Since the 1880s, there have been blue gum eucalyptus trees growing on San Francisco’s Mount Sutro, which lies just south of Golden Gate Park. Recently, the University of California, San Francisco, which owns most of Mount Sutro, has been trying to thin the dense eucalyptus forest. The reason is fire control — eucalyptus trees are “fire intensive,” shedding a lot of debris and burning with unusual volatility. But the effort to cull the Mount Sutro forest has been met with strident protest by residents who want to see the eucalyptus left untouched.

By the standard of the California Native Plant Society, eucalyptus, which were brought from Australia, are officially nonnative trees because they were introduced after the first European contact with the New World. But the trees on Mount Sutro have been there within the memory of every living San Franciscan, and to the generations who have grown up within view of them, it seems almost perverse to insist that they are aliens.

To keep a clear distinction between native and nonnative species requires nearly geologic memory. But humans, like most species, don’t live in the past, where the distinction originates. In the present, the difference is largely immaterial. Native or nonnative, California’s eucalyptus trees, like the starlings of Central Park, have come to seem original just because they predate us. Of course, the vast majority of nonnative species have not been intentionally introduced, as the Mount Sutro eucalyptus were, but have been distributed accidentally, unnoticed baggage in the wanderings of our species.

Some species — invasive ones like kudzu, Japanese knotweed, rabbits and rats — find almost unlimited room for expansion in their new environs, often overwhelming native species. But not all introduced species are invasive, and pose a threat only when they outcompete native species.