A review of competitive resource sharing under constraints in sticklebacks

The best strategy for an animal competing intraspecifically for food depends on its relative competitive ability, its needs, and on the strategies its competitors are using. Three different investigations using threespined sticklebacks, Gasterosteus aculeatus , as predators and Daphnia or Tubifex worms as prey are reviewed: (1) six sticklebacks differing in competitive ability compete for two food patches, of which one is twice as profitable as the other; (2) two sticklebacks differing in competitive ability compete in one patch for two types of prey, of which one is three times as profitable as the other; (3) parasitised and healthy sticklebacks have to decide alone, and in competition with each other, how close they dare approach on their own to a live predator waiting close to profitable food. The best strategy from the point of view of the stickleback is different from that of its parasite. The two parasite species studied, Schistocephalus solidus and Glugea anomala, ought to influence their host's behaviour in opposite directions.     

Stoichiometric relationships among producers,consumers and nutrient cycling in pelagic ecosystems

Most ecosystem models consolidate members of food-webs, e.g. species, into a small number of functional components. Each of these is then described by a single state variable such as biomass. When a multivariate approach incorporating multiple substances within components is substituted for this univariate one, a stoichiometric model is formed. Here we show that the Nitrogen:Phosphorus ratio within zooplankton herbivores varies substantially intraspecifically but not intraspecifically. By using stoichiometric theory and recent measurements of the N:P ratio within different zooplankton taxa, we calculate large differences in ratios of nutrients recycled by different zooplankton species. Finally, we demonstrate that N:P stoichiometry can successfully account for shifts in N- and P-limitation previously observed in whole-lake experiments. Species stoichiometry merges food-web dynamics with biogeochemical cycles to yield new insights.Abbreviations b N:P in zooplankton biomass - f N:P in algal biomass - L maximum accumulation eficiency - N:P ratio of nitrogen to phosphorus (moles:moles) - s N:P supply ratio from grazers - TN Total nitrogen = seston N + dissolved N (µmoles/liter) - TP Total phosphorus = seston P + dissolved P (µmoles/liter)     

Synergistic effects of microbial and animal decomposers on plant and herbivore performance

Decomposers drive essential ecosystem functions, such as organic matter turnover and nutrient cycling, thereby functioning as key determinants of soil fertility and nutrient uptake by plants. However, knowledge of interacting effects of functional dissimilar decomposer groups, such as microorganisms and animals, on aboveground functions is scarce.We set up a microcosm experiment to investigate single and combined effects of microbial (the fungus Fusarium graminearum) and animal decomposers (the earthworm Aporrectodea caliginosa) on the performance of winter wheat (Triticum aestivum) and aphids (Rhopalosiphum padi) in a full factorial design. We tested the shape of response of every variable in order to explore if interacting impacts of decomposers are under-additive (logarithmic fit), additive (linear fit) or over-additive (quadratic and exponential fit).Both microbial and animal decomposers increased the majority of the studied plant and herbivore performance parameters. While decomposers had additive effects on five plant performance variables they had over-additive effects on seven plant variables and three herbivore variables.The dominance of over-additive effects suggests positive interactions between microbial and animal decomposers. Facilitation in the decomposition process most likely synergistically increased nutrient supply for plants and food availability and quality for aphids.The present study indicates that functionally dissimilar decomposer groups of different kingdoms synergistically impact plant performance. Further, these beneficial effects propagated to herbivores suggesting that belowground functional diversity and positive interactions alter essential aboveground ecosystem functions over several trophic levels.     

Effects of decomposers and herbivores on plant performance and aboveground plant-insect interactions

Most ecologists acknowledge that plants are subject to complex interactions between both below- and aboveground dwelling animals. However, these complex interactions are seldomly investigated simultaneously. In a factorial common garden experiment we tested single and combined effects of decomposers, root herbivores and leaf herbivores on the growth, flower visitation, and abundance of naturally colonizing aphids and parasitoids on wild mustard ( Sinapis arvensis ). We found that the individual presence of either root herbivores or decomposers resulted in increased aphid abundance, demonstrating that the same aboveground plant–insect interaction can be released by different belowground processes. Enhanced aphid densities caused higher numbers of parasitoids. Furthermore, decomposers increased plant growth and plant fitness (measured as the number of seeds produced), indicating that mustard may benefit from nutrients provided by decomposers, regardless whether plants are attacked by root herbivores or leaf herbivores, or both simultaneously. More flower visits were observed in plants attacked by root herbivores but without leaf herbivores than in plants with both herbivores, suggesting that root herbivory can modify flower attractivity to pollinators. Our results suggest that patterns in plant–insect interactions above the ground are not only affected by aboveground factors but also by a wealth of different belowground processes mediated by the plant.     

