Propagule pressure from invasive plant species in gardens in low-density suburban areas of the Costa Brava (Spain)

A substantial proportion of the cultivated plants in urban domestic gardens in Europe are exotic species. Among these species, a large number may become invasive, causing negative impacts on natural areas. To prevent this situation, the early detection of invasive species and the assessment of propagule pressure play a key role. In this study, we analyse the flora of 258 domestic gardens in the Costa Brava to explore the importance of these factors in these urban ecosystems. Of the 635 taxa identified, 68% were exotic (77% considering only cultivated plants). Moreover, 39 species were considered invasive in Spain, although only 25 were present within the limits of the adjacent Aiguamolls de l'Empordà Natural Park (AENP). The results from multiple regression models showed that all plant biodiversity parameters (overall plant richness and exotic and native plant richness) were strongly related to the garden area, the occupancy rate of the house and the different socio-economic and cognitive characteristics of the household members. A distance-based redundancy analysis (dbRDA) showed that the invasive species composition was related to the garden area, the age of the building, the income level and the proportion of non-working residents. We also detected that garden centres were by far the most used source of horticultural species, although garden plants were replaced and/or renewed after relatively long periods of time. We conclude that influencing homeowners' preferences by providing more detailed information in garden centres and nurseries may lead to the creation and restructuring of more native and environmentally friendly private landscapes.