Abstract
The cumulative ecological risk from invasions is assumed to increase as more species invade. However, we propose that cumulative risk can saturate when multiple, functionally similar species are present, limiting the marginal contribution of newcomers. We explored this idea by quantifying the spatial footprint of climatic suitability for 12 fire-promoting grasses in Hawaii using species distribution models, using the total climatically suitable area as a landscape-scale measure of fire risk. We examined how the risk footprint changed under current and future climates and by modeling well-established invaders versus incipient invaders. We found that well-established species already make most of the islands suitable for invasion, and neither climate change nor further invasion by incipient species expands the risk footprint by more than 10%. The contribution of each successive species to the cumulative risk declines steeply, revealing a saturating risk accumulation curve. This suggests that, in heavily invaded systems, preventing new invasions may yield diminishing returns unless established invaders are suppressed simultaneously. We propose that risk saturation may be an emerging feature of multi-species invasions, reflecting the same redundancy-driven saturation documented in biodiversity–ecosystem function studies.