But if we set a date thousands of years back, safely before any humans arrived, we run into another problem. Ecosystems are always changing, whether humans are involved or not. Ancient forests with trees thousands of years old may feel timeless to us. We are a short-lived species with a notoriously bad grasp of timescales longer than a few of our own generations. But from the point of view of a geologist, a paleoecologist, or some other expert with "deep time eyes," ecosystems are in a constant dance, as their components compete, react, evolve, migrate, and form new communities. Geological upheaval, evolution, climatic cycles, fire, storms, and population dynamics see to it that nature is always changing. On Hawaii, volcanic activity wipes the slate clean on any given slope every few hundred years, and occasional new arrivals to the islands, washed ashore or drifting in on the wind, adapt to their new home and find a place for themselves in its ecosystems.

Once we pick a date from amid this muddle, another problem emerges. Even when we use all the scientific tools available to look backward in time, from fossil pollen records to the climate information enshrined in tree rings, we don't always know what places looked like thousands or even hundreds of years ago.

The final and perhaps most vexing issue with prehuman baselines is that they are increasingly impossible to achieve--either through restoration or management of wild areas. Every ecosystem, from the deepest heart of the largest national park to the weeds growing behind the local big-box store, has been touched by humans. We have stirred the global pot, moved species around, turned up the thermometer, domesticated a handful of plants and animals, and driven extinct many more. We have definitively changed the entire planet, and it is getting increasingly difficult to undo all these changes at any one place.

In Hawaii, the lush tropical plants out the hotel window looked gorgeous, but I knew that many of them had been introduced by people and were now considered a threat to the native species. I also knew that Hawaii has been called "the extinction capital of the world," and that a whole list of beautiful birds are either gone or near gone. And yet the islands are thick with conservationists who have not given up on the ideal of Hawaii as it once was.

My first stop was a group of experimental field plots testing the feasibility of restoring lowland forests on the Big Island's wet side. The plots are hidden in a forest on the Hawaii Army National Guard Keaukaha Military Reservation. Growing on flat land with plenty of rain, most forests of this type had been cleared for agriculture. What was left, or what grew back, is now dominated by plants from places other than Hawaii.

Rebecca Ostertag of the University of Hawaii at Hilo explained why these "invaders" are so prevalent on Hawaii. Hawaiian plants, having evolved in isolation for up to 30 million years, generally grow slowly and use resources less efficiently than continental plants, which evolved with more competition. Similarly, Hawaiian birds and animals are mostly helpless against introduced diseases. Avian malaria has knocked off many bird species; there were no mosquitoes on the islands until recently, so birds there never evolved any defenses to the mosquito-borne disease. Hawaiian raspberries and roses have even lost their thorns, and Hawaiian mints their minty defense chemicals, because there were no plant-eating animals around to fend off. Such mellow Hawaiian species are pushovers for the scrappier mainland species that humans brought to the islands. Today half of the plants in Hawaii are nonnative. In many lowland forests only the large trees are native; under them grows a carpet of introduced seedlings, just waiting for the day the giant natives fall. Some ecologists call such places "forests of the living dead."