The ecology of marginality—linking the informal settlement of mangrove forests and the rise of ecological novelty

• Socioeconomic exclusion shapes urban settlement patterns in mangrove ecosystems. • Urbanization reshapes biodiversity, producing novel ecological assemblages. • Loss of foundation species marks a pivotal shift in Caribbean mangrove urbanization. • Anthropogenic waste and loss of foundation species drive ecological novelty. • Understanding ecological novelty is critical for planning and restoration in coastal cities. The human habitation of mangrove forests in coastal cities of Latin America has historically be seen a signature of the marginalization of low-income ethnic minorities, but the ecological consequences of this dwelling practice is far from being understood. This study examines the impact of chronic stress from expanding low-income urban settlements on plant and intertidal macrofaunal communities in Colombian Caribbean mangroves commonly dominated by Rhizophora mangle , comparing urban, rural, and wild mangrove stands. In urbanized mangrove forests, R. mangle was replaced by Laguncularia racemosa , a hallmark of urbanization in Caribbean mangroves. Intriguingly, urban forests experienced a significant increase in species richness compared to wild counterparts. This suggests reconsidering this variable as indicator of ecosystem degradation. Urbanized mangroves host novel assemblages of mangrove species, alien species, utilized plants, generalist weeds or pests, and freshwater species. These assemblages reflect typical urbanization processes in low-income coastal areas from the Global South, with wastewater runoff shaping biotic structure and providing food subsidies for opportunistic species. The use of rubble and timber for landfilling in urban mangroves alters tidal regimes and flood patterns, adding stress to remaining forest patches and promoting the proliferation of utilized plants and alien freshwater species representing a threat for human health. This study highlights the complex interplay between ecological and social processes in creating ecological novelty.