Anthropogenic nitrogen addition interrupts seasonal connectivity and structures of plant–pollinator networks

Floral visitation alone has been typically used to characterize plant–pollinator interaction networks even though it ignores differences in the quality of floral visits (e.g. transport of pollen) and thus may overestimate the number and functional importance of pollinating interactions. However, how network structural properties differ between floral visitation and pollen transport networks is not well understood. Furthermore, the strength and frequency of plant–pollinator interactions may vary across fine temporal scales (within a single season) further limiting our predictive understanding of the drivers and consequences of plant–pollinator network structure. Thus, evaluating the structure of pollen transport networks and how they change within a flowering season may help increase our predictive understanding of the ecological consequences of plant–pollinator network structure. Here we compare plant–pollinator network structure using floral visitation and pollen transport data and evaluate within-season variation in pollen transport network structure in a diverse plant–pollinator community. Our results show that pollen transport networks provide a more accurate representation of the diversity of plant–pollinator interactions in a community but that floral visitation and pollen transport networks do not differ in overall network structure. Pollen transport network structure was relatively stable throughout the flowering season despite changes in plant and pollinator species composition. Overall, our study highlights the need to improve our understanding of the drivers of plant–pollinator network structure in order to more fully understand the process that govern the assembly of these interactions in nature.

Simple SummaryComparisons of plant and insect pollinator networks along elevational gradients can help predict future impacts of changing climate on pollinator distribution on local scales. We compare the pollination network structure along the altitudinal gradient of the San Francisco Peaks in Arizona. We evaluate shifts in network connectance, nestedness, modularity, and overall generalization with increased elevation. We conclude that plant–pollinator networks become more nested and generalized with elevation and identify the insect pollinator species most critical for network stability at the higher elevation pollination community. The variation in plant–pollinator network structure at different elevation zones of the San Francisco Peaks helps unveil which local communities currently support the most stable systems in the face of climate change.The structural patterns comprising bimodal pollination networks can help characterize plant–pollinator systems and the interactions that influence species distribution and diversity over time and space. We compare network organization of three plant–pollinator communities along the altitudinal gradient of the San Francisco Peaks in northern Arizona. We found that pollination networks become more nested, as well as exhibit lower overall network specialization, with increasing elevation. Greater weight of generalist pollinators at higher elevations of the San Francisco Peaks may result in plant–pollinator communities less vulnerable to future species loss due to changing climate or shifts in species distribution. We uncover the critical, more generalized pollinator species likely responsible for higher nestedness and stability at the higher elevation environment. The generalist species most important for network stability may be of the greatest interest for conservation efforts; preservation of the most important links in plant–pollinator networks may help secure the more specialized pollinators and maintain species redundancy in the face of ecological change, such as changing climate.

Anthropogenic environmental change disrupts interactions between plants and their animal pollinators. To assess the importance of different drivers, baseline information is needed on interaction networks and plant reproductive success around the world. We conducted a systematic literature review to determine the state of our knowledge on plant–pollinator interactions and the ecosystem services they provide for European ecosystems. We focussed on studies that published information on plant–pollinator networks, as a community-level assessment of plant–pollinator interactions and pollen limitation, which assesses the degree to which plant reproduction is limited by pollinator services. We found that the majority of our knowledge comes from Western Europe, and thus there is a need for baseline assessments in the traditional landscapes of Eastern Europe. To address this data gap, we quantified plant–pollinator interactions and conducted breeding system and pollen supplementation experiments in a traditionally managed mountain meadow in the Western Romanian Carpathians. We found the Romanian meadow to be highly diverse, with a healthy plant–pollinator network. Despite the presence of many pollinator-dependent plant species, there was no evidence of pollen limitation. Our study is the first to provide baseline information for a healthy meadow at the community level on both plant–pollinator interactions and their relationship with ecosystem function (e.g. plant reproduction) in an Eastern European country. Alongside the baseline data, we also provide recommendations for future research, and the methodological information needed for the continued monitoring and management of Eastern European meadows.

