Joint Impacts of Drought and Habitat Fragmentation on Native Bee Assemblages in a California Biodiversity Hotspot

Identifying protected areas in biodiversity hotspots at risk from climate and human-induced compound events for conserving threatened species

Anthropogenic pressure in areas of biodiversity importance erodes the integrity of the ecosystems they harbour, making features of biodiversity less buffered against extreme climatic events. We define the combination of these disturbances as compound events. We assessed compound event risk in protected areas (PAs) applying a spatial framework guided by criteria and quantitative thresholds associated with exposure to cyclones, drought, and intense human pressure. This assessment was used in a relational matrix to classify PAs with different risk of compound event occurrence. We identified PAs of higher conservation concern by quantifying the extent of human pressure in their surrounding landscape while harbouring large numbers of threatened vertebrate species. Of the 39,694 PAs assessed, very high risk of compound events was determined for 6965 PAs (17.5 %) related to cyclones and human pressure (mainly island hotspots), 6367 PAs (16 %) related to droughts and human pressure (island and continental hotspots), and 2031 PAs (5 %) to cyclones, drought and human pressure (mainly in island hotspots). From the subset of 2031 PAs assessed at very high risk, we identified 239 PAs of higher conservation concern distributed predominantly in the Caribbean Islands, Japan, North America Coastal Plain, Philippines, and Southwest Australia. Our work highlights PAs in the biodiversity hotspots where high risk of compound event occurrence poses a greater threat to species. We encourage researchers to adapt and apply this framework across other globally significant sites for conserving biodiversity to identify high risk-prone areas, and prevent further biodiversity decline.

Heat and desiccation tolerances predict bee abundance under climate change

Climate change could pose an urgent threat to pollinators, with critical ecological and economic consequences. However, for most insect pollinator species, we lack the long-term data and mechanistic evidence that are necessary to identify climate-driven declines and predict future trends. Here we document 16 years of abundance patterns for a hyper-diverse bee assemblage1 in a warming and drying region2, link bee declines with experimentally determined heat and desiccation tolerances, and use climate sensitivity models to project bee communities into the future. Aridity strongly predicted bee abundance for 71% of 665 bee populations (species × ecosystem combinations). Bee taxa that best tolerated heat and desiccation increased the most over time. Models forecasted declines for 46% of species and predicted more homogeneous communities dominated by drought-tolerant taxa, even while total bee abundance may remain unchanged. Such community reordering could reduce pollination services, because diverse bee assemblages typically maximize pollination for plant communities3. Larger-bodied bees also dominated under intermediate to high aridity, identifying body size as a valuable trait for understanding how climate-driven shifts in bee communities influence pollination4. We provide evidence that climate change directly threatens bee diversity, indicating that bee conservation efforts should account for the stress of aridity on bee physiology.

Ecological Drivers and Consequences of Bumble Bee Body Size Variation

Body size is arguably one of the most important traits influencing the physiology and ecology of animals. Shifts in animal body size have been observed in response to climate change, including in bumble bees (Bombus spp. [Hymenoptera: Apidae]). Bumble bee size shifts have occurred concurrently with the precipitous population declines of several species, which appear to be related, in part, to their size. Body size variation is central to the ecology of bumble bees, from their social organization to the pollination services they provide to plants. If bumble bee size is shifted or constrained, there may be consequences for the pollination services they provide and for our ability to predict their responses to global change. Yet, there are still many aspects of the breadth and role of bumble bee body size variation that require more study. To this end, we review the current evidence of the ecological drivers of size variation in bumble bees and the consequences of that variation on bumble bee fitness, foraging, and species interactions. In total we review: (1) the proximate determinants and physiological consequences of size variation in bumble bees; (2) the environmental drivers and ecological consequences of size variation; and (3) synthesize our understanding of size variation in predicting how bumble bees will respond to future changes in climate and land use. As global change intensifies, a better understanding of the factors influencing the size distributions of bumble bees, and the consequences of those distributions, will allow us to better predict future responses of these pollinators.

Community Science Reveals High Diversity of Nectaring Plants Visited by Painted Lady Butterflies (Lepidoptera: Nymphalidae) in California Sage Scrub

California's sage scrub habitats support a diversity of nectar and host plants for migrating and resident populations of painted lady butterflies (Vanessa cardui) throughout all seasons. North America experiences spring V. cardui migrations involving butterflies totaling in the millions in some years. These irruptive years are thought to be driven by winter weather patterns at breeding grounds near the US-Mexico border and due to their irregularity, it is difficult to study floral resource use along the migration route. Here we used the community science platform iNaturalist to quantify patterns in V. cardui nectar resource use in sage scrub over time and space during irruptive and nonirruptive years. We identified over 329 different nectaring plant species of varying functional types (72% native to California) visited by adult V. cardui, 195 of which had not been previously identified as known nectar plants for V. cardui. Vanessa cardui butterflies were observed in similar locations regardless of whether an irruptive migration occurred, indicating the presence of either sparse migrants or resident populations across California. Moreover, irruptive years were positively correlated with warmer and wetter local conditions at observation locations. Our results provide new insights into patterns of floral resource use by North American V. cardui by harnessing the power of community science data and while highlighting the factors associated with its North American migration.

Occurrence of Honey Bee (Apis mellifera L.) Pathogens in Wild Pollinators in Northern Italy

Diseases contribute to the decline of pollinator populations, which may be aggravated by the interspecific transmission of honey bee pests and pathogens. Flowers increase the risk of transmission, as they expose the pollinators to infections during the foraging activity. In this study, both the prevalence and abundance of 21 honey bee pathogens (11 viruses, 4 bacteria, 3 fungi, and 3 trypanosomatids) were assessed in the flower-visiting entomofauna sampled from March to September 2021 in seven sites in the two North-Italian regions, Emilia-Romagna and Piedmont. A total of 1,028 specimens were collected, identified, and analysed. Of the twenty-one pathogens that were searched for, only thirteen were detected. Altogether, the prevalence of the positive individuals reached 63.9%, with Nosema ceranae, deformed wing virus (DWV), and chronic bee paralysis virus (CBPV) as the most prevalent pathogens. In general, the pathogen abundance averaged 5.15 * 10⁶ copies, with CBPV, N. ceranae, and black queen cell virus (BQCV) as the most abundant pathogens, with 8.63, 1.58, and 0.48 * 10⁷ copies, respectively. All the detected viruses were found to be replicative. The sequence analysis indicated that the same genetic variant was circulating in a specific site or region, suggesting that interspecific transmission events among honey bees and wild pollinators are possible. Frequently, N. ceranae and DWV were found to co-infect the same individual. The circulation of honey bee pathogens in wild pollinators was never investigated before in Italy. Our study resulted in the unprecedented detection of 72 wild pollinator species as potential hosts of honey bee pathogens. Those results encourage the implementation of monitoring actions aiming to improve our understanding of the environmental implications of such interspecific transmission events, which is pivotal to embracing a One Health approach to pollinators’ welfare.

Soil moisture affects plant-pollinator interactions in an annual flowering plant

Many environmental factors impact plant and pollinator communities. However, variation in soil moisture and how it mediates the plant-pollinator interactions has yet to be elucidated. We hypothesized that long-term variation in soil moisture can exert a strong selective pressure on the floral and vegetative traits of plants, leading to changes in pollinator visitation. We demonstrated that there are three phenotypic populations of Gentiana aristata in our study alpine region in the Qinghai-Tibetan Plateau that vary in floral colour and other traits. Pink (dry habitat) and blue (intermediate habitat) flower populations are visited primarily by bumblebees, and white (wet habitat) flower populations are visited by flies. These patterns of visitation are driven by vegetative and floral traits and are constant when non-endemic plants are placed in the intermediate habitats. Additionally, the floral communities in different habitats vary, with more insect-pollinated forbs in the dry and intermediate habitats versus the wet habitats. Through a common garden and reciprocal transplant experiment, we demonstrated that plant growth traits, pollinator attractiveness and seed production are highest when the plant population is raised in its endemic habitat. This suggests that these plant populations have evolved to pollinator communities associated with habitat differences. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Diversity and turnover of wild bee and ornamental plant assemblages in commercial plant nurseries

In human-modified landscapes, understanding how habitat characteristics influence the diversity and composition of beneficial organisms is critical to conservation efforts and modeling ecosystem services. Assessing turnover, or the magnitude of change in species composition across sites or through time, is crucial to said efforts, yet is often overlooked. For pollinators such as wild bees, variables influencing temporal turnover, particularly across seasons within a year, remain poorly understood. To investigate how local and landscape characteristics correlate with bee diversity and turnover across seasons, we recorded wild bee and flowering ornamental plant assemblages at 13 plant nurseries in California between spring and autumn over 2 years. Nurseries cultivate a broad diversity of flowering plant species that differ widely across sites and seasons, providing an opportunity to test for correlations between turnover and diversity of plants and bees. As expected, we documented strong seasonal trends in wild bee diversity and composition. We found that local habitat factors, such as increased cultivation of native plants, were positively associated with bee diversity in sweep netting collections, whereas we detected moderate influences of landscape level factors such as proportion of surrounding natural area in passive trap collections. We also detected a moderate positive correlation between the magnitude of turnover in plant species and that of bee species (as number of taxa gained) across consecutive seasons. Our results have implications for the conservation of wild bees in ornamental plant landscapes, and highlight the utility of plant nurseries for investigating hypotheses related to diversity and turnover in plant-pollinator systems.

Maine's Bumble Bee (Hymenoptera: Apidae) Assemblage-Part 1: Composition, Seasonal and Regional Distribution, and Resource Use

Global declines of bumble bees place natural and agricultural ecosystems at risk. Given bumble bees importance to Maine's major agricultural crops, we conducted a statewide, quantitative survey of bumble bee species seasonal and ecoregional abundance, richness, diversity, and floral resource use. We recorded 11 Bombus species at 40 survey sites across Maine's three ecoregions, with Bombus ternarius Cresson, 1863 and Bombus impatiens Cresson, 1863 being the most common and Bombus citrinus Smith, 1854 the least commonly encountered. Bumble bee species richness did not differ as a function of ecoregion, but did decline over the season, while species diversity differed by ecoregion and also declined over the season. Multiple response permutation procedure (MRPP) indicated ecoregional differences in species composition of bumble bee assemblages and nonmetric multidimensional scaling produced a stable ordination suggesting assemblage differences were associated with survey site variables including forage plant cover, forage plant richness, elevation, development, and deciduous forest cover. Both MRPP and correspondence analysis also revealed differences in the floral resources utilized by bumble bee species in each ecoregion. Low connectance and nestedness levels indicated low stability pollinator networks in each ecoregion, suggesting Maine bumble bee assemblages may be at risk of decline in response to additional external perturbations.