Patterns of functional and phylogenetic homogenization of plant diversity at the scale of entire cities are well established. However, fewer studies have investigated how shifts in the composition of urban species pools may alter spatial variation in community assembly across patches within a city. We compared plant diversity both within and between vacant lots in a single urban neighborhood and asked: (1) how do heterogeneous human legacies of land use within vacant lots structure plant community diversity and composition and (2) what is the importance of human legacies for structuring community composition, relative to spatial and environmental variation?
Urban residential neighbourhood in Baltimore, MD, US (39°17′ N, 76°38′ W).
We surveyed herbaceous plant species identity and abundance in 24 unmanaged vacant lots. We constructed functional and phylogenetic diversity metrics from databases and the literature and compared taxonomic, functional and phylogenetic diversity between sections of the lot where a residential building was once located (building footprint), and the area that was originally the garden or backyard (remnant garden). We term these different ‘human legacy’ groups. We partitioned variation in plant community composition between human legacy groups as well as across measured environmental and spatial gradients.
We report significant plant community compositional divergence between human legacy groups. Beta-diversity, in particular, was significantly higher in remnant garden sections compared to building footprint sections for all metrics of biodiversity. Plant community compositional variation was primarily explained by differences in human legacies. We found no measurable effects of selected metrics of local environmental variation (abiotic soil characteristics) or environmental context (lot area, proximity to other vacant lots and tree canopy) on compositional variation. Geographic distance between sites, however, did interact with human legacy variation to structure phylogenetic composition.
Our findings suggest that human land-use legacies have complex but measurable effects on both the patterns and the processes by which species co-existence is maintained across the urban landscape. Within a single habitat type (here, residential vacant lots), variation in legacies of land use may have a stronger plant community structuring effect than contemporary environmental variation.