always been most influential in aquatic, especially marine biogeochemistry, where the focus is usually on nitrogen, phosphorus and carbon, though other elements, for example iron, also have key roles to play (Tagliabue et al., 2017).
cellulases, which most animals lack
The large amounts of fixed carbon in plant materials mean that they are potentially rich sources of energy. Other components of the diet (e.g. nitrogen) are more likely to be limiting. Yet most of that energy is only directly available to consumers if they have enzymes capable of mobilising cellulose and lignins. An increasing number of species, especially insects, have been shown to have these enzymes themselves (Watanabe & Tokuda, 2010), but the overwhelming majority of species in both the plant and animal kingdoms lack them, the latter relying instead on the cellulases produced by gut‐inhabiting, cellulolytic prokaryotes with which they form intimate, ‘mutualistic’ relationships, discussed further, for both vertebrate and invertebrate herbivores, in Chapter 13.
Because most animals lack cellulases, the cell wall material of plants hinders the access of digestive enzymes to the contents of plant cells. The acts of chewing by the grazing mammal, cooking by humans and grinding in the gizzard of birds allow digestive enzymes to reach cell contents more easily. The carnivore, by contrast, can more safely gulp its food.
Of course, one big difference between the resources of autotrophs and those of heterotrophs, at least those that consume living prey, is that the resources of the heterotrophs can fight back – on both ecological and evolutionary timescales. We pick up the story of prey defence in Chapter 9.
3.8 A classification of resources, and the ecological niche
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 these 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 replaced 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 3.30).
Figure 3.30 Resource‐dependent growth isoclines. Each of the growth isoclines represents the amounts of two resources (R1 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 – has a negative growth rate, isocline zero a zero growth rate and isocline + a positive growth rate. In the respective figures, resources are (a) essential, (b) perfectly substitutable, (c) complementary, (d) antagonistic and (e) inhibitory.
Source: After Tilman (1982).
zero net growth isoclines
In this classification, the concentration or quantity of one resource is plotted on the x‐axis, and that of the other resource on the y‐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 3.30, line ‘zero’ 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 ‘−’ isoclines, then, with less resources than the zero line, join combinations giving the same negative growth rate; whilst the ‘+’ isoclines, with more resources than the zero line, join combinations giving the same positive growth rate. As we shall see, the shapes of the isoclines vary with the nature of the resources.
3.8.1
