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

(Poster)

Anna Persson
anna.persson@cec.lu.se

Center for Environment and Climate research, Lund University, Sweden
,
Richard Fuller
School of Biological Sciences, University of Queensland, Australia
,
Henrik Smith
Center for Environment and Climate research, Lund University, Sweden

DOI: 10.17011/conference/eccb2018/107250

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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.

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INTRO:
The value of urban areas for pollinator conservation, and the characteristics of cities affecting pollinators are not well understood. This study aims, for its part, filling the existing knowledge gap. Pollinator occurrence and pollination of experimental plants was studied in domestic gardens along gradients of surrounding vegetation cover and population density in Malmö and Brisbane. In addition, in Malmö, gardens in the city were compared to gardens in the nearby rural areas. Preliminary results differ between different species of experimental plants, pollinator groups and cities. However, at least one plant species produced more seeds in urban than rural gardens, and some wild pollinators tended to increase in abundance with increasing vegetation cover in the cities.
MERITS:
The topic is important and little studied compared to pollinators in agricultural areas. Study design is excellent, and sample size respectable (ca 100 gardens in total). Pollination of experimental plants was measured with several complementary methods (visitation rates, fruit weight, seed set). The background and methods of the study are described well in the abstract.
CRITIQUE:
The last part of the abstract includes many detailed results, but it's difficult to see what is most important. Could you try to summarize the main points in one or two sentences, and maybe remove some of the details? In addition, in the results, you mention three plants (knapweed, bellflower, strawberry), but based on the methods paragraph, I understood that you had more plant species in the experiment. So, it's unclear if you found any differences between city and rural areas, or any effect of vegetation cover or population density for the other plant species. The limited space doesn't allow reporting detailed results for every species, but I think the other species would still be good to mention.
DISCUSSION:
Have you tried to combine data from all experimental plant species and the both cities? It would be super interesting to see if there is any global effect of vegetation cover and population density on pollinator abundance and pollination success of plants. These results would improve the abstract and help to make general conclusions.

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INTRO:
This study investigated resident insect pollinators in urban green spaces along gradients of surrounding vegetation cover and population density in Sweden and Australia. Further, they analysed pollinator efficiency (insect visitation and seed set) and fruit quality using experimental plantings of three flowering plant species. Plant-pollinator-landscape relationships were highly plant-specific: In knapweed, seed set varied between urban and rural areas, whereas in bellflower, visitation rates of hoverflies and bees had conflicting responses. In strawberry, there was no effect of land use on any metric. Trends were also not generalizable between urban areas and highlight the context-specific responses of insect pollinators to anthropogenic land uses.
MERITS:
This abstract describes an interesting study looking at insect pollinators in urban green spaces. The two environmental gradients used: vegetation cover and population density, both represent sound hypotheses in terms of factors influencing insect pollinators. The sample size is large for an ecological study and the use of experimental plantings is methodologically robust as it allowed for accurate comparison in different sites. The metrics used for quantifying pollinator are well-regarded and widely used.
CRITIQUE:
I think this abstract could be edited significantly. It is currently convoluted and difficult to follow. In particular, the introduction makes up 1/2 of the abstract. Further, the results could be made more concise and focus on the main trends (although I do admit this may be difficult given the variety of different results obtained for each plant species).
DISCUSSION:
I think this research is important as it is crucial we understand how much urban areas can be reservoirs for insect pollinators, and thus aid their conservation. I would like to know more about the pollinator communities observed in this study, in particular, if there were trends related to the richness and diversity of pollinators along the land-use gradients or between urban-rural areas. As an extension to this, the use of community compositional metrics may be interest to the authors. Lastly, little is described about the plant community of each garden in which plantings took place, it would be of interest to know if insect pollinators were observed on other co-flowering plants, and whether or not there were disparate patterns in utilisitation of the experimental plant species and other garden plants by different pollinting taxa.

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