Canopy-forming ecosystem engineers are commonly assumed to increase local species richness and diversity. However, such effects might depend on the environmental context. We tested this notion by investigating the canopy effects of the same macroalgal species on understory communities along rocky intertidal elevation gradients. Because of tides, such gradients exhibit a pronounced increase in environmental stress from low to high elevations. Through a manipulative experiment done in Nova Scotia (Canada), we created canopy (100 % cover) and no-canopy (0 % cover) treatments at the low, middle, and high intertidal zones and removed all understory species from replicate quadrats, aiming to see the effects on richness and diversity after one year of recolonization. Through a mensurative study, we determined the natural relationship between canopy cover (0-100 %) and understory species richness and diversity for those three elevation zones by surveying pristine intertidal areas that summarized years of canopy effects. We predicted that canopy effects would change from being small or neutral at low elevations to significant and positive at high elevations.
Results/Conclusions
For the experiment, after recolonization, species richness and diversity (considering all algae and invertebrates) were higher under full canopy cover than in the absence of canopies at the high and middle intertidal zones, but no effects occurred at the low zone. Similarly, species composition (which considers the identity of species in addition to their relative abundance) was affected by canopies at the high and middle zones, but not at the low zone. The mensurative study yielded the same results, as richness and diversity increased with canopy cover at the high and middle zones (approaching stabilization towards high cover values), but no effects occurred at the low zone. The lack of canopy effects at low elevations is related to locally mild habitat conditions, which canopies are unable to modify, while the positive effects at higher elevations relate to the capacity of canopies to ameliorate conditions in understory habitats during low tides. This is the first time that a combined experimental and mensurative approach shows that the same bioengineer species affect overall species richness, diversity, and composition differently along an environmental stress gradient. Overall, protecting canopy-forming bioengineers to preserve biodiversity should be most effective in stressful habitats.