Plant invasions:Early and continuing expressions of global change

Abstract

Through prolific, current use and tacit agreement ‘global change’ refers to permanent alteration of the Biosphere as caused by humans. In practice, most emphasis in predicting global change stems from the on-going rise in levels of greenhouse gases in the atmosphere and their myriad environmental consequences (National Academy of Sciences 1988). So defined and practised, it has been treated as a subject with immensely important present and future implications but little past; parts of the Biosphere have been drastically and repeatedly altered, but humans’ historic role has been largely viewed as restricted to non-global alterations (e.g. forest-clearing and burning, diversion of rivers, land-filling and flooding) (Turner et al. 1990). I contend that this conventional view, whether explicitly or implicitly stated, overlooks the role humans have already played in at least one important arena of global change — plant invasions (Drake et al. 1989). Unlike other forms of current or projected global change, humans have altered the composition of vegetation for several thousand years through their deliberate or accidental dispersal of plants beyond their native ranges (di Castri 1989). As I hope to demonstrate, there are important lessons to be gleaned from plant invasions, both as examples of global change in their own right and as phenomena that will complicate (and confound) predictions about the consequences of other forms of global change, including those stemming from increases in levels of greenhouse gases. I concentrate here on four such topics; in my estimation, all are either under- or unappreciated in the current concern about global change, although D’Antonio and Vitousek (1992) provide a related assessment of the role of alien grasses as agents of global change.