Global Drivers of Plant-Pollinator Interaction Specialization in Gardens

The frequency of plant-pollinator interactions is shaped by abiotic (e.g., climate and land use characteristics) and biotic factors (e.g., morphological traits, evolutionary history). When applied to gardens size, the degree of urbanization and climate likely influence species richness and interaction specialization. We hypothesize that specialization will be higher in gardens with high species richness and warmer, wetter climates. Additionally, phylogenetically related plants would show similar specialization levels. We further predict that both plant and pollinator richness increase in larger and less urbanized sites. To test these predictions, we analyzed a global dataset of plant-pollinator interactions sampled in garden environments. We considered garden characteristics such as size and type (urban, suburban, rural), and annual mean temperature and precipitation within a causal framework. Additionally, we examined how species richness and phylogenetic relatedness influenced plant specialization (d'). Our analysis of 40 plant-pollinator networks revealed that plant species richness was significantly influenced only by garden size and the degree of urbanization, with larger gardens supporting higher richness, and suburban gardens hosting more plant species than both rural and highly urbanized ones. Plant richness and precipitation positively influenced pollinator richness, but no association was found between specialization and environmental variables. Furthermore, the high species-specific variation in specialization with no phylogenetic signal implies that other plant traits than phylogeny could be driving plant-pollinator specialization in these systems. Our results suggest distinct factors drive species diversity and interaction specialization in urban gardens.