Proteomic characterization of the major arthropod associates of the carnivorous pitcher plant Sarracenia purpurea

The array of biomolecules generated by a functioning ecosystem represents both a potential resource for sustainable harvest and a potential indicator of ecosystem health and function. The cupped leaves of the carnivorous pitcher plant, Sarracenia purpurea, harbor a dynamic food web of aquatic invertebrates in a fully functional miniature ecosystem. The energetic base of this food web consists of insect prey, which is shredded by aquatic invertebrates and decomposed by microbes. Biomolecules and metabolites produced by this food web are actively exchanged with the photosynthesizing plant. In this report, we provide the first proteomic characterization of the sacrophagid fly (Fletcherimyia fletcheri), the pitcher plant mosquito (Wyeomyia smithii), and the pitcher-plant midge (Metriocnemus knabi). These three arthropods act as predators, filter feeders, and shredders at distinct trophic levels within the S. purpurea food web. More than 50 proteins from each species were identified, ten of which were predominantly or uniquely found in one species. Furthermore, 19 peptides unique to one of the three species were identified using an assembled database of 100 metazoan myosin heavy chain orthologs. These molecular signatures may be useful in species monitoring within heterogeneous ecosystem biomass and may also serve as indicators of ecosystem state.     

Species richness and trophic diversity increase decomposition in a co-evolved food web

Ecological communities show great variation in species richness, composition and food web structure across similar and diverse ecosystems. Knowledge of how this biodiversity relates to ecosystem functioning is important for understanding the maintenance of diversity and the potential effects of species losses and gains on ecosystems. While research often focuses on how variation in species richness influences ecosystem processes, assessing species richness in a food web context can provide further insight into the relationship between diversity and ecosystem functioning and elucidate potential mechanisms underpinning this relationship. Here, we assessed how species richness and trophic diversity affect decomposition rates in a complete aquatic food web: the five trophic level web that occurs within water-filled leaves of the northern pitcher plant, Sarracenia purpurea. We identified a trophic cascade in which top-predators--larvae of the pitcher-plant mosquito--indirectly increased bacterial decomposition by preying on bactivorous protozoa. Our data also revealed a facultative relationship in which larvae of the pitcher-plant midge increased bacterial decomposition by shredding detritus. These important interactions occur only in food webs with high trophic diversity, which in turn only occur in food webs with high species richness. We show that species richness and trophic diversity underlie strong linkages between food web structure and dynamics that influence ecosystem functioning. The importance of trophic diversity and species interactions in determining how biodiversity relates to ecosystem functioning suggests that simply focusing on species richness does not give a complete picture as to how ecosystems may change with the loss or gain of species.     

Bacterial diversity in three distinct sub-habitats within the pitchers of the northern pitcher plant, Sarracenia purpurea

Pitcher plants have been widely used in ecological studies of food webs; however, their bacterial communities are poorly characterized. Pitchers of Sarracenia purpurea contain several distinct sub-habitats, namely the bottom sediment, the liquid, and the internal pitcher wall. We hypothesized that those three sub-habitats within pitcher plants are inhabited by distinct bacterial populations. We used denaturing gradient gel electrophoresis and 16S rRNA gene sequencing to characterize bacterial populations in pitchers from three bogs. DGGE and sequencing revealed that in any given pitcher, the three sub-habitats contain significantly different bacterial populations. However, there was significant variability between bacterial populations inhabiting the same type of habitat in different pitchers, even at the same site. Therefore, no consistent set of bacterial populations was enriched in any of the three sub-habitats. All sub-habitats appeared to be dominated by alpha- and betaproteobacteria in differing proportions. In addition, sequences from the Bacteroidetes and Firmicutes were obtained from all three sub-habitats. We conclude that container aquatic habitats such as the pitchers of S.?purpurea possess a very high bacterial diversity, with many unique bacterial populations enriched in individual pitchers. Within an individual pitcher, populations of certain bacterial families may be enriched in one of the three studied sub-habitats.     