Pollination for the people

Pollinator declines pose a significant threat to ecosystem services, making effective monitoring methods critical for conservation efforts. Current research on pollination interactions remains constrained by traditional methods such as direct observations, which have limited spatial and temporal coverage and are inherently biased toward diurnal interactions. Moreover, the presence of human observers can alter pollinator behaviour, and field notebooks serve as the only permanent records, restricting data accessibility and reproducibility. To overcome these challenges, we developed an Automated Camera System (ACS) that integrates Raspberry Pi hardware with YOLOv5, a deep learning‐based object detection model, to detect pollinators in plant communities. The system was trained on video recordings of plant–pollinator interactions collected over 4 years across four different islands. Detected pollinators were subsequently classified to the species level by an entomologist, who also determined which individuals had landed on floral structures. We also conducted 13 field campaigns across six study sites over two spring seasons to monitor all flowering plants using both direct observations and multiple ACS units. This approach allowed us to compare the plant–pollinator data acquired by each method. The resulting datasets derived from ACS and direct observations shared similar characteristics (e.g. species richness and Shannon diversity ) and core interactions. However, ACS recorded higher pollinator visitation rates per individual and species than direct observations, likely due to the absence of human observers. Additionally, ACS detected a greater proportion of interactions, particularly low‐frequency ones, which significantly influenced network metrics and lead to higher connectance and specialisation and lower robustness . Despite these advantages, ACS had difficulties in detecting small‐bodied pollinators (<5 mm), highlighting an area for future refinement. This is the first study to introduce an open‐source tool for automatically detecting both nocturnal and diurnal plant–pollinator interactions across natural plant communities and to cross‐validate it with direct observations. We foresee that the proposed system has broad applications in research and conservation, providing valuable insights into pollination networks across diverse species and ecosystems.

Fire has a major impact on the structure and function of many ecosystems globally. Pyrodiversity, the diversity of fires within a region (where diversity is based on fire characteristics such as extent, severity, and frequency), has been hypothesized to promote biodiversity, but changing climate and land management practices have eroded pyrodiversity. To assess whether changes in pyrodiversity will have impacts on ecological communities, we must first understand the mechanisms that might enable pyrodiversity to sustain biodiversity, and how such changes might interact with other disturbances such as drought. Focusing on plant-pollinator communities in mixed-conifer forest with frequent fire in Yosemite National Park, California, we examine how pyrodiversity, combined with drought intensity, influences those communities. We find that pyrodiversity is positively related to the richness of the pollinators, flowering plants, and plant-pollinator interactions. On average, a 5% increase in pyrodiversity led to the gain of approximately one pollinator and one flowering plant species and nearly two interactions. We also find that a diversity of fire characteristics contributes to the spatial heterogeneity (β-diversity) of plant and pollinator communities. Lastly, we find evidence that fire diversity buffers pollinator communities against the effects of drought-induced floral resource scarcity. Fire diversity is thus important for the maintenance of flowering plant and pollinator diversity and predicted shifts in fire regimes to include less pyrodiversity compounded with increasing drought occurrence will negatively influence the richness of these communities in this and other forested ecosystems. In addition, lower heterogeneity of fire severity may act to reduce spatial turnover of plant-pollinator communities. The heterogeneity of community composition is a primary determinant of the total species diversity present in a landscape, and thus, lower pyrodiversity may negatively affect the richness of plant-pollinator communities across large spatial scales.

Introduction Invasive alien species are a major concern in the management and conservation of habitats and species worldwide (Crooks 2002; Bax et al. 2003; Levine et al. 2003; Vila et al. 2010). The direct effects of these species may further cascade in the ecosystem and affect inter- and intraspecific ecological interactions. The introduction of alien plants and animals can have severe consequences, not only for individual native plant and pollinator species, but also for their ecological interactions through plant–pollinator networks (Morales and Traveset 2009; Dohzono and Yokoyama 2010; Schweiger et al. 2010). Integration of alien plant and pollinator species into pollination networks inevitably creates new interactions and may also affect the strength and quality of existing ones. These changes are open niches for novel evolutionary adaptations of both alien and native species (Mooney and Cleland 2001). However, research in this topic is very limited, and has focused mostly on adaptations of alien plant species to pollinator-independent reproduction modes (Barrett et al. 2008). We know of no study investigating adaptations of native plant and pollinator species to invaders, and the ecological and possibly evolutionary consequences of these adaptations in the context of plant–pollinator networks. Such adaptations might have far-reaching ecological and evolutionary implications, as has been shown in plant–herbivore and predator–prey interactions (Cox 2004). Here we outline the main effects of species invasions on plant–pollinator interactions, and deduce the main adaptive mechanisms that native plant species can exhibit in response to changes in their pollination regime. Finally, we explore the characteristics of plant populations that are likely to affect their probability of exhibiting such adaptations and their conservation implications. Effects of alien plant and animal species on native plant pollination Several groups of alien organisms have been shown to affect native plant pollination. Most research has focused on alien plants (Morales and Traveset 2009) and flower visitors (Lach 2003; Dohzono and Yokoyama 2010); however, other groups, such as alien herbivores and predators, can also be influential (Traveset and Richardson 2006). In the following, we explore the possible effects of different groups of alien organisms on pollination of native plants.

