Drivers of woody plant phylogenetic and taxonomic beta diversity across an urban density gradient

Abstract

Urban plant diversity patterns are influenced by socioeconomic factors, yet our understanding of these relationships in Global South cities remains limited. We investigated how taxonomic and phylogenetic beta diversity of woody plants vary across Harare's socioeconomic gradient, examining patterns for indigenous and exotic species. We surveyed 300 household yards across 15 suburbs representing low-, medium-, and high-density areas, characterized by differences in property values, population density, and yard size. For combined species, both taxonomic and phylogenetic beta diversity showed significant positive relationships with property value and yard area, supporting the 'luxury effect' hypothesis. However, patterns differed when analysing species groups separately. Exotic species demonstrated strong socioeconomic filtering, with greater taxonomic diversity in wealthy areas but consistently low phylogenetic turnover across suburbs, suggesting selection of evolutionarily related species. Indigenous species showed unexpected resilience in high-density areas, maintaining greater phylogenetic diversity than affluent suburbs. While species turnover dominated overall beta-diversity patterns, phylogenetic nestedness contributed substantially more than taxonomic nestedness, particularly for exotic species. These findings challenge conventional urban ecology paradigms and highlight the importance of considering both taxonomic and phylogenetic dimensions when assessing urban biodiversity. The retention of phylogenetically diverse indigenous communities in high-density areas presents opportunities for biodiversity conservation in African cities.