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.     

Seed‐dispersal networks respond differently to resource effects in open and forest habitats

While patterns in species diversity have been well studied across large‐scale environmental gradients, little is known about how species’ interaction networks change in response to abiotic and biotic factors across such gradients. Here we studied seed‐dispersal networks on 50 study plots distributed over ten different habitat types on the southern slopes of Mt Kilimanjaro, Tanzania, to disentangle the effects of climate, habitat structure, fruit diversity and fruit availability on different measures of interaction diversity. We used direct observations to record the interactions of frugivorous birds and mammals with fleshy‐fruited plants and recorded climatic conditions, habitat structure, fruit diversity and availability. We found that Shannon interaction diversity (H) increased with fruit diversity and availability, whereas interaction evenness (E_H) and network specialization (H₂) responded differently to changes in fruit availability depending on habitat structure. The direction of the effects of fruit availability on E_H and H₂ differed between open habitats at the mountain base and structurally complex habitats in the forest belt. Our findings illustrate that interaction networks react differently to changes in environmental conditions in different ecosystems. Hence, our findings demonstrate that future projections of network structure and associated ecosystem functions need to account for habitat differences among ecosystems.     

Abundance drives broad patterns of generalisation in plant–hummingbird pollination networks

Abundant pollinators are often more generalised than rare pollinators. This could be because abundant species have more chance encounters with potential interaction partners. On the other hand, generalised species could have a competitive advantage over specialists, leading to higher abundance. Determining the direction of the abundance–generalisation relationship is therefore a ‘chicken‐and‐egg’ dilemma. Here we determine the direction of the relationship between abundance and generalisation in plant–hummingbird pollination networks across the Americas. We find evidence that hummingbird pollinators are generalised because they are abundant, and little evidence that hummingbirds are abundant because they are generalised. Additionally, most patterns of species‐level abundance and generalisation were well explained by a null model that assumed interaction neutrality (interaction probabilities defined by species relative abundances). These results suggest that neutral processes play a key role in driving broad patterns of generalisation in animal pollinators across large spatial scales.     

Landscape genetic analyses reveal fine‐scale effects of forest fragmentation in an insular tropical bird

Within the framework of landscape genetics, resistance surface modelling is particularly relevant to explicitly test competing hypotheses about landscape effects on gene flow. To investigate how fragmentation of tropical forest affects population connectivity in a forest specialist bird species, we optimized resistance surfaces without a priori specification, using least‐cost (LCP) or resistance (IBR) distances. We implemented a two‐step procedure in order (i) to objectively define the landscape thematic resolution (level of detail in classification scheme to describe landscape variables) and spatial extent (area within the landscape boundaries) and then (ii) to test the relative role of several landscape features (elevation, roads, land cover) in genetic differentiation in the Plumbeous Warbler (Setophaga plumbea). We detected a small‐scale reduction of gene flow mainly driven by land cover, with a negative impact of the nonforest matrix on landscape functional connectivity. However, matrix components did not equally constrain gene flow, as their conductivity increased with increasing structural similarity with forest habitat: urban areas and meadows had the highest resistance values whereas agricultural areas had intermediate resistance values. Our results revealed a higher performance of IBR compared to LCP in explaining gene flow, reflecting suboptimal movements across this human‐modified landscape, challenging the common use of LCP to design habitat corridors and advocating for a broader use of circuit theory modelling. Finally, our results emphasize the need for an objective definition of landscape scales (landscape extent and thematic resolution) and highlight potential pitfalls associated with parameterization of resistance surfaces.     

Phenological phases of the host plant shape plant–treehopper interaction networks

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Context‐dependency and anthropogenic effects on individual plant–frugivore networks

Anthropogenic activities, such as grazing by domestic animals, are considered drivers of environmental changes that may influence the structure of interaction networks. The study of individual‐based networks allows testing how species‐level interaction patterns emerge from the pooled interaction modes of individuals within populations. Exponential random graph models (ERGMs) examine the global structure of networks by allowing the inclusion of specific node (i.e. interacting partners) properties as explanatory covariates. Here we assessed the structure of individual plant–frugivore interaction networks and the ecological variables that influence the mode of interactions under different land‐use (grazed versus ungrazed protected areas). We quantified the number of visits, the number of fruits removed per visit and the interaction strength of mammal frugivore species at each individual tree. Additionally we quantified ecological variables at the individual, microhabitat, neighborhood and habitat scales that generated interaction network structure under the different land uses. Individual plant–frugivore networks were significantly modular in both land uses but the number of modules was higher in the grazed areas. We found interaction networks for grazed and ungrazed lands were structured by phenotypic traits of individual trees, by the microhabitat beneath the tree canopy and were affected by habitat modifications of anthropogenic origin. The neighborhood surrounding each individual plant influenced plant–frugivore interactions only at the grazed‐land trees. We conclude that anthropogenic land uses influence the topological patterns of plant–frugivore networks and the frugivore visitation to trees through modification of both habitat complexity and the ecological traits underlying interactions between individual plants and frugivore species.     

