Emissions of methane (CH4), over time, at sites in northern Sweden at various stages of thawing from permafrost, as indicated. Bars are SEs. (b) The isotopic signatures of those methane emissions, δ13C‐CH4, measured as the relative difference in the ratio of 13C to 12C in the methane, compared with an international standard material, expressed as parts per thousand. Bars are SEs. (c) The composition of the microbial community in each case as inferred from those isotopic signatures, subdivided into bacteria and Archaea further subdivided into hydrogenotrophic and acetoclastic methanogens, and ‘others’.
Source: After McCalley et al. (2014).
3.7 Organisms as food resources
predators, grazers and parasites
‘True’ predators predictably kill their prey. Examples include a mountain lion consuming a rabbit but also consumers that we may not refer to as predators in everyday speech: a water flea consuming phytoplankton cells, a squirrel eating an acorn (both herbivorous predators), and even a pitcher plant drowning a mosquito. Grazing can also be regarded as a type of predation, but the food (prey) organism is not killed. Only part of the prey is taken, leaving the remainder to live on with the potential to reproduce or regenerate. Also, grazers feed on (or from) many prey during their lifetime. Cattle and sheep are grazers of plants, but blood‐sucking flies, for example, are carnivorous grazers. True predation and grazing are discussed in detail in Chapter 9. Parasitism, too, is a form of predation in which the consumer usually does not kill its food organism, but unlike a grazer, a parasite feeds from only one or a very few host organisms in its lifetime. Chapter 12 is devoted to parasitism.
specialists and generalists
An important distinction amongst animal consumers is whether they are specialised or generalised in their diet. Generalists (polyphagous species) take a wide variety of prey species, though they very often have clear preferences and a rank order of what they will choose when there are alternatives available. Some specialists consume only particular parts of their prey though they range over a number of species. This is most common among herbivores because, as we saw in Figure 3.26 and shall see again in Figure 3.29, different parts of plants are quite different in their composition. Thus, many birds specialise in eating seeds though they are seldom restricted to a particular species. Other specialists, however, may feed on only a narrow range of closely related species or even just a single species (when they are said to be monophagous). Examples are caterpillars of the cinnabar moth (which eat the leaves, flower buds and very young stems of species of ragwort, Senecio) and many species of host‐specific parasites.
Figure 3.29 The composition of various plant parts and of the bodies of animals that serve as food resources for other organisms.
Data from various sources.
the importance of lifespan
Many of the resource‐use patterns found among animals reflect the different lifespans of the consumer and what it consumes. Individuals of long‐lived species are likely to be generalists: they cannot depend on one food resource being available throughout their life. Specialisation is increasingly likely if a consumer has a short lifespan. Evolutionary forces can then shape the timing of the consumer’s food demands to match the timetable of its prey. Specialisation also allows the evolution of structures that make it possible to deal very efficiently with particular resources. This is especially the case with mouthparts. Darwin’s hawkmoth, Xanthopan morganii praedicta, with its 20 cm long proboscis, is alone in being able to take nectar and pollen from the Madagascan orchid, Angraecum sesquipidale, with its near‐30 cm long nectary. (It is called Darwin’s hawkmoth, because Charles Darwin predicted its existence on seeing the flower 20 years before the moth itself was discovered.) This can be interpreted as an exquisite product of the evolutionary process that has given the moth access to a valuable food resource – or as an example of the ever‐deepening rut of specialisation that has constrained what the moth can feed on. The more specialised the food resource required by an organism, the more it is constrained to live in patches of that resource or to spend time and energy in searching for it among a mixture of resources. This is one of the costs of specialisation. We return to food preferences and diet widths in Section 9.2.
3.7.1 The nutritional contents of plants and animals and their extraction
C : N ratios in animals and plants
As a ‘package’ of resources, the body of a green plant is quite different from the body of an animal. This has a marked effect on the value of these resources as potential food (Figure 3.29). The most important contrast is that plant cells are bounded by walls of cellulose, lignin and/or other structural materials. It is these cell walls that give plant material its high fibre content. The presence of cell walls is also largely responsible for the high fixed carbon content of plant tissues and the high ratio of carbon to other important elements. For example, the carbon : nitrogen (C : N) ratio of plant tissues commonly exceeds 40 : 1.
In contrast, the C : N ratios in bacteria, fungi and animals are approximately 10 : 1. When plant parts are decomposed, the microbes multiplying on the decaying plant withdraw nitrogen and other mineral resources from their surroundings and build them into their own microbial bodies. Thus, plant material with a high carbon content is converted to microbial bodies with a relatively low carbon content. For this reason, and because microbial tissue is more readily digested and assimilated, plant detritus that has been richly colonised by microorganisms is generally preferred by detritivorous animals. Unlike plants, animal tissues contain no structural carbohydrate or fibre component but are rich in fat and, in particular, protein.
different plant parts represent very different resources …
The various parts of a plant, however, have very different compositions (Figure 3.29) and so offer quite different resources. Bark, for example, is largely composed of dead cells with corky and lignified walls and is quite useless as a food for most herbivores (even species of ‘bark beetle’ specialise on the nutritious cambium layer just beneath the bark, rather than on the bark itself). The richest concentrations of plant proteins (and hence of nitrogen) are in the meristems in the buds at shoot apices and in leaf axils. Not surprisingly, these are usually heavily protected with bud scales and defended from herbivores by thorns and spines. Seeds are usually dried, packaged reserves rich in starch or oils as well as specialised storage proteins. And the very sugary and fleshy fruits are resources provided by the plant as 'payment' to the animals that disperse the seeds. Very little of the plants’ nitrogen is spent on these rewards.
The dietary value of different tissues and organs is so different that it is no surprise to find that most small herbivores are specialists – not only on particular species or plant groups, but on particular plant parts: meristems, leaves, roots, stems, etc. The smaller
