Despite the negative impacts of urbanization on biodiversity, there is evidence that cities can sustain fairly rich pollinator communities, most notably regarding bees (Hall et al. 2017). In particular, Baldock et al. (2019) emphasize the major importance of private gardens and allotments in supporting pollinators, compared to parks and other public greenspaces. Nevertheless, in densely populated cities like Paris, private gardens and allotments are scarce, while parks cover larger areas (Shwartz et al. 2013). In such cities, public greenspace management practices are critical to maintaining pollinator biodiversity; and it is necessary to understand which ones best promote plant-pollinator interactions (Mata et al. 2021).
In private gardens and urban parks, much of the available flower resources are provided by ornamental plants, either native or exotic, that are highly variable in their attractiveness to pollinators (Garbuzov and Ratnieks 2014; Garbuzov et al. 2017; Erickson et al. 2019). The geographic origin of these garden plant species raises concerns since exotic plants can affect pollinator community composition (Pardee and Philpott 2014; Threlfall et al. 2015), whereas native plants are key elements to sustain rich and functionally diverse insect communities (Mata et al. 2021; Cecala and Wilson Rankin 2021). Yet, the relative contribution of native and exotic plants to urban pollinator communities remains debated (Majewska and Altizer 2020). Exotic flowering plants contribute substantially to the supply of nectar and pollen in urban landscapes (Tew et al. 2021; Casanelles-Abella et al. 2021), thus potentially supporting pollinator communities by increasing overall resources (Tasker et al. 2020; Staab et al. 2020). Indeed, while at the plant community level natives might receive more visits, at the species level some exotics can be very attractive (Lowenstein et al. 2019; da Rocha‐Filho et al. 2021). In addition, most of the mentioned studies were conducted in private gardens, allotments and nurseries; but seldom in public greenspaces within high density cityscapes. The relative attractiveness of exotics and natives to pollinators may depend on urbanization levels, due to possible effects of urban environmental stressors on pollinator foraging choices (Buchholz and Kowarik 2019). This issue has yet to be assessed in the context of public greenspaces in densely urbanized landscapes.
Furthermore, studies rarely explore how exotic garden plants integrate into pollination networks and affect their structure. Yet network structure is essential in maintaining stability against disturbance (Thebault and Fontaine 2010). By allowing to quantify single-species specialization levels (Blüthgen et al. 2006), network approaches can also help reconcile the contradictory levels of attractiveness observed at community and species levels for exotic plants. Such differences in plant specialization may rely on pollinator preferences (Salisbury et al. 2015), as exotic plants often fail to appeal to specialist pollinator species (Erickson et al. 2019). Looking at invasive plants, some studies have shown that these species often successfully integrate pollination networks, occupying a central place therein (Vilà et al. 2009; Thompson and Knight 2018). The consequences on insect communities vary greatly depending on the context (Stout and Tiedeken 2017; Davis et al. 2018). Invasive plants tend to attract more generalist pollinator species, while specialist pollinators are more strictly dependent on native plants (Parra-Tabla and Arceo‐Gómez 2021). As a result, invasive plants display different species-level properties in networks compared to native plants (Arroyo‐Correa et al. 2020). They have been found to generate less specialized pollination networks (Seitz et al. 2020) and create profound topological changes in interactions (Albrecht et al. 2014; Larson et al. 2016). Invasive plants act as super-generalists, notably raising network nestedness (Bartomeus et al. 2008; Russo et al. 2019). In an urban context, it is important to determine whether these results could be applied to exotic garden plants, and how that would affect the structure of pollination networks.
Recent works on plant and pollinator communities emphasize the importance of seasonal dynamics on pollination networks structure and species persistence (Guzman et al. 2021). Considering the respective phenologies of plants and pollinators, it is essential to examine interactions from a month-to-month perspective to assess short-term variations in network properties (CaraDonna and Waser 2020). Interestingly, the seasonal dynamics of exotic plants in pollination networks may be different from those of native plants (Larson et al. 2016; Arroyo-Correa et al. 2020; Seitz et al. 2020). Typically, exotic flora has been shown to complement native flora by providing late season resources for pollinators (Salisbury et al. 2015; Staab et al. 2020). While native floral cover fluctuates over time, exotic plants are often selected for their extended and complementary flowering, that can be kept constant by gardening practices (Erickson et al. 2019, 2021). However, these results still need to be confirmed in high-density city contexts where species phenologies can be broader and seasonality less marked than in natural landscapes (Uchida et al. 2018; Zaninotto et al. 2020). There, the seasonal dynamics of pollination networks and their impacts on pollinator diversity are still poorly understood.
Here we present the results of a two-year replicated monitoring of insect pollinator activity in the greenspaces of a densely urbanized landscape: the city of Paris (France). We investigated plant-pollinator interactions both at the plant species level and the plant community level. We examined pollination networks on a monthly basis from March to October, assessing the respective roles of the native and exotic floras. We addressed the following questions: 1) How attractive are exotic and native floras to pollinators, both at the plant community and plant species level? Based on the literature, we hypothesize that native plants attract more pollinators in general, with wide variation among plant species. 2) How does this attractiveness to pollinators vary over the seasons? We expect exotic plants, at the species and community level, to be visited more often in late season. 3) How do these species fit into pollination networks and what are the implications for seasonal network dynamics? We expect exotic plants to be more generalist and contribute more to network nestedness than native plants, leading to more nested and generalist networks in the late season.