Overall, we found that there was no dominant landscape variable that could solely explain variation in communities of avocado pollinators. For avocado pollinator management, there is no single solution that guarantees the enhancement of diversity, abundance, biomass and size. Instead, a combination of variables seems to play a role, as indicated by the resulting averaged models that incorporate up to six predictors. Our study is not the first to find that various attributes of vegetation within the landscape are likely to have a role in shaping pollinator communities. For instance, meta analyses have demonstrated that wild bees appear to respond to many different aspects of landscape configuration (Kennedy et al. 2013). Taken together, our findings suggest that the complexity of the multiple interacting factors at the landscape scale might present difficulties in pinpointing specific features that can be managed for the enhancement of pollinator biodiversity and ecosystem functioning (Rundlöf et al. 2008).
The prevailing view in the restoration of agroecosystems is that native vegetation plays a crucial role, and the selection of native plants is often recommended to enhance bee and insect pollinator populations (Nicholls and Altieri 2013). However, our results challenge this notion, indicating that the prominence of woody vegetation rather than native plants specifically, had a more significant impact on pollinators. This requirement for both native and exotic vegetation by pollinators could occur for several reasons. When exotic plants are present, they may provide supplementary resources, such as nectar and pollen, when resources from native plants are seasonally scarce or do not coincide with avocado flowering. A broader temporal range of flowering times can help sustain pollinator populations and diversity (Staab et al. 2020). For example, avocado flowering in New Zealand occurs in October and November (Pattemore et al. 2018) whereas an exotic plant species such as gorse (Ulex europaeus) flowers from May through to November (Northland Regional Council, n.d.), and can therefore provide resources to some pollinators when avocado trees are not flowering. Indeed, past research has shown that exotic plants can be important for maintaining pollinator diversity, as exotic species can fill otherwise vacant ‘coevolutionary niches’ which provide opportunities for specialist pollinators to interact with these plants (Stouffer et al. 2014). Although the presence of exotic species in agricultural landscapes could provide additional benefits to native species, the overdominance of exotic species versus native plant species has been found to reduce pollinator abundances (Salisbury et al., 2015). Thus, a mixture of both native and exotic plant species within the agroecosystem is likely to be important for supporting pollinating insect communities, particularly in landscapes with already minimal vegetation cover.
The landscape variables in this study did not strongly predict pollinator richness; however, the percentage of woody vegetation cover seemed to be an important variable being included in all the final models at the local scales (250 m and 500 m buffers), regardless of whether honeybees were present or not. Results from this study suggest that increasing the percentage of woody vegetation cover within 500 meters of avocado orchards could be the best approach for enhancing the diversity of avocado pollinators. These findings are congruent with past studies that showed that increasing the size of semi-natural habitats increased the diversity of the pollination insects visiting flowers on blueberries and that these relationships were limited to more localised scales (250 m) (Steffan-Dewenter, 2003). This was thought to be due to the larger patches containing more abundance and diverse resources meaning a more diverse community of pollinators can be supported (Steffan-Dewenter 2003; Blaauw and Isaacs 2014).
We found that the predictability of pollinator evenness was greatly dependent on whether honeybees were included in the analyses. One of the reasons that pollinator evenness could be better explained by landscape-level features of vegetation when honeybees were excluded from analyses could be due to honeybees being numerically dominant, meaning they overshadow any variation in other pollinator species. In this study, honeybees were present at all of the orchards and made up 67.5% of the total abundance, and other studies have found they can contribute to over 90% of flower visits in New Zealand avocado orchards (Sagwe et al., 2021; Read et al., 2017). Although honeybees dominate avocado orchard flower visitation, they are considered to be less effective pollinators compared to flies and wasps and tend to reduce their visits when there are other flowering plants in the surrounding environment (Ish-Am and Eisikowitch 1993; Ish-Am et al. 1999; Perez-Balam et al. 2012).
Increasing the connectivity of the surrounding landscape was found to decrease pollinator evenness therefore promoting species dominance when honeybees were excluded. This was the opposite relationship to that expected based on previous research, suggesting that increasing habitat connectivity should promote the movement of individuals between fragments and reduce the degree of dominance of superior competitors (Leibold et al. 2004; Marini et al. 2014). Therefore, it is expected that greater connectivity between fragments would lead to higher evenness compared to isolated fragments, primarily due to increased immigration (Marini et al. 2014). This follows the principles of island biogeography theory where increased connectivity is thought to increase colonization rates (MacArthur and Wilson 1969). In contrast, we found the opposite relationship whereby increasing connectivity reduced pollinator species evenness. The theory of increasing connectivity causing an increase in evenness assumes that there are no trade-offs between competition and dispersal across species (Amarasekare et al. 2004; Cadotte 2006). The negative effect of increasing connectivity in our study could be attributed to the increased facilitation of colonization of patches by superior competitors displacing previously established inferior competitors, which potentially could lead to a decline in evenness (Orrock et al., 2011). This could explain the negative effect of cohesion in plant cover on insect pollinator evenness, whereby superior competitors dominate available resources and out-compete other species for food and nesting sites (Cadotte 2006; Marini et al. 2014).