植物-传粉者相互作用面临人类活动的威胁。在青藏高原地区，放牧是一项主要的人类活动干扰，过度放牧导致高寒草甸植被严重退化。然而在该区域放牧如何影响植物-传粉者相互作用网络还不得而知。在青藏高原东部的高寒草甸选取了两个研究样点，每个样点包括一块禁牧样地（生长季禁牧）和放牧样地（全年放牧）。在2016年至2018年每年的生长季（7月和8月）进行了连续3年观测，共构建16个传粉网络。结果发现，在研究区域的高寒草甸生态系统中，传粉者群落的物种组成以双翅目昆虫为主。放牧后双翅目、鳞翅目以及鞘翅目传粉者的种类数减少，但膜翅目传粉者的种类数未受到放牧的影响。放牧干扰显著降低了群落中植物、传粉者以及它们之间相互作用的多样性，但对传粉网络的嵌套性和特化程度（H₂'）没有显著影响，说明网络的稳定性和恢复力没有受到放牧的影响。探讨了放牧对传粉网络的影响，发现区域放牧强度过大，降低了传粉昆虫和传粉网络的多样性。未来需要进一步深入研究高寒草甸生态系统中放牧强度对传粉网络的影响模式，以期为合理的放牧制度模式提供理论依据。;Plant-pollinator interactions are threatened by human activities. In the Tibetan Plateau, grazing is one of the main anthropic disturbances, and overgrazing has led to degradation of alpine meadow. However, it is still unclear how grazing affects plant-pollinator interaction network of alpine meadows. Here we collected data from two locations in the eastern Tibetan Plateau to evaluate the effects of grazing on the pollination networks in alpine meadow. Each location comprised one ungrazed plot (fenced during growing season) and one grazed plot (grazed all year). We conducted surveys during growing seasons (July and August) in three consecutive years (2016-2018), and constructed 16 pollination networks in total. The plants-pollinators interactions were surveyed using timed observation method by cameras. We found that the pollinator communities of alpine meadow were dominated by Diptera species. The richness of Diptera, Lepidoptera and Coleoptera pollinators were decreased after grazing, while the richness of Hymenoptera pollinators was unaffected by grazing. Grazing reduced the diversity of plants, pollinators and their interactions. The generalism of plants was also decreased by grazing, but the nestedness and specialization (H₂') of networks were similar between treatments, indicated that the stability and resilience of pollination networks are unaffected by grazing. Furthermore, the network metrics were similar between different months, and grazing had similar impacts on pollination networks in July and August. Our study firstly investigated the effect of the grazing on the pollination networks of alpine meadows in the Tibetan Plateau. The results highlighted that the grazing intensity of study area is exceeded the optimum intensity as it decreased the diversity of pollinators and pollination networks. Further studies should focus on how pollination networks response to grazing intensity in alpine meadow to find a reasonable grazing regime.

