The importance of interannual variation and bottom–up nitrogen enrichment for plant–pollinator networks

Striking changes in food web structure occur with alterations in resource supply. Like predator–prey interactions, many mutualisms are also consumer–resource interactions. However, no studies have explored how the structure of plant–pollinator networks may be affected by nutrient enrichment. For three years, we enriched plots of subalpine plant communities with nitrogen and observed subsequent effects on plant–pollinator network structure. Although nitrogen enrichment affects floral abundance and rates of pollinator visitation, we found no effects of nitrogen enrichment on the core group of generalist plants and pollinators or on plant–pollinator network structure parameters, such as network topology (the identity and frequency of interactions) and the degree of nestedness. However, individual plant and pollinator taxa were packed into the nested networks differently among nitrogen treatments. In particular, pollinators visited different numbers and types of plants in the nested networks, suggesting weak, widespread effects of nitrogen addition on individual taxa. Independent of nitrogen enrichment, there were large interannual differences in network structure and interactions, due to species turnover among years and flexibility in interacting with new partners. These data suggest that the community structure of small‐scale mutualistic networks may be relatively robust to short‐term bottom–up changes in the resource supply, but sensitive to variation in the opportunistic behavior and turnover of plant and pollinator species among years.     

Pollinator community responses to the spatial population structure of wild plants: A pan-European approach

Land-use changes can alter the spatial population structure of plant species, which may in turn affect the attractiveness of flower aggregations to different groups of pollinators at different spatial scales. To assess how pollinators respond to spatial heterogeneity of plant distributions and whether honeybees affect visitation by other pollinators we used an extensive data set comprising ten plant species and their flower visitors from five European countries. In particular we tested the hypothesis that the composition of the flower visitor community in terms of visitation frequencies by different pollinator groups were affected by the spatial plant population structure, viz. area and density measures, at a within-population (‘patch’) and among-population (‘population’) scale. We found that patch area and population density were the spatial variables that best explained the variation in visitation frequencies within the pollinator community. Honeybees had higher visitation frequencies in larger patches, while bumblebees and hoverflies had higher visitation frequencies in sparser populations. Solitary bees had higher visitation frequencies in sparser populations and smaller patches. We also tested the hypothesis that honeybees affect the composition of the pollinator community by altering the visitation frequencies of other groups of pollinators. There was a positive relationship between visitation frequencies of honeybees and bumblebees, while the relationship with hoverflies and solitary bees varied (positive, negative and no relationship) depending on the plant species under study. The overall conclusion is that the spatial structure of plant populations affects different groups of pollinators in contrasting ways at both the local (‘patch’) and the larger (‘population’) scales and, that honeybees affect the flower visitation by other pollinator groups in various ways, depending on the plant species under study. These contrasting responses emphasize the need to investigate the entire pollinator community when the effects of landscape change on plant–pollinator interactions are studied.     

Quantitative predictions of pollinators’ abundances from qualitative data on their interactions with plants and evidences of emergent neutrality

Making quantitative predictions of the effects of human activities on ecological communities is crucial for their management. In the case of plant–pollinator mutualistic networks, despite the great progress in describing the interactions between plants and their pollinators, the capability of making quantitative predictions is still lacking. Here, in order to estimate pollinator species abundances and their niche distribution, I propose a general method to transform a plant–pollinator network into a competition model between pollinator species. Competition matrices were obtained from ‘first principles’ calculations, using qualitative interaction matrices compiled for a set of 38 plant–pollinator networks. This method is able to make accurate quantitative predictions for mutualistic networks spanning a broad geographic range. Specifically, the predicted biodiversity metrics for pollinators – species relative abundances, Shannon equitability and Gini–Simpson indices – agree quite well with those inferred from empirical counts of visits of pollinators to plants. Furthermore, this method allows building a one‐dimensional niche axis for pollinators in which clusters of generalists are separated by specialists thus rendering support to the theory of emergent neutrality. The importance of interspecific competition between pollinator species is a controversial and unresolved issue, considerable circumstantial evidence has accrued that competition between insects does occur, but a clear measure of its impact on their species abundances is still lacking. I contributed to fill this gap by quantifying the effect of competition between pollinators. Particular applications of our analysis could be to estimate the quantitative effects of removing a species from a community or to address the fate of populations of native organisms when foreign species are introduced to ecosystems far beyond their home range.     

