Size area, patch heterogeneity and plant species richness across archaeological sites of Rome: different patterns for different guilds

In this study we analyzed species-area relationships in vascular plant assemblages occurring in a set of archaeological sites in Rome. The relationship was investigated both for total species richness and for richness of habitat-related guilds, also controlling the role of habitat heterogeneity at site scale. By floristic sampling, we obtained 585 plant species, about 50% of the spontaneous flora of Rome. The power equation between total site area and total species number showed a weak relationship (R2 = 0.36) but when considering the relationship between total site area and species number of each habitat guild, the regression value increased as human disturbance decreased (woods and uncultivated lands vs synanthropic and cultivated lands). When controlling for patch heterogeneity, we observed significant correlations between species number of three guilds linked to woods, shrubs and uncultivated lands and their site areas. The investigated archaeological sites, despite their spatial arrangement dispersed in an urban matrix, do not respond to classical rules of insular biogeography theory, at least when we consider all species. Species-area relationships were significant only in semi-natural habitat guilds, especially when we control for habitat heterogeneity. The patterns observed in both total species richness and more synanthropic habitat guilds are probably affected less by biogeographic processes when compared to stochastic processes at patch scale (e.g., site-specific anthropic management) that locally may drive within-patch habitat heterogeneity.