Differential impacts of urbanization on multiple components of common
Milkweed (Asclepias syriaca) pollination success
Abstract
Urbanization is one the main threats to biodiversity and its effects are
projected to increase in magnitude, hence stressing the importance to
better understand how urbanization may affect vital ecosystem services,
such as pollination. Studies on the impact of urbanization have largely
focused on only one aspect of the pollination process (e.g. pollinator
visitation, pollen deposition), potentially misrepresenting the full
complexity of urbanization effects on pollination services. Evaluating
impacts across multiple components of pollination can also help pinpoint
the underlying mechanisms driving urbanization effects. This study
evaluates how urbanization affects multiple aspects of the pollination
process in common milkweed populations, including pollinator community
composition and visitation rate, pollen removal and deposition on
stigmas. Urbanization altered floral visitor composition, with small
bees having a larger presence in urbanized areas. Insect visitation rate
and pollen removal were also higher in more urbanized areas, perhaps
suggesting a positive impact of urbanization as predicted by the
intermediate disturbance hypothesis. However, pollen deposition on
stigmas, which is a key component of fertilization success, was not
different between natural and urbanized sites. Our findings highlight
the complexity evaluating urbanization effects on pollination, as these
likely depend on the specific aspect of pollination evaluated and on the
of the intensity of disturbance. Our study thus stresses the importance
of evaluating multiple components of the pollination process in order to
fully understand overall effects and the mechanisms underlying
urbanization effects on this vital ecosystem service