Drivers of plant individual-based pollinator visitation network topology in an arid ecosystem

Interactions with pollinators underlie the structure and function of plant communities. Network analysis is a valuable tool for studying plant-pollinator interactions, but these networks are most frequently built by aggregating interactions at the species level. Interactions are between individuals and an advantage of individual-based networks is the ability to integrate inter-individual variation in traits and environmental context within complex ecological networks. We studied the influence of inter-individual variation on pollinator sharing among foundation shrubs and cactus in a desert ecosystem using plant individual-based pollinator visitation networks. We hypothesized that the traits that alter attractiveness of plants to pollinators will also influence an individual plant's role within the visitation network. Foundation plants growing with higher densities of nearby blooming shrubs had higher pollinator visitation rates and had greater access to the conspecific mating pool, suggesting widespread and diffuse pollination facilitation within this community. Further, shrub density influenced the role of betweenness centrality and the effective number of partners (e^H). Floral display size also influenced the effective number of interaction partners but did not directly influence the centrality measures for individual plants or other measures of network structure despite increasing visitation rates. The individual-based visitation networks were significantly modular and module membership was predicted by species identity and pollinator visitation rates. Ecological and individual context mediate the outcome of pollinator-mediated interactions and are fundamental drivers of whole community structure. This study shows that the density of immediate neighbours can influence the overall structure of plant-pollinator interaction networks. Exploring the contribution of intraspecific variation to community interaction networks will improve our understanding of drivers of community-level ecological dynamics.     

Detecting phylogenetic signal in mutualistic interaction networks using a Markov process model

Ecological interaction networks, such as those describing the mutualistic interactions between plants and their pollinators or between plants and their frugivores, exhibit non‐random structural properties that cannot be explained by simple models of network formation. One factor affecting the formation and eventual structure of such a network is its evolutionary history. We argue that this, in many cases, is closely linked to the evolutionary histories of the species involved in the interactions. Indeed, empirical studies of interaction networks along with the phylogenies of the interacting species have demonstrated significant associations between phylogeny and network structure. To date, however, no generative model explaining the way in which the evolution of individual species affects the evolution of interaction networks has been proposed. We present a model describing the evolution of pairwise interactions as a branching Markov process, drawing on phylogenetic models of molecular evolution. Using knowledge of the phylogenies of the interacting species, our model yielded a significantly better fit to 21% of a set of plant–pollinator and plant–frugivore mutualistic networks. This highlights the importance, in a substantial minority of cases, of inheritance of interaction patterns without excluding the potential role of ecological novelties in forming the current network architecture. We suggest that our model can be used as a null model for controlling evolutionary signals when evaluating the role of other factors in shaping the emergence of ecological networks.     

Flower-visitor and pollen transport networks in a large city: structure and properties

Plant species and their pollinators are linked by their mutualistic interactions, which form the basis of pollination networks. The use of a network approach allows one to take into account all interactions between a group of plants and its animal pollinators, and to reveal the structure of these connections. We analysed pollination interactions for urban habitat fragments located within the Warsaw city environment. We compared two similar, ruderal communities (phytosociological order Onopordetalia acanthii) located in distant parts of the city of Warsaw (Poland) that differed with the surrounding ecosystems. The aim of this study was to define the structures and properties of flower-visitor (visitation) and pollen transport networks (based on analysis of pollen loads carried by insects) and to assess the differences between the studied sites. Although the sites differed in insect relative abundance (Diptera dominated one study site, whereas Hymenoptera dominated the other), network size and structure were similar for both communities. In both cases, networks contained moderately specialized species (based on H ₂′ index); however, networks were dominated by apparently ecologically generalized insect taxa as well as those represented by a single specimen. Networks based on pollen transport indicated greater generality of insect species (more links) than those based on our samples of visitation. The most highly linked plant species represented were either the most abundant (Fabaceae) or phenotypically generalized taxa (Daucus carota). We conclude that plant–pollinator interactions in such highly disturbed and isolated habitats are composed mostly of ecologically generalised species. Moreover, we stress the usefulness of pollen load analysis in the development and verification of visitation data.     

Flower‐visitor networks only partially predict the function of pollen transport by bees

Mutualistic networks display distinct structural and organizational features such as nestedness, power‐law degree distribution and asymmetric dependencies. Attention is now focused on how these structural properties influence network function. Most plant‐pollinator networks are constructed using records of animals contacting flowers, which is based on the assumption that all visitors to flowers are pollinators; however, animals may visit flowers as nectar robbers, florivores, or to prey upon other visitors. To differentiate potential pollinator interactions from other interaction types, we examined individual bees that had visited flowers to detect if they carried pollen. Using these data, we constructed visitation and pollen‐transport networks for a spinifex‐dominated arid zone grassland. To determine how the structure of the visitation network reflects pollen transport, we compared the two networks using a null model approach to account for differences in network size. Differences in number of species, nestedness and connectance observed between the visitation and pollen‐transport networks were within expected ranges generated under the null model. The pollen‐transport network was more specialized, had lower interaction evenness, and fewer links compared to the visitation network. Almost half the number of species of the visitation network participated in the pollen‐transport network, and one‐third of unique visitation interactions resulted in pollen transport, highlighting that visitation does not always result in pollination. Floral visitor data indicate potential pollen transporters, but inferring pollination function from visitation networks needs to be performed cautiously as pollen transport resulted from both common and rare interactions, and depended on visitor identity. Although visitation and pollen‐transport networks are structurally similar, the function of all species cannot be predicted from the visitation network alone. Considering pollen transport in visitation networks is a simple first step towards determining pollinators from non‐pollinators. This is fundamental for understanding how network structure relates to network function.     

The temporal dimension in individual‐based plant pollination networks

The pollination success of animal‐pollinated plants depends on the temporal coupling between flowering schedules and pollinator availability. Within a population, individual plants exhibiting disparate flowering schedules will be exposed to different pollinators when the latter exhibit temporal turnover. The temporal overlap between individual plants and pollinators will result in a turnover of interactions, which can be analyzed through a network approach. We have explored the temporal dynamics of individual‐based plant networks resulting from pairwise similarities in pollinator composition. During two flowering seasons, we surveyed the phenology and pollinator fauna of the individual plants from a population of Erysimum mediohispanicum (Brassicaceae). We analyzed the topology of these networks by means of their modularity, clustering, and core–periphery structure. These metrics are related to network functional properties such as cohesion, transitivity and centralization respectively. Afterwards, we analyzed the influence of each pollinator functional group on network topology. We found that network topology varied widely over time as a consequence of the differences in plant phenology and the idiosyncratic and contextual effect of pollinators. When integrating all temporary networks, the network became cohesive (non modular), transitive (locally clusterized), and centralized (core–periphery topology). These topologies could entail important consequences for plant reproduction. Our results highlight the importance of considering the entire flowering season and the necessity of making comprehensive temporal sampling when trying to build reliable interaction networks.     

Interactive effects of elevation,species richness and extreme climatic events on plant–pollinator networks

Plant–pollinator interactions are essential for the functioning of terrestrial ecosystems, but are increasingly affected by global change. The risks to such mutualistic interactions from increasing temperature and more frequent extreme climatic events such as drought or advanced snow melt are assumed to depend on network specialization, species richness, local climate and associated parameters such as the amplitude of extreme events. Even though elevational gradients provide valuable model systems for climate change and are accompanied by changes in species richness, responses of plant–pollinator networks to climatic extreme events under different environmental and biotic conditions are currently unknown. Here, we show that elevational climatic gradients, species richness and experimentally simulated extreme events interactively change the structure of mutualistic networks in alpine grasslands. We found that the degree of specialization in plant–pollinator networks (H2′) decreased with elevation. Nonetheless, network specialization increased after advanced snow melt at high elevations, whereas changes in network specialization after drought were most pronounced at sites with low species richness. Thus, changes in network specialization after extreme climatic events depended on climatic context and were buffered by high species richness. In our experiment, only generalized plant–pollinator networks changed in their degree of specialization after climatic extreme events. This indicates that contrary to our assumptions, network generalization may not always foster stability of mutualistic interaction networks.     

Direct and indirect effects of habitat area and matrix composition on species interactions among flower-visiting insects

Traditionally, ecologists have focused on direct effects of habitat area and arrangement on individual species or pairwise species interactions. Indirect effects of habitat heterogeneity on multiple interacting species are often neglected or lack experimental support. In a factorial field experiment, we explored the direct and indirect effects of habitat area, fragmentation, and matrix composition on a community of flower-visiting insects in red clover. Habitat area, fragmentation, and density of inflorescences of red clover all influenced the insect pollinators and, in turn, the production of clover seeds. The strongest direct effect was on pollinator visitation, which was substantially higher in small clover patches surrounded by bare-ground than in larger patches embedded within grass. Structural equation modeling indicated that the observed matrix-dependent changes in pollinator visitation propagated across a tri-trophic system: higher visitation rates positively correlated with a higher seed set, which in turn was positively correlated with abundances of seed predators and their parasitoids. Therefore, this study suggests that habitat area and matrix composition can strongly influence the structure of species interaction webs through indirect effects, and also emphasizes that these effects can be propagated through mutualistic as well as trophic interactions.     

Community structure of pollination webs of Mauritian heathland habitats

Pollination webs have recently deepened our understanding of complex ecosystem functions and the susceptibility of biotic networks to anthropogenic disturbances. Extensive mutualistic networks from tropical species-rich communities, however, are extremely scarce. We present fully quantitative pollination webs of two plant–pollinator communities of natural heathland sites, one of which was in the process of being restored, on the oceanic island of Mauritius. The web interaction data cover a full flowering season from September 2003 to March 2004 and include all flowering plant and their pollinator species. Pollination webs at both sites were dominated by a few super-abundant, disproportionately well-connected species, and many rare and specialised species. The webs differed greatly in size, reflecting higher plant and pollinator species richness and abundance at the restored site. About one fifth of plant species at the smaller community received <3 visits. The main pollinators were insects from diverse taxonomic groups, while the few vertebrate pollinator species were abundant and highly linked. The difference in plant community composition between sites appeared to strongly affect the associated pollinator community and interactions with native plant species. Low visitation rate to introduced plant species suggested little indirect competition for pollinators with native plant species. Overall, our results indicated that the community structure was highly complex in comparison to temperate heathland communities. We discuss the observed differences in plant linkage and pollinator diversity and abundance between the sites with respect to habitat restoration management and its influence on pollination web structure and complexity. For habitat restoration to be successful in the long term, practitioners should aim to maintain structural diversity to support a species-rich and abundant pollinator assemblage which ensures native plant reproduction.     

Plant species roles in pollination networks: an experimental approach

Pollination is an important ecosystem service threatened by current pollinator declines, making flower planting schemes an important strategy to recover pollination function. However, ecologists rarely test the attractiveness of chosen plants to pollinators in the field. Here, we experimentally test whether plant species roles in pollination networks can be used to identify species with the most potential to recover plant–pollinator communities. Using published pollination networks, we calculated each plant's centrality and chose five central and five peripheral plant species for introduction into replicate experimental plots. Flower visitation by pollinators was recorded in each plot and we tested the impact of introduced central and peripheral plant species on the pollinator and resident plant communities and on network structure. We found that the introduction of central plant species attracted a higher richness and abundance of pollinators than the introduction of peripheral species, and that the introduced central plant species occupied the most important network roles. The high attractiveness of central species to pollinators, however, did not negatively affect visitation to resident plant species by pollinators. We also found that the introduction of central plant species did not affect network structure, while networks with introduced peripheral species had lower centralisation and interaction evenness than networks with introduced central species. To our knowledge, this is the first time species network roles have been tested in a field experiment. Given that most restoration projects start at the plant community, being able to identify the plants with the highest potential to restore community structure and functioning should be a key goal for ecological restoration.     

The effects of herbivore-induced plant volatiles on interactions between plants and flower-visiting insects

Plants are faced with a trade-off between on the one hand growth, development and reproduction and on the other hand defence against environmental stresses. Yet, research on insect-plant interactions has addressed plant-pollinator interactions and plant-attacker interactions separately. Plants have evolved a high diversity of constitutive and induced responses to attack, including the systemic emission of herbivore-induced plant volatiles (HIPVs). The effect of HIPVs on the behaviour of carnivorous insects has received ample attention for leaf-feeding (folivorous) species and their parasitoids and predators. Here, we review whether and to what extent HIPVs affect the interaction of plants in the flowering stage with mutualistic and antagonistic insects. Whereas the role of flower volatiles in the interactions between plants and insect pollinators has received increased attention over the last decade, studies addressing both HIPVs and pollinator behaviour are rare, despite the fact that in a number of plant species herbivory is known to affect flower traits, including size, nectar secretion and composition. In addition, folivory and florivory can also result in significant changes in flower volatile emission and in most systems investigated, pollinator visitation decreased, although exceptions have been found. Negative effects of HIPVs on pollinator visitation rates likely exert negative selection pressure on HIPV emission. The systemic nature of herbivore-induced plant responses and the behavioural responses of antagonistic and mutualistic insects, requires the study of volatile emission of entire plants in the flowering stage. We conclude that approaches to integrate the study of plant defences and pollination are essential to advance plant biology, in particular in the context of the trade-off between defence and growth/reproduction.     

Construction,validation, and application of nocturnal pollen transport networks in an agro‐ecosystem: a comparison using light microscopy and DNA metabarcoding

1. Moths are globally relevant as pollinators but nocturnal pollination remains poorly understood. Plant–pollinator interaction networks are traditionally constructed using either flower‐visitor observations or pollen‐transport detection using microscopy. Recent studies have shown the potential of DNA metabarcoding for detecting and identifying pollen‐transport interactions. However, no study has directly compared the realised observations of pollen‐transport networks between DNA metabarcoding and conventional light microscopy. 2. Using matched samples of nocturnal moths, we constructed pollen‐transport networks using two methods: light microscopy and DNA metabarcoding. Focussing on the feeding mouthparts of moths, we developed and provide reproducible methods for merging DNA metabarcoding and ecological network analysis to better understand species interactions. 3. DNA metabarcoding detected pollen on more individual moths, and detected multiple pollen types on more individuals than microscopy, although the average number of pollen types per individual was unchanged. However, after aggregating individuals of each species, metabarcoding detected more interactions per moth species. Pollen‐transport network metrics differed between methods because of variation in the ability of each to detect multiple pollen types per moth and to separate morphologically similar or related pollen. We detected unexpected but plausible moth–plant interactions with metabarcoding, revealing new detail about nocturnal pollination systems. 4. The nocturnal pollination networks observed using metabarcoding and microscopy were similar yet distinct, with implications for network ecologists. Comparisons between networks constructed using metabarcoding and traditional methods should therefore be treated with caution. Nevertheless, the potential applications of metabarcoding for studying plant–pollinator interaction networks are encouraging, especially when investigating understudied pollinators such as moths.     

The role of the endemic and critically endangered Colorful Puffleg Eriocnemis mirabilis in plant‐hummingbird networks of the Colombian Andes

Ecological network approaches may contribute to conservation practices by quantifying within‐community importance of species. In mutualistic plant‐pollinator systems, such networks reflect potential pollination of the plants and a considerable portion of the energy consumption by the pollinators, two key components for each party. Here, we used two different sampling approaches to describe mutualistic plant‐hummingbird networks from a cloud forest in the Colombian Western Andes, home to the Colorful Puffleg Eriocnemis mirabilis, an endemic and critically endangered hummingbird. We contrast networks between two localities (a protected area inside a National park vs. its buffer zone) and across sampling methods (floral visitation vs. pollen loads) to assess how the network structure and the importance of each hummingbird species within the networks may change. Visitation networks were characterized as having higher sampling completeness, yet pollen load network recorded more pollen types than plant species recorded by visitation. Irrespective of the sampling methods, the Colorful Puffleg was one of the most important hummingbird species in the network within the protected area inside the National park, but not in the buffer zone. Moreover, most species‐level network indices were related to hummingbirds’ abundance. This suggests that conservation initiatives aimed at the endangered Colorful Puffleg may both help on the survival of this endangered hummingbird, as well as on maintaining its key role in the mutualistic interaction network inside the National Park. Our study illustrates how conservation practitioners could assess the local importance of endangered species using interaction network approaches.     

The topological differences between visitation and pollen transport networks: a comparison in species rich communities of the Himalaya–Hengduan Mountains

Pollination networks are usually constructed and assessed by direct field observations which commonly assume that all flower visitors are true pollinators. However, this assumption is often invalid and the use of data based on mere visitors to flowers may lead to a misunderstanding of intrinsic pollination networks. Here, using a large dataset by both sampling floral visitors and analyzing their pollen loads, we constructed 32 networks pairs (visitation versus pollen transport) across one flowering season at four elevation sites in the Himalaya–Hengduan Mountains region. Pollen analysis was conducted to determine which flower visitors acted as potential pollinators (pollen vectors) or as cheaters (those not carrying pollen of the visited plants). We tested whether there were topological differences between visitation and pollen transport networks and whether different taxonomic groups of insect visitors differed in their ability to carry pollen of the visited plants. Our results indicated that there was a significantly higher degree of specialization at both the network and species levels in the pollen transport networks in contrast to the visitation networks. Modularity was lower but nestedness was higher in the visitation networks compared to the pollen transport networks. All the cheaters were identified as peripheral species and most of them contributed positively to the nested structure. This may explain in part the differences in modularity and nestedness between the two network types. Bees carried the highest proportion of pollen of the visited plants. This was followed by Coleoptera, other Hymenoptera and Diptera. Lepidoptera carried the lowest proportion of pollen of the visited plants. Our study shows that the construction of pollen transport networks could provide a more in‐depth understanding of plant–pollinator interactions. Moreover, it suggests that detecting and removing cheater interactions when studying the topology of other mutualistic networks might be also important.     

