Wild Bees and Urban Agriculture: Assessing Pollinator Supply and Demand Across Urban Landscapes
Total Page：16
File Type：pdf, Size：1020Kb
Urban Ecosystems https://doi.org/10.1007/s11252-019-0826-6 Wild bees and urban agriculture: assessing pollinator supply and demand across urban landscapes Chang Zhao1 & Heather A. Sander1 & Stephen D. Hendrix2 # Springer Science+Business Media, LLC, part of Springer Nature 2019 Abstract Growing interest in urban agriculture has increased demand for pollination services. Most studies map pollination supply broadly, and do not consider the impacts of fine-scale urban land-use practices on the dynamics of pollination delivery, leaving a critical gap in our understanding of the pollinator supply-demand balance in urban landscapes. This study demonstrates a spatially- explicit framework, using Iowa City, IA (USA) as the case study region, for assessing the capacity of urban ecosystems to produce pollinator services in support of demand from urban agriculture. We estimate pollinator supply using the InVEST pollination model with detailed land-cover data produced through field survey and Bayesian hierarchical analysis, and we validate modeling results with bee abundance and richness data. We map social demand for pollinators using a kernel density estimation of urban agricultural sites and evaluate supply-demand budgets through spatial overlay analysis. Our results show that incorporating high-thematic-resolution urban land-use data substantially improves the performance of pollination supply model- ing. Pollinator supply meets demand in 72% of the city. Surpluses occur in natural areas and heavily-vegetated, established residential neighborhoods, whereas deficits occur in resource-poor lawns. Our mapping framework stresses the key role of humans in modifying resource availability and pollinator services, and demonstrates the effectiveness of using disaggregated socio-economic data in urban land-cover classification for predicting pollinator supply. Our improved ability to identify spatial congruence and disparities in urban pollinator supply and demand can be used to inform pollinator conservation to support sustainable urban agriculture. Keywords Urban agriculture . Wild bees . Pollination . Ecosystem service mapping . Ecosystem service supply . Ecosystem service demand Introduction Electronic supplementary material The online version of this article Urban agriculture involves the development and transforma- (https://doi.org/10.1007/s11252-019-0826-6) contains supplementary tion of urbanized land to support food production through material, which is available to authorized users. community and private gardens, allotments, edible landscap- ing, and other productive uses. Urban agriculture is well- * Heather A. Sander recognized in ensuring urban food security and proper nutri- [email protected] tion, supporting local economies, reducing waste and pollu- tion, and enhancing social cohesion (Armar-Klemesu 2000; Chang Zhao [email protected] Lovell 2010). In addition to honey bees kept by citizens, wild bees are particularly important in supplying urban agriculture Stephen D. Hendrix with pollination services given that a wide variety of plants [email protected] and fruits (e.g., tomatoes, apples, strawberries) in urban gar- 1 Department of Geographical and Sustainability Sciences, University dens and farms depend on animal pollinators to set fruit and of Iowa, 316 Jessup Hall, Iowa City, IA 52242, USA seeds (Klein et al. 2007). The diverse wild bees that occur in 2 Department of Biology, University of Iowa, 425 Biology Building, large American cities such as Phoenix (Cane et al. 2006), New Iowa City, IA 52242, USA York (Matteson et al. 2008), San Francisco (Potter and Urban Ecosyst LeBuhn 2015) and Chicago (Lowenstein et al. 2015) are the have high accuracy. However, field-sampling of bees across primary pollinators for urban agriculture (Matteson et al. urban landscapes is costly in time and effort and is rarely 2008; Matteson and Langellotto 2010;Lowensteinetal. applied in mapping urban pollination services. 2015; Tonietto et al. 2011). Where pollinator occurrence data are unavailable, a second Both the surrounding landscape context and local site approach is used that relies on extrapolation techniques to attributes influence wild bee occurrence and foraging pat- estimate pollination based on generalized relationships be- terns (Ahrne et al. 2009;Wojcik2011; Tonietto et al. tween land cover, interpatch distance and pollination success 2011). In highly heterogeneous landscapes, these factors (Maes et al. 2011; Schulp and Alkemade 2011;Schulpetal. combine so that the distribution of pollinators and hence, 2014). One such study of farm sites identified an exponential pollination services, are uneven. Such relationships are less decay function describing the negative association between straightforward in urban ecosystems than agricultural or pollinator visitation rates and distances from suitable pollina- natural systems (Ricketts et al. 2008; Kennedy et al. tor habitat using data across multiple pollinator communities, 2013) because landscape heterogeneity occurs at a much crop species and biomes in rural agricultural systems (Ricketts finer grain. Gardens are often interspersed with areas of et al. 2008). Although this relationship facilitated continental impervious surfaces, potentially altering bee community assessments of crop pollination (Maes et al. 2011;Schulpetal. diversity and abundance. 