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Land-use intensification and loss of semi-natural habitats have induced a severe decline of bee diversity in agricultural landscapes. Semi-natural habitats like calcareous grasslands are among the most important bee habitats in central Europe, but they are threatened by decreasing habitat area and quality, and by homogenization of the surrounding landscape affecting both landscape composition and configuration. In this study we tested the importance of habitat area, quality and connectivity as well as landscape composition and configuration on wild bees in calcareous grasslands. We made detailed trait-specific analyses as bees with different traits might differ in their response to the tested factors. Species richness and abundance of wild bees were surveyed on 23 calcareous grassland patches in Southern Germany with independent gradients in local and landscape factors. Total wild bee richness was positively affected by complex landscape configuration, large habitat area and high habitat quality (i.e. steep slopes). Cuckoo bee richness was positively affected by complex landscape configuration and large habitat area whereas habitat specialists were only affected by the local factors habitat area and habitat quality. Small social generalists were positively influenced by habitat area whereas large social generalists (bumblebees) were positively affected by landscape composition (high percentage of semi-natural habitats). Our results emphasize a strong dependence of habitat specialists on local habitat characteristics, whereas cuckoo bees and bumblebees are more likely affected by the surrounding landscape. We conclude that a combination of large high-quality patches and heterogeneous landscapes maintains high bee species richness and communities with diverse trait composition. Such diverse communities might stabilize pollination services provided to crops and wild plants on local and landscape scales.
Animal pollinators are globally threatened by anthropogenic land use change and agricultural intensification. The yield of many food crops is therefore negatively impacted because they benefit from biotic pollination. This is especially the case in the tropics. For instance, fruit set of Coffea arabica has been shown to increase by 10–30% in plantations with a high richness of bee species, possibly influenced by the availability of surrounding forest habitat. Here, we performed a global literature review to (1) assess how much animal pollination enhances coffee fruit set, and to (2) examine the importance of the amount of forest cover, distance to nearby forest and forest canopy density for bee species richness and coffee fruit set. Using a systematic literature review, we identified eleven case studies with a total of 182 samples where fruit set of C. arabica was assessed. We subsequently gathered forest data for all study sites from satellite imagery. We modelled the effects of open (all forest with a canopy density of ≥25%), closed (≥50%) and dense (≥75%) forests on pollinator richness and fruit set of coffee. Overall, we found that animal pollination increases coffee fruit set by ~18% on average. In only one of the case studies, regression results indicate a positive effect of dense forest on coffee fruit set, which increased with higher forest cover and shorter distance to the forest. Against expectations, forest cover and distance to open forest were not related to bee species richness and fruit set. In summary, we provide strong empirical support for the notion that animal pollinators increase coffee fruit set. Forest proximity had little overall influence on bee richness and coffee fruit set, except when farms were surrounded by dense tropical forests, potentially because these may provide high-quality habitats for bees pollinating coffee. We, therefore, advocate that more research is done to understand the biodiversity value of dense forest for pollinators, notably assessing the mechanisms underlying the importance of forest for pollinators and their pollination services.
