As cities continually expand to accommodate growing populations, the challenges of conserving and creating greenspaces for people and pollinators are growing (Desa, 2018). With 87% of plants(Ollerton et al., 2011)and 70% of agricultural crops (Klein et al., 2007) dependent on pollinating animals, pollination is an essential ecosystem service for native and cultivated ecosystems. However, in the context of global pollinator declines driven by habitat loss, pesticides, and monoculture farming, cities provide a surprising opportunity for pollinator conservation (Baldock et al., 2019; Goulson et al., 2015). Although urbanisation itself can lead to a loss in insect biodiversity (Newbold et al., 2015; Seibold et al., 2019), the heterogeneous matrix of urban greenspaces in cities can act as pollinator ‘refuges’ as they offer a diversity of generally well-irrigated floral resources, and environments lower in pesticides compared with intensive agriculture (Hall et al., 2017; Šlachta et al., 2020; Threlfall et al., 2015). Consequently, an abundance of pollinating insects has been observed in cities around the world (reviewed in Wenzel et al., 2020), which is sometimes even greater than rural surrounds (Bates et al., 2011; Hall et al., 2017).
Whilst cities are characterised by vast areas of paved surfaces and dense buildings, they also include a mosaic of constructed and semi-natural greenspaces which may act as hotspots of urban biodiversity (Baldock et al., 2019). Studies of urban pollinators have identified rich biodiversity in urban greenspaces such as home gardens (Fukase & Simons, 2016; Pardee & Philpott, 2014), community gardens (Makinson et al., 2017; Tasker et al., 2020), public parks (Banaszak-Cibicka et al., 2018), cemeteries(Bates et al., 2011) and botanical gardens (Banaszak-Cibicka et al., 2018). Other studies have found that different pollinators thrive under different levels of urbanisation across greenspace types (Banaszak-Cibicka & Żmihorski, 2012). However, few studies have assessed the interaction between habitats for flower visiting insects, such as pollinator ‘spillover’. Understanding the relative role and interaction between these greenspaces is important for evidence-based planning that allows urban pollinator conservation.
Spillover is the flow of insects across habitat boundaries and occurs when proximity to a habitat results in increased abundance and diversity in another focal habitat (Rand et al., 2006). Insect spillover may consequently increase diversity and alter ecosystem functioning of pollination, herbivory, and parasitism across adjoining environments (Blitzer et al., 2012). In the case of pollination, this can manifest as enhanced pollination of the adjoining habitat (beneficial spillover; Hagen & Kraemer, 2010; Hanley et al., 2011; Westphal et al., 2003) or decreased pollination by drawing potential flower visitors away from an adjoining habitat(negative spillover; Chittka & Schürkens, 2001; Grab et al., 2017; Holzschuh et al., 2011). For example, many bee species forage in managed agricultural fields, but nest in adjoining semi-natural habitats (Blitzer et al., 2012; Williams & Kremen, 2007). Wild solitary bees (but not wasps) have been found to take advantage of the abundant food resource of mass flowering crops when they are in bloom (Pereira-Peixoto et al., 2014). As tilling and crop rotation in agricultural systems creates frequent soil disturbance, ground nesting pollinators in particular must return to less disturbed ‘natural’ habitats during their reproduction cycle (Greenleaf et al., 2007; Holzschuh et al., 2007; Kremen et al., 2007). Subsequently, in seasons where forests have lower flower density, plant-species rich farmland that adjoins monoculture farms can supplement bees’ foraging needs (Hagen & Kraemer, 2010). As such, adjoining forests act as a source of pollinating insects for coffee (de Marco & Coelho, 2004; Klein et al., 2003; Ricketts et al., 2008), Macadamia (Blanche et al., 2006), and Longan (Blanche et al., 2006). In cities, patches of remnant and restored urban forest may act similarly to forests bordering agricultural lands. Urban forests create habitat corridors of mostly native plants, flower seasonally, and likely offer more quality nesting sites than surrounding greenspaces for insect pollinators. Due to the heterogeneity of urban greenspaces with a diversity of resources, there is reason to believe that spillover of insect pollinators may also occur within cities, but this has yet to be empirically tested.
Community gardens may offer a steady flow of floral resources as they are buffered by exotic plants that flower most of the year in tropical-temperate climates (Kaluza et al., 2016). Community gardens are public spaces where the public can rent allotments to grow food and/or grow communally. A large difference to conventional mono-agriculture is that community gardeners often use low pesticide or organic methods, and plant a high diversity of flowering fruit, vegetables, and herbs which flower most of the year (Hou et al., 2009; Middle et al., 2014; Tasker et al., 2020). This abundance of floral resources (all-year-round in warmer climates) may be especially important late in the native flowering season when native plants are sparsely flowering (Staab et al., 2020). Consequently, similarly to agricultural fields, the abundance of flowers in community gardens may act as “sources” or “magnets” for pollinators (Gilpin et al., 2019; Montero-Castaño et al., 2016). This may enhance the pollination of nearby bushland (beneficial spillover) (Hagen & Kraemer, 2010; Hanley et al., 2011; Westphal et al., 2003) or draw away from it (negative spillover) (Chittka & Schürkens, 2001; Grab et al., 2017; Holzschuh et al., 2011).
Here we studied whether spillover occurred between the community gardens and forest fragments within Sydney basin, Australia. We tested whether adjoined community garden-bushland pairs had higher or lower insect diversity and abundance than isolated gardens or isolated forest fragments. We expected to find the greatest abundance, species richness and diversity in adjoining rather than isolated greenspaces as taxa would be able to take advantage of resources in both habitats.