Linking Demographic Processes of Juvenile Corals to Benthic Recovery Trajectories in Two Common Reef Habitats

Abstract

Tropical reefs are dynamic ecosystems that host diverse coral assemblages with different life-history strategies. Here, we quantified how juvenile (<50 mm) coral demographics influenced benthic coral structure in reef flat and reef slope habitats on the southern Great Barrier Reef, Australia. Permanent plots and settlement tiles were monitored every six months for three years in each habitat. These environments exhibited profound differences: the reef slope was characterised by 95% less macroalgal cover, and twice the amount of available settlement substrata and rates of coral settlement than the reef flat. Consequently, post-settlement coral survival in the reef slope was substantially higher than that of the reef flat, and resulted in a rapid increase in coral cover from 7 to 31% in 2.5 years. In contrast, coral cover on the reef flat remained low (~10%), whereas macroalgal cover increased from 23 to 45%. A positive stock-recruitment relationship was found in brooding corals in both habitats; however, brooding corals were not directly responsible for the observed changes in coral cover. Rather, the rapid increase on the reef slope resulted from high abundances of broadcast spawning Acropora recruits. Incorporating our results into transition matrix models demonstrated that most corals escape mortality once they exceed 50 mm, but for smaller corals mortality in brooders was double those of spawners (i.e. acroporids and massive corals). For corals on the reef flat, sensitivity analysis demonstrated that growth and mortality of larger juveniles (21–50 mm) highly influenced population dynamics; whereas the recruitment, growth and mortality of smaller corals (<20 mm) had the highest influence on reef slope population dynamics. Our results provide insight into the population dynamics and recovery trajectories in disparate reef habitats, and highlight the importance of acroporid recruitment in driving rapid increases in coral cover following large-scale perturbation in reef slope environments.

Post-bleaching coral community change on southern Maldivian reefs: is there potential for rapid recovery?

“…Biomass was calculated using length data and lengthweight relationships and multiplied by fish abundance. Juvenile coral data were collected at each site in March 2017, with data on genera and size (defined as corals up to 50 mm in maximum diameter, following Doropoulos et al 2015) collected from 60 replicate 1-m 2 quadrats (10 per 10 m transect) at each site.…”

Demographic Mechanisms of Reef Coral Species Winnowing from Communities under Increased Environmental Stress

“…While differential resilience to environmental stresses has been documented, exact demographic mechanisms of persistence or extinction are less clear (but see Edmunds and Elahi, 2007;Edmunds, 2010;Roth et al, 2010;Doropoulos et al, 2015). Dynamics of PAG species, with respect to potential winnowing from the community, fell into two categories.…”

The Dynamics and Resilience of Coral Reefs

“…In reality, reinforcing feedback mechanisms are important drivers of coral reef dynamics (Mumby & Steneck 2008) and tend to generate nonlinear interactions among processes (Birkeland 2004). For example, the outcome of having a 'high' value for coral larval supply depends strongly on the processes that influence settlement and post-settlement mortality, such as macroalgal cover (Dixon et al 2015;Doropoulos et al 2015) and corallivory (Doropoulos et al 2016). The nonlinear and context-dependencies among processes are more easily dealt with using mechanistic models.…”

“…While higher-level taxonomic data are more time consuming to collect, the data can be combined into functional groups if desired. Detailed benthic documentation is important even when simplified to major lifeforms, especially when results are linked to ecosystem functioning such as the critical role of plating corals in driving rapid reef recovery in the Pacific (Halford et al 2004;Ortiz et al 2014;Doropoulos et al 2015) and the importance of branching Acropora as a nursery habitat for juvenile fishes (Floros & Schleyer 2016). The attempt to make resilience assessments logistically more feasible with lower resolution data may limit the insights that can be drawn with respect to reef functioning.…”

“…While many reefs still exhibit prolific recovery (Connell 1997;Adjeroud et al 2009;Roff & Mumby 2012;, combinations of chronic stressors and frequent disturbances have reduced the average reef health of ecosystems as far afield as the Caribbean and the Great Barrier Reef (Pandolfi et al 2003;Gardner et al 2005;De'ath et al 2012). Yet, unpacking the relative importance of different stressors is challenging and undoubtedly varies among physical environments and community structures (Tomascik & Sander 1987;Doropoulos et al 2015;. The majority of research into coral reef ecosystems has focused on relatively clear-water ecosystems, generally located in areas of high water quality but extensive reefs also occur in areas that are either naturally or unnaturally turbid (Perry & Larcombe 2003).…”