Floral resources and insect pollinator populations in urban landscapes

Abstract

A diverse assortment of insects act as pollinators for plants, performing a valuable role in maintaining healthy functioning ecosystems as well as improving the yields of many agricultural crops. Given the overwhelming importance of pollination to plants and people, pollinator declines are of widespread concern and have led to an increased focus on pollinator conservation. One of the main drivers of pollinator loss in the countryside is the decrease in floral resources which has accompanied intensive farming practices. Meanwhile, urbanised landscapes contain some flower-rich green spaces, such as parks and gardens, which can support diverse pollinator communities, but have received less research attention. These urban pollinators could act as source populations for subsequent spread into the surrounding countryside, but even within urban landscapes they deliver an important service by pollinating wild plants and food crops. The first aim of this thesis is to quantify floral resources in urban landscapes. Combining nectar sugar measurements with pre-existing floral abundance data, I compared nectar supplies between urban areas, farmland and nature reserves in the UK. Although the magnitude of nectar sugar production did not differ significantly among the three landscape types, urban nectar supplies were more diverse in floral origin and predominantly derived from non-native plants. Within cities, gardens provided an average of 85% of all nectar sugar and, along with allotments, produced the most diverse supplies. This shows that urban landscapes are hotspots of floral resource diversity, and that residential gardens play a critical role in feeding urban pollinator communities. Following on from these findings, I focused in detail on UK residential gardens, measuring nectar supplies through the year. I found substantial variation in the magnitude of nectar sugar production in different gardens, and this was not significantly predicted by their size. Temporal patterns in nectar supply also varied from garden to garden, but complementarity among different flowering periods led to a relatively smooth and continuous overall supply of nectar through time. These results show that the independent management of many small gardens scales up to provide a food supply for urban pollinators which is both diverse and stable through the year. The second aim was to compare pollinator populations between urban and agricultural landscapes. Focusing on the buff-tailed bumblebee (Bombus terrestris), a widespread and commercially important crop pollinator in the UK, I estimated colony density using molecular markers to identify bees from the same nest. Colony density was twice as high in urbanised sites than farmed sites, with urban bees also significantly greater in body size. These results indicate that towns and cities act as population hotspots for this bumblebee species within regions dominated by intensive agriculture. Overall, this thesis demonstrates the value of urban landscapes, and in particular of residential gardens, in providing floral resources to insect pollinators, emphasising the importance of including these areas when planning pollinator conservation strategies.

Date of Award	21 Jun 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Katherine Baldock (Supervisor), Jane Memmott (Supervisor), Stephanie Bird (Supervisor) & Ian P Vaughan (Supervisor)