Influence of taxonomic resolution on mutualistic network properties

1. Ecologists are increasingly interested in plant–pollinator networks that synthesize in a single object the species and the interactions linking them within their ecological context. Numerous indices have been developed to describe the structural properties and resilience of these networks, but currently, these indices are calculated for a network resolved to the species level, thus preventing the full exploitation of numerous datasets with a lower taxonomic resolution. Here, we used datasets from the literature to study whether taxonomic resolution has an impact on the properties of plant–pollinator networks.
2. For a set of 41 plant–pollinator networks from the literature, we calculated nine network index values at three different taxonomic resolutions: species, genus, and family. We used nine common indices assessing the structural properties or resilience of networks: nestedness (estimated using the nestedness index based on overlap and decreasing fill [NODF], weighted NODF, discrepancy [BR], and spectral radius [SR]), connectance, modularity, robustness to species loss, motifs frequencies, and normalized degree.
3. We observed that modifying the taxonomic resolution of these networks significantly changes the absolute values of the indices that describe their properties, except for the spectral radius and robustness. After the standardization of indices measuring nestedness with the Z‐score, three indices—NODF, BR, and SR for binary matrices—are not significantly different at different taxonomic resolutions. Finally, the relative values of all indices are strongly conserved at different taxonomic resolutions.
4. We conclude that it is possible to meaningfully estimate the properties of plant–pollinator interaction networks with a taxonomic resolution lower than the species level. We would advise using either the SR or robustness on untransformed data, or the NODF, discrepancy, or SR (for weighted networks only) on Z‐scores. Additionally, connectance and modularity can be compared between low taxonomic resolution networks using the rank instead of the absolute values.

     

Quantifying keystone species complexes: Ecosystem-based conservation management in the King George Island (Antarctic Peninsula)

A keystone species complex (KSC) is a small set of interacting species that play an outstandingly important role in community organization. Two KSC indices are suggested and have been calculated in the coastal benthic/pelagic ecosystem of Fildes Bay, King George Island (Antarctica). These indices of keystoneness emerge after considering: (1) functional indices based on steady-state and dynamic quantitative trophic models (using bottom-up, mixed and top-down control flow mechanisms); (2) structural indices including bottom-up and top-down control mechanisms, (3) semi-quantitative (qualitative) keystone indices using loop analysis (under mixed control); and (4) topological key player indices based on the centrality of node sets in the network. The models constructed and analyzed describe the interactions of the most abundant species and functional groups inhabiting the coastal ecological systems of Fildes Bay. Although our results only represent the transient dynamics of these ecological systems, the KSC indices identified the following trophically connected common core of components: the functional groups of Seastars (top-predators), the herbivorous sea urchin species Sterechinus neumayeri and the Phytoplankton (primary producers). The KSC indices for Fildes Bay could facilitate the design and assessment of conservation monitoring, especially when the Antarctic ecosystems are being severely stressed by the direct effects of global warming and UV radiation. A more holistic view of conservation remains difficult because the traditional view is based principally on single species. This imposes an even greater challenge, for global changes accompany the network of interacting species, co-varying with the variables of the natural system.     

The genetic network of greater sage‐grouse: Range‐wide identification of keystone hubs of connectivity

Genetic networks can characterize complex genetic relationships among groups of individuals, which can be used to rank nodes most important to the overall connectivity of the system. Ranking allows scarce resources to be guided toward nodes integral to connectivity. The greater sage‐grouse (Centrocercus urophasianus) is a species of conservation concern that breeds on spatially discrete leks that must remain connected by genetic exchange for population persistence. We genotyped 5,950 individuals from 1,200 greater sage‐grouse leks distributed across the entire species’ geographic range. We found a small‐world network composed of 458 nodes connected by 14,481 edges. This network was composed of hubs—that is, nodes facilitating gene flow across the network—and spokes—that is, nodes where connectivity is served by hubs. It is within these hubs that the greatest genetic diversity was housed. Using indices of network centrality, we identified hub nodes of greatest conservation importance. We also identified keystone nodes with elevated centrality despite low local population size. Hub and keystone nodes were found across the entire species’ contiguous range, although nodes with elevated importance to network‐wide connectivity were found more central: especially in northeastern, central, and southwestern Wyoming and eastern Idaho. Nodes among which genes are most readily exchanged were mostly located in Montana and northern Wyoming, as well as Utah and eastern Nevada. The loss of hub or keystone nodes could lead to the disintegration of the network into smaller, isolated subnetworks. Protecting both hub nodes and keystone nodes will conserve genetic diversity and should maintain network connections to ensure a resilient and viable population over time. Our analysis shows that network models can be used to model gene flow, offering insights into its pattern and process, with application to prioritizing landscapes for conservation.     

