Biotic homogenization of urban and non urban marine benthic assemblages

Biotic homogenization refers to the process by which ecological communities become increasingly similar over time in terms of species, functional traits and genetic diversity as a consequence of the loss of native species, the introduction and spread of non-native species, and the environmental changes associated with human activities that favor the same set of tolerant or opportunistic taxa across different locations. While this phenomenon has received growing attention in terrestrial urban environments, it remains comparatively understudied in marine ecosystems, despite the proliferation of poorly planned and often unregulated artificial structure in the sea, a process known as ocean urban sprawl. This study aimed to address this important knowledge gap by addressing whether and how homogenization occurs in marine urban systems, such as those represented by harbors. We sampled benthic assemblages on harbor pontoons and seawalls, as well as on natural rocky reefs, across three distinct eco-regions along the Italian coast. Based on predictions from terrestrial urban systems and marine urban habitats we hypothesized 1) significant compositional and functional differences in the benthos among pontoons, seawalls and rocky shores; 2) similar species richness despite the differences and 3) increasing taxonomical and functional homogenization from rocky reefs to seawalls to pontoons reflecting progressively more standardized and constraining conditions of the three habitats. Organisms were identified to species level and assigned to trophic guilds using dichotomous keys and existing literature. Using univariate and multivariate analyses and β-diversity metrics, we observed that the three habitats supported distinct communities, with natural rocky reefs dominated by Rhodophytas, sea walls and pontoons increasingly characterized by filter-feeding organisms and a reduced variety of algal species. However, contrary to our expectations, species richness decreased from rocky shores to seawalls to pontoons. Further, we did not detect taxonomic homogenization in pontoon or seawalls compared to natural reefs, while there was indication of functional homogenization in pontoon habitats, due to a significant dominance of filter feeders. To explain this apparent discrepancy, a multi-factor filtering framework is proposed, in which abiotic and biotic filters progressively constrain the range of viable functional traits from a stochastic species pool. The findings highlight the need for more comprehensive research into the ecological effects of ocean sprawl and underscore the importance of examining multiple dimensions of homogenization to understand how urbanized marine environments are reshaping coastal biodiversity.