A primer of host-plant specialization in bees

Insect pollination in deep time

Inferring insect pollination from compression fossils and amber inclusions is difficult because of a lack of consensus on defining an insect pollinator and the challenge of recognizing this ecological relationship in deep time. We propose a conceptual definition for such insects and an operational classification into pollinator or presumed pollinator. Using this approach, we identified 15 insect families that include fossil pollinators and show that pollination relationships have existed since at least the Upper Jurassic (~163 Ma). Insects prior to this can only be classified as presumed pollinators. This gives a more nuanced insight into the origin and evolution of an ecological relationship that is vital to the establishment, composition and conservation of modern terrestrial ecosystems.

Generalism in nature: a community ecology perspective

Life on Earth is complex and generally abounds in food webs with other living organisms in terms of an ecological community. Besides such complexity, and the fact that populations of most living organisms have never been studied in terms of their molecular ecology, it is best to tread carefully when describing a given species as a ‘generalist’, more especially in terms of dietary and habitat breadth. We very much doubt that population homogeneity ever exists—because populations are always undergoing molecular-genetic changes, sometimes rapid, in response to various ecological challenges (e.g. climate, intra- and interspecific competition). In any case, a population may already have begun to undergo cryptic speciation. Such entities can occupy different habitats or exhibit different dietary breadths as a result of various ecological interactions formed over different spatial scales. These scales include everything from local (including islands) to geographic. The fossil evidence reveals that specialisations have existed over vast swathes of time. Besides, as is well documented, evolution only occurs as a result of adaptations leading to specialisation, and ultimately, specialist entitles, i.e. species and lower levels of ecological-evolutionary divergence. Here, focusing on diet, we posit that the terms mono-, oligo-and polyphagous are more accurate in relation to the dietary breadth of animals, with omnivory adopted in the case of organisms with very different food items. Thus, we strongly urge that the dubious and unscientific term ‘generalism’ be dropped in favour of these more precise and scientifically accurate terms directly relating to levels of phagy.

The Native Bees of Lolland (Denmark) Revisited after 100 Years: The Demise of the Specialists

There is a global concern over insect declines, including both species loss and population declines. In particular, declines of species, such as bees that anchor trophic interactions and shoulder many of the essential ecosystem services, have been the focus of broad public concern. However, our understanding of what characterizes those species that are lost because of declines over long periods is limited by a scarcity of comparative studies. We here compare the species composition from a collection of bees sampled over two decades (2000–2019) from the island of Lolland in Denmark, with a collection also sampled over two decades (1900–1919), but a century ago by Jørgensen and his contemporaries. We further test if (1) the probability that bee species that were sampled a century ago are also found today depends on their degree of floral specialization; (2) and use information from pollen samples from bees from the historical records to assess if certain floral resources have been lost. In total, 203 species were recorded in the two periods, but less than half, 92 species, occurred in both sampling periods. A total of 174 species of bees were recorded from 1900–1919, and 121 species were recorded from 2000–2019, including 29 species not reported in 1900–1919. Notably, we report a reduction in the species composition among forage specialist bees from 26.4% to 15.7% of the bee fauna, a consistent and highly significant decline both when correcting for parasitic and social species. Pollen swabs from bees collected in the first period, 1900–1919, did not identify any plants that are not available today but documented a series of plants that were important to bees back then. These plants are still common today, such as Taraxacum and Salix. Our findings highlight the importance of local and historical faunistic studies, such as that of Jørgensen, who was a resident schoolteacher on the island of Lolland in southern Denmark, for documenting how changes over time affect the species composition in bee communities.

Are Bumblebees Relevant Models for Understanding Wild Bee Decline?