The Bacterial Composition within the Sarracenia purpurea Model System: Local Scale Differences and the Relationship with the Other Members of the Food Web

The leaves of the carnivorous pitcher plant, Sarracenia purpurea, contain a microscopic aquatic food web that is considered a model system in ecological research. The species identity of the intermediate and top trophic level of this food web, as well the detritivore midge, are highly similar across the native geographic range of S. purpurea and, in some cases, appear to have co-evolved with the plant. However, until recently, the identity, geographic variation, and diversity of the bacteria in the bottom trophic level of this food web have remained largely unknown. This study investigated bacterial community composition inside the leaves of S. purpurea to address: 1) variation in bacterial communities at the beginning of succession at the local scale in different areas of the plant’s native geographic range (southern and mid-regional sites) and 2) the impacts of bacterial consumers and other members of the aquatic food web (i.e., insects) on bacterial community structure. Communities from six leaves (one leaf per plant) from New York and Florida study sites were analyzed using 16S ribosomal RNA gene cloning. Each pitcher within each site had a distinct community; however, there was more overlap in bacterial composition within each site than when communities were compared across sites. In contrast, the identity of protozoans and metazoans in this community were similar in species identity both within a site and between the two sites, but abundances differed. Our results indicate that, at least during the beginning of succession, there is no strong selection for bacterial taxa and that there is no core group of bacteria required by the plant to start the decomposition of trapped insects. Co-evolution between the plant and bacteria appears to not have occurred as it has for other members of this community.     

Predicting food‐web structure with metacommunity models

Synthesis Metacommunity theory aims to elucidate the relative influence of local and regional‐scale processes in generating diversity patterns across the landscape. Metacommunity research has focused largely on assemblages of competing organisms within a single trophic level. Here, we test the ability of metacommunity models to predict the network structure of the aquatic food web found in the leaves of the northern pitcher plant Sarracenia purpurea. The species‐sorting and patch‐dynamics models most accurately reproduced nine food web properties, suggesting that local‐scale interactions play an important role in structuring Sarracenia food webs. Our approach can be applied to any well‐resolved food web for which data are available from multiple locations. The metacommunity framework explores the relative influence of local and regional‐scale processes in generating diversity patterns across the landscape. Metacommunity models and empirical studies have focused mostly on assemblages of competing organisms within a single trophic level. Studies of multi‐trophic metacommunities are predominantly restricted to simplified trophic motifs and rarely consider entire food webs. We tested the ability of the patch‐dynamics, species‐sorting, mass‐effects, and neutral metacommunity models, as well as three hybrid models, to reproduce empirical patterns of food web structure and composition in the complex aquatic food web found in the northern pitcher plant Sarracenia purpurea. We used empirical data to determine regional species pools and estimate dispersal probabilities, simulated local food‐web dynamics, dispersed species from regional pools into local food webs at rates based on the assumptions of each metacommunity model, and tested their relative fits to empirical data on food‐web structure. The species‐sorting and patch‐dynamics models most accurately reproduced nine food web properties, suggesting that local‐scale interactions were important in structuring Sarracenia food webs. However, differences in dispersal abilities were also important in models that accurately reproduced empirical food web properties. Although the models were tested using pitcher‐plant food webs, the approach we have developed can be applied to any well‐resolved food web for which data are available from multiple locations.     