Functional importance vs keystoneness: Reformulating some questions in theoretical biocenology

Abstract Two concepts relating to the influence of individual species on the biocenoses in which they occur are reviewed. The first, the general functional importance of a species, is denned as the sum, over all species, of the changes (sign ignored) in productivity which would occur on removal of the particular species from the biocenosis. General functional importance is calculated as: where P_j is the productivity of the jth species before (t= 0) and after (t= 1) removal of the particular (ith) species being evaluated. Though I_i values cannot be determined empirically, this concept raises provocative questions for theoretical biocenology. The second concept reviewed is that of the keystone species. Never having been precisely or operationally defined,‘keystone’ has come to mean little more than ‘important for something.’ Moreover, there is no empirical or theoretical foundation for the idea that there exists in any biocenosis a natural dichotomy corresponding to the verbal one of keystone and non-keystone species. Some investigators have implied that such a dichotomy is suggested by the frequency distributions of experimentally determined values of interaction strength. The patterns they refer to are, however, artifacts resulting from small sample sizes and the plotting of frequency distributions on arithmetic rather than logarithmic scales. As a casual metaphor ‘keystone species’ was appealing and harmless; but the pretence that it is a well-defined concept or phenomenon has had a stultifying effect on ecological thought and argument.     

Using food network unfolding to evaluate food–web complexity in terms of biodiversity: theory and applications

Food–web complexity often hinders disentangling functionally relevant aspects of food–web structure and its relationships to biodiversity. Here, we present a theoretical framework to evaluate food–web complexity in terms of biodiversity. Food network unfolding is a theoretical method to transform a complex food web into a linear food chain based on ecosystem processes. Based on this method, we can define three biodiversity indices, horizontal diversity (D_H), vertical diversity (D_V) and range diversity (D_R), which are associated with the species diversity within each trophic level, diversity of trophic levels, and diversity in resource use, respectively. These indices are related to Shannon's diversity index (H′), where H′ = D_H + D_V ? D_R. Application of the framework to three riverine macroinvertebrate communities revealed that D indices, calculated from biomass and stable isotope features, captured well the anthropogenic, seasonal, or other within‐site changes in food–web structures that could not be captured with H′ alone.     

Topological keystone species: measures of positional importance in food webs

The local extinction or large fluctuation in abundance of a species may seriously affect other species in the community. The effects spread through the community by direct and indirect interactions. The network perspective on ecology can help map the pathways of these effects, for food webs, the pathways of indirect trophic interactions. Indirect interactions typically decay in intensity as they spread. Therefore, there is a conceptual maximum range in topological space beyond which interactions have no effects, even though all species remain connected. Neither the local characteristics of species, nor the global characteristics of entire webs, suitably quantify this range. We therefore apply intermediate scale indices that reflect the limitations imposed by effect damping in networks. We present a complex analysis of the topological positional importance of species in the Chesapeake Bay web. This web is a carbon-flow network that represents trophic interactions. We present several different indices reflecting different properties and discuss which questions the different indices best answer. We look for the best indices for identifying the key players in ecosystem functioning. Our study contributes to the quantification of relative species importance and provides an exact and a priori determination of a class of candidate keystone species that can inform applied and conservation ecology as well as theoretical concerns.     

Dispersal limitation and geographical distributions of mammal species

Aim To relate the dispersal limitation of endemic terrestrial mammals in Mexico to species life‐history traits and latitude. Location Mexico. Methods We modelled species ecological niches projected as potential distributions (P) using point occurrence data and 19 environmental variables for 89 endemic mammal species, and compared the areas covered by these ecological niche models with maps of species actual distributions (R) based on minimum convex polygons connecting marginal records based on museum specimens. We correlated body mass, food habits (herbivore, omnivore, insectivore, frugivore/granivore), volant and non‐volant (fossorial, arboreal, terrestrial) habits and mean latitude to the proportion of occupancy of species potential distributional areas (R/P). Results R and P were significantly positively correlated, with an overall average R/P ratio of 0.49. Less than half of the endemics (41 species) had a high occupancy (R/P values ranging from 0.50 to 0.90); a few (four species) showed full occupancy (> 0.90). Body mass and food habits were not correlated with R/P, but latitude showed significant correlations with R/P; volant mammals tended to show higher R/P values than non‐volant mammals. Main conclusions Few species filled most of the spatial extent of their ecological niches. Life‐history traits were generally poor predictors of proportional occupancy of species potential distributions. Endemics occurring at higher latitudes showed higher occupancy, suggesting that abiotic factors are likely to limit their distributions. Conversely, species at lower latitudes showed lower occupancy, suggesting that their distributions are limited by biotic factors and/or by geographical or historical barriers that prevent dispersal. The dispersal abilities of volant compared with non‐volant endemics can explain the higher occupancy in species potential distributions in the former group. These trends provide a baseline for exploring the importance of life‐history traits and abiotic versus biotic factors in limiting species distributions.     

