Local variations in environmental parameters govern 50 years of the decline of Fucus guiryi populations in the Canary Islands (eastern Atlantic)

Fucus guiryi is a species strongly threatened by climate change, and the populations in its southern distribution limit are fast disappearing. However, the extinction of this species is not widespread, since there are small places where some populations still survive. In this study we explore the general regression trend of F. guiryi populations in Tenerife (Canary Islands), and compare the changes in the thallus morphology, abundance of individuals and biomass that have occurred in two of its populations over the last decades. Our results show that populations on the North and South of the island have followed different trajectories. Populations of F. guiryi in the North showed a slight regression, the thallus still have a similar size to the past and have numerous receptacles. On the contrary, those of the South are in a critical state, with a strong reduction in plant size and loss of reproductive capacity. The biomass of the southern populations has also been reduced by 90%. The increase in wind speed, sea surface temperature and air temperature were negatively related to the development of F. guiryi populations. Our results suggest a 'site effect', where the geography and local climatic conditions interact with the current global warming trend. In this sense, some places in the North of the island, with climatic conditions less adverse to F. guiryi growth, are acting as refugia.