Geographic variation in network structure of a nearctic aquatic food web

Aim The network structure of food webs plays an important role in the maintenance of diversity and ecosystem functioning in ecological communities. Previous research has found that ecosystem size, resource availability, assembly history and biotic interactions can potentially drive food web structure. However, the relative influence of climatic variables that drive broad‐scale biogeographic patterns of species richness and composition has not been explored for food web structure. In this study, we assess the influence of broad‐scale climatic variables in addition to known drivers of food web structure on replicate observations of a single aquatic food web, sampled from the leaves of the pitcher plant (Sarracenia purpurea), at different geographic sites across a broad latitudinal and climatic range. Location Using standardized sampling methods, we conducted an extensive ‘snapshot’ survey of 780 replicated aquatic food webs collected from the leaves of the pitcher plant S. purpurea at 39 sites from northern Florida to Newfoundland and westward to eastern British Columbia. Methods We examined correlations of 15 measures of food web structure at the pitcher and site scales with geographic variation in temperature and precipitation, concentrations of nutrients from atmospheric nitrogen deposition, resource availability, ecosystem size and the abundance of the pitcher plant mosquito (Wyeomyia smithii), a potential keystone species. Results At the scale of a single pitcher plant leaf, linkage density, species richness, measures of chain length and the proportion of omnivores in a web all increased with pitcher volume. Linkage density and species richness were greater at high‐latitude sites, which experience low mean temperatures and precipitation and high annual variation in both of these variables. At the site scale, variation in 8 of the 15 food web metrics decreased at higher latitudes, and variation in measures of chain length increased with the abundance of mosquitoes. Main conclusions Ecosystem size and climatic variables related to latitude were most strongly correlated with network structure of the Sarracenia food web. However, in spite of large sample sizes, thorough standardized sampling and the large geographic extent of the survey, even the best‐fitting models explained less than 40% of the variation in food web structure. In contrast to biogeographic patterns of species richness, food web structure was largely independent of broad‐scale climatic variables. The large proportion of unexplained variance in our analyses suggests that stochastic assembly may be an important determinant of local food web structure.     

From broadscale patterns to fine-scale processes: habitat structure influences genetic differentiation in the pitcher plant midge across multiple spatial scales

The spatial scale at which samples are collected and analysed influences the inferences that can be drawn from landscape genetic studies. We examined genetic structure and its landscape correlates in the pitcher plant midge, Metriocnemus knabi, an inhabitant of the purple pitcher plant, Sarracenia purpurea, across several spatial scales that are naturally delimited by the midge’s habitat (leaf, plant, cluster of plants, bog and system of bogs). We analysed 11 microsatellite loci in 710 M. knabi larvae from two systems of bogs in Algonquin Provincial Park (Canada) and tested the hypotheses that variables related to habitat structure are associated with genetic differentiation in this midge. Up to 54% of variation in individual‐based genetic distances at several scales was explained by broadscale landscape variables of bog size, pitcher plant density within bogs and connectivity of pitcher plant clusters. Our results indicate that oviposition behaviour of females at fine scales, as inferred from the spatial locations of full‐sib larvae, and spatially limited gene flow at broad scales represent the important processes underlying observed genetic patterns in M. knabi. Broadscale landscape features (bog size and plant density) appear to influence oviposition behaviour of midges, which in turn influences the patterns of genetic differentiation observed at both fine and broad scales. Thus, we inferred linkages among genetic patterns, landscape patterns and ecological processes across spatial scales in M. knabi. Our results reinforce the value of exploring such links simultaneously across multiple spatial scales and landscapes when investigating genetic diversity within a species.     

Permanent Presence of Grazing-Resistant Bacteria in a Hypertrophic Lake

The size structure of planktonic bacteria from a hypertrophic lake was investigated at 5- to 15-day intervals by means of a semiautomatic image analysis system during 1 year. Characteristic of this bacterial assemblage was the permanent presence of large filamentous bacteria and small cocci with cell sizes of <0.01 (mu)m(sup3). These filamentous bacteria, sometimes longer than 200 (mu)m and with cell volumes of up to 276 (mu)m(sup3), are larger than nanoflagellates (<20 (mu)m) and, even, metazoans living in the lake. Although they account for only 4 to 16% of bacterial abundance, their contribution to total bacterial biovolume was between 45 and 86%. An analysis of the food web structure indicates that this particular bacterial size structure may be the consequence of a strong bacterivory pressure by nanoflagellates and the absence of other larger bacterivores. The persistence of bacterial forms resistant to grazing has important consequences for the carbon flow within the microbial food web.     