Mutualistic interactions between plants and animal pollinators are increasingly under threat through anthropogenic change, and it is critical to understand how temporal changes affect the structure and function of these ecologically important interactions. Because the responses of plant–pollinator interactions to anthropogenic change may take place over decades, historical collections that store information across long time horizons contribute uniquely to our understanding. In this article, we highlight several key questions related to long‐term changes in the structure and function of plant–pollinator interactions. We articulate how research could proceed rapidly via new techniques, greater integration of resources in museum collections along with coincident use of a single data source. We acknowledge the challenges that come with using historical collections and discuss how to minimize them. We provide suggestions that will allow for full utilization of museum resources for addressing a variety of issues regarding plant–pollinator interactions. This perspective paper aims to stimulate new integrative research aimed at understanding temporal patterns in plant–pollinator interactions. Read the free Plain Language Summary for this article on the Journal blog.

AimUnderstanding how climate conditions influence plant–pollinator interactions at the global scale is crucial to understand how pollinator communities and ecosystem function respond to environmental change. Here, we investigate whether climate drives differences in network roles of the main insect pollinator orders: Diptera, Coleoptera, Lepidoptera and Hymenoptera.LocationGlobal.Time period1968–2020.Major taxa studiedDiptera, Coleoptera, Lepidoptera and Hymenoptera.MethodsWe collated plant–pollinator networks from 26 countries and territories across the five main Köppen–Geiger climate zones. In total, we compiled data from 101 networks that included >1500 plant species from 167 families and >2800 pollinator species from 163 families. We assessed differences in the composition of plant–pollinator interactions among climate zones using a permutational ANOVA. We calculated standard network metrics for pollinator taxonomic groups and used Bayesian generalized mixed models to test whether climate zone influenced the proportion of pollinator network links and the level of pollinator generalism.ResultsWe found that climate is a strong driver of compositional dissimilarities between plant–pollinator interactions. Relative to other taxa, bees and flies made up the greatest proportion of network links across climate zones. When network size was accounted for, bees were the most generalist pollinator group in the tropics, whereas non‐bee Hymenoptera were the most generalist in arid zones, and syrphid flies were the most generalist in polar networks.Main conclusionsWe provide empirical evidence at the global scale that climate strongly influences the roles of different pollinator taxa within networks. Importantly, non‐bee taxa, particularly flies, play central network roles across most climate zones, despite often being overlooked in pollination research and conservation. Our results identify the need for greater understanding of how global environmental change affects plant–pollinator interactions.

Species morphological and behavioural traits are key determinants of which pollinator species interact with which plant species. However, individuals within species are not identical in their traits and this diversity could help us understand plant–pollinator interaction patterns. Using three independent data sets, we assessed whether bee intraspecific body size variation (ITV) and sociality influenced pollinator interaction specialisation, intraspecific niche partitioning, centrality in the interaction network and phylogenetic diversity of the plants visited. We found that solitary pollinators were more specialised in their interactions with plants and had lower intraspecific niche partitioning compared to social pollinators. Furthermore, solitary pollinators with higher ITV had higher centrality in the network and visited a higher phylogenetic diversity of plants compared to solitary species with lower ITV, whereas the opposite pattern emerged for social pollinators. Pollinator ITV did not differ between social and solitary bee species. Our findings show that the effect of pollinator body size variation on plant–pollinator interactions depends on pollinator species sociality. Specifically, solitary pollinators with higher ITV and social pollinators with lower ITV seem to be the most important contributors to maintaining the evolutionary diversity of the plant community, and also the species with the largest potential to affect (via cascade effects) the entire plant–pollinator network. Read the free Plain Language Summary for this article on the Journal blog.

Land use change is a major pressure on pollinator abundance, diversity and plant–pollinator interactions. Far less is known about how land‐use alters the structure of plant–pollinator networks and their robustness to plant–pollinator coextinctions. We analysed the structure of plant–pollinator networks sampled in 12 landscapes along an urbanisation and agricultural intensity gradient, from early spring to late summer 2021, and used a stochastic coextinction model to correlate plant–pollinator coextinction risk with network structure (species and network‐level metrics) and landscape context. Networks in intensively managed (i.e., agricultural and urban) landscapes had a lower risk of initiating a coextinction cascade, while networks in less intensively managed landscapes may be less robust. Network structure modulated the frequency and severity of coextinctions and species loss, while the strength of species interactions increased robustness. Urban networks were more species rich and symmetrical due to the high diversity of ornamental plants, while intensively managed agricultural landscapes had smaller, more tightly connected and nested networks. Network structure modulated the frequency of extinctions, which was decreased by greater linkage density, interaction asymmetry and interaction dependence in the networks, while once an extinction occurred, nestedness and linkage density propagated the degree of the coextinction cascade and species loss. At the species level, species strength was inversely correlated with extinction risk, implying that generalist species with a high number of interactions with specialists had the lowest extinction risk. An interplay between land‐use and network structure affects community robustness to coextinctions with implications for pollination services and plant reproduction. Land‐use change or other global change pressures by reorganising species interactions can alter communities and their potential functioning. Read the free Plain Language Summary for this article on the Journal blog.