Scaling up keystone effects from simple to complex ecological networks

Predicting the consequences of species loss requires extending our traditional understanding of simpler dynamic systems of few interacting species to the more complex ecological networks found in natural ecosystems. Especially important is the scaling up of our limited understanding of how and under what conditions loss of ‘keystone’ species causes large declines of many other species. Here we explore how these keystone effects vary among simulations progressively scaled up from simple to more complex systems. Simpler simulations of four to seven interacting species suggest that species up to four links away can strongly alter keystone effects and make the consequences of keystone loss potentially indeterminate in more realistically complex communities. Instead of indeterminacy, we find that more complex networks of up to 32 species generally buffer distant influences such that variation in keystone effects is well predicted by surprisingly local ‘top‐down’, ‘bottom‐up’, and ‘horizontal‘ constraints acting within two links of the keystone subsystem. These results demonstrate that: (1) strong suppression of the competitive dominant by the keystone may only weakly affect subordinate competitors; (2) the community context of the target species determines whether strong keystone effects are realized; (3) simple, measurable, and local attributes of complex communities may explain much of the empirically observed variation in keystone effects; and (4) increasing network complexity per se does not inherently make the prediction of strong keystone effects more complicated.     

Identifying keystone trophic groups in benthic ecosystems: Implications for fisheries management

Many species inhabiting the benthic marine ecosystems of the central and northern Chilean coast have been intensively harvested and this exploitation has increased considerably in recent years. Despite this harvest pressure, few studies have attempted to establish a more holistic, systems-based management plan. On the contrary, research continues to rely on population models in which the species of interest are isolated from their ecological context. This work offers several keystone indices in order to help multispecies fisheries management. The indices used are: (1) functional indices based on steady-state and dynamic trophic models; (2) structural indices based on bottom-up and top-down control mechanisms; and (3) qualitative keystone species indices using loop models (mixed control). The quantitative trophic models were constructed using Ecopath with Ecosim (EwE; v. 5.0) software, and the qualitative model was analysed using Loop Analysis. All models describe the interactions of the most representative species and functional groups inhabiting the benthic ecosystems of Tongoy Bay, La Rinconada Marine Reserve (Antofagasta Bay), and the kelp forest of Mejillones Peninsula (Antofagasta). Even though our results only represent the short-term dynamics of these systems, we have found keystoneness properties of several species and functional groups, including primary producers, herbivores, and top predators. Despite this wide variability of groups, we detected a different core set of species or functional groups, each of which contained prey–predator and plant–herbivore relationships. Because the traditional keystone concept of a single species is difficult to apply, we suggest shifting away from this view towards a more holistic alternative such as that of a keystone species complex. This kind of approach would facilitate the design and assessment of sustainable management strategies for ecological marine ecosystems. Despite the ecological relevance of our results, further experimental studies and modelling using other theoretical frameworks should be performed.     

Feeding strategy,dietary overlap and resource partitioning among four mesopredatory catfishes of a tropical estuary

The feeding strategy, overlap and trophic interactions of four demersal catfishes inhabiting Cochin Estuary, Arius maculatus (n = 67), A. subrostratus (n = 63), A. arius (n = 21) and Mystus gulio (n = 69) were investigated through gut-content analyses. The indices of relative importance revealed that the prey items in their stomachs were dominated by crustaceans except in A. arius where molluscs constituted the major prey item. The highest diet and niche breadth values (3.93, 0.36 respectively) were recorded in A. subrostratus, while the lowest was recorded in A. arius (2.64, 0.23 respectively). Pianka's overlap, calculated through null models constructed by Ecosim 7.0, revealed significant niche overlap between A. maculatus and A. subrostratus (O–0.91, P < 0.001), between A. maculatus and M. gulio (O–0.72, P < 0.05) and between A. subrostratus and M.gulio (O–0.64 P < 0.05). However, no overlap was recorded between A. arius and other species. The trophic niche breadth along with prey specific abundance confirmed that these fishes are highly specific feeders. The present results on trophic level status indicate that these catfishes are mid-level carnivores that can be considered as moderate benthic mesopredators and are specialist feeders unlike most catfishes.     