Direct and indirect food web regulation of microbial decomposers in headwater streams

The direct and indirect regulation of primary productivity has been well established in autotrophic‐based ecosystems; however, less is known about the processes affecting decomposers in detrital‐based ecosystems. Because, small headwater, woodland streams are a dominate feature in most ecosystems and are tightly linked to terrestrial detritus, understanding decomposer‐mediated functions in these systems is critical for understanding carbon processes across the landscape. In this light, we conducted a microcosm and mesocosm experiment to test the direct and indirect food web effects on decomposers in small stream ecosystems. The results from the microcosm experiment supported an existing literature, demonstrating that nutrients directly stimulate decomposers and that microbivores directly reduce decomposers. Based on well‐founded food web theory in autotrophic systems, we predicted that fishes from different trophic‐functional guilds would indirectly stimulate decomposers by enhancing dissolved nutrients and by reducing microbivore densities. Our mesocosm experiment partially supported these predictions. Specifically, we found that fishes that consumed mostly terrestrial foods increased decomposers from the bottom–up by enhancing allochthonous nutrient loading into the stream ecosystems. Contrary to our predictions, however, predatory fishes that consume microbivores did not increase decomposers from the top–down. Rather, in streams with the predatory fish species, microbivores increased (rather than decreased) on leaf litter. This may have resulted from an experimental artifact associated with refuge provided by leaf packs. In conclusion, our data demonstrate that decomposers are regulated by similar direct and indirect processes important in autotrophic‐based ecosystems. This provides further evidence that food web processes can regulate leaf decomposition and flux of detrital carbon through ecosystems.     

Experimentally simulated global warming and nitrogen enrichment effects on microbial litter decomposers in a marsh

Atmospheric warming and increased nitrogen deposition can lead to changes of microbial communities with possible consequences for biogeochemical processes. We used an enclosure facility in a freshwater marsh to assess the effects on microbes associated with decomposing plant litter under conditions of simulated climate warming and pulsed nitrogen supply. Standard batches of litter were placed in coarse-mesh and fine-mesh bags and submerged in a series of heated, nitrogen-enriched, and control enclosures. They were retrieved later and analyzed for a range of microbial parameters. Fingerprinting profiles obtained by denaturing gradient gel electrophoresis (DGGE) indicated that simulated global warming induced a shift in bacterial community structure. In addition, warming reduced fungal biomass, whereas bacterial biomass was unaffected. The mesh size of the litter bags and sampling date also had an influence on bacterial community structure, with the apparent number of dominant genotypes increasing from spring to summer. Microbial respiration was unaffected by any treatment, and nitrogen enrichment had no clear effect on any of the microbial parameters considered. Overall, these results suggest that microbes associated with decomposing plant litter in nutrient-rich freshwater marshes are resistant to extra nitrogen supplies but are likely to respond to temperature increases projected for this century.     

Determining resource intake of a nonnative fish highlights potential predatory and competitive interactions

Biological Invasions - Nonnative species are often perceived to cause the decline or impede management and recovery of native species, yet the ability to quantify the ecological impacts of...     

The effects of interspecific interactions on resource use and behavior in a desert ant

We examined how interspecific competition in ants affects resource use and behavior. To test how neighboring Myrmecocystus colonies influence the desert ant Aphaenogaster cockerelli, we placed temporary enclosures around Myrmecocystus spp. colonies and recorded the resources collected by A. cockerelli and the numbers of A. cockerelli ants engaged in various tasks outside the nest. When neighbors were enclosed, A. cockerelli colonies collected a significantly higher proportion of termites and significantly less plant matter than when neighbors were active. The numbers of A. cockerelli ants engaged in foraging behavior and nest maintenance work increased when Myrmecocystus colonies were enclosed. Interspecific interactions thus can affect the behavior and resource use of A. cockerelli colonies and may influence colony fitness.     

Evolution of resource cycling in ecosystems and individuals

Background  

Resource cycling is a defining process in the maintenance of the biosphere. Microbial communities, ranging from simple to highly diverse, play a crucial role in this process. Yet the evolutionary adaptation and speciation of micro-organisms have rarely been studied in the context of resource cycling. In this study, our basic questions are how does a community evolve its resource usage and how are resource cycles partitioned?     