Direct and plant‐mediated effects of climate on bird diversity in tropical mountains

AimAlthough patterns of biodiversity across the globe are well studied, there is still a controversial debate about the underlying mechanisms and their generality across biogeographic scales. In particular, it is unclear to what extent diversity patterns along environmental gradients are directly driven by abiotic factors, such as climate, or indirectly mediated through biotic factors, such as resource effects on consumers.LocationAndes, Southern Ecuador; Mt. Kilimanjaro, Tanzania.MethodsWe studied the diversity of fleshy‐fruited plants and avian frugivores at the taxonomic level, that is, species richness and abundance, as well as at the level of functional traits, that is, functional richness and functional dispersion. We compared two important biodiversity hotspots in mountain systems of the Neotropics and Afrotropics. We used field data of plant and bird communities, including trait measurements of 367 plant and bird species. Using structural equation modeling, we disentangled direct and indirect effects of climate and the diversity of plant communities on the diversity of bird communities.ResultsWe found significant bottom‐up effects of fruit diversity on frugivore diversity at the taxonomic level. In contrast, climate was more important for patterns of functional diversity, with plant communities being mostly related to precipitation, and bird communities being most strongly related to temperature.Main conclusionsOur results illustrate the general importance of bottom‐up mechanisms for the taxonomic diversity of consumers, suggesting the importance of active resource tracking. Our results also suggest that it might be difficult to identify signals of ecological fitting between functional plant and animal traits across biogeographic regions, since different species groups may respond to different climatic drivers. This decoupling between resource and consumer communities could increase under future climate change if plant and animal communities are consistently related to distinct climatic drivers.     

Net primary productivity allocation and cycling of carbon along a tropical forest elevational transect in the Peruvian Andes

The net primary productivity, carbon (C) stocks and turnover rates (i.e. C dynamics) of tropical forests are an important aspect of the global C cycle. These variables have been investigated in lowland tropical forests, but they have rarely been studied in tropical montane forests (TMFs). This study examines spatial patterns of above‐ and belowground C dynamics along a transect ranging from lowland Amazonia to the high Andes in SE Peru. Fine root biomass values increased from 1.50 Mg C ha^?1 at 194 m to 4.95 ± 0.62 Mg C ha^?1 at 3020 m, reaching a maximum of 6.83 ± 1.13 Mg C ha^?1 at the 2020 m elevation site. Aboveground biomass values decreased from 123.50 Mg C ha^?1 at 194 m to 47.03 Mg C ha^?1 at 3020 m. Mean annual belowground productivity was highest in the most fertile lowland plots (7.40 ± 1.00 Mg C ha^?1 yr^?1) and ranged between 3.43 ± 0.73 and 1.48 ± 0.40 Mg C ha^?1 yr^?1 in the premontane and montane plots. Mean annual aboveground productivity was estimated to vary between 9.50 ± 1.08 Mg C ha^?1 yr^?1 (210 m) and 2.59 ± 0.40 Mg C ha^?1 yr^?1 (2020 m), with consistently lower values observed in the cloud immersion zone of the montane forest. Fine root C residence time increased from 0.31 years in lowland Amazonia to 3.78 ± 0.81 years at 3020 m and stem C residence time remained constant along the elevational transect, with a mean of 54 ± 4 years. The ratio of fine root biomass to stem biomass increased significantly with increasing elevation, whereas the allocation of net primary productivity above‐ and belowground remained approximately constant at all elevations. Although net primary productivity declined in the TMF, the partitioning of productivity between the ecosystem subcomponents remained the same in lowland, premontane and montane forests.     

Seeing through the static: the temporal dimension of plant–animal mutualistic interactions

Most studies of plant–animal mutualistic networks have come from a temporally static perspective. This approach has revealed general patterns in network structure, but limits our ability to understand the ecological and evolutionary processes that shape these networks and to predict the consequences of natural and human‐driven disturbance on species interactions. We review the growing literature on temporal dynamics of plant–animal mutualistic networks including pollination, seed dispersal and ant defence mutualisms. We then discuss potential mechanisms underlying such variation in interactions, ranging from behavioural and physiological processes at the finest temporal scales to ecological and evolutionary processes at the broadest. We find that at the finest temporal scales (days, weeks, months) mutualistic interactions are highly dynamic, with considerable variation in network structure. At intermediate scales (years, decades), networks still exhibit high levels of temporal variation, but such variation appears to influence network properties only weakly. At the broadest temporal scales (many decades, centuries and beyond), continued shifts in interactions appear to reshape network structure, leading to dramatic community changes, including loss of species and function. Our review highlights the importance of considering the temporal dimension for understanding the ecology and evolution of complex webs of mutualistic interactions.     