基于SURF指数识别海州湾食物网的关键饵料生物

关键种(keystone species)在生态系统中发挥着不可替代的重要作用,对于群落结构的稳定与演替起着决定性的作用。基于2011年3—12月在海州湾及其邻近海域进行的渔业资源底拖网调查资料以及胃含物分析数据和参考历史文献数据,以物种间的摄食关系作为基础,采用SURF(Supportive Role to Fishery ecosystems)指数识别海州湾食物网中的关键饵料生物。在矩阵中分析每个物种作为饵料生物为捕食者提供的摄食比例及捕食者数量,计算每个物种的关键指标(SURFi)的值,结果表明前五位数值较高的物种是细鳌虾(Leptochela gracilis)、毛虾(Acetessp.)、疣背宽额虾(Latreutes planirostris)、日本鼓虾(Alpheus japonicus)和鳀鱼(Engraulis japonicus),它们是海州湾的关键饵料生物,在整个食物网中起到关键的控制作用,它们的数量波动会对海州湾其他物种产生直接或间接的影响。加强关键饵料生物的保护,对于维持海州湾生态系统的健康和稳定至关重要。     

Node centrality indices in food webs: Rank orders versus distributions

Network analysis examines the role of species in ecological communities. The most common approach involves measurement of centrality of species or other groups of individuals based on their topological positions in food webs, followed by establishing the rank order of importance of these groups. However, ranking may differ considerably with indices of centrality and therefore comparison of rank orders is essential to obtain more meaningful results on species performance. Since ranking ignores absolute differences between centrality values, species orders may neglect important structural information in food webs. Consequently, simultaneous examination of the distribution of index values is inevitable. Hierarchical clustering and consensus generation revealed that rank orders of centrality exhibit a similar pattern over six example food webs, while distributions differ not only with indices because their relationships are largely inconsistent with food webs as well. Therefore, optimal analysis of networks and the selection of keystone species in any ecological study should rely upon both of these procedures. Similar conclusions are drawn from the detailed evaluation of a sample food web from the Florida Bay.     

Effect of flood pulses on the trophic ecology of four piscivorous fishes from the eastern Amazon

This study investigated the effect of hydrological periods on the feeding activity and trophic interactions of four piscivorous fishes from the middle Xingu River, Brazil: pike‐characid Boulengerella cuvieri, dogtooth characin Hydrolycus armatus, dogtooth characin Hydrolycus tatauaia and South American silver croaker Plagioscion squamosissimus. Repletion Index (I_R%), Alimentary Index (I_Ai%) and food web properties were calculated for each species. A total of 825 specimens were collected. The I_R showed changes in feeding intensity of B. cuvieri, H. armatus and H. tatauaia among hydrological periods. Flood pulse showed no influence on composition and importance of food items consumed. Trophic connections showed that connectivity ranged from 0.025 to 0.038. The highest number of trophic connections (75) occurred in the high‐water period, when 51 food items were recorded and the lowest number of trophic connections (43) occurred in receding water, with 31 food items. In all food webs, over 45% of food items were consumed by only one species (ultra‐peripheral items), which is common in piscivorous fishes.     

Born among the ice: first morphological observations on two developmental stages of the Antarctic silverfish Pleuragramma antarcticum, a key species of the Southern Ocean

The Antarctic silverfish Pleuragramma antarcticum is a keystone species in the Southern Ocean ecosystem, providing one of the major links between lower and higher trophic levels. Despite the importance of this species, surprisingly little is known of its early development. The first spawning area for the silverfish has been recently identified in the near-shore of Terra Nova Bay (Ross Sea). Evidence indicates that spawning and embryo development occurs in the cryopelagic environment, below the seasonal pack-ice. In order to contribute to the knowledge of the life cycle of this very important Antarctic species, we carried out the first histological characterization on pre-hatching embryos and newly hatched larvae. Embryonated eggs and larvae of P. antarcticum were collected between late October and November 2005 at TNB through holes drilled into the sea ice. Embryonic stage just before hatching and the first post-hatching stage were the most abundant within our samples and thus were analysed using both macroscopic and histological approaches. Early life stages of the Antarctic silverfish revealed interesting features: the sensory system, foraging apparatus and heart appeared well developed, whereas the liver and gills were underdeveloped. Morphological details of the organogenesis were performed, providing the first substantial information on the development of P. antarcticum and representing a further steps towards the knowledge of the life cycle of this important Antarctic key species. An erratum to this article can be found at     

