Mixed effects of urbanization on pollination services among four native plant species

The pollination services provided by insects maintain ecosystem
functioning and services in urban green spaces. Most research investigates
rural to urban gradients, where it is found that pollination services are
often greater in urban green spaces than in rural counterparts. However,
as urbanization intensifies, the extent to which pollination services are
maintained as urban green spaces are replaced and become increasingly
fragmented requires further research. The purpose of this study was to
examine pollination services across gradients of within-city urbanization,
urban green space fragmentation, and local floral richness. We deployed
four plant indicator (phytometer) species across ten community gardens
that varied in urban cover and urban green space edge density. We measured
visitor rate and α-diversity and related these factors to plant
reproductive outcome for each phytometer species. While we predicted a
negative effect of urbanization, we found limited evidence that
pollination services decline across the sampled within-city urbanization
gradient. Contrary to our expectation, Lobelia siphilitica showed
increased visitor α-diversity and higher mass per seed in more urbanized
gardens, suggesting positive urbanization effects. Fragmentation effects
were mixed, with edge density positively correlated with one of the four
phytometer species and negatively correlated with another species. Local
garden richness did not influence pollination services, likely due to
uniformly high floral richness across gardens. Our findings suggest that
in some cases, highly urbanized community gardens can not only maintain
pollination services but also outperform less urbanized gardens. Both
urbanization and fragmentation effects were highly species-specific,
highlighting the need for further research on how landscape heterogeneity
and plant traits interact to influence urban pollination services.