Variance in composition of inquiline communities in leaves of Sarracenia purpurea L. on multiple spatial scales

A survey of the abundances of species that inhabit the water-bearing leaves of the pitcher plant Sarracenia purpurea was conducted at several different spatial scales in northern Florida. Individual leaves are hosts to communities of inquiline species, including mosquitoes, midges, mites, copepods, cladocerans, and a diverse bacterial assemblage. Inquiline communities were quantified from four pitchers per plant, three plants per subpopulation, two subpopulations per population, and three populations. Species varied in abundance at different spatial scales. Variation in the abundances of mosquitoes and copepods was not significantly associated with any spatial scale. Midges varied in abundance at the level of populations; one population contained significantly more midges than the other two. Cladocerans varied at the level of the subpopulation, whereas mites varied at the level of the individual plants. Bacterial communities were described by means of Biolog plates, which quantify the types of carbon media used by the bacteria in each pitcher. Bacterial communities were found to vary significantly in composition among individual plants but not among populations or subpopulations. These results suggest that independent factors determining the abundances of individual species are important in determining community patterns in pitcher-plant inquilines.     

Population growth and production of Habrotrocha rosa Donner (Rotifera: Bdelloidea) and its contribution to the nutrient supply of its host,the northern pitcher plant,Sarracenia purpurea L. (Sarraceniaceae)

The population growth and biomass production of the pitcher-plant (Sarracenia purpurea L.) inquiline, Habrotocha rosa Donner (Rotifera: Bdelloidea), its consumption by other pitcher-plant inqulines, and its excretion of phosphorus (PO4–P) and nitrogen (NO3–N and NH4–N), were investigated in laboratory experiments. Observed population growth and production rate of H. rosa were higher at pH 4 (2.3 rotifers d-1) than at pH 3 (1.3 rotifers d-1), 5 (1.9 rotifers d-1), or 6 (0.8 rotifers d-1). Populations of H. rosa are an abundant and reliable food source for larvae of the dipteran inqulines Wyeomyia smithii (Coq.) and Blaesoxipha fletcheri (Aldrich) that co-occur with H. rosa in S. purpurea pitchers. Abundance of H. rosa within a pitcher is negatively associated with abundance of dipteran larvae, and these larvae consume rotifers in direct proportion to rotifer density (Type I functional response). Habrotrocha rosa may also account for the majority of the plant's supply of N and P. An average population of rotifers in the field (∼400 per pitcher) can excrete ∼5.2 μg NO3-N, ∼3.91 μg NH4-N, and ∼18.4 μg PO4–P per day into a single leaf, and excretion rate is independent of water pH. Over the six-month growing season of pitcher-plants in Massachusetts, U.S.A., we estimate that rotifers could supply 8.8–43 mg of N and 18.2–88 mg of P. These values far exceed the amount of N and P previously estimated to be supplied annually to the plants through insect capture or rainfall. This revised version was published online in September 2006 with corrections to the Cover Date.     

Local‐ to continental‐scale variation in the richness and composition of an aquatic food web

Aim We investigated patterns of species richness and composition of the aquatic food web found in the liquid‐filled leaves of the North American purple pitcher plant, Sarracenia purpurea (Sarraceniaceae), from local to continental scales. Location We sampled 20 pitcher‐plant communities at each of 39 sites spanning the geographic range of S. purpurea– from northern Florida to Newfoundland and westward to eastern British Columbia. Methods Environmental predictors of variation in species composition and species richness were measured at two different spatial scales: among pitchers within sites and among sites. Hierarchical Bayesian models were used to examine correlates and similarities of species richness and abundance within and among sites. Results Ninety‐two taxa of arthropods, protozoa and bacteria were identified in the 780 pitcher samples. The variation in the species composition of this multi‐trophic level community across the broad geographic range of the host plant was lower than the variation among pitchers within host‐plant populations. Variation among food webs in richness and composition was related to climate, pore‐water chemistry, pitcher‐plant morphology and leaf age. Variation in the abundance of the five most common invertebrates was also strongly related to pitcher morphology and site‐specific climatic and other environmental variables. Main conclusions The surprising result that these communities are more variable within their host‐plant populations than across North America suggests that the food web in S. purpurea leaves consists of two groups of species: (1) a core group of mostly obligate pitcher‐plant residents that have evolved strong requirements for the host plant and that co‐occur consistently across North America, and (2) a larger set of relatively uncommon, generalist taxa that co‐occur patchily.     

