Passage A

Until recently, conservationists were often complacent about the effect of nonindigenous plant and animal species on the ecosystems they invade. Many shared Charles Elton's view, introduced in his 1958 book on invasive species, that disturbed habitats are most vulnerable to new arrivals because they contain fewer or less vigorous native species. Now, however, ecologists realize that when humans introduce new species into existing ecosystems, even pristine, species-rich habitats are threatened. The rapidly increasing conservation problems and high damage and control costs generated by these invasions merit serious concern.

Invasive plants profoundly affect ecosystems and threaten biodiversity throughout the world. For example, to the untrained eye, the Everglades National Park in Florida appears wild and natural. Yet this and other unique ecosystems are being degraded as surely as if by chemical pollution. In Florida, forests are growing where none existed before. Traditionally, saw grass dominated large regions of Florida's marshes, providing habitat for unique Everglades wildlife. Although saw grass grows over 9 feet tall, introduced Australian melaleuca trees, typically 70 feet tall, now outcompete marsh plants for sunlight. As melaleuca trees grow and form dense stands, their leaf litter increases soil elevations, inhibiting normal water flow. Wildlife associated with saw grass marshes declines. Similarly, in Australia, the introduction of Scotch broom plants led to the disappearance of a diverse set of native reptiles.

Passage B

The real threat posed by so-called invasive species isn't against nature but against humans' ideas of what nature is supposed to be. Species invasion is not a zero-sum game, with new species replacing old ones at a one-to-one ratio. Rather, and with critical exceptions, it is a positive-sum game, in which ecosystems can accept more and more species. Indeed, in both marine and terrestrial ecosystems, ecologists have found that invasions often increase biodiversity at the local level: if you add many new species and lose few or no native species, the overall species count goes up.

Invasions don't cause ecosystems to collapse. Invasions may radically alter the components of an ecosystem, perhaps to a point at which the ecosystem becomes less valuable or engaging to humans. But 50 years of study has failed to identify a clear ecological difference between an ecosystem rich in native species and one chock-full of introduced species. Unlike ecosystem destruction—clear cutting of forests, for example—invasions don't make ecosystems shrink or disappear. They simply transform them into different ecosystems.

When the issue is phrased as one of ecosystem destruction, the stakes are stark: we choose between nature's life and nature's death. In actuality, introduced species present a continuum. A few species do cause costly damage and tragic extinctions. But most plant and animal species simply blend in harmlessly. The issue they present for humans is not whether we will be surrounded by nature but rather what kind of nature we will have around us.