Are Invasive Species the Drivers or Passengers of Change in Degraded Ecosystems?

Abstract

Few invaded ecosystems are free from habitat loss and disturbance, leading to uncertainty whether dominant invasive species are driving community change or are passengers along for the environmental ride. The “driver” model predicts that invaded communities are highly interactive, with subordinate native species being limited or excluded by competition from the exotic dominants. The “passenger” model predicts that invaded communities are primarily structured by noninteractive factors (environmental change, dispersal limitation) that are less constraining on the exotics, which thus dominate. We tested these alternative hypotheses in an invaded, fragmented, and fire‐suppressed oak savanna. We examined the impact of two invasive dominant perennial grasses on community structure using a reduction (mowing of aboveground biomass) and removal (weeding of above‐ and belowground biomass) experiment conducted at different seasons and soil depths. We examined the relative importance of competition vs. dispersal limitation with experimental seed additions. Competition by the dominants limits the abundance and reproduction of many native and exotic species based on their increased performance with removals and mowing. The treatments resulted in increased light availability and bare soil; soil moisture and N were unaffected. Although competition was limiting for some, 36 of 79 species did not respond to the treatments or declined in the absence of grass cover. Seed additions revealed that some subordinates are dispersal limited; competition alone was insufficient to explain their rarity even though it does exacerbate dispersal inefficiencies by lowering reproduction. While the net effects of the dominants were negative, their presence restricted woody plants, facilitated seedling survival with moderate disturbance (i.e., treatments applied in the fall), or was not the primary limiting factor for the occurrence of some species. Finally, the species most functionally distinct from the dominants (forbs, woody plants) responded most significantly to the treatments. This suggests that relative abundance is determined more by trade‐offs relating to environmental conditions (long‐term fire suppression) than to traits relating to resource capture (which should most impact functionally similar species). This points toward the passenger model as the underlying cause of exotic dominance, although their combined effects (suppressive and facilitative) on community structure are substantial.

Impacts of biological invasions: what's what and the way forward

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Alien species in a warmer world: risks and opportunities

“…Alien species can be viewed as drivers and passengers of change in biological communities [69,70]. Many invasive species exert strong impacts on invaded communities and ecosystems [71] and transform ecosystem properties [10], which inevitably leads to changes in biological communities.…”

Seaweed invasions: introduction and scope

“…This sounds self-evident, but several authors have pointed out that, for example, putative claims of the impacts of alien invasive species are often unsupported by data or critical analysis (Gurevitch and Padilla 2004, Didham et al 2005, MacDougall and Turkington 2005, and a recent review concludes there is little evidence that many alien invasive species cause the impacts and problems attributed to them (Bruno et al 2005). While it is clear that some invasive marine species do have large impacts on the structure and dynamics of the systems in which they proliferate (e.g., Nichols et al 1990, Carlton 1996, Shiganova 1998, Daskalov 2002, Ross et al 2003, this review provides an opportunity to carefully examine available evidence of invasion processes and impacts for invasive seaweeds.…”