Coral rubble dynamics in the Anthropocene and implications for reef recovery

Abstract

With rubble predicted to increase on coral reefs worldwide, we review the physical, biological, and ecological dynamics of rubble beds, with a focus on how rubble generation, mobilization, binding, and coral recruitment is expected to change on future reefs. Major disturbances, including storms and coral bleaching, are predicted to increase in intensity and frequency, and-like localized impacts including blast fishing and ship groundingsgenerate large quantities of coral rubble. Reefs will have increasingly smaller recovery windows between successive disturbances, leading to persistence of unstable rubble beds on reefs. With more severe storms and increased bioerosion on future reefs, rubble mobilization thresholds will be met more often as smaller, less complex rubble pieces are generated. If rubble remains stable for adequate time, it can be bound by organisms including sponges and coralline algae, and eventually be cemented. However, increasing rubble mobilization frequencies will reduce the time available for binding, while changing ocean chemistry could reduce the efficacy of calcifying binders. Ultimately, increased rubble cover will negatively impact coral recruitment into rubble beds. Rubble mobilization abrades and smothers corals, and rubble beds typically experience altered environmental and ecological conditions to the coral frameworks that precede them. Several knowledge gaps exist in relation to improved rubble mobilization thresholds, binding rates and strengths, and coral survival in varying rubble bed types and hydrodynamic regimes. Addressing these knowledge gaps will improve our ability to predict the recovery trajectory of rubble beds and assess the need for stabilization interventions.

Juvenile waiting stage crown‐of‐thorns sea stars are resilient in heatwave conditions that bleach and kill corals

“…Juvenile herbivorous COTS have a flexible diet being able to survive on a range of algal food sources (Deaker, Mos, et al, 2020). With the high cover of algae that characterises the coral infrastructure and rubble habitat (Kenyon et al, 2023; Klumpp & McKinnon, 1989, 1992), they are unlikely to be food limited in nature. This may help make the juveniles robust to stress, including thermal perturbations.…”

“…In recent years, bleaching events have increased in intensity, extent and frequency causing collapse of coral reef structure and associated communities (Baum et al, 2023; Hughes et al, 2017, 2018; Stuart‐Smith et al, 2018). This ecological disruption can lead to a shift from coral‐dominated to algal‐dominated coral rubble ecosystems (Hughes, 1994; Kenyon et al, 2023). Impacts on associated species have been documented, especially for species that depend on coral for food and/or shelter with flow‐on effects for the ecosystem (Jones et al, 2004; Przeslawski et al, 2008; Rice et al, 2019; Stella et al, 2022).…”

“…cyclones and bioeroders) accelerate breakdown of the skeleton into rubble. While thermal stress and changing habitat are deleterious for live coral-associated species, the breakdown of dead coral into rubble gives rise to a diversity of crypto-benthic invertebrates and fishes that rapidly avail of this newly created habitat (Kenyon et al, 2023;Stella et al, 2022;Timmers et al, 2021;Wolfe et al, 2020Wolfe et al, , 2021.…”

Hierarchical drivers of cryptic biodiversity on coral reefs

“…Here, we take a hierarchical approach to the study of cryptofauna on coral reefs. Cryptofauna are generally associated with rubble that results from the structural degradation of live coral (Kenyon et al, 2023; Wolfe et al, 2021). As observed along gradients of wood decay in terrestrial systems (Andringa et al, 2019), invertebrate abundance and diversity can increase markedly along profiles of reef erosion from live coral to rubble (Enochs et al, 2011; Fraser et al, 2021a; Nelson et al, 2016; Stella et al, 2022).…”

“…As modern reefs transition from dominance by live coral toward degraded states (Alvarez‐Filip et al, 2009; Hughes et al, 2018; Steneck, 1994; Wolff et al, 2015), it is timely to address the features and functions of coral rubble biomes that may provide a window into future reef states (Kenyon et al, 2023; Wolfe et al, 2021). The importance of rubble in supporting small benthic organisms, including sessile (e.g., algae, sponges and ascidians) and motile (e.g., mollusks, crustaceans and worms) cryptofauna, is increasingly recognized (Wolfe et al, 2021).…”