Geographic variation and the evolution of reproductive allocation in the pitcher-plant mosquito, Wyeomyia smithii

We measured the egg size of six geographic populations of the pitcher-plant mosquito, Wyeomyia smithii, from Florida (30 degrees N) to Ontario (49 degrees N). Populations from northern latitudes produced larger eggs than populations from southern latitudes. Egg size increased with increasing latitude more rapidly when larvae were reared under low rather than high density. One southern (30 degrees N) and one northern (49 degrees N) population of W. smithii that persisted through 10 generations of selection for increased persistence under conditions of chronic thermal- and nutrient-limiting stress (conditions similar to southern rather than northern habitats) produced smaller eggs more rapidly than unselected control lines. However, there were no differences in lifetime fecundity or fertility between control and selected lines. Thus, laboratory evolution in an environment representative of extreme southern latitudes caused evolutionary changes consistent with geographic patterns of egg size. These results implicate temperature as a selective factor influencing the geographic variation of egg size in W. smithii, and demonstrate a novel trade-off in reproductive allocation between egg size and egg maturation time.     

Epistasis underlying a fitness trait within a natural population of the pitcher-plant mosquito, Wyeomyia smithii

We selected on divergent photoperiodic response in three separate lines from a natural population of the pitcher-plant mosquito, Wyeomyia smithii. Line crosses reveal that there exists within a population, diverse epistatic variation for a fitness trait that could contribute to adaptive potential following founder events or rapid climate change.     

农田土壤食物网管理的原理与方法

土壤食物网在维持生态系统生产力和健康等方面起着重要作用,但现代农业中,化肥农药等外部投入已经改变或部分替代了土壤食物网的功能,由此也造成一系列的环境问题。为了协调作物高产与环境保护的利益,需要对土壤食物网进行管理,使土壤食物网符合作物生长的需要,即建立健康土壤食物网。管理土壤食物网有两种方式:(1)直接方式,即通过调节食物网各个功能群的组成来管理土壤食物网;(2)间接方式,即根据农田土壤食物网以自下而上调控方式为主、强调低营养阶层的资源限制的原理,通过调节碎屑的数量和质量来管理食物网。在这两种调控方式中,都需要对被管理的食物网进行监测,监测的方式也分两种,一种是直接测定食物网各功能群的数量和生物量,另外一种方式即以线虫为工具来反应土壤食物网的结构。     

