Agustín Sáez

A global quantitative synthesis of local and landscape effects on wild bee pollinators in agroecosystems

Bees provide essential pollination services that are potentially affected both by local farm management and the surrounding landscape. To better understand these different factors, we modelled the relative effects of landscape composition (nesting and floral resources within foraging distances), landscape configuration (patch shape, interpatch connectivity and habitat aggregation) and farm management (organic vs. conventional and local-scale field diversity), and their interactions, on wild bee abundance and richness for 39 crop systems globally. Bee abundance and richness were higher in diversified and organic fields and in landscapes comprising more high-quality habitats; bee richness on conventional fields with low diversity benefited most from high-quality surrounding land cover. Landscape configuration effects were weak. Bee responses varied slightly by biome. Our synthesis reveals that pollinator persistence will depend on both the maintenance of high-quality habitats around farms and on local management practices that may offset impacts of intensive monoculture agriculture.

Extremely frequent bee visits increase pollen deposition but reduce drupelet set in raspberry

Summary 1. Production of many flowering crops often benefits from elevated pollinator diversity and abundance. Nevertheless, the opposite relationship may arise if bees impair fruit or seed production and/or quality by damaging flowers during visitation, despite transferring pollen. 2. We assessed pollination and drupelet set (i.e. the number of drupelets per fruit) in 16 raspberry Rubus idaeus fields along a gradient of bee abundance in north-west Patagonia, Argentina. Using pollen supplementation, we also tested whether drupelet set was pollen limited in a subset of six fields. 3. Managed Apis mellifera and the invasive bumblebee Bombus terrestris accounted for 50% and 45% of all bee visits, respectively, to raspberry flowers. Pollen loads on stigmas increased with visit frequency of all bees combined and particularly with visitation by A. mellifera, but not by B. terrestris. Drupelet set was not pollen limited along the gradient of bee abundance. 4. Instead, drupelet set decreased with the proportion of damaged styles, which varied more strongly with the frequency of visits by B. terrestris than by A. mellifera. In fields with the highest bee frequency of visits ( � 300 visits flower � 1 day � 1 ), � 80% of styles were damaged in flowers and these developed into fruits with � 30% fewer drupelets compared to flowers in fields with the lowest bee visitation rates ( � 4 visits flower � 1 day � 1 ). 5. Synthesis and applications. Extreme bee visitation, particularly by Bombus terrestris, damaged the styles of raspberry flowers, precluding ovule fertilization by deposited pollen and limiting crop production by reducing drupelet set. Only a few bee visits are required to maximize fruit production in raspberry plants, therefore, pollinator management in north-west Patagonia should focus principally on reducing the abundance of the invasive bumblebee B. terrestris and secondarily controlling the number of honeybee hives in nearby cultivated fields. Although mainstream pollinator management relies on the assumption that more visits enhance fruit set, high bee visitation rates can be detrimental for fruit development and, consequently, for crop yield.

Interactive effects of large- and small-scale sources of feral honey-bees for sunflower in the Argentine Pampas.

Pollinators for animal pollinated crops can be provided by natural and semi-natural habitats, ranging from large vegetation remnants to small areas of non-crop land in an otherwise highly modified landscape. It is unknown, however, how different small- and large-scale habitat patches interact as pollinator sources. In the intensively managed Argentine Pampas, we studied the additive and interactive effects of large expanses (up to 2200 ha) of natural habitat, represented by untilled isolated “sierras”, and narrow (3–7 m wide) strips of semi-natural habitat, represented by field margins, as pollinator sources for sunflower (Helianthus annus). We estimated visitation rates by feral honey-bees, Apis mellifera, and native flower visitors (as a group) at 1, 5, 25, 50 and 100 m from a field margin in 17 sunflower fields 0–10 km distant from the nearest sierra. Honey-bees dominated the pollinator assemblage accounting for >90% of all visits to sunflower inflorescences. Honey-bee visitation was strongly affected by proximity to the sierras decreasing by about 70% in the most isolated fields. There was also a decline in honey-bee visitation with distance from the field margin, which was apparent with increasing field isolation, but undetected in fields nearby large expanses of natural habitat. The probability of observing a native visitor decreased with isolation from the sierras, but in other respects visitation by flower visitors other than honey-bees was mostly unaffected by the habitat factors assessed in this study. Overall, we found strong hierarchical and interactive effects between the study large and small-scale pollinator sources. These results emphasize the importance of preserving natural habitats and managing actively field verges in the absence of large remnants of natural habitat for improving pollinator services.

Disruption of Pollination Services by Invasive Pollinator Species

Plant–pollinator interactions and associated pollination services are essential for crop production and the integrity of terrestrial ecosystem services. Introduced pollinators, in particular social bees such as honeybees and bumblebees, have become invaders in many regions of the world, strongly affecting the pollination of native, cultivated, and non-native plants. These effects can be direct, when invaders interact with local flowering plants, or indirect, when invaders modify the interaction of native pollinators with flowering plants. Direct effects on pollination depend on whether the plant benefits from the flower visits are greater than their costs, a relationship that can be density dependent. Shifts from mutualism to antagonism occur when invasive pollinators reach extremely high densities, because the interaction costs exceed the benefits. Indirect effects depend on whether pollinator invaders alter the benefit–cost ratio of native pollinator visits, displace them, or trigger reductions in native pollinator diversity. Through a literature review, we found that the impacts of invasive pollinators on pollination were predominantly negative for native plants, mixed for crops, and positive for invasive plants. Furthermore, they can synergistically interact with other stressors on pollination such as climate change and habitat disturbance. Although invasive pollinators can back up pollination of some native plants in highly disturbed habitats, and some crops in intensively modified agro-ecosystems, they cannot replace the role of a diverse pollinator assemblage for wild plant reproduction and crop yield. Hence, managing agro-ecosystems for enhancing wild pollinator diversity, and avoiding further introductions of non-native pollinators, are realistic cost-effective measures for the provision and stability of pollination services.

Coordinated species importation policies are needed to reduce serious invasions globally: The case of alien bumblebees in South America

Fil: Aizen, Marcelo Adrian. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; Argentina