The nutritional content of plants and animals as food

As a 'package' of resources, the body of a green plant is quite different from the body of an animal. This has a tremendous effect on the value of these resources as potential food (Figure 3.23). 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 fiber 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 to the ratios of approximately 10:1 in bacteria, fungi and animals. Unlike plants, animal tissues contain no structural carbohydrate or fiber component but are rich in fat and, in particular, protein.

The various parts of a plant have very different compositions (Figure 3.23) 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' specialize 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 specialized 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 the herbivore, the finer is the scale of heterogeneity of saprotrophs, predators, grazers and parasites specialists and generalists

C : N ratios in animals and plants different plant parts represent very different resources...

To midgut

Food being sucked from phloem

Vein in leaf

Leaf section

To midgut

Food being sucked from phloem

Vein in leaf

Leaf section

Aphids And Phloem

HH Stylet track with stylet Empty stylet tracks

Figure 3.22 The stylet of an aphid penetrating the host tissues and reaching the sugar-rich phloem cells in the leaf veins. (a) Aphid mouthparts and cross-section of a leaf. (b) A stylet, showing its circuitous path through a leaf. (After Tjallingii & Hogen Esch, 1993.)

the plant on which it may specialize. Extreme examples can be found in the larvae of various species of oak gall wasps, some of which may specialize on young leaves, some on old leaves, some on vegetative buds, some on male flowers and others on root tissues.

Although plants and their parts may differ widely in the resources they offer to potential consumers, 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 parceled (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 utilize 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. This is the realm of ecological stoichiometry (Elser & Urabe 1999): the analysis of constraints and consequences in ecological interactions of the mass balance of multiple chemical elements (particularly the ratios of carbon to nitrogen and of carbon to phosphorus -see Sections 11.2.4 and 18.2.5). The main waste products of organisms that consume plants are carbon-rich compounds: CO2, fiber, and in the case of aphids, for example, carbon-rich honey-dew 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 differential in C : N ratios between plants and microbial decomposers also means that the long-term effects of CO2 enhancement (see Section 3.3.4) are not as straightforward as might be imagined (Figure 3.24): that is, it is not necessarily the case that plant

... but the composition of all herbivores is remarkably similar

C : N ratios and the effects of CO2 enhancement

Wood

Bark

Softwood

Wood

Softwood

Hardwood
Buckwheat

Softwood

Hardwood

Seeds

Brazil nut

Bark

Softwood

Hardwood

Petiole Leaf

Cabbage

Phloem sap

( Yucca flaccida)

Lettuce

Fruit

Plum

Fruit

Plum

Lettuce

Fungus

(Agaricus campestris)

Tuber

Mung bean

Mung bean

Kidney

Sesame

Kidney

Heart

Sesame

Potato

Tuber

Potato

Fish

Heart

Catfish

Fish

Catfish

Minerals

Fat

Carbohydrate

Fiber

Protein

Xylans and other wood chemicals

Shrimp

Goose

Liver

T-bone

T-bone

Shrimp

Figure 3.23 The composition of various plant parts and of the bodies of animals that serve as food resources for other organisms. (Data from various sources.)

Figure 3.24 Potential positive and negative feedback from elevated CO2 concentrations to plant growth, to microbial activity and back to plant growth. The arrows between descriptors indicate causation; the black arrows alongside descriptors indicate increases or decreases in activity. The dashed arrow from elevated [CO2] to plant growth indicates that any effect may be absent as a result of nutrient-limitation. (After Hu et al., 1999.)

Time

Time

Figure 3.24 Potential positive and negative feedback from elevated CO2 concentrations to plant growth, to microbial activity and back to plant growth. The arrows between descriptors indicate causation; the black arrows alongside descriptors indicate increases or decreases in activity. The dashed arrow from elevated [CO2] to plant growth indicates that any effect may be absent as a result of nutrient-limitation. (After Hu et al., 1999.)

biomass is increased. If the microbes themselves are carbon-limited, then increased CO2 concentrations, apart from their direct effects on plants, might stimulate microbial activity, making other nutrients, especially nitrogen, available to plants, further stimulating plant growth. Certainly, short-term experiments have demonstrated this kind of effect on decomposer communities. On the other hand, though, decomposers may be nitrogen-limited, either initially or following a period of enhanced plant growth during which nitrogen accumulates in plant biomass and litter. Then, microbial activity would be depressed, diminishing the release of nutrients to plants and potentially preventing their enhanced growth in spite of elevated CO2 concentrations. These, though, are longer term effects and to date very few data have been collected to detect them. The more general issue of local and global 'carbon budgets' is taken up again in Section 18.4.6.

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