Standardized diet compositions and trophic levels of skates (Chondrichthyes: Rajiformes: Rajoidei)

Skates by virtue of their abundance and widespread occurrence appear to play an influential role in the food webs of demersal marine communities. However, few quantitative dietary studies have been conducted on this elasmobranch group. Therefore, to better understand the ecological role of skates, standardized diet compositions and trophic level (TL) values were calculated from quantitative studies, and compared within and among skate and shark taxa. Prey items were grouped into 11 general categories to facilitate standardized diet composition and TL calculations. Trophic level values were calculated for 60 skate species with TL estimates ranging from 3.48 to 4.22 (mean TL = 3.80 ± 0.02 SE). Standardized diet composition results revealed that decapods and fishes were the main prey taxa of most skate species followed by amphipods and polychaetes. Correspondingly, cluster analysis of diet composition data revealed four major trophic guilds, each dominated by one of these prey groups. Fish and decapod guilds were dominant comprising 39 of 48 species analyzed. Analysis of skate families revealed that the Arhynchobatidae and Rajidae had similar TL values of 3.86 and 3.79 (t-test, P = 0.27), respectively. The Anacanthobatidae were represented by a single species, Cruriraja parcomaculata, with a TL of 3.53. Statistical comparison of TL values calculated for five genera (Bathyraja, Leucoraja, Raja, Rajella, Rhinoraja) revealed a significant difference between Bathyraja and Rajella (t-test, P = 0.03). A positive correlation was observed between TL and total length (L _T) with larger skates (e.g. >100 cm L _T) tending to have a higher calculated TL value (>3.9). Skates were found to occupy TLs similar to those of several co-occurring demersal shark families including the Scyliorhinidae, Squatinidae, and Triakidae. Results from this study support recent assertions that skates utilize similar resources to those of other upper trophic-level marine predators, e.g. seabirds, marine mammals, and sharks. These preliminary findings will hopefully encourage future research into the trophic relationships and ecological impact of these interesting and important demersal predators.     

Keystone Interactions: Salmon and Bear in Riparian Forests of Alaska

The term “keystone species” is used to describe organisms that exert a disproportionately important influence on the ecosystems in which they live. Analogous concepts such as “keystone mutualism” and “mobile links” illustrate how, in many cases, the interactions of two or more species produce an effect greater than that of any one species individually. Because of their role in transporting nutrients from the ocean to river and riparian ecosystems, Pacific salmon (Oncorhynchus spp.) and brown bear (Ursus arctos) have been described as keystone species and mobile links, although few data are available to quantify the importance of this interaction relative to other nutrient vectors. Application of a mass balance model to data from a southwestern Alaskan stream suggests that nitrogen (N) influx to the riparian forest is significantly increased in the presence of both salmon and bear, but not by either species individually. The interactions of salmon and bear may provide up to 24% of riparian N budgets, but this percentage varies in time and space according to variations in salmon escapement, channel morphology and watershed vegetation characteristics, suggesting interdependence and functional redundancy among N sources. These findings illustrate the complexity of interspecific interactions, the importance of linkages across ecosystem boundaries and the necessity of examining the processes and interactions that shape ecological communities, rather than their specific component parts.     

山西文峪河上中游森林群落多样性

基于山西文峪河上中游森林群落的野外调查数据,选取植物生活型、生活史、固氮类型、传粉途径、种子传播途径等14个植物功能性状,计算丰富度指数(R)、多样性指数(H')、均匀度指数(E)等物种多样性指数和功能丰富度指数(FRic)、功能均匀度指数(FEve)、功能分歧度指数(FDiv)等功能多样性指数,并用TWINSPAN对森林群落进行分类,Spearman秩相关分析多样性指数间及其与环境因子间的相关性,对山西文峪河上中游森林群落多样性进行研究。结果表明:青杄林种数最多(R=27),辽东栎油松林和油松林种数最少(R=16);白桦林的H'和E最大,油松林的H'和E最小。山杨白桦林的FRic值最大,白杄林的FRic值最小;青杄林的FEve值最大,山杨白桦林的FEve值最小;山杨林的FDiv值最大,白桦林的FDiv值最小。文峪河上中游森林群落物种多样性指数与功能多样性指数间相关性不显著(P0.05),仅FDiv与H'呈显著负相关关系(P0.05);物种多样性指数间呈极显著相关关系(P0.01),功能多样性指数间相关性不显著(P0.05),仅FRic与FEve呈显著负相关关系(P0.05);随着海拔增加,物种多样性指数增加(P0.05或P0.01),但功能多样性指数减小(P0.01或P0.05)。     

