Pollinators and Pollination in Urban Environments

The immense increase in human population numbers over the 20th century has had pronounced consequences on virtually all ecosystems. Firstly, vast amounts of land were taken intocultivation to feed the rising population. Secondly, all these new peopleneeded houses to live in which also led to large amounts of land being adaptedto meet the housing needs of humans. Both forms of land use significantlyaltered and for the most part destroyed native ecosystems and replaced themwith new and mostly degraded ones.Research on the effects of these land uses on ecosystems and ecosystem functions has long been focused primarily on agricultural land use as the dramatic impacts of new chemicals and pesticidesthat were widely used in modern agriculture became apparent soon after theyfirst started being used. The other major form of anthropogenic land use urban land use has long been relatively neglected by ecological scientists.Yet, this land use also takes up large amounts of land all over the globe andhas in the last decades been shown to have drastic impacts (both similar anddistinct from the ones originating from agricultural land use) on ecosystemsand ecosystem processes. Moreover, urbanization is ever increasing andknowledge on its impacts on ecosystems is crucial for sustaining ecosystemfunctioning in existing urban areas and for sustainable planning in newlyurbanizing regions.One very important ecosystem process is pollination. Most plant species rely on pollinators for successful reproduction and the pollinators in turn rely on the plants to provide them with resourcesto feed themselves and their offspring. As this interaction between organismgroups is of vital importance to sustaining ecosystem functioning, it isimportant to gain insights into how both organism groups are affected by urbanland use. An obvious aspect of urban land use is the extreme fragmentation thatit causes leaving behind small, remnant patches of (native?) vegetation thatare strongly isolated in space by an inhospitable matrix. For plants, thismeans that the gene flow through transport of pollen is likely to get moredifficult and for pollinators this means that they will have to travel furtherto obtain the resources they need.In this research we looked into the effects of urban land use on both organism groups in gradients of urban to rural land use. These gradients were laid out mainly around Leuven (but also Ghent,Hasselt, and Mechelen) and ranged from the inner city core over the cityoutskirts to the rural countryside around the city. We also looked further intothe nature of the rural countryside as different types of rural can exertdifferent influences on pollinator communities and consequently on plantpollination. This, of course, is an abstraction as the reality of landscapes isa continuum in which various land uses occur alongside each other in varyingamounts. The position of urban areas in terms of pollinator communities within theurban-rural continuum remains unclear. We studied pollinator communities of thetwo main pollinating insect groups in our region bees (Apidae) and hoverflies(Syrphidae) along two river systems which cross a gradient of urban toagricultural rural to semi-natural rural areas. We grouped the sites into threesite types (urban, rural-agricultural, and rural-natural) based on thesurrounding land use. We compared pollinator communities between these threesite types in general terms of abundance, alpha and beta diversity, but alsoassessed differences in species composition. Both abundance and diversity ofthe two pollinator groups was reduced in agricultural and urban sites comparedto the more (semi-)natural sites. Wealso observed that hoverflies and bees respond differently to different landuses. These differences stress the importance of incorporating all types ofland use practice to get a clear view on how urban land use may affect aspecific pollinator group.Urban areas consist mainly of a matrix that is inhospitable to bees but they also contain large public green spaces that are a potential habitat for bees if managed properly and therefore provide agood opportunity for bee conservation. Since urban green spaces are usually notdesigned to optimize resources for bees, we studied public green spaces in an urban-peri-urbangradient to detect which variables (local and/or regional) drive differences inabundance, diversity, and species composition of bee communities. Our resultsindicated that overall abundance is mainly driven by the cover of Fabaceae inpublic green spaces; these are largely made up of lawns in which Trifolium repens is the mainrepresentative of the Fabaceae. Increasingly urban land use decreased beediversity and also altered the species composition of the communities. However,the negative effects of urban land use were counteracted by the management ofthe public green space with (partly) extensively managed sites scoring overallbetter for bee abundance and diversity than the traditional, highly manicuredsites. We concluded that adapting the management of public green spaces toallow for diverse plant communities that ensure continuous flowering couldgreatly improve the urban bee communities and could lead to public green spacesin urban areas becoming favorable bee habitats.Changes in pollinator faunas due to urban land use are expected to also cause changes in both plant-pollinator interactions and increased pollen limitation in plants (seed set limited by amount of pollendeposited and therefore suboptimal). We therefore investigated the effects of urban land useon pollinator diversity, flower visitation rates to and reproductive success of Digitalis purpurea (self-incompatible) in experimental populations in three urban-rural gradients and contrasted theseagainst two large, natural populations. In the experimental populations wequantified the visitation rates of pollinators to the flowers and appliedsupplemental hand-pollinations to determine the level of pollen limitation onseed set. We found no impact of urban land use on pollinator diversity at any of the locationsand also the visitation rates did not differ significantly between urban andrural populations. Reproductive success appeared, however, pollen limited inall sites, even in the large and optimal reference populations. This led us toconclude that the pollen limitation was caused by a bet-hedging strategy of thespecies rather than by insufficient pollination. This species is monocarpic and only flowers for one season.Therefore on days with high pollinator visitation rates this bet-hedgingstrategy allows the plant to compensate for days with low pollinator visitationrates.We then studied bee abundance, flowervisitation rates to, and seed set of Trifoliumrepens in public lawns in an urban-peri-urban gradient around Leuven,Belgium, to check in how far these experimental results applied to real-worldplant population. T. repens (white clover) is an obligately outcrossing plant and seed set is therefore sensitive to pollen limitation. We explored relationships between visitation rates andseed set on the one hand with both lawn and matrix variables on the other hand.The two variables that best explained visitation rates to flowers were theabundance of the plant species in the lawns and the amount of green areas(gardens, parks, grasslands) in the surroundings. Surprisingly, an increasingamount of green areas in the surroundings had a negative effect on both flowervisitation rates to, and seed set of T. repens. Bumblebees (the main pollinators) responded positively to urban land use resulting in more flower visits by this group and an increased seed set inthe more urban sites. This was probably caused by a concentration effect of thepollinators in our study sites in the city center as these were the major foodsources for them due to lack of alternatives. Responses will likely differ forother bee and plant species, but this shows that at least for this species,pollination is not compromised by urbanization. <w:latentstyles deflockedstate="false" defunhidewhenused="true"