Biodiversity is in rapid decline worldwide. These declines are more pronounced in areas that are currently biodiversity rich, but economically poor – essentially describing many tropical regions in the Global South where landscapes are dominated by smallholder agriculture. Agriculture is an important driver of biodiversity decline, through habitat destruction and unsustainable practices. Ironically, agriculture itself is dependent on a range of ecosystem services, such as pollination and pest control, provided by biodiversity. Biodiversity on fields and the delivery of ecosystem services to crops is often closely tied to the composition of the surrounding landscape – complex landscapes with a higher proportion of (semi-)natural habitats tend to support a high abundances and biodiversity of pollinators and natural enemies that are beneficial to crop production. However, past landscape scale studies have focused primarily on industrialized agricultural landscapes in the Global North, and context dependent differences between regions and agricultural systems are understudied. Smallholder agriculture supports 2 billion people worldwide and contributes to over half the world’s food supply. Yet smallholders, particularly in sub-Saharan Africa, are underrepresented in research investigating the consequences of landscape change and agricultural practices. Where research in smallholder agriculture is conducted, the focus is often on commodity crops, such as cacao, and less on crops that are directly consumed by smallholder households, though the loss of services to these crops could potentially impact the most vulnerable farmers the hardest. Agroecology – a holistic and nature-based approach to agriculture, provides an alternative to unsustainable input-intensive agriculture. Agroecology has been found to benefit smallholders through improved agronomical and food-security outcomes. Co-benefits of agroecological practices with biodiversity and ecosystem services are assumed, but not often empirically tested. In addition, the local and landscape effects on biodiversity and ecosystem services are more commonly studied in isolation, but their potentially interactive effects are so far little explored. Our study region in northern Malawi exemplifies many challenges experienced by smallholder farmers throughout sub-Saharan Africa and more generally in the Global South. Malawi is located in a global biodiversity hotspot, but biodiversity is threatened by rapid habitat loss and a push for input-intensive agriculture by government and other stakeholders. In contrast, agroecology has been effectively promoted and implemented in the study region. We investigated how land-use differences and the agroecological practices affects biodiversity and ecosystem services of multiple taxa in a maize-bean intercropping system (Chapter 2), and pollination of pumpkin (Chapter 3) and pigeon pea (Chapter 4). Additionally, the effects of local and landscape scale shrub- to farmland habitat conversion was investigated on butterfly communities, as well as the potential for agroecology to mitigate these effects (Chapter 5).
The East Himalaya is one of the world’s most biodiverse ecosystems. However, very little is known about the abundance and distribution of many plant and animal taxa in this region. Bumble bees are a group of cold-adapted and high elevation insects that fulfil an important ecological and economical function as pollinators of wild and agricultural flowering plants and crops. The Himalayan mountain range provides ample suitable habitats for bumble bees. Systematic study of Himalayan bumble bees began a few decades ago and the main focus has centred on the western region, while the eastern part of the mountain range has received little attention and only a few species have been verified. During a three-year survey, more than 700 bumble bee specimens of 21 species were collected in Arunachal Pradesh, the largest of the north-eastern states of India. The material included a range of species that were previously known from a limited number of collected specimens, which highlights the unique character of the East Himalayan ecosystem. Our results are an important first step towards a future assessment of species distribution, threat, and conservation. Clear elevation patterns of species diversity were observed, which raise important questions about the functional adaptations that allow bumble bees to thrive in this particularly moist region in the East Himalaya.
Specialization of plant-pollinator interactions increases with temperature at Mt. Kilimanjaro
(2020)
Aim: Species differ in their degree of specialization when interacting with other species, with significant consequences for the function and robustness of ecosystems. In order to better estimate such consequences, we need to improve our understanding of the spatial patterns and drivers of specialization in interaction networks.
Methods: Here, we used the extensive environmental gradient of Mt. Kilimanjaro (Tanzania, East Africa) to study patterns and drivers of specialization, and robustness of plant–pollinator interactions against simulated species extinction with standardized sampling methods. We studied specialization, network robustness and other network indices of 67 quantitative plant–pollinator networks consisting of 268 observational hours and 4,380 plant–pollinator interactions along a 3.4 km elevational gradient. Using path analysis, we tested whether resource availability, pollinator richness, visitation rates, temperature, and/or area explain average specialization in pollinator communities. We further linked pollinator specialization to different pollinator taxa, and species traits, that is, proboscis length, body size, and species elevational ranges.
Results: We found that specialization decreased with increasing elevation at different levels of biological organization. Among all variables, mean annual temperature was the best predictor of average specialization in pollinator communities. Specialization differed between pollinator taxa, but was not related to pollinator traits. Network robustness against simulated species extinctions of both plants and pollinators was lowest in the most specialized interaction networks, that is, in the lowlands.
Conclusions: Our study uncovers patterns in plant–pollinator specialization along elevational gradients. Mean annual temperature was closely linked to pollinator specialization. Energetic constraints, caused by short activity timeframes in cold highlands, may force ectothermic species to broaden their dietary spectrum. Alternatively or in addition, accelerated evolutionary rates might facilitate the establishment of specialization under warm climates. Despite the mechanisms behind the patterns have yet to be fully resolved, our data suggest that temperature shifts in the course of climate change may destabilize pollination networks by affecting network architecture.
