Two Decades of Community Responses Reveal a Consistent Increase in Species Occurrences in South-Eastern Pacific Rocky Shores

Aim: Understanding how diverse communities respond to environmental fluctuations is a central challenge in ecology. Here, we assessed how communities responded to environmental variation over the past 23 years and evaluated the extent to which these responses can be associated with taxonomic relationships, as well as biological and ecological species traits. Location: Central portion of the Humboldt Upwelling Ecosystem, with 22 rocky shore survey sites spanning 8° of latitude (28° S–36° S). Time Period: 2000–2023. Major Taxa Studied: Intertidal zone communities. Methods: We used joint species distribution models to integrate quantitative survey data, satellite sea surface temperature (SST) as a proxy for environmental conditions, taxonomic information and species biological traits along a latitudinal gradient with heterogeneous thermal conditions. Results: Our proxy for environmental variation revealed weak and non-significant SST cooling at central sites, whereas sites near 30° S showed slight warming trends. Taxonomic relationships and individual species traits were weakly associated with their collective responses to SST variability. However, we identified a consistent increase in the occurrence of both macroalgal and invertebrate species across the region. The occurrence of macroalgal species was more sensitive to SST variation than invertebrates, with responses shifting from positive at equatorward sites to increasingly negative at poleward sites. Patterns of species co-occurrence were strongly dependent on spatial scale, particularly among invertebrates. Main Conclusions: Species occurrences increased across the region, but these responses were not significantly associated with taxonomic relatedness or with easily assigned species traits. This pattern likely indicates comparatively low niche conservatism within these communities in relation to SST responses, while other structuring processes—such as species interactions—are not well captured by the traits examined. As other studies have detected slight cooling trends over the past two decades, our results suggest that the lack of community-wide reorganisation reflects the absence of a clear environmental driver.