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In The Coal Question (1865), the economist William Stanley Jevons advanced a claim that still strikes many readers as counterintuitive: improvements in the efficiency with which coal is used would not reduce Britain’s consumption of coal but increase it. The reasoning is that greater efficiency lowers the effective cost of the work coal performs, and cheaper work invites new uses: machines that were once too costly to run become profitable, and activities that once relied on other sources of power switch to coal.

Modern economists call this phenomenon the rebound effect, and they generally agree that some rebound accompanies most gains in efficiency. What remains disputed is its size. Jevons’s own claim was stronger: that the rebound would exceed the original savings, so that consumption would actually rise. That outcome occurs only under particular conditions, chiefly when demand for the service in question is highly responsive to its price. It would therefore be a mistake either to dismiss energy-efficiency policies as self-defeating or to assume that they will deliver the full savings that engineering estimates predict.

Many tree species, including oaks and beeches, reproduce in a pattern that ecologists call masting: in most years a population produces few seeds, but every few years nearly all of its members fruit heavily at once. Because a tree that fruited when its neighbors did not would seem to gain an advantage, facing less competition for the animals that disperse its seeds, the synchrony itself demands explanation.

The best-known account, the predator-satiation hypothesis, holds that masting starves seed predators in lean years, keeping their populations small, and then overwhelms them in mast years with more seeds than they can consume. A second account emphasizes pollination: for wind-pollinated species, flowering in synchrony greatly increases the chance that pollen will reach a receptive flower, so heavy, coordinated flowering yields disproportionately more fertile seed. Consistent with this second account, masting is, on average, more pronounced in wind-pollinated species than in species pollinated by animals.

It is tempting to treat these accounts as rivals and to ask which one is correct. Yet the two are not mutually exclusive, and evidence for one does not count against the other. A species may have begun to mast because synchronous flowering improved pollination and only later come to depend on satiating predators whose own life cycles had adjusted to its rhythms. The more productive question is not which mechanism explains masting but how much each contributes in a given species, a question that only long-term studies of particular populations can answer.