Body size is an integral functional trait that underlies pollination-related ecological processes, yet it is often impractical to measure directly. Allometric scaling laws have been used to overcome this problem. However, most existing models rely upon small sample sizes, geographically restricted sampling and have limited applicability for non-bee taxa. Allometric models that consider biogeography, phylogenetic relatedness, and intraspecific variation are urgently required to ensure greater accuracy. We measured body size as dry weight and intertegular distance (ITD) of 391 bee species (4,035 specimens) and 103 hoverfly species (399 specimens) across four biogeographic regions: Australia, Europe, North America, and South America. We updated existing models within a Bayesian mixed-model framework to test the power of ITD to predict interspecific variation in pollinator dry weight in interaction with different co-variates: phylogeny or taxonomy, sexual dimorphism, and biogeographic region. In addition, we used ordinary least squares regression to assess intraspecific dry weight ~ ITD relationships for ten bees and five hoverfly species. Including co-variates led to more robust interspecific body size predictions for both bees and hoverflies relative to models with the ITD alone. In contrast, at the intraspecific level, our results demonstrate that the ITD is an inconsistent predictor of body size for bees and hoverflies. The use of allometric scaling laws to estimate body size is more suitable for interspecific comparative analyses than assessing intraspecific variation. Collectively, these models form the basis of the dynamic R package, “pollimetry,” which provides a comprehensive resource for allometric pollination research worldwide.
Avocado (Persea americana Mill.) is a major horticultural crop that relies on insect mediated pollination. In avocado production, a knowledge gap exists as to the importance of insect pollination, especially in East African smallholder farms. Although it is evident that pollination improves the yield of avocado fruits, it is still unclear if pollination has benefits on fruit quality and the nutritional profile, particularly oils. Prior studies have shown that honey bees increase avocado’s fruit set and yield. However, an avocado flower is being visited by various insect species. Therefore, determining pollination efficiency will allow a comparison of the relative importance of the different insect species to optimize crop pollination for increased fruit set and crop yield and pollinator conservation. This study was conducted in a leading smallholder avocado production region in Kenya, first I assessed the dependence of avocado fruit set on insect pollination and whether current smallholder production systems suffer from a deficit in pollination services. Furthermore, I assessed if supplementation with colonies of the Western honey bee (Apis mellifera L.) to farms mitigated potential pollination deficits. The results revealed a very high reliance of avocado on insect pollinators, with a significantly lower fruit set observed for self- and wind-pollinated (17.4%) or self-pollinated flowers (6.4%) in comparison with insect-pollinated flowers (89.5%). I found a significant pollination deficit across farms, with hand-pollinated flowers on average producing 20.7% more fruits than non-treated open flowers prior to fruit abortion. This pollination deficit could be compensated by the supplementation of farms with A. mellifera colonies. These findings suggest that pollination is limiting fruit set in avocado and that A. mellifera supplementation on farms is a potential option to increase fruit yield. Secondly, I investigated the contribution of insect pollination to fruit and seed weight, oil, protein, carbohydrate, and phytochemicals contents (flavonoids and phenolics), and whether supplementation with pollinators (honey bee) could improve these fruit parameters was assessed. This was through pollinator-manipulative pollination treatments: hand, open, pollinator exclusion experiments. The results showed that avocado fruit weight was significantly higher in open and hand-pollinated than pollinator exclusion treatments, indicating that flower visitors/pollinators contribute to avocado yields and enhance marketability. Furthermore, insect pollination resulted in heavier seeds and higher oil contents, indicating that insect pollination is beneficial for the fruit’s high seed yield and quantity of oil. Honey bee supplementation also enhanced the avocado fruit weight by 18% more than in control farms and slightly increased the avocado oil content (3.6%). Contrarily, insect pollination did not influence other assayed fruit quality parameters (protein, carbohydrates, and phytochemicals). These results indicate that insect pollinators are essential for optimizing avocado yields, nutritional quality (oils), and thus marketability, underscoring the value of beehive supplementation to achieve high-quality avocado fruits and improved food security. Thirdly, pollinator efficiency based on pollen deposition after single visits by different pollinator species in avocado flowers was tested, and their frequency was recorded. The estimated pollination efficiency was highest in honey bees (Apis mellifera), followed by the hoverfly species (Phytomia incisa). These two species had the highest pollen deposition and more pollen grains on their bodies. In addition, honey bees were the most frequent avocado flower visitors, followed by flies. The findings from this study highlight the higher pollination efficiency of honey bees and Phytomia incisa. Hence, management practices supporting these species will promote increased avocado fruit yield. Additionally, these results imply that managed honey bees can be maintained to improve avocado pollination, particularly in areas lacking sufficient wild pollinators.
