Has an ecological sleeper at it southern invasive limits gone back to sleep? If so, why?

Austrominius modestus, a species of barnacle native to Australasia, has been present in European waters for over 75 years. Towards its northern invasive range, there were concerns that climatic warming in the 21st century allowed this ‘ecological sleeper’ to increase in abundance and spread. Less research has been undertaken towards its southern limits in Europe. Semi-quantitative and quantitative surveys were carried out between 2014 and 2025, at intertidal sites in the Algarve in Portugal and Andalusia in southern Spain. It was not found in Andalusia in 2018, thus Farol on the Ilha da Culatra, in the Ria Formosa Nature Reserve in the Algarve, Portugal is, as far as we are aware, the most southerly site where it has been recorded as established in Europe. In the Algarve, abundances peaked at four of the sites in 2014, being recorded as common (10-100 barnacles per 100cm2), but by 2025 it had decreased in abundance, with only a few or no individuals being found within the search times at the 12 sites that were re-surveyed. Sites were dominated by the native barnacle Chthamalus montagui. Low abundances of the native Chthamalus stellatus and non-native Amphibalanus amphitrite were also present, but only at some sites. At the southern European limit of A. modestus, it is hypothesised that mortality and stress, due to desiccation or other environmental factors, may be controlling its abundance and spread, since it is known to be less tolerant to high temperatures than C. montagui and A. amphitrite, but this needs further research.