Interactions between alien species and restoration of large-river ecosystems.

The large-river ecosystems that are the subject of restoration efforts also typically are heavily invaded by alien species. We use our experience on the Hudson River estuary, New York (USA), to illustrate some important interactions that link alien species and river restoration. Most obviously, restoration may be aimed at controlling populations of undesirable alien species. Alien species like common reed (Phragmites communis), water-chestnut (Trapa natans), and purple loosestrife (Lythrum salicaria) have been the targets of eradication programs in the Hudson. Such eradication efforts sometimes are poorly motivated, have non-target effects, have had mixed success in controlling the target species, or are entirely infeasible. Second, the presence of alien species may complicate restoration programs designed to address other goals. The presence of alien species may limit the range of available options, make the results of restoration activities more difficult to predict, and make it difficult to define or reach reference conditions. Finally, restoration programs may bring in or favor alien species. For example, materials used in restoration projects may be contaminated with the propagules of alien species. Consequently, restoration projects in large rivers often are intertwined with alien species in one way or another.