Macroalgae size refuge from herbivory promotes alternative stable states on coral reefs

Abstract

Coral reef systems can undergo rapid transitions from coral-dominated to macroalgae-dominated states following disturbances, and models indicate that these may sometimes represent shifts between alternative stable states. While several mechanisms may lead to alternate stable states on coral reefs, only a few have been investigated theoretically. We explore a model that illustrates that reduced vulnerability of macroalgae to herbivory as macroalgae grow and mature could be an important mechanism: when macroalgae are palatable to herbivores as juveniles, but resistant as adults, coral-dominated and algae-dominated states are bistable across a wide range of parameter space. We compare two approaches to global sensitivity analysis to rank the relative importance of the model parameters in determining the presence and magnitude of alternative stable states, and find that the two most influential parameters are the death rate of coral and the rate of maturation of algae out of the vulnerable stage. The Random Forest approach for global sensitivity analysis, recently adopted by ecologists, provides a more efficient method for ranking the relative importance of parameters than a variance-based approach that has been used frequently by computer scientists and engineers. Our results suggest that managing reefs to reduce chronic stressors that cause coral mortality and/or enhance the growth rates of algae can help prevent reefs from becoming locked in a macroalgae-dominated state.

Landscape‐scale patterns of nutrient enrichment in a coral reef ecosystem: implications for coral to algae phase shifts

“…Note that N is consistently enriched at the fringing reef sites relative to the back reef and reef crest. Also note that LTER sites 1, 2, and 3 tend to be enriched relative to sites 4, 5, and 6. et al 2017, Briggs et al 2018). In this study, we found that several species of macroalgae have recently increased in abundance on back reef habitats of Moorea, but only where levels of N are relatively high.…”

“…Nutrient enrichment can allow for the proliferation of algae that are physiologically unable to grow under nutrient poor conditions. In addition, excess nutrients can also facilitate algae indirectly through the loss of top‐down control if adding nutrients causes algal production to outpace herbivory (Scheffer et al 2008, Arias‐González et al 2017, Briggs et al 2018). In this study, we found that several species of macroalgae have recently increased in abundance on back reef habitats of Moorea, but only where levels of N are relatively high.…”

Microtopographic refuges enhance recruitment and survival but inhibit growth of propagules of the tropical macroalga Sargassum swartzii

“…may similarly reduce propagule survival through grazing of the EAM even though detrital aggregates may be the primary dietary target (Wilson 2000, Brandl et al 2018. Although our results suggest that cryptobenthic blennies contribute to the mortality of small Sargassum propagules (<2 mm, and up to 25 days post-settlement), it is likely that Microhabitats enhance propagule survival Sargassum and other macroalgal propagules will quickly reach a size refuge from blennies as they grow (Lubchenco 1983, Briggs et al 2018.…”

“…To date, studies investigating spatial and temporal variation in rates of herbivory on coral reef macroalgae have primarily focused on the consumption of adult thalli (Hay 1981, Lewis 1986, Loffler et al 2015), yet it is often assumed that macroalgal biomass is largely Microhabitats enhance propagule survival controlled through the grazing of early life stages of macroalgae (Done 1992, Diaz-Pulido & McCook 2003, Bellwood et al 2004. Spatial and temporal refuges may facilitate the early post-settlement survival of macroalgae by providing a refuge from herbivory through reductions in grazing pressure (Brandl et al 2014, Brandl & Bellwood 2016, potentially allowing propagules to grow to a size more resistant to grazing (Lubchenco 1983, Stiger et al 2004, Briggs et al 2018. In this study, therefore, we investigate how reef microtopography (i.e., crevices) influences the recruitment, growth and survival of propagules of the common tropical macroalga Sargassum swartzii.…”

Filamentous calcareous alga provides substrate for coral-competitive macroalgae in the degraded lagoon of Dongsha Atoll, Taiwan

“…On the other hand, the association with the unpalatable Galaxaura may provide a refuge from herbivory for certain palatable algae [40,59,60], and facilitate their establishment on the reef, increasing macroalgae biodiversity [61]. The facilitation of harmful, allelopathic algal types could decrease the resilience and promote alternative stable states on coral reefs [62]. Some of the identified G .…”