滇西北退化高寒草甸植物群落结构对刈割的响应

为探究滇西北不同退化级别高寒草甸植物群落结构对外界干扰的响应敏感性,以香格里拉市的典型高寒草甸为研究对象,于2018-2020年在三个退化梯度上（严重退化,S1;中度退化,S2;轻度退化,S3）开展控制刈割实验,进而分析草甸植物物种丰富度、群落组成相似性、群落复杂度和关键种的变化规律。结果表明：（1）刈割后,S1的物种丰富度显著增加（P<0.05）,S2和S3的物种丰富度未发生显著变化（P>0.05）;（2）相较于S2和S3,S1梯度的植物群落组成变化最大;（3） S1、S2和S3的植物群落复杂度在刈割后均呈先下降后增加的趋势,但S1的植物群落复杂度变化幅度高于S2和S3;（4）刈割导致各退化草甸植物群落的关键种发生了变化,2018、2019和2020年S1梯度的关键种在豆科（Leguminsae）和蔷薇科（Rosaceae）之间变化,S2梯度的关键种在禾本科（Gramineae）和菊科（Compositae）之间变化,S3梯度的关键种在蔷薇科（Rosaceae）、菊科（Compositae）和禾本科（Gramineae）之间转变。研究表明,滇西北高寒草甸植物物种丰富度、群落组成和群落复杂度对外界干扰响应的敏感性可能随退化加剧而上升,但群落关键种的响应过程较复杂。     

Phytoplankton as an Indicator of the Water Quality of the Deep Lakes South of the Alps

This paper offers a synoptic account of studies on the phytoplankton communities in the deep southern subalpine lakes (DSL) Garda, Iseo, Como, Lugano and Maggiore. The main cause of the degradation of the water quality in the DSL is eutrophication. The euphotic layers of these lakes are trophically different, ranging from the oligo-mesotrophy of lakes Maggiore and Garda to the meso-eutrophy of lakes Iseo and Lugano. The trophic status as estimated by using total phosphorus and chlorophyll a has provided consistent results in agreement with the models proposed by OECD (1982. Eutrophication of Waters. Monitoring, Assessment and Control, OECD, Paris). Though related with chlorophyll a and TP, the Secchi disk depths have significantly underestimated the trophic status of the DSL. Two trophic indices using the algal orders (PTI_orders) and species (PTI_species) were drawn up on the basis of the distribution of phytoplankton along a trophic gradient defined by the application of multivariate methods; the scores emerging from these indices were used to make a definitive ecological classification of water bodies on a scale from 1 to 5, in accordance with the Water Framework Directive. A third index (PTI_OE) was computed as the ratio between the annual mean values of the cumulative biovolumes of two groups of algal orders with opposite trophic characteristics. The three PTI indices were highly correlated, providing a consistent classification of the water bodies. The indices proposed in this work were specifically adopted for use in the DSL. However, the criteria for their implementation constitute a robust and impartial tool for assessing similar indices in other lake typologies and for evaluating the degree of specificity of the trophic indicator values assigned to the single phytoplankton orders and species.     

A commensal network of epiphytic orchids and host trees in an Atlantic Forest remnant: A case study revealing the important role of large trees in the network structure

Application of metrics derived from network theory could elucidate the structural organization of orchid assemblages, and help identify the host tree species on which they depend, as well as predicting the impacts of removing host tree species. In this study, we used nestedness, modularity, connectance and robustness, to identify the factors that predict the structure of a quantitative orchid–host tree network in a remnant of Atlantic Forest in Brazil. The network exhibited low nestedness (NODF = 14.07; P = 0.03; WNODF = 5.3; P = 0.02) and no modularity. It was highly robust to the random elimination of host tree species, but showed low robustness when host trees with more interactions started to be eliminated. The nested pattern found was attributed to a combination of two main host tree traits, height and diameter (given by DBH measure). Thus, our analyses reflect the importance of tall and large host tree species, which seem to play an important role in the network structure, providing a substrate for orchid species with different habitat associations and representing a decisive factor in both nested and robust patterns found.