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Persistence of tropical herbivores in temperate reefs constrain kelp resilience to cryptic habitats

Citation

Zarco-Perello, Salvador; Bosch, Nestor; Wernberg, Thomas (2021), Persistence of tropical herbivores in temperate reefs constrain kelp resilience to cryptic habitats, Dryad, Dataset, https://doi.org/10.5061/dryad.9ghx3ffgj

Abstract

1. Global warming is facilitating the range-expansion of tropical herbivores, causing a tropicalization of temperate marine ecosystems, where tropical herbivores can suppress habitat-forming macrophytes, supporting the resilience of canopy-free ecosystem states. However, currently we lack a thorough understanding of the mechanisms that, on one hand, support the persistence of tropical herbivores and on the other support the recovery of temperate foundation species in tropicalized ecosystems, a required knowledge to predict potential regime-shifts and reversals to the baseline state of the ecosystem. 2. This study tested processes behind the persistence of the tropicalization of temperate reefs which experienced a complete loss of their kelp forests and an influx of tropical herbivores following a marine heatwave in 2011. For this, we assessed the feedback mechanisms that maintain turf-dominated states (recruitment of tropical herbivores, browsing and grazing rates and turf cover) and those that resist it (kelp recruitment, survival, and reproductiveness). 3. We found that the reefs remained tropicalized with high cover of turf and high abundance of tropical herbivores after nine years from the regime shift. The most important herbivores, the rabbitfish Siganus fuscescens and the chub Kyphosus bigibbus, persisted with high abundances. This was supported by the adjacent reef lagoon, where seagrass meadows and the backreef habitats hosted juveniles of both species, particularly rabbitfish. Tropical herbivores exerted a strong top-down control on turf seaweed and kelp during herbivory assays, rapidly consuming kelp individuals in open areas. However, in topographical refuges in the reefs, herbivory was low and we found kelp individuals that had re-established, with some having reproductive tissue. 4. Synthesis. Our findings incorporate the importance of nursery grounds for tropical herbivores and herbivory refugia for kelp individuals into the tropicalization model, where the former increases the resilience of canopy-free states and the latter might facilitate recovering kelp populations. The restoration of abundant warm-resistant kelp populations in shelters could provide local sources of propagules to recolonize open spaces, however, our results suggest that the reduction of herbivory and the provision of turf-free substratum would be necessary to boost the recovery of kelp forests.

Funding

The Holsworth Wildlife Research Endowment & The Ecological Society of Australia , Award: RA/1/411/101

The Sea World Research & Rescue Foundation, Award: SWR/14/2019