Intermediate-consumer identity and resources alter a food web with omnivory

1. Omnivory is an important interaction that has been the centre of numerous theoretical and empirical studies in recent years. Most of these studies examine the conditions necessary for coexistence between an omnivore and an intermediate consumer. Trait variation in ecological interactions (competition and predator tolerance) among intermediate consumers has not been considered in previous empirical studies despite the evidence that variation in species-specific traits can have important community-level effects. 2. I conducted a multifactorial microcosm experiment using species from the Sarracenia purpurea phytotelmata community, organisms that inhabit the water collected within its modified leaves. The basal trophic level consisted of bacterial decomposers, the second trophic level (intermediate consumers) consisted of protozoa and rotifers, and the third trophic level (omnivore) were larvae of the pitcher plant mosquito Wyeomyia smithii. Trophic level number (1, 2 and 3), resources (low and high), omnivore density (low and high) and intermediate consumer (monoculture of five protozoa and rotifers) identity were manipulated. Abundance of the basal trophic level, intermediate consumers, and growth of the omnivore were measured, as well as time to extinction (intermediate consumers) and time to pupation (mosquito larvae). 3. The presence of different intermediate consumers affected both bacteria abundance and omnivore growth. At high resource levels, Poteriochromonas, Colpidium and Habrotrocha rosa reduced bacteria densities greater than omnivore reduction of bacteria. Mosquito larvae did not pupate at low resource levels except when Poteriochromonas and Colopoda were present as intermediate consumers. Communities with H. rosa were the only ones consistent with the prediction that omnivores should exclude intermediate consumers at high resources. 4. These results had mixed support for predictions from omnivory food web theory. Intermediate consumers responded and affected this community differently under different community structures and resource levels. Consequently, variation in species-specific traits can have important population- and community-level effects and needs to be considered in food webs with omnivory.     

Local and Regional-Scale Food Web Structure in Nepenthes alata Pitchers1

Tropical Nepenthes pitcher plants provide small, isolated aquatic habitats. We examined inter-pitcher variation in the community structure of the inhabitants of Nepenthes alata Blanco in West Sumatra, focusing on the conditions of the pitchers, bacterial density in the pitcher fluid, density and biomass of metazoan inhabitants, and the frequencies of interspecific encounters. Older pitchers contained more insect carcasses. The bacterial density increased with the age of the pitchers, but decreased in withered pitchers that contained finely decomposed detritus. In live pitchers, the bacterial density, the density, mass and species richness of metazoa, and the number of trophic levels per pitcher were positively correlated with detrital mass, which was correlated with volume of pitcher fluid. The metazoan fauna from N. alata consisted of 4 predators and 12 saprophages, among the richest known for Nepenthes species. However, each individual pitcher harbored a limited numbers of species, owing to (1) the low incidence of many species, and (2) the aggregated distribution and different temporal colonization pattern of major species. Six dipteran taxa (one predator and five saprophages) accounted for the bulk of metazoan inhabitant biomass. Of 48 combinations of predator-prey encountered, only four occurred frequently (in > 30% of pitchers), which included two predators and three saprophages. Thus, individual pitchers harbored relatively simple communities despite the regional species richness, and only limited kinds of predator-prey encounters seemed to occur frequently in the regional food web. The local-scale properties of the subdivided communities presented here provide the basic information for understanding the maintenance of regional species richness and food web complexity.     

Patterns of diversity for aphidiine (Hymenoptera: Braconidae) parasitoid assemblages on aphids (Homoptera)

We used aphids (Aphidae) as a representative hemimetabolous host family to investigate patterns of parasitoid (Aphidiine) assemblage size. The aphidiine assemblages from 477 aphid species were used to estimate average assemblage size and the influence of eight ecological and taxonomic variables. Aphids species support an average of 1.7 aphidiine species. Aphid subfamily and invasion status (native or exotic) were the most important determinants of parasitoid richness, explaining 28% of the deviance in aphidiine assemblage size. Aphids within the largest aphid subfamily, the Aphidinae, support larger parasitoid assemblages than those in other subfamilies. Parasitoid diversity was also highest on exotic aphid hosts (within the Aphidinae) and on hosts in developed habitats (agricultural or urban), though the latter effect is weak. Patterns related to aphid food plant architecture were influenced by an interaction with aphid invasion status; parasitoid diversity drops with increasing architectural complexity on exotic aphids, whereas the diversities on native aphid hosts are similar on different plant types. Weak effects were also found for aphid food plant alternation (whether or not aphids switch hosts seasonally) and climate (annual range in temperature); alternating aphids support more parasitoids than non-alternating hosts, and parasitoid assemblage size is lowest in warm climates. Taxonomic isolation of aphids at the generic level showed no significant relationship with parasitoid diversity. Finally, in contrast to parasitoid assemblages on holometabolous hosts, sample size effects were weak for aphids, possibly due to the narrow host ranges of aphidiines. Received: 22 November 1997 / Accepted: 7 March 1998     