尖峰岭热带山地雨林根部真菌-植物互作网络结构特征

不同功能群的根部真菌可能会与植物差异性地互作, 并进一步影响地下真菌与植物群落构建。本研究采用Illumina Miseq测序方法检测了海南尖峰岭热带山地雨林中常见植物的根部真菌; 采用网络分析法比较了丛枝菌根(AM)真菌、外生菌根(ECM)真菌, 以及所有根部真菌与植物互作的二分网络(bipartite networks)结构特性。从槭树科、番荔枝科、夹竹桃科、冬青科、棕榈科、壳斗科、樟科和木犀科等8科植物的根系中, 检测到297,831条真菌ITS1序列, 这些序列被划为1,279个真菌分类单元(OTUs), 其中子囊菌门748个、担子菌门354个、球囊菌亚门80个, 以及未知真菌97个。核心根部真菌群落(420个OTUs)中, 至少有三类不同生态功能的真菌常见, 即丛枝菌根真菌(40个OTUs, 占总序列数23.4%)、外生菌根真菌(48个OTUs, 13.9%)和腐生型真菌(83个OTUs, 19.8%)。尖峰岭山地雨林根部真菌-植物互作网络结构特性的指标普遍显著高于/低于假定物种随机互作的零模型期待值。在群落水平, 不同功能型的根部真菌-植物互作网络表现出不同或相反的结构特性, 如丛枝菌根互作网络表现为比零模型预测值高的嵌套性和连接性, 以及比零模型低的专一性, 而外生菌根互作网络呈现出比零模型预测值低的嵌套性和连接性, 以及比零模型高的专一性。在功能群水平, 植物的生态位重叠度在AM互作网络高, 而ECM互作网络低; 真菌的生态位宽度在ECM互作网络窄, 而在AM互作网络较宽。共现(co-occurrence)网络分析进一步揭示, ECM群落的物种对资源的高度种间竞争(植物、真菌高C-score), 以及AM群落的物种无明显种间竞争(低C-score), 可能分别是形成反嵌套ECM互作网络及高嵌套AM互作网络结构的原因。上述结果说明, 尖峰岭山地雨林中至少有两种及以上的种间互作机制调节群落构建: 驱动AM互作网络冗余(nestedness)及ECM互作网络的高生态位分化(专一性)。本研究在同一个森林内探讨了不同功能型的真菌-植物互作特性, 对深入理解热带森林的物种共存机制和生态恢复具有重要意义。     

The phylogenetic structure of plant–pollinator networks increases with habitat size and isolation

Similarity among species in traits related to ecological interactions is frequently associated with common ancestry. Thus, closely related species usually interact with ecologically similar partners, which can be reinforced by diverse co‐evolutionary processes. The effect of habitat fragmentation on the phylogenetic signal in interspecific interactions and correspondence between plant and animal phylogenies is, however, unknown. Here, we address to what extent phylogenetic signal and co‐phylogenetic congruence of plant–animal interactions depend on habitat size and isolation by analysing the phylogenetic structure of 12 pollination webs from isolated Pampean hills. Phylogenetic signal in interspecific interactions differed among webs, being stronger for flower‐visiting insects than plants. Phylogenetic signal and overall co‐phylogenetic congruence increased independently with hill size and isolation. We propose that habitat fragmentation would erode the phylogenetic structure of interaction webs. A decrease in phylogenetic signal and co‐phylogenetic correspondence in plant–pollinator interactions could be associated with less reliable mutualism and erratic co‐evolutionary change.     

Seed dispersal networks in tropical forest fragments: Area effects,remnant species,and interaction diversity

Seed dispersal interactions involve key ecological processes in tropical forests that help to maintain ecosystem functioning. Yet this functionality may be threatened by increasing habitat loss, defaunation, and fragmentation. However, generalist species, and their interactions, can benefit from the habitat change caused by human disturbance while more specialized interactions mostly disappear. Therefore, changes in the structure of the local, within fragment, networks can be expected. Here we investigated how the structure of seed dispersal networks changes along a gradient of increasing habitat fragmentation. We analyzed 16 bird seed dispersal assemblages from forest fragments of a biodiversity-rich ecosystem. We found significant species–, interaction–, and network–area relationships, yet the later was determined by the number of species remaining in each community. The number of frugivorous bird and plant species, their interactions, and the number of links per species decreases as area is lost in the fragmented landscape. In contrast, network nestedness has a negative relationship with fragment area, suggesting an increasing generalization of the network structure in the gradient of fragmentation. Network specialization was not significantly affected by area, indicating that some network properties may be invariant to disturbance. Still, the local extinction of partner species, paralleled by a loss of interactions and specialist–specialist bird–plant seed dispersal associations, suggests the functional homogenization of the system as area is lost. Our study provides empirical evidence for network–area relationships driven by the presence/absence of remnant species and the interactions they perform.