Turf wars: Competition between foundation and turf-forming species on temperate and tropical reefs and its role in regime shifts

Shifts in competitive balance between key functional groups may drive regime shifts in tropical and temperate marine ecosystems. On shallow reefs, regime shifts increasingly involve changes from spatial dominance by foundation species (e.g. reefbuilding corals, canopy-forming algae) to dominance by turf-forming algae differing in structural complexity. To disentangle competitive inter actions fromother processes that may contribute to these shifts, we conducted a global meta-analysis of manipulative competition experiments between foundation and turf-forming species. Canopy- forming algae had consistently negative effects on abundance of turfforming algae, particularly on subtidal reefs, but with a tendency towards larger effects on delicate filamentous forms compared to articulated coralline and corticated/coarsely branching turf. Competitive effects of turf-forming algae on canopy species were limited to early life-history stages, and similarly varied between turf functional groups and between subtidal and intertidal reefs. Conversely, shorter filamentous turf assemblages typical of tropical reefs had no significant effect on settlement and survival of coral larvae. Interactions between turf-forming algae and established coral colonies were negative overall, but variable in magnitude. Mean effect sizes indicated that corals suppress turf abundance, but not vice versa. However, turf-forming algae significantly im - pacted coral growth and tissue mortality. We suggest reefs with extensive cover of foundation species are resistant to proliferation of turf algae, but competition will inhibit recovery of reefs following disturbances that enable turf algae to establish. Therefore, competitive effects of foundation and turf-forming species must be accounted for to effectively evaluate the stability of these undesirable regime shifts and recovery potential under alternative climate and management scenarios.