ÐÓ°ÉÔ­´´

MY SCIENTIFIC specialty is herbivory, the process of foliage consumption. I could thus be described as a professional leaf detective. The major herbivores are insects – such as beetles and caterpillars – and mammals – koalas, sloths and other vegetarians.

The topic of herbivory is complex. Why do insects prefer some leaves to others? Do they like to feed at different heights in the canopy, or on leaves of a certain age? Do they eat by day or by night? In my opportunistic world of canopy research, I sample many leaves, branches, seasons, years, trees and forests in order to address the question of how herbivory varies over space and time. I have measured nearly 250,000 leaves in my quest to understand defoliation in forests. My colleagues have jokingly appointed me honorary president of the Leaf Lovers Club.

Why is herbivory so important? First, the process whereby insects consume leaves and in turn transfer this energy to the forest floor via their bodies and their frass (droppings) is critical to effective nutrient cycling among the various layers of the forest. Herbivory also creates a catalyst for the production of plant chemicals that defend foliage from animals that would otherwise eat it. Complex relationships have sometimes developed between specific herbivores and their ability to digest the chemicals in a certain type of foliage. This chicken-and-egg scenario is never resolved. Leaves evolve toxins that deter foliage feeders, and herbivores evolve ways to digest those toxins. As ecologists, we become detectives seeking clues to who eats what, what camouflages whom, whose tissue is defended against whom, and which different plant or animal defences are triggered by chemical or physical traits.

I accidentally stumbled on this ecological battleground. When first undertaking my studies of leaves, I never intended to study insects. I only wanted to find out about leaf growth and turnover of photosynthetic material within the canopy. But the fact that creatures kept nibbling on the marked leaves piqued my curiosity.

I began looking at the herbivores in tandem with the leaves and became intrigued by their complex interactions. I also found out that measuring leaves in forest canopies is no simple task. What appeared to be a matter of merely selecting 10 leaves and measuring their defoliation became a complex chronology of birth, growth, survival, struggle and death in the treetops. Most insects eat young leaves, but a few also eat older tissue; some insects feed on lower canopies and others on the uppermost foliage; some herbivores take big bites and others simply suck juices.

In no time I had amassed a data set of over 100,000 leaves. Even in my initial canopy research in Australia, some leaves lived 19 years, extending beyond the duration of my anticipated fieldwork. I faithfully checked all marked leaves throughout their lifetimes each and every month, including when I was pregnant. This long span of data collection showed me that leaf-area losses in forest canopies approach 15 to 25 per cent a year, a fairly high turnover of green material falling from the treetops via insect digestion and becoming part of the soil below. ÐÓ°ÉÔ­´´s measure these processes of energy transfer to understand what keeps forests healthy, just as a doctor measures blood pressure and heart rate.