Abstract
Foundation species, such as kelp, exert disproportionately strong community effects and persist, in part, by dominating taxa that inhibit their regeneration. Human activities which benefit their competitors, however, may reduce stability of communities, increasing the probability of phase-shifts. We tested whether a foundation species (kelp) would continue to inhibit a key competitor (turf-forming algae) under moderately increased local (nutrient) and near-future forecasted global pollution (CO2). Our results reveal that in the absence of kelp, local and global pollutants combined to cause the greatest cover and mass of turfs, a synergistic response whereby turfs increased more than would be predicted by adding the independent effects of treatments (kelp absence, elevated nutrients, forecasted CO2). The positive effects of nutrient and CO2 enrichment on turfs were, however, inhibited by the presence of kelp, indicating the competitive effect of kelp was stronger than synergistic effects of moderate enrichment of local and global pollutants. Quantification of physicochemical parameters within experimental mesocosms suggests turf inhibition was likely due to an effect of kelp on physical (i.e. shading) rather than chemical conditions. Such results indicate that while forecasted climates may increase the probability of phase-shifts, maintenance of intact populations of foundation species could enable the continued strength of interactions and persistence of communities.
Highlights
A few strong interactions often contribute disproportionately to maintaining the composition and function of an ecosystem by modifying both the physical conditions and species interactions within [1,2,3]
The inhibition of competitors associated with contrasting physical conditions and species interactions, enhances the stability of systems centered on these foundation species [5]
A synergistic interaction occurred between the simultaneous effects of kelp loss and multiple pollutants (i.e. CO2 and nutrients), with these treatments resulting in percentage covers (Figure 2; mean 6 SE; 69.2565.88%) which cannot be predicted from the independent effects of kelp in the absence of elevated CO2 and nutrients, future CO2 in the absence of kelp and elevated nutrients and elevated nutrients in the absence of kelp and elevated CO2
Summary
A few strong interactions often contribute disproportionately to maintaining the composition and function of an ecosystem by modifying both the physical conditions and species interactions within [1,2,3]. Altered land use and ensuing discharges to the marine environment elevate nutrient concentrations at local scales, with the extent of change ranging from strong enrichment in urban areas to little or no change in agricultural and natural systems [8,9,10]. These waters will absorb approximately 30 percent of the atmospheric CO2 produced by human populations globally, leading to gradual ocean acidification [11,12]. While there is recent recognition that these alterations of the physical environment will affect species interactions [13,14,15,16] experiments to date have not progressed sufficiently to identify how they will affect biological communities dominated by foundation species such as kelp
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