Ecology raids the techniques of chemistry

BIOLOGISTS are sometimes accused of physics envy—and there is truth in this accusation. The essential fuzziness of biological systems can never be captured by the precise, mathematically based experiments of something like the Large Hadron Collider. Between physics and biology on the spectrum of fuzziness, though, lies chemistry. And a group of researchers led by Carolyn Nersesian of the University of Sydney has just borrowed one of chemistry's most elegant techniques, titration, to answer a pressing ecological question: how do animals choose where to feed?

Titration, to remind those who dozed through their chemistry lessons, is a way of working out the concentration of a substance in a solution. A reagent of known concentration is dripped (or titrated, to use the term of art) into the unknown solution in the presence of an appropriate indicator molecule (for example litmus, in the case of a reaction between an acid and an alkali). When the tipping point comes, and all of the unknown reagent has reacted, the solution changes colour. The unknown concentration can then be calculated from the amount of known reagent used.

In Dr Nersesian's titration the litmus was a species called the brushtail possum. The two reagents were predators and poisons. In the wild, possums feed mainly on eucalyptus trees. Feeding in trees this way also keeps them safe from ground-based predators, such as foxes. They frequently have to shift from tree to tree in search of non-poisonous leaves, though, because a tree that is being browsed starts manufacturing toxins. Dr Nersesian reasoned that there is probably a measurable point at which a plant becomes so toxic that possums prefer to take their chances with the predators on the ground—and she realised this was a perfect opportunity to do an ecological titration.

As she reports in the Journal of Animal Ecology, she attempted to imitate the distinction between trees and ground by giving eight possums the opportunity to feed in either of two enclosures. In one, cover was created with eucalyptus branches and light levels were kept low, mimicking an arboreal habitat. Here, the food was sometimes spiked with cineole—a poison often found in eucalyptus leaves. In the other enclosure, the food was always untarnished. However, no cover was provided and fox scent, in the form of faeces and urine, was scattered liberally around, mimicking conditions on the ground.

To start with, when the level of cineole was low, possums preferred the first enclosure. But as the drip of the titration went up, from 0% to 1% to 2% to 5% and ultimately to 10% of the food being cineole, their behaviour changed. It was not quite the sudden shift from litmus red to blue of an acid being neutralised by an alkali, but it was not far off.

When the food was toxin-free, the possums spent an average of 40 minutes of every hour eating safely under treelike cover and only 20 minutes in the risky, ground-like enclosure, and that scarcely changed for 1% and 2% cineole. The switch began at 5%, and by the time the cineole level had reached 10% the ratio of time the animals spent in the two enclosures had reversed itself—presumably reflecting, though this remains to be tested, the point at which they change trees in the wild.

Though foxes are of recent introduction in Australia (having been brought for sport by British settlers), many marsupial predators, now extinct, were present before the arrival of man in both his Aboriginal and his European forms, so the ecological system of plants, herbivores and carnivores in the eucalyptus forests would probably not have been that different in the evolutionary past. A neat illustration, then, of co-evolution between three different parts of an ecosystem—and of the value, even in biology, of precise measurement.