Land cover associations of wild bees visiting flowers in apple orchards across three geographic regions of southeast Australia

Abstract

The conversion of natural vegetation to agriculture is a leading cause of biodiversity decline globally, and can impact negatively on ecosystem services such as pollination. Global meta-analyses find that crop visitation by wild pollinators increases with the amount of natural or semi-natural vegetation in the surrounding landscape. However, these studies typically test the effect of one land cover type, rather than comparing multiple land cover types, and so do not provide information about the land cover arrangements that maximize crop visitation by wild pollinators. We sampled wild bee visitors to apple flowers in 2017 and 2018, and weeds and native plants in apple orchards in 2018, along landscape gradients of native vegetation and non-crop agricultural cover (open grassy areas, grazed or ungrazed) across three widely-separate agricultural regions of southeast Australia. We compared different land cover types as predictors of wild bee visitation to apple orchards, classifying non-crop land cover as: 1) ‘natural vegetation’ (NV), 2) ‘open grassy areas’ (OGA), and 3) ‘natural vegetation plus open grassy areas’ (NVOGA). The dominant flower-visiting wild bees in apple orchards in all regions were soil-nesting species of Halictidae that appear to be capable of exploiting open areas cleared for agriculture; however, even these taxa were rare or absent from orchards in some regions and years. Wild bee visitation to apples was best predicted by OGA in 2017 (positive association), but no land cover type in 2018, while visitation to weeds and native plants increased with both OGA and NV. Comparing different ways of classifying non-crop land cover is important for identifying land management strategies that maximize crop pollination services. However, managing land cover for wild bees may have negligible impacts on apple pollination in southeast Australia where wild bees are often rare in orchards, exhibit between-year variation in land cover associations, and are vastly outnumbered by honeybees (> 90% of visits to apple flowers).