The Role of Exotic Marine Ecosystem Engineers

Of all invader impacts, those likeliest to have the most wide-reaching consequences are alterations to ecosystems, as they can essentially “change the rules of existence” for broad suites of resident biota (Vitousek 1990). One often-considered class of ecosystem-level effects of exotics is the disruption of energy or material fluxes (Chap. 17, Grosholz and Ruiz). For example, the initiation of trophic cascades, which can be trig gered by events such as the invasion of new predators, can dramatically alter energy flow within ecosystems (e.g., Spencer et al. 1991). Similarly, the cycling of nutrients through biogeochemical pathways can be affected by exotics (e.g., Larned 2003). This can occur through the invasion of species that differ from natives in their utilization of nutrients, such as when nitrogen-fixing plants invade nitrogen-poor soils (Vitousek et al. 1987). In addition to directly affecting the cycling of energy or nutrients, exotics also can alter the actual physical or chemical nature of the ecosystem itself. Such organisms have been called ecosystem engineers (Jones et al. 1994, 1997).

Reduced to its essence, ecosystem engineers affect other biota via alterations to the abiotic environment (Fig. 16.1). These species create, destroy, or otherwise modify habitats, and thereby affect resources or stressors (e.g., living space, sedi ment, and ambient temperature) that affect other organisms (Jones et al. 1994; Crooks 2002). The beaver is the classic example of an ecosystem engineer. By cre ating dams out of trees, beavers dramatically change the nature of the ecosystem by converting forests to ponds, thereby benefiting aquatic species at the expense of ter restrial ones. Beavers have effects beyond pond creation, however. The trees they fell are themselves ecosystem engineers that create shade, provide structure for nests, and dampen winds, and the loss of these engineering functions alters forest areas.