Thermal stress induces persistently altered coral reef fish assemblages

Abstract

Ecological communities are reorganizing in response to warming temperatures. For continuous ocean habitats this reorganization is characterized by large‐scale species redistribution, but for tropical discontinuous habitats such as coral reefs, spatial isolation coupled with strong habitat dependence of fish species imply that turnover and local extinctions are more significant mechanisms. In these systems, transient marine heatwaves are causing coral bleaching and profoundly altering habitat structure, yet despite severe bleaching events becoming more frequent and projections indicating annual severe bleaching by the 2050s at most reefs, long‐term effects on the diversity and structure of fish assemblages remain unclear. Using a 23‐year time series spanning a thermal stress event, we describe and model structural changes and recovery trajectories of fish communities after mass bleaching. Communities changed fundamentally, with the new emergent communities dominated by herbivores and persisting for >15 years, a period exceeding realized and projected intervals between thermal stress events on coral reefs. Reefs which shifted to macroalgal states had the lowest species richness and highest compositional dissimilarity, whereas reefs where live coral recovered exceeded prebleaching fish richness, but remained dissimilar to prebleaching compositions. Given realized and projected frequencies of bleaching events, our results show that fish communities historically associated with coral reefs will not re‐establish, requiring substantial adaptation by managers and resource users.

Changing role of coral reef marine reserves in a warming climate

“…5b, c) [170, 312]). Gains in herbivorous reef fish following coral mortality have been documented before 6,22 , and here we demonstrate the role marine reserves play in greatly enhancing these fish groups in novel postdisturbance coral reef configurations 2 .…”

“…The recovering reefs were characterised by increases in keystone structure provided by branching corals, which favours survivorship and increases in small-sized fishes and small-size classes of larger species 28 . Further, the increases in biomass of scraping herbivores and mixed-diet carnivores described above, also resulted in increases in species richness for these groups 22 . In sum, while the expectation 9 of reserves enhancing fish biomass was generally maintained post-bleaching, their role in enhancing species richness relative to fished reefs was only retained on macroalgal-dominated reefs.…”

“…Following the 1998 bleaching event, Seychelles' reefs underwent one of two well-documented trajectories. While 12 of the 21 reefs surveyed recovered to hard coral-dominance, the other nine underwent regime shifts to fleshy macroalgae 5,22 . These post-bleaching trajectories are apparent on both fished and protected reefs, with no difference in cover of coral or macroalgae between marine reserves and fished reefs in the years post-bleaching ( Fig.…”

Marine Heatwave Hotspots in Coral Reef Environments: Physical Drivers, Ecophysiological Outcomes, and Impact Upon Structural Complexity

“…Similarly, herbivory is a core driver of reef recovery (Bellwood et al, 2006;Graham et al, 2011) and is promoted by structural complexity (Vergés et al, 2011). Indeed, a loss of structural complexity can effectively predict the probability of a coral-algal phase shift following mass mortality (Graham et al, 2015), as well as the long term recovery trajectory of coral populations (Robinson et al, 2019a) in part due to concurrent changes in the composition of fish communities (Graham et al, 2015;Robinson et al, 2019b) and the level of larval recruitment (Gilmour et al, 2013;Graham et al, 2015).…”

Bleaching-driven reef community shifts drive pulses of increased reef sediment generation

“…The global coral bleaching event of 2014-2017 had widespread impacts on coral reefs [1]. These impacts included extensive coral mortality over large spatial scales [2] and at some sites to depths of in excess of 20 m [3]; associated shifts in coral community assemblages resulting from selective coral mortality [4] and alterations to reef-associated fish communities [5,6]. The net effects of bleaching have thus been significant, leading to changes in abundance of species that contribute to reef ecological structure and diversity.…”