The sudden collapse of pollinator communities

Declines in pollinator populations may harm biodiversity and agricultural productivity. Little attention has, however, been paid to the systemic response of mutualistic communities to global environmental change. Using a modelling approach and merging network theory with theory on critical transitions, we show that the scale and nature of critical transitions is likely to be influenced by the architecture of mutualistic networks. Specifically, we show that pollinator populations may collapse suddenly once drivers of pollinator decline reach a critical point. A high connectance and/or nestedness of the mutualistic network increases the capacity of pollinator populations to persist under harsh conditions. However, once a tipping point is reached, pollinator populations collapse simultaneously. Recovering from this single community‐wide collapse requires a relatively large improvement of conditions. These findings may have large implications for our view on the sustainability of pollinator communities and the services they provide.     

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.     

The relative importance of positive and negative interactions for pollinator attraction in a plant community

Plant–pollinator interactions provide ideal frameworks for studying interactions in plant communities. Despite the large potential influence of such interactions on plant community structure, biodiversity and evolutionary processes, we know surprisingly little about the relative importance of positive and negative interactions among plant species for pollinator attraction. Therefore, we explored the relationships between conspecific and heterospecific floral densities and the flower visitation rates of nine plant species mainly visited by bumble bees, and six plant species mainly visited by flies, in a temperate grassland, through stepwise multiple regressions. Significant relationships were interpreted as interactions for pollinator attraction. Our results revealed that positive intra- and interspecific interactions for pollinator attraction were far more frequent than negative ones. Seventeen interspecific interactions were revealed of which 14 were significantly positive, whereas three of four significant intraspecific interactions were positive. Seven species experienced only positive interactions and two species experienced only negative interactions. The results presented here indicate that negative interactions are not necessarily the dominant ecological interaction for pollination among plants within a community, and the study represents a straightforward approach to study intra- and interspecific interactions among multiple species within a community. We discuss which mechanisms may drive the positive interactions for pollinator attraction and whether this may result in facilitative effects on reproductive success. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Network structural properties mediate the stability of mutualistic communities

Key advances are being made on the structures of predator–prey food webs and competitive communities that enhance their stability, but little attention has been given to such complexity–stability relationships for mutualistic communities. We show, by way of theoretical analyses with empirically informed parameters, that structural properties can alter the stability of mutualistic communities characterized by nonlinear functional responses among the interacting species. Specifically, community resilience is enhanced by increasing community size (species diversity) and the number of species interactions (connectivity), and through strong, symmetric interaction strengths of highly nested networks. As a result, mutualistic communities show largely positive complexity–stability relationships, in opposition to the standard paradox. Thus, contrary to the commonly-held belief that mutualism's positive feedback destabilizes food webs, our results suggest that interplay between the structure and function of ecological networks in general, and consideration of mutualistic interactions in particular, may be key to understanding complexity–stability relationships of biological communities as a whole.     

Effects of species interactions on community organization along a trampling gradient

Abstract. Effects of interspecific interactions on the organization of two trampled communities, Eragrostio ferrugineae - Plantaginetum asiaticae (EP) and Eleusino indicae - Digitarietum violascentis (ED), were examined by 4-yr field experiments. We compared changes in the relative abundance of the main component species of the communities in monoculture and mixed culture along a trampling gradient. At no trampling in mixed culture, Ambrosia artemisiifolia var. elatior and Erigeron annuus (roadside herbs, RH) predominated and excluded the trampled community species. Severe trampling markedly reduced the predominance of these roadside herbs, promoted Eragrostis ferruginea (a perennial grass of EP), but suppressed Eleusine indica (an annual grass of ED). These results suggest that the differentiation of trampled and roadside herb communities under heavy trampling are caused by asymmetric competition between their main species. Several species pairs showed a constant rank order of relative cover at high trampling levels. Pennisetum alopecuroides (a perennial grass of both RH and EP) and Eragrostis coexisted, indicating symmetric competition. Eragrostis and Plantago asiatica (a perennial herb of EP) or Poa annua (a winter-annual grass of both EP and ED) coexisted through separation in phenology. Eragrostis promoted the survival of Plantago by moderating unpredictable drought (commensalistic positive interaction). This suggests that community composition is maintained by several interspecific interactions.     

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.     

