Urban and rural pollinators and pollination – When is the city green enough?

Abstract

The declines of insect pollinators have largely been attributed to human induced land use changes such as farming intensification and concomitant landscape changes have been identified as major drivers1. Declines in pollinators are negative for biodiversity per se and pose a risk to pollination of native plants and crops. Urban areas have been suggested as potential refuges for insect pollinators compared to farmland2. The suitability of cities for pollinators likely depend on the extent and quality of urban green spaces3. Consequently, these factors could also benefit urban pollination, but this has not yet been thoroughly investigated. Cities are increasingly used for local crop production, stressing the need for pollination. Cities also provide space for increasing numbers of humans and grow both by expansion and infill development, and both directions of growth may be negative for pollinators. To advice on how to build cities to benefit biodiversity while accommodating people, it is therefore essential to consider effects of both human population density and vegetation cover on biodiversity.

We investigated insect pollinators and pollination of native plants and garden crops in two urban areas: Malmö, Sweden, and Brisbane, Australia. We selected 40-45 domestic gardens per city along gradients of surrounding vegetation cover and population density. In Malmö, an additional 14 gardens in rural areas surrounding the city were included as a comparison. We surveyed pollinators and placed experimental plants (3 native species and 2 crops in Malmö, 3 crops in Brisbane) in gardens. We measured insect visitation rates to plants, fruit weight and seed set.

Preliminary results from a subset of the experimental plants species show contrasting effects among plants, and for bees and hoverflies. For an early flowering bellflower we found higher visitation rates of hoverflies at rural sites, while visitation rates by bees were higher at urban sites. For the early flowering strawberry we found no effect of land use on fruit weight. Seed set of the late flowering knapweed was higher in urban than in rural sites in Malmö, but we detected no difference in insect visitation rates. Results on the vegetation and population gradients show a near-significant trend for surrounding vegetation cover to positively affect hoverfly visitation rates to bellflower, while we found no such effect on seed set, insect visitation to knapweed, or strawberry weight. In Brisbane, we found a non-significant trend for vegetation cover to positively affect abundance of non-managed pollinators combined (hover flies and solitary bees). Potentially, contrasting effects of bees and hover flies, as was shown for Malmö, partly mask effects.

1. Potts S.G., et al. (2016) Nature, 540, 220-229.

2. Hall D.M., et al. (2017) Conserv Biol, 31, 24-29.

3. Beninde J., Veith M. & Hochkirch A. (2015) Ecol Lett, 18, 581-592.