Abstract
Agricultural intensification has led to the conversion of natural habitats into agricultural fields, increased field sizes and simplified crop rotations. The resulting homogenisation of the landscape has led to a decline in bees, which provide an essential ecosystem service to agriculture. It has been suggested that an increase in landscape crop diversity supports higher biodiversity by providing more diverse and continuous resources without taking land out of agricultural production. We selected 14 faba bean (Vicia faba minor L.) fields in southern Sweden along uncorrelated gradients of landscape crop diversity and proportion of semi-natural habitat within 1.5 km radii surrounding focal fields. Pollinator surveys and pollinator exclusion experiments were conducted to assess whether landscape crop diversity affected pollinator densities, pollinator foraging behaviour (i.e. legitimate flower visitation, nectar robbing or extra-floral nectary visitation), pollination and yield formation. Landscape crop diversity enhanced bumble bee densities. Insect-pollinated faba bean plants produced, on average, 27 % higher bean weight per plant than bagged plants and the insect pollination benefit decreased with increasing semi-natural habitat cover. Bumble bee and honey bee densities, the proportion of nectar robbing bees as well as faba bean yield increased with increasing proportion of semi-natural habitat. Pollinator densities were not the driver of high yields associated with higher proportions of semi-natural habitat because the observed yield increase was unrelated to pollinator densities and driven by bagged plants that were excluded from pollinator visits. Insect pollination, however, clearly decreased the yield gap associated with low proportions of semi-natural habitat in the landscape. Our results highlight that agri-environmental policies should promote the retention of existing semi-natural habitats and encourage landscape crop diversity to provide pollinators with sufficient food and nesting resources.
Highlights
A main component of intensified crop production over the last cen tury has been the conversion of natural habitats into agricultural fields, increased field sizes and simplified crop rotations (Robinson and Sutherland, 2002)
Bumble bee densities increased with increasing crop diversity (Fig. 2a) and proportion of semi-natural habitats (SNH) (Fig. 2b)
The proportion of nectar robbing increased with SNH (Fig. 2d) but we found no effect of landscape on the proportion of legitimate pollinator visits nor EFN visits (Table 2)
Summary
A main component of intensified crop production over the last cen tury has been the conversion of natural habitats into agricultural fields, increased field sizes and simplified crop rotations (Robinson and Sutherland, 2002). Alarming are the reported wild bee declines (Bies meijer et al, 2006; IPBES, 2016; Zattara and Aizen, 2020), whose pollination provision increases yield in many economically important crops (Aizen et al, 2009; Klein et al, 2007). The loss of nearby nesting and foraging resources in homogenous agricultural landscapes is considered a key driver of bee declines (Biesmeijer et al, 2006; Goulson et al, 2008). Retention of semi-natural habitats (SNH) such as forests and grasslands can support higher bee densities (Kennedy et al, 2013; Pro esmans et al, 2019; Svensson et al, 2000; Williams and Kremen, 2007) and enhance crop pollination (Garibaldi et al, 2011; Ricketts et al, 2008). Given global demand for agricultural products (Alex andratos and Bruinsma, 2012; Godfray et al, 2010; Tilman et al, 2011), taking land out of crop production might become increasingly chal lenging to realise (Hodge et al, 2015)
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