Working Through Ecosystem Problems
The hard questions in this chapter are rarely about a fact you have not met. They are about applying a small number of methods cleanly. Each method below is stated once and then used.
1. Productivity algebra. The single identity is GPP - R = NPP. Given any two of the three quantities, the third follows. Keep the meanings distinct: GPP is the total rate of organic matter produced during photosynthesis; R is the amount consumed by the plants themselves in respiration; NPP is what remains, and it is the biomass actually available to heterotrophs. Secondary productivity - the rate of formation of new organic matter by consumers - is a separate quantity and does not appear anywhere in this identity. A consumer's own budget is worked out on its own terms: of what an animal ingests, the part voided as faeces never enters its body economy at all, and part of what is assimilated is spent in respiration. Only what is left is laid down as new tissue.
2. Chaining the ten per cent law. Count transfers, not levels. Producer to primary consumer is one transfer; producer to secondary consumer is two; producer to tertiary consumer is three. So the energy available at the nth trophic level is the producer value multiplied by 0.1 raised to the power (n - 1). Using the level number itself as the exponent gives an answer that is out by a factor of ten - it is the most common arithmetic slip in this chapter.
3. Reading unit expressions. Settle first whether the quantity named is a rate or an amount held at one instant, then check that the unit agrees with it. A rate has to carry a time term; an amount does not. Nothing else in the expression decides the matter - the same substance, the same area basis and the same energy or mass unit turn up in both kinds - and brackets are only typography, so (kcal m-2) yr-1 and kcal m-2 yr-1 are one unit written two ways. Read the time term first and the rest of the unit second.
4. Deciding whether a pyramid inverts. Ask what the tiers are measuring - a stock or a flow. Numbers and biomass are stocks held at an instant, and a fast-turning-over stock can be smaller than what it supports, so those pyramids may invert. Energy in a pyramid of energy is a flow per unit time, and a flow shrinks at every transfer, so that pyramid stays upright.
5. Placing a species in a web. In a food web a species occupies as many trophic levels as it has feeding modes. A fox eating berries is feeding as a primary consumer; the same fox eating a rabbit is feeding as a secondary consumer. Assign the level from the meal, not from the animal.
6. Comparing decomposition rates. Two independent levers control the rate. The first is substrate chemistry, the composition of the detritus itself. The second is climate, the temperature of the site together with its water and oxygen status. Both are set out in the decomposition set, and both must be checked before any comparison is answered. Find out which lever the question has held constant, and reason about the other. When both levers move at once do not assume the substrate settles it: the two can pull in opposite directions, and only the evidence in front of you decides which one has prevailed.
7. Separating the paired terms. Four pairs in this chapter are routinely swapped for one another.
Detritivore and decomposer - a detritivore such as an earthworm ingests detritus and breaks it into smaller particles; a decomposer secretes enzymes onto the substrate, digests it outside the body and then absorbs the products.
Standing crop and standing state - standing crop is the living biomass present in a trophic level at a given time; standing state is the amount of inorganic nutrients present in the soil at a given time.
Production and productivity - production is an amount, productivity is a rate, and the unit tells you which is meant.
Grazing food chain and detritus food chain - a GFC begins at a living producer, a DFC begins at dead organic matter, and the share of an ecosystem's energy that each of them carries is not the same in every ecosystem. The two are connected: organisms of the DFC are prey to some GFC animals.