2014), it remains unclear whether such empirical knowledge Studies of the effects of urbanization on urban wild bees can be applied to quantify pollination services in heteroge- disagree with respect to their magnitude and direction. Some neous urban landscapes. It is likely that the simplification of studies suggest that bee abundance and diversity increase with urban landscapes into binary land categories (i.e., natural and urban green space coverage and that bee diversity declines semi-natural habitat vs. agricultural land) required in model- with increased impervious surface coverage in surrounding ing this relationship could fail to account for the complexity of landscapes (Ahrne et al. 2009; Tonietto et al. 2011). In con- fine landscape elements that provide resources for bees trast, other studies show wild bee foraging dynamics to be (Kennedy et al. 2013). strongly resource-driven with little if any effect of the urban A third approach to mapping urban pollination supply uses matrix on species diversity and pollination efficiency (Winfree habitat models based on expert knowledge to identify land- et al. 2008;Wojcik2011; Potter and LeBuhn 2015). The scape suitability for bees. The Integrated Valuation of patchy distribution of floral and nesting resources introduced Ecosystem Services and Tradeoffs (InVEST) pollination mod- by mosaics of trees, shrubs, gardens and other greenspaces in el exemplifies such an approach (Lonsdorf et al. 2009;Sharp urban neighborhoods may contribute to this finding, for exam- et al. 2016). It combines land-cover data, expert assessments of ple, by providing stepping-stone habitat to support bees mov- nesting and floral resource availability, and pollinator life- ing through urban landscapes. Several studies suggest that history characteristics (i.e., nesting type, flight ranges) to gen- ornamental plants in urban residential neighborhoods can sup- erate a pixel-level bee abundance index. This model has been port bee diversity and abundance and, in turn, pollination ser- applied in numerous contexts and geographic locations, in- vices by mitigating the otherwise negative impacts of urban cluding global cropping systems (Kennedy et al. 2013), the development (Frankie et al. 2009; Matteson and Langellotto entire conterminous US (Koh et al. 2016), European Union 2010; Lowenstein et al. 2014). Landscape heterogeneity thus croplands (Zulian et al. 2013), and different agricultural set- poses both opportunities and threats to urban wild bee com- tings in US states and Costa Rica (Lonsdorf et al. 2009;Chapin munities and to associated pollination services. While small 2014; Groff et al. 2016). While these studies provide spatially- urban habitat patches can host rich wild bee communities, they explicit estimates of pollination services, their focus on broad may also experience regular disturbance that changes nesting extents, coarse-resolution data and rural ecosystems tells us site and food plant availability and/or accessibility. The patchy little about the applicability of this approach in urban areas. distribution of high-quality habitat may allow bees to persist in Recent studies applied the InVEST model in identifying some urban settings, but in locations that may be distant from urban pollinator supply (Grafius et al. 2016; Davis et al. urban agriculture. Spatially-explicit estimation of the state, 2017; Stange et al. 2017). These studies assessed the capaci- dynamics and availability of pollination services for urban ties of urban land covers to provide pollination services agriculture could enhance understanding of these factors. through scenario analysis (Davis et al. 2017)aswellasinves- Mapping pollination services across landscapes generally tigations of the influence of spatial (Grafius et al. 2016)and follows one of three approaches. First, when spatially-explicit thematic resolution (Stange et al. 2017) on pollination models environmental and species occurrence data are available, em- in urban areas. These studies improved our understanding of pirical data-driven species-distribution models can be used to urban pollination supply, but did not quantify pollination de- predict