Assembly rules for New England ant assemblages

Community assembly rules specify patterns of species co-occurrence and morphology dictated by interspecific competition. We collected data on the occurrence of ground-foraging ant species in 22 ombrotrophic bogs and adjacent forest plots of New England to test two general assembly rules: reduced co-occurrence of species among communities, and even spacing of body sizes of species within communities. We used null models to generate random communities unstructured by competition and evaluated patterns at regional and local spatial scales. At the regional scale, species co-occurrence in forests, but not bogs, was less than expected by chance, whereas, at the local scale, co-occurrence in both habitats was not different from random. At the regional scale, spacing of body size distributions was random (in bogs) or aggregated (in forests). At the local scale, body size patterns were weakly segregated in bogs, but random or weakly aggregated in forests. In bogs, size ratio constancy was accompanied by greater generic diversity than expected. Although assembly rules were originally developed for vertebrate communities, they successfully explained some patterns in New England ant assemblages. However, the patterns were contingent on spatial scale, and were distinctly different for bog and forest communities, despite their close proximity and the presence of many shared species in both assemblages. The harsh physical conditions of bogs may act as a habitat filter that alters community assembly rules.     

Relationship between assemblages of mycorrhizal fungi and bacteria on grass roots

Soils support an enormous microbial diversity, but the ecological drivers of this diversity are poorly understood. Interactions between the roots of individual grass species and the arbuscular mycorrhizal (AM) fungi and bacteria in their rhizoplane were studied in a grazed, unimproved upland pasture. Individual root fragments were isolated from soil cores, DNA extracted and used to identify plant species and assess rhizoplane bacterial and AM fungal assemblages, by amplifying part of the small-subunit ribosomal RNA gene, followed by terminal restriction fragment length polymorphism analysis. For the first time we showed that AM fungal and bacterial assemblages are related in situ and that this relationship occurred at the community level. Principal coordinate analyses of the data show that the AM fungi were a major factor determining the bacterial assemblage on grass roots. We also report a strong influence of the composition of the plant community on AM fungal assemblage. The bacterial assemblage was also influenced by soil pH and was spatially structured, whereas AM fungi were influenced neither by the bacteria nor by soil pH. Our study shows that linkages between plant roots and their microbial communities exist in a complex web of interactions that act at individual and at community levels, with AM fungi influencing the bacterial assemblage, but not the other way round.     

Food Webs in Relation to Variation in the Environment and Species Assemblage: A Multivariate Approach

The abiotic environment has strong influences on the growth, survival, behavior, and ecology of aquatic organisms. Biotic interactions and species life histories interact with abiotic factors to structure the food web. One measure of food-web structure is food-chain length. Several hypotheses predict a linear relationship between one environmental variable (e.g., disturbance or ecosystem size) and food-chain length. However, many abiotic and biotic variables interact in diverse ways to structure a community, and may affect other measures of food web structure besides food-chain length. This study took a multivariate approach to test the influence of several important environmental variables on four food-web characteristics measured in nine ponds along a hydroperiod gradient over two years. This approach allowed for testing the ecosystem size and dynamic constraints hypotheses while in context of other possibly interacting environmental variables. The relationship between amphibian and invertebrate communities and pond habitat variables was assessed to understand the underlying food-web structure. Hydroperiod and pond area had a strong influence on amphibian and invertebrate communities, trophic diversity and δ¹⁵N range. The range in δ¹³C values responded strongly to dissolved oxygen. Food-chain length responded to multiple environmental variables. Invertebrate and amphibian communities were structured by pond hydroperiod which in turn influenced the trophic diversity of the food web. The results of this study suggest food-chain length is influenced by environmental variation and species assemblage and that a multivariate approach may allow us to better understand the dynamics within and across aquatic food webs.