The variation in abundance and biomass of pollinators in avocado orchards were less accurately described by aspects of the landscape when honeybees were removed, suggesting that they contribute largely to these measures. This could be due to the generalist foraging behaviour of honeybees as they can exploit a diverse range of floral resources and landscape variables may therefore impose more consistent effects on honey bee abundance compared to more specialist pollinators (Sponsler et al. 2020). For both abundance and biomass of pollinator insects, woody functional divergence stood out with the highest variable importance at all spatial scales. This suggests that adding vegetation in the surrounding landscape, such as hedgerows, with functionally diverse native and exotic plant species could increase the abundance and biomass of pollinators in orchards. Hedgerow plantings in avocado orchards have been shown to increase native bee populations in California (Frankie & Faber, 2020).
Increasing plant species richness in the surrounding landscape was found to be negatively associated with the abundance of pollinators. This relationship, like that found for pollinator evenness, contradicted our expectations based on past research. For example, Kennedy et al., (2013) found that wild bee abundance was higher when there was more diverse habitats surrounding the fields. This is believed to be caused by the addition of floral resources other than mass-flowering crops which provide essential food resources for pollinator populations during periods when the cultivated crops are not in flower, helping to ensure their continued survival (Holzschuh et al. 2008; Rundlöf et al. 2008). our findings could be a result of the orchards in this study that had the lowest plant diversity also being those that were close to the coast, where mangrove forests were often present in the buffers. Thus, while plant diversity may typically be a strong predictor of insect abundance, the presence of mangroves could override total plant diversity effects. This is because mangroves are known to be important habitats for pollinating insects and are often overlooked hotspots for insect diversity (Yeo et al. 2021). Currently, to the best of our knowledge, there are no studies looking at how mangroves influence crop production, though this would be a valuable avenue of research in the future. Another potential explanation is that increased plant diversity could dilute pollinator abundance caught in the pan traps by distracting the pollinators (Holzschuh et al. 2016). While recent studies have emphasized the importance of pollinator movements from semi-natural areas to intensively managed agricultural lands (Aizen and Feinsinger 1994; Klein et al. 2007; Ricketts et al. 2008; Garibaldi et al. 2011; Blitzer et al. 2012), a notable gap in our understanding remains on the impacts of pollinator movement in the opposite direction (Blitzer et al. 2012). Honeybees, despite dominating avocado flower visitation have been known to reduce their avocado visitation rates when there is other flowering planting in the surrounding environment (Ish-Am and Eisikowitch 1993; Ish-Am et al. 1999; Perez-Balam et al. 2012). The movement of pollinators away from avocados may have a negative effect on yield and warrants the next for future research.
Pollinator average body size
Variation in the CWM body size of pollinators on avocado orchards was better explained when bees were excluded from analysis, suggesting that honeybees may obscure the effects of landscape factors on the average body size of other pollinator species. Without honeybees, the percentage of woody vegetation cover was the most important variable for understanding variation among orchards in pollinator body size. (Greenleaf and Kremen 2006; Librán-Embid et al. 2021)Conversely, we found that when orchards contained proportionally more woody vegetation within the surrounding buffers, there were on average larger-bodied pollinating insects. The correlation between CWM pollinator’s body size and woody vegetation cover could be due to several reasons. Firstly, increasing the percentage of woody vegetation in the landscape could also increase the availability of floral resources, which can support the higher energy demands of larger-bodied pollinators (Wray et al. 2014). This was shown to be the case in a study that looked at the importance of woody species for supporting insect pollinator food resources and found that the continuity of the food supply over the foraging season is important for sustaining pollinators through the off-season (Bożek et al. 2023). Secondly, larger insect pollinators may be able to avoid some of the negative impacts of invasive plants due to their ability to cover longer distances (Greenleaf et al. 2007). From a management perspective, these findings suggest that if growers increase the percentage of mixed assembly plantings around orchards, they could increase the mean body size of the pollinators on their orchards. Larger pollinators are thought to have higher rates of successful fertilisation by depositing more pollen onto stigmas increasing the probability of contact with the reproduction organs of the flowers (Ramalho et al. 1998; Willmer and Finlayson 2014; Földesi et al. 2021).