Discussion
Land-use change can have major impacts on pollinator communities, with knock-on effects to ecosystem function, yet limited knowledge of these relationships hinders understanding to inform management priorities for conservation. Here we show how an interaction-site network approach can be used to identify keystone sites, taxa and interactions that are important in the focal landscape (Tylianakis et al.2008; Schleuning et al. 2015). We demonstrate that measures of interaction diversity, along with specialisation (Berlow et al.1999; Weiner et al. 2014), are powerful for informing management across mosaic agricultural landscapes to prioritize the conservation of important community interactions.
Changes in land-use intensity often drive changes in abundance and diversity of plant and pollinating insect taxa (Clough et al.2014; Harrison et al. 2017; Stavert et al. 2017). Here, we found that relatively low intensity land-uses—small intact forest patches and avocado orchards—consistently supported fewer individuals across the four main insect orders that we sampled, compared with more intensively managed dairy and cropping landscapes. Further, these intensive land-uses had a greater richness and number of unique interactions (both from a qualitative and quantitative perspective). Other studies have reported high solitary bee and syrphid fly abundance in such landscapes, but not necessarily greater species diversity (Kleinet al. 2002; Haenke et al. 2009, 2014; Williams et al. 2010; Mogren et al. 2016; Stavert et al. 2018). Conversely, phytophagous beetle species, such as carabids, are more abundant and speciose in agricultural landscapes (da Silva et al.2008). Our results demonstrate greater pollinator abundance in more intensively managed land-uses and indicate that a subset of these taxa is important to conserving unique plant-pollinator interactions.
Several factors likely explain the greater insect abundance and number of plant interactions in more intensively managed land-uses. Many pollinating insects frequently forage in open landscapes, such as grasslands, meadows or forest glades (Memmott 1999; Weiner et al.2011; Hanula et al. 2016). Floral abundance is also an important feature of open modified land-use types, particularly due to increases in mass flowering crops and wildflower enhancements (Westphal et al. 2003; Williams et al. 2015). It is unclear why field-scale management was found to influence plant-pollinator interactions more than landscape-scale, as other studies show conflicting results on the benefits of each for different pollinator taxa (Westphal et al.2003; Ferreira et al. 2013; Williams & Winfree 2013; Kremen & M’Gonigle 2015). However, one possible explanation is that insects tend to forage on what is nearby (Pasquet et al. 2008; Zurbuchenet al. 2010; Rader et al. 2011), so we are likely detecting pollen from plants most recently visited, rather than from habitat in surrounding land-uses. Although we did not measure plant species richness in our study landscapes, high pollen richness is often observed in land-use types that experience greater turnover of floral sources (i.e. those experiencing heavy cattle grazing or frequent cropping) and could also be associated with a high number of weeds in arable systems (Brenchley & Warington 1933; Marshall et al.2003). Weeds can also be an important pollinator food resource, particularly between periods of crop flowering (Marshall et al.2003; Bretagnolle & Gaba 2015; Requier et al. 2015).
Interestingly, we found that the majority of pollen from different plant families was carried by flies, including non-syrphid Diptera, which are often overlooked in pollination studies (Orford et al. 2015; Rader et al. 2020). Our pollen samples were also dominated by grass (Poaceae) pollen. These results are significant from the pollinator perspective. Grasses and three other plant families sampled here (Casuarinaceae, Cyperaceae and Pinaceae) are considered to be anemophilous (wind pollinated) (Friedman & Barrett 2009). Some studies outright dismiss wind-dispersed plant taxa as irrelevant to pollinators, both as a food source and because pollinating insects are not directly involved in their reproduction (Dupont et al. 2009; Decourtyeet al. 2010). Other studies have identified insects either carrying or foraging upon anemophilous pollen, or recorded its presence in hives (Sabugosa-Madeira et al. 2008; Reemer & Rotheray 2009; Saunders 2018). We found the amount of pollen on insects from wind-pollinated species varied by land-use type, with more being transported in dairy and cropping land-uses. The abundance and presence of pollen from Poaceae and other wind-pollinated plant families in these landscapes indicate that wind pollinated taxa require greater attention in future pollination studies, as they are being carried by insects across multiple land-use types and may be an important pollen source for many pollinator groups.
Although the majority of pollinator interactions are often reported among generalist species (Waser et al. 1996; Bosch et al.2009), we found a number of interactions occurred far more frequently (i.e. were specialised) in particular land-uses. For instance, the interaction between syrphid flies and three plant families were specialised to different land-uses: Myrtaceae in forest, Amaranthaceae in dairy and Asteraceae in cropping land-uses. It is therefore possible that while insect taxa do not rely on these specialised interactions, the plant requires that particular pollinator at the time of flowering. Our approach provides an intuitive way to identify specialised interactions to a particular land-use and highlights the importance of maintaining all land-uses, including forest sites, despite these not being as critical to interaction diversity than more intensively managed land-uses. Unlike the specialisation of interactions to dairy and cropping landscapes, where the plant families likely comprise crop or weed species, interactions between pollinators and native plant families in forested areas (e.g., Myrtaceae) require careful management.
The interaction-site network approach could be further improved in a number of ways. First, data relating to the richness and identity of plant species at the site level would increase our understanding of floral availability in the context of pollen carried by different taxa. Whilst we would predict that most insects in the potato cropping landscapes would carry Solanaceae pollen and those from avocado farms would carry Lauraceae pollen, we found little evidence that this was the case. This raises the question of whether pollinators trapped at a certain location were indeed carrying pollen from that location. In this study, only 20-25% of all insects that we trapped were carrying pollen. Whilst static traps are useful for collecting abundant data for multiple taxa (Saunders & Luck 2013; Hall 2018; Hall & Reboud 2019), different communities are often detected using transect or sweep netting methods (Gibbs et al. 2017). It is possible the proportion of pollen carriers would have been greater using sweep netting, as individuals would have been collected whilst physically visiting flowers, rather than whilst visiting a particular site where they may not have been engaged in pollination flights. Recording the pollen carried specifically by flower visitors (as opposed to flight intercept traps) would provide greater detail in this regard.