Beyond connectedness: why pairwise metrics cannot capture community stability

The connectedness of species in a trophic web has long been a key structural characteristic for both theoreticians and empiricists in their understanding of community stability. In the past decades, there has been a shift from focussing on determining the number of interactions to taking into account their relative strengths. The question is: How do the strengths of the interactions determine the stability of a community? Recently, a metric has been proposed which compares the stability of observed communities in terms of the strength of three‐ and two‐link feedback loops (cycles of interaction strengths). However, it has also been suggested that we do not need to go beyond the pairwise structure of interactions to capture stability. Here, we directly compare the performance of the feedback and pairwise metrics. Using observed food‐web structures, we show that the pairwise metric does not work as a comparator of stability and is many orders of magnitude away from the actual stability values. We argue that metrics based on pairwise‐strength information cannot capture the complex organization of strong and weak links in a community, which is essential for system stability.     

Patterns in the assembly of an island plant community

Aim  To test for patterns in the assembly of an island plant community.
Location  Islands off the west coast of Vancouver Island, British Columbia, Canada.
Methods  Twenty-seven islands were visited by boat, and the abundance of six woody angiosperm species was quantified. Null models were then used to test whether: (1) some species co-occur less than expected by chance (i.e. co-occurrence assembly rule), (2) the incidence and abundance of some species are inversely related to the abundance of other species (i.e. incidence assembly rule), and (3) support for assembly rules precludes evidence for nestedness, which refers to a pattern in species composition in which the species present on depauperate islands form regular subsets of those occurring on progressively more diverse islands.
Results  Most species co-occurred with other species at frequencies expected by chance. However, one species ( Sambucus racemosa ) co-occurred with other species less frequently than randomized expectations. The observed incidence and abundance patterns of most species were also consistent with randomized patterns. However, the incidence and abundance of S. racemosa declined with the abundance of other plant species. Weak, variable support was found for nestedness of the total plant community. However, stronger, consistent support was found after removing S. racemosa from the matrix prior to analyses.
Main conclusions  Most species were assembled on islands in a manner consistent with randomized expectations. However, non-random distributional patterns were observed in one species whose distribution was consistent with the hypothesis that competition limits the assembly of island communities.     

Intraspecific variation in species interactions promotes the feasibility of mutualistic assemblages

Patterns of resource use observed at the species level emerge from the way individuals exploit the range of available resources. Hence, accounting for interindividual differences in resource use, such as pollinator use by plants, is essential to advance our understanding of community assembly and persistence. By using finely resolved data on plant–pollinator interactions, we evaluated how interindividual plant variation in pollinator use scales up to affect community structure and dynamics. All co-occurring plant species comprised specialists interacting with proper subsets of pollinators that visited generalists, and differences in interaction patterns were driven by among-individual trait variation. Furthermore, the nested structure and feasibility of plant–pollinator communities were maximised at higher levels of interindividual plant variation in traits and pollinator use. Our study sheds light on how pervasive properties of community structure arise from individual-level processes and contributes to elucidate the importance of preserving intraspecific variation in traits and resource use within populations.     

Plant interactions for pollinator visits: a test of the magnet species effect

Summary From 1985–1987, patterns of fruit and seed set were studied in a population of mayapple (Podophyllum peltatum), a clonal, self-incompatible herb found in deciduous woods in eastern North America. Mayapple flowers do not produce nectar, but depend on infrequent visits by nectar-seeking queen bumble bees for pollination. In all years female reproductive success in mayapple colonies was influenced by colony size (number of flowers), by the distance to neighbouring colonies and by proximity to lousewort plants (Pedicularis canadensis), a prolific nectar producer heavily visited by bumble bees. In all years fruit and seed set were greater in mayapple colonies <25 m from lousewort flowers than in matched colonies which were >50 m from lousewort. In 1985 and 1987 the frequency of queen bumble bee visits to flowers in colonies close to lousewort was about four times greater than to distant colonies. In 1986 I removed about 80% of lousewort flowers to test whether the enhanced fruit and seed set in mayapples close to lousewort was pollinator mediated. Mayapple colonies close to flowerless lousewort patches did not differ in fruit or seed set from matched colonies >50 m from lousewort. In contrast, mayapples close to flowering lousewort patches had greater fruit and seed set compared with distant colonies. Over all years, a larger proportion of mayapples close to flowering lousewort patches had enhanced fruit and seed set compared with colonies close to louseworts without flowers. Though rarely documented, this type of facilitative interaction between plants that are highly attractive to pollinators (magnet species), and co-flowering species that are rarely visited by pollinators, may be widespread in plant communities.     