Global warming and the disruption of plant-pollinator interactions

Anthropogenic climate change is widely expected to drive species extinct by hampering individual survival and reproduction, by reducing the amount and accessibility of suitable habitat, or by eliminating other organisms that are essential to the species in question. Less well appreciated is the likelihood that climate change will directly disrupt or eliminate mutually beneficial (mutualistic) ecological interactions between species even before extinctions occur. We explored the potential disruption of a ubiquitous mutualistic interaction of terrestrial habitats, that between plants and their animal pollinators, via climate change. We used a highly resolved empirical network of interactions between 1420 pollinator and 429 plant species to simulate consequences of the phenological shifts that can be expected with a doubling of atmospheric CO₂. Depending on model assumptions, phenological shifts reduced the floral resources available to 17–50% of all pollinator species, causing as much as half of the ancestral activity period of the animals to fall at times when no food plants were available. Reduced overlap between plants and pollinators also decreased diet breadth of the pollinators. The predicted result of these disruptions is the extinction of pollinators, plants and their crucial interactions.     

Comparing the conservatism of ecological interactions in plant–pollinator and plant–herbivore networks

Conservatism in species interaction, meaning that related species tend to interact with similar partners, is an important feature of ecological interactions. Studies at community scale highlight variations in conservatism strength depending on the characteristics of the ecological interaction studied. However, the heterogeneity of datasets and methods used prevent to compare results between mutualistic and antagonistic networks. Here we perform such a comparison by taking plant–insect communities as a study case, with data on plant–herbivore and plant–pollinator networks. Our analysis reveals that plants acting as resources for herbivores exhibit the strongest conservatism in species interaction among the four interacting groups. Conservatism levels are similar for insect pollinators, insect herbivores and plants as interacting partners of pollinators, although insect pollinators tend to have a slightly higher conservatism than the two others. Our results thus clearly support the current view that within antagonistic networks, conservatism is stronger for species as resources than for species as consumer. Although the pattern tends to be opposite for plant–pollinator networks, our results suggest that asymmetry in conservatism is much less pronounced between the pollinators and the plant they interact with. We discuss these differences in conservatism strength in relation with the processes structuring plant–insect communities.     

Invariant properties in coevolutionary networks of plant–animal interactions

Plant–animal mutualistic networks are interaction webs consisting of two sets of entities, plant and animal species, whose evolutionary dynamics are deeply influenced by the outcomes of the interactions, yielding a diverse array of coevolutionary processes. These networks are two‐mode networks sharing many common properties with others such as food webs, social, and abiotic networks. Here we describe generalized patterns in the topology of 29 plant–pollinator and 24 plant–frugivore networks in natural communities. Scale‐free properties have been described for a number of biological, social, and abiotic networks; in contrast, most of the plant–animal mutualistic networks (65.6%) show species connectivity distributions (number of links per species) with a power‐law regime but decaying as a marked cut‐off, i.e. truncated power‐law or broad‐scale networks and few (22.2%) show scale‐invariance. We hypothesize that plant–animal mutualistic networks follow a build‐up process similar to complex abiotic nets, based on the preferential attachment of species. However, constraints in the addition of links such as morphological mismatching or phenological uncoupling between mutualistic partners, restrict the number of interactions established, causing deviations from scale‐invariance. This reveals generalized topological patterns characteristic of self‐organized complex systems. Relative to scale‐invariant networks, such constraints may confer higher robustness to the loss of keystone species that are the backbone of these webs.     

Predicting the effect of competition on secondary plant extinctions in plant–pollinator networks

What are the limitations of models that predict the behavior of an ecological community based on a single type of species interaction? Using plant–pollinator network models as an example, we contrast the predicted vulnerability of a community to secondary extinctions under the assumption of purely mutualistic interactions versus mutualistic and competitive interactions. We find that competition among plant species increases the risk of secondary extinctions and extinction cascades. Simulations over a number of different network structures indicate that this effect is stronger in larger networks, more strongly connected networks and networks with higher plant:pollinator ratios. We conclude that efforts to model plant–pollinator communities will systematically over‐estimate community robustness to species loss if plant competition is ignored. However, because the effect of plant competition depends on network architecture, and because characterization of plant competition is work intensive, we suggest that efforts to account for plant competition in plant–pollinator network models should be focused on large, strongly connected networks with high plant:pollinator ratios.