Urbanization affects pollinator diversity and plant–pollinator networks by changing resource availability locally and in the surrounding landscape. We experimentally established (N = 12) standardized plant communities in farmland, villages, and cities to identify the relative role of local and landscape effects on plant–pollinator communities along this urbanization gradient. We found that the number of flower visits by solitary bees, but not bumblebees, was highest in cities and lowest in farmland, with villages being intermediate, whereas syrphid flies exhibited lowest numbers in cities. Villages supported the richest pollinator communities, as they appeared to benefit from both farmland and city communities. Plant–pollinator network metrics such as robustness, interaction evenness, and interaction diversity decreased with increasing urbanization, although local plant richness increased toward urban areas. In conclusion, pollinator communities were most diverse and stable in farmland and village sites, despite the high plant richness in cities. The different composition of pollinator communities along the urbanization gradient suggests considering all three landscape types for conservation schemes.

Roads represent one of the most widespread forms of human infrastructure. Their presence, along with associated vehicle traffic, fragments ecosystems and disrupts plant–pollinator interactions. However, the precise mechanisms through which these disturbances cascade to reorganise plant–pollinator interaction networks and ultimately affect plant reproduction remain poorly understood. In this study, we investigated how traffic volume (low vs. high) affects plant–pollinator communities, interaction networks and plant reproductive success along spatial gradients extending from roads in alpine meadows of the Tibetan Plateau. Using piecewise structural equation modelling, we found that high traffic volume reduced flower richness, which subsequently lowered pollinator abundance and restructured interaction networks—characterised by decreased connectance, increased nestedness and heightened specialisation—ultimately leading to lower community‐level plant reproductive success. These adverse impacts diminished with increasing distance from roads, suggesting a spatial buffering effect in roadside ecosystems. Synthesis . Our findings demonstrate that high traffic volume triggers cascading effects that disrupt mutualistic networks and undermine pollination services in fragile ecosystems. To conserve biodiversity and sustain ecosystem resilience, it is essential to incorporate pollination dynamics into roadside management. Practical measures such as establishing vegetative buffer zones and actively enhancing floral resources should be prioritised across existing and expanding road networks.

Climate change can disrupt ecosystem services such as pollination. Warming increases the frequency and scale of wildfires, leading to smoke‐induced reductions in incident solar radiation, which together may affect plants, pollinators and their interactions. To determine the emergent effects of these factors on pollination, we exposed experimental communities comprising a solitary bee species and three annual wildflower species to treatments representing combinations of warming and reduced solar radiation conditions. We measured floral traits, plant–pollinator interactions and reproduction. Slight warming did not affect floral traits or plant–pollinator interactions, whereas a higher level of warming combined with reduced solar radiation stressed both partners of the mutualism. Compared to ambient conditions, warming combined with reduced solar radiation significantly lowered floral abundance and caused a 91.5% reduction in nectar volume, likely by depressing primary productivity. This limitation of floral resources translated into less caloric intake for bees and lowered flower visitation rates, lengthened flower handling times and increased diet breadth. Slight increases in temperature above ambient allowed bees to produce 48.2% more offspring, likely by bringing bees closer to their thermal optimum, but the reduction in solar radiation negated this positive effect. Conversely, seeds produced under warming combined with reduced solar radiation had the highest germination success. Our results suggest that warming and reduced solar radiation have synergistic, negative effects on pollination through a range of mechanisms including changes in primary productivity and nectar production, and bee attraction and behaviour. The reduced strength of these plant–pollinator mutualisms raises concern for the future of biodiversity and pollination services in a warmer world with more frequent wildfires. Read the free Plain Language Summary for this article on the Journal blog.