The nested structure of a scavenger community

Scavenging is a widespread phenomenon in vertebrate communities which has rarely been accounted for, in spite of playing an essential role in food webs by enhancing nutrient recycling and community stability. Most studies on scavenger assemblages have often presented an oversimplified view of carrion foraging. Here, we applied for the first time the concept of nestedness to the study of a species-rich scavenger community in a forest ecosystem (Białowieża Primeval Forest, Poland) following a network approach. By analysing one of the most complete datasets existing up to now in a pristine environment, we have shown that the community of facultative scavengers is not randomly assembled but highly nested. A nested pattern means that species-poor carcasses support a subset of the scavenger assemblage occurring at progressively species-rich carcasses. This result contradicts the conventional view of facultative scavenging as random and opportunistic and supports recent findings in scavenging ecology. It also suggests that factors other than competition play a major role in determining community structure. Nested patterns in scavenger communities appear to be promoted by the high diversity in carrion resources and consumers, the differential predictability of the ungulate carcass types and stressful environmental conditions.     

Intraspecific functional diversity of common species enhances community stability

Common species are fundamental to the structure and function of their communities and may enhance community stability through intraspecific functional diversity (iFD). We measured among‐habitat and within‐habitat iFD (i.e., among‐ and within‐plant community types) of two common small mammal species using stable isotopes and functional trait dendrograms, determined whether iFD was related to short‐term population stability and small mammal community stability, and tested whether spatially explicit trait filters helped explain observed patterns of iFD. Southern red‐backed voles (Myodes gapperi) had greater iFD than deer mice (Peromyscus maniculatus), both among habitats, and within the plant community in which they were most abundant (their “primary habitat”). Peromyscus maniculatus populations across habitats differed significantly between years and declined 78% in deciduous forests, their primary habitat, as did the overall deciduous forest small mammal community. Myodes gapperi populations were stable across habitats and within coniferous forest, their primary habitat, as was the coniferous forest small mammal community. Generalized linear models representing internal trait filters (e.g., competition), which increase within‐habitat type iFD, best explained variation in M. gapperi diet, while models representing internal filters and external filters (e.g., climate), which suppress within‐habitat iFD, best explained P. maniculatus diet. This supports the finding that M. gapperi had higher iFD than P. maniculatus and is consistent with the theory that internal trait filters are associated with higher iFD than external filters. Common species with high iFD can impart a stabilizing influence on their communities, information that can be important for conserving biodiversity under environmental change.     

Examining the effects of species richness on community stability: an assembly model approach

We build dynamic models of community assembly by starting with one species in our model ecosystem and adding colonists. We find that the number of species present first increases, then fluctuates about some level. We ask: how large are these fluctuations and how can we characterize them statistically? As in Robert May's work, communities with weaker interspecific interactions permit a greater number of species to coexist on average. We find that as this average increases, however, the relative variation in the number of species and return times to mean community levels decreases. In addition, the relative frequency of large extinction events to small extinction events decreases as mean community size increases. While the model reproduces several of May's results, it also provides theoretical support for Charles Elton's idea that diverse communities such as those found in the tropics should be less variable than depauperate communities such as those found in arctic or agricultural settings.     

Niche partitioning due to adaptive foraging reverses effects of nestedness and connectance on pollination network stability

Much research debates whether properties of ecological networks such as nestedness and connectance stabilise biological communities while ignoring key behavioural aspects of organisms within these networks. Here, we computationally assess how adaptive foraging (AF) behaviour interacts with network architecture to determine the stability of plant–pollinator networks. We find that AF reverses negative effects of nestedness and positive effects of connectance on the stability of the networks by partitioning the niches among species within guilds. This behaviour enables generalist pollinators to preferentially forage on the most specialised of their plant partners which increases the pollination services to specialist plants and cedes the resources of generalist plants to specialist pollinators. We corroborate these behavioural preferences with intensive field observations of bee foraging. Our results show that incorporating key organismal behaviours with well‐known biological mechanisms such as consumer‐resource interactions into the analysis of ecological networks may greatly improve our understanding of complex ecosystems.