Leaf litter diversity alters microbial activity,microbial abundances,and nutrient cycling in a subtropical forest ecosystem

Human activities affect both tree species composition and diversity in forested ecosystems. This in turn alters the species diversity of plant litter and litter quality, which may have cascading effects on soil microbial communities and their functions for decomposition and nutrient cycling. We tested microbial responses to litter species diversity in a leaf litter decomposition experiment including monocultures, 2-, and 4-species mixtures in the subtropical climate zone of southeastern China. Soil microbial community composition was assessed by lipid analysis, and microbial functions were measured using extracellular enzyme activity and gross rates of nitrogen mineralization. We observed a positive relationship between litter species diversity and abundances of mycorrhizal fungi and actinomycetes. Alternatively, enzyme activities involved in carbon and phosphorus acquisition, and enzyme indices of relative carbon limitation, were higher only in the 4-species mixtures. This suggests that the minimum basal substrate level for enzyme production was reached, or that limitation was higher, at the highest diversity level only. Responses to litter diversity also changed over time, where phosphatase responses to litter diversity were strongest early in decomposition and the indices of carbon limitation relative to other nutrients showed stronger responses later in decomposition. Enzyme activities were related to lipid biomarker data and the mass of litter remaining at the third time point, but relationships between enzyme activity and the mass of litter remaining were not consistent across other time points. We conclude that litter species richness will likely only reduce microbial functions at key intervals of diversity loss while microbial growth is more sensitive to incremental diversity loss, with no clear relationships between them or to ecosystem functions. The observed litter diversity effects on soil microbial biomass and enzyme activity indicate interactions of aboveground and belowground communities, and together with environmental conditions they are important for maintaining ecosystem functions.     

Frequency-dependent competitive abilities and differential resource use in competition between Drosophila hydei and D. melanogaster

Several experiments, each involving competition between Drosophila melanogaster and D. hydei in population cages, were set up and allowed to run for up to 50 weeks. The population sizes of both species, and hence the species frequencies, were monitored once a fortnight, i.e. approximately once per generation. Coexistence of the two species was observed in cages containing resource bottles with 5 g of food medium; cages whose resource bottles contained only 1.5 g resulted in competitive exclusion of D. hydei. Competitive abilities were frequency-dependent in the former case but not in the latter. Tests of larval depth distributions revealed that D. hydei larvae feed at a deeper level in the food medium than larvae of D. melanogaster. The explanation of the contrasting results of competition when bottles contained 5 g and 1.5 g of resources lies in the production of frequency-dependent competitive abilities by larval resource partitioning in the bottles with 5 g, and the preclusion of such partitioning in the 1.5 g bottles because of the very limited depth of food medium then available. The relevance of these results to a model of competition is discussed, and the potential generality of differential resource use as a stabilizing mechanism in both interspecific and intergenotypic competition is noted.     

The use of genetic markers in the analysis of competitive interactions in Drosophila melanogaster

A range of long established inbred lines derived from the TEXAS population of Drosophila melanogaster have been used to elucidate the nature of the competitive interactions which occur in genetically heterogeneous mixtures. A prerequisite for this type of investigation is the ability to distinguish the genotypes which compete in mixed culture. Specific marker alleles are generally used to achieve this distinction although in the past little attention has been given to the possibility of competitive bias introduced by the marker alleles themselves. For the experiments reported in this paper two specific marker alleles (y ² and w ^a) have been introduced independently into the TEXAS inbred lines. In this way the original wild type inbred lines could be compared with similar series of genotypes marked with either y ² or w ^a and the effects of the marker alleles determined.The results indicated that the body colour mutation (y ²) was neutral in its effect on the competitive interaction of recipient strains. The introduction of the white apricot eye colour mutation (w ^a) however, had a pronounced and deleterious effect on competitive ability. This effect was to render genotypes less able to compete effectively in mixed culture by depressing inter-genotypic competitive ability. These effects were found to be consistent over a range of genotypes and for each of two characters measuring competitive success.     

Stoichiometric flexibility as a regulator of carbon and nutrient cycling in terrestrial ecosystems under change

Ecosystems across the biosphere are subject to rapid changes in elemental balance and climatic regimes. A major force structuring ecological responses to these perturbations lies in the stoichiometric flexibility of systems - the ability to adjust their elemental balance whilst maintaining function. The potential for stoichiometric flexibility underscores the utility of the application of a framework highlighting the constraints and consequences of elemental mass balance and energy cycling in biological systems to address global change phenomena. Improvement in the modeling of ecological responses to disturbance requires the consideration of the stoichiometric flexibility of systems within and across relevant scales. Although a multitude of global change studies over various spatial and temporal scales exist, the explicit consideration of the role played by stoichiometric flexibility in linking micro-scale to macro-scale biogeochemical processes in terrestrial ecosystems remains relatively unexplored. Focusing on terrestrial systems under change, we discuss the mechanisms by which stoichiometric flexibility might be expressed and connected from organisms to ecosystems. We suggest that the transition from the expression of stoichiometric flexibility within individuals to the community and ecosystem scales is a key mechanism regulating the extent to which environmental perturbation may alter ecosystem carbon and nutrient cycling dynamics.     

Biogeochemical cycling and microbial diversity in the thrombolitic microbialites of Highborne Cay,Bahamas

Thrombolites are unlