Kelp forests collapse reduces understory seaweed β-diversity.

BACKGROUND AND AIMS

Kelps are the primary foundation species in temperate subtidal rocky shores worldwide. However, global change is causing their decline with consequences for the organisms that rely on them. An accurate assessment of these consequences may depend on which attributes of the associated community are considered. This study shows that conventional α-diversity approaches may overlook some of these consequences compared to spatially explicit approaches such as β-diversity.

METHODS

A 1-year seasonal study was conducted to compare the macroalgal understory between healthy reefs with a Laminaria ochroleuca canopy and degraded reefs where the canopy collapsed years ago due to excessive fish herbivory. At each reef, the understory seaweed assemblage was recorded in five replicate quadrats to estimate α (total richness, species density, Shannon index) and β-diversity (intra-reef and inter-reef scale).

KEY RESULTS

The understory assemblage exhibited a distinct seasonal dynamic in both healthy and degraded reefs. α-diversity attributes increased in spring and summer; turf-forming algae were particularly dominant in degraded reefs during summer. β-diversity also showed seasonal variability, but mostly due to the changes in degraded reefs. None of the α-diversity estimates differed significantly between healthy and degraded reefs. In contrast, spatial β-diversity was significantly lower in degraded reefs.

CONCLUSIONS

Although the loss of the kelp canopy affected the composition of the macroalgal understory, none of the conventional indicators of α-diversity detected significant differences between healthy and degraded reefs. In contrast, small-scale spatial β-diversity decreased significantly as a result of deforestation, suggesting that the loss of kelp canopy may not significantly affect the number of species but still have an effect on their spatial arrangement. Our results suggest that small-scale β-diversity may be a good proxy for a more comprehensive assessment of the consequences of kelp forest decline.

Informations de copyright

© The Author(s) 2023. Published by Oxford University Press on behalf of the Annals of Botany Company.