The unsustainable use of ecosystems by human societies has put global biodiversity in peril. Bees are, in this context, a popular example of a highly diversified group of pollinators whose collapse is a major concern given the invaluable ecosystem services they provide. Amongst them, bumblebees (Bombus) have increasingly drawn the attention of scientists due to their dramatic population declines globally. This regression has converted them into popular conservation entities, making them the second most studied group of bees worldwide. However, in addition to have become relevant models in the fields of ecology, evolution and biogeography, bumblebees have also been used as models for studying wild bee decline and conservation worldwide. Integrating evidence from the comparative ecology and resilience of bumblebees and wild bees, I discuss the relevance of using Bombus as radars for wild bee decline worldwide. Responses of bumblebees to environmental changes are generally not comparable with those of wild bees because of their relatively long activity period, their inherent sensitivity to high temperatures, their relatively generalist diet breadth and many aspects arising from their eusocial behavior. Moreover, important differences in the available historical data between bumblebees and other bees make comparisons of conservation status even more arduous. Overall, these results reinforce the need for conservation actions that consider a higher level of understanding of ecological diversity in wild bees, highlight the need for an updated and more extensive sampling of these organisms, and emphasize that more caution is required when extrapolating trends from model species.

Bee flowers drive macroevolutionary diversification in long-horned bees

The role of plant–pollinator interactions in the rapid radiation of the angiosperms have long fascinated evolutionary biologists. Studies have brought evidence for pollinator-driven diversification of various plant lineages, particularly plants with specialized flowers and concealed rewards. By contrast, little is known about how this crucial interaction has shaped macroevolutionary patterns of floral visitors. In particular, there is currently no empirical evidence that floral host association has increased diversification in bees, the most prominent group of floral visitors that essentially rely on angiosperm pollen. In this study, we examine how floral host preference influenced diversification in eucerine bees (Apidae, Eucerini), which exhibit large variations in their floral associations. We combine quantitative pollen analyses with a recently proposed phylogenetic hypothesis, and use a state speciation and extinction probabilistic approach. Using this framework, we provide the first evidence that multiple evolutionary transitions from host plants with accessible pollen to restricted pollen from ‘bee-flowers’ have significantly increased the diversification of a bee clade. We suggest that exploiting host plants with restricted pollen has allowed the exploitation of a new ecological niche for eucerine bees and contributed both to their colonization of vast regions of the world and their rapid diversification.

Pollen sterols are associated with phylogeny and environment but not with pollinator guilds

Phytosterols are primary plant metabolites that have fundamental structural and regulatory functions. They are also essential nutrients for phytophagous insects, including pollinators, that cannot synthesize sterols. Despite the well-described composition and diversity in vegetative plant tissues, few studies have examined phytosterol diversity in pollen. We quantified 25 pollen phytosterols in 122 plant species (105 genera, 51 families) to determine their composition and diversity across plant taxa. We searched literature and databases for plant phylogeny, environmental conditions, and pollinator guilds of the species to examine the relationships with pollen sterols. 24-methylenecholesterol, sitosterol and isofucosterol were the most common and abundant pollen sterols. We found phylogenetic clustering of twelve individual sterols, total sterol content and sterol diversity, and of sterol groupings that reflect their underlying biosynthesis pathway (C-24 alkylation, ring B desaturation). Plants originating in tropical-like climates (higher mean annual temperature, lower temperature seasonality, higher precipitation in wettest quarter) were more likely to record higher pollen sterol content. However, pollen sterol composition and content showed no clear relationship with pollinator guilds. Our study is the first to show that pollen sterol diversity is phylogenetically clustered and that pollen sterol content may adapt to environmental conditions.

Evaluating competition for forage plants between honey bees and wild bees in Denmark