Agricultural biodiversity and associated ecosystem functions are declining at alarming rates due to widespread land use intensification. They can only be maintained through targeted landscape management that supports species with different habitat preferences, dispersal capacities and other functional traits that determine their survival. However, we need better understanding whether short-term measures can already improve functional diversity in European agroecosystems.
We investigated spatio-temporal responses of bees (solitary bees, bumblebees and honey bees), hoverflies, carabid beetles and spiders to newly established grassland strips in Lower Austria over 3 years, and along a distance gradient to old grasslands. Specifically, we asked if new grasslands, compared to old grasslands and cereal fields, serve as temporal dispersal habitat or corridor, and how species-specific traits affect dispersal patterns. Using a trait-based functional diversity approach, we investigated year and distance effects for nine selected key traits per taxon (e.g. body size, feeding guild and habitat preferences).
Our results show that the functional diversity of predators and pollinators (i.e. functional richness and evenness), as well as community-weighted means of selected key traits in new grasslands significantly differed from adjacent cereal fields, but only slowly adjusted to adjacent old grasslands. These effects significantly decreased with increasing distance to old grasslands for carabids and spiders, but not for mobile bees and hoverflies.
Synthesis and applications. Over 3 years, newly established grassland strips supported larger sized and actively foraging/hunting species in the agricultural landscape. Adjacent crops likely benefit from such measures through enhanced functional diversity and related ecosystem services. However, our results also suggest that 3-year period is too short to enhance the occurrence of pollinators and epigeic predators in new grasslands. Agri-environment measures need to be complemented by the conservation of permanent habitats to effectively maintain species and functional diversity. Our findings should be acknowledged by European policy and agricultural decision makers for the design of more effective agri-environment schemes, taking into account trait-dependent species responses to land use change.
Mimicking female insects to attract male pollinators is an important strategy in sexually deceptive orchids of the genus Ophrys, and some species possess flowers with conspicuous labellum patterns. The function of the variation of the patterns remains unresolved, with suggestions that these enhance pollinator communication. We investigated the possible function of the labellum pattern in Ophrys heldreichii, an orchid species in which the conspicuous and complex labellum pattern contrasts with a dark background. The orchid is pollinated exclusively by males of the solitary bee, Eucera berlandi. Comparisons of labellum patterns revealed that patterns within inflorescences are more similar than those of other conspecific plants. Field observations showed that the males approach at a great speed and directly land on flowers, but after an unsuccessful copulation attempt, bees hover close and visually scan the labellum pattern for up to a minute. Learning experiments conducted with honeybees as an accessible model of bee vision demonstrated that labellum patterns of different plants can be reliably learnt; in contrast, patterns of flowers from the same inflorescence could not be discriminated. These results support the hypothesis that variable labellum patterns in O. heldreichii are involved in flower-pollinator communication which would likely help these plants to avoid geitonogamy.
Sexually deceptive orchids mimic signals emitted by female insects in order to attract mate-searching males. Specific attraction of the targeted pollinator is achieved by sex pheromone mimicry, which constitutes the major attraction channel. In close vicinity of the flower, visual signals may enhance attraction, as was shown recently in the sexually deceptive orchid Ophrys heldreichii. Here, we conducted an in situ manipulation experiment in two populations of O. heldreichii on Crete to investigate whether the presence/absence of the conspicuous pink perianth affects reproductive success in two natural orchid populations. We estimated reproductive success of three treatment groups (with intact, removed and artificial perianth) throughout the flowering period as pollinaria removal (male reproductive success) and massulae deposition (female reproductive success). Reproductive success was significantly increased by the presence of a strong visual signal—the conspicuous perianth—in one study population, however, not in the second, most likely due to the low pollinator abundance in the latter population. This study provides further evidence that the coloured perianth in O. heldreichii is adaptive and thus adds to the olfactory signal to maximise pollinator attraction and reproductive success.