A recurrent concern in nature conservation is the potential competition for forage plants between wild bees and managed honey bees. Specifically, that the highly sophisticated system of recruitment and large perennial colonies of honey bees quickly exhaust forage resources leading to the local extirpation of wild bees. However, different species of bees show different preferences for forage plants. We here summarize known forage plants for honey bees and wild bee species at national scale in Denmark. Our focus is on floral resources shared by honey bees and wild bees, with an emphasis on both threatened wild bee species and foraging specialist species. Across all 292 known bee species from Denmark, a total of 410 plant genera were recorded as forage plants. These included 294 plant genera visited by honey bees and 292 plant genera visited by different species of wild bees. Honey bees and wild bees share 176 plant genera in Denmark. Comparing the pairwise niche overlap for individual bee species, no significant relationship was found between their overlap and forage specialization or conservation status. Network analysis of the bee-plant interactions placed honey bees aside from most other bee species, specifically the module containing the honey bee had fewer links to any other modules, while the remaining modules were more highly inter-connected. Despite the lack of predictive relationship from the pairwise niche overlap, data for individual species could be summarized. Consequently, we have identified a set of operational parameters that, based on a high foraging overlap (>70%) and unfavorable conservation status (Vulnerable+Endangered+Critically Endangered), can guide both conservation actions and land management decisions in proximity to known or suspected populations of these species.

Potential regional declines in species richness of tomato pollinators in North America under climate change

About 70% of the world's main crops depend on insect pollination. Climate change is already affecting the abundance and distribution of insects, which could cause geographical mismatches between crops and their pollinators. Crops that rely primarily on wild pollinators (e.g., crops that cannot be effectively pollinated by commercial colonies of honey bees) could be particularly in jeopardy. However, limited information on plant-pollinator associations and pollinator distributions complicate the assessment of climate change impacts on specific crops. To study the potential impacts of climate change on pollination of a specific crop in North America, we use the case of open-field tomato crops, which rely on buzz pollinators (species that use vibration to release pollen, such as bumble bees) to increase their production. We aimed to (1) assess potential changes in buzz pollinator distribution and richness, and (2) evaluate the overlap between areas with high densities of tomato crops and high potential decrease in richness. We used baseline (1961-1990) climate and future (2050s and 2080s) climatic projections in ecological niche models fitted with occurrences of wild bees, documented in the literature as pollinators of tomatoes, to estimate the baseline and future potential distribution of suitable climatic conditions of targeted species and to create maps of richness change across North America. We obtained reliable models for 15 species and found important potential decreases in the distribution of some pollinators (e.g., Lasioglossum pectorale and Augochlorella aurata). We observed geographical discrepancies in the projected change in species richness across North America, detecting important declines in the eastern United States (up to 11 species decrease for 2050s). After overlapping the maps of species richness change with a tomato crop map for the United States, we found spatial correspondence between richness declines and areas with high concentration of tomato crops. Disparities in the effects of climate change on the potential future distribution of different wild pollinators and geographical variation in richness highlight the importance of crop-specific studies. Our study also emphasizes the challenges of compiling and modeling crop-specific pollinator data and the need to improve our understanding of current distribution of pollinators and their community dynamics under climate change.

New directions in pollinator research: diversity, conflict and response to global change

Interactions between pollinators and their plant hosts are central to maintaining global biodiversity and ensuring our food security. In this special issue, we compile reviews that summarize existing knowledge and point out key outstanding research areas to understand and safeguard pollinators, pollinators-host plant interactions and the pollination ecosystem services they provide. The vast diversity of the pollinator-plant interactions that exists on this planet still remains poorly explored, with many being associations involving a specialist pollinator partner, although historically most focus has been given to generalist pollinators, such as the honeybee. Two areas highlighted here are the ecology and evolution of oligolectic bee species, and the often-neglected groups of pollinators that forage solely at night. Advances in automated detection technologies could offer potential and complementary solutions to the current shortfall in knowledge on interactions occurring between less well-documented plant-pollinator associations, by increasing the collection range and capacity of flower visitation data over space and time. Pollinator-host plant interactions can be affected by external biotic factors, with herbivores and pathogens playing particularly important roles. Such interactions can be disrupted by modifying plant volatile and reward chemistry, with possible effects on pollinator attraction and pollination success. Mechanisms which underpin interactions between plants and their pollinators also face many anthropogenic disturbances. Reviews in this issue discuss threats from parasites and climate change to pollinator populations and plant-pollinator networks, and suggest new ways to mitigate these threats. While the protection of existing plant-pollinator networks will be a crucial goal for conservation biology, more research is needed to understand how lost interactions in degraded habitats may be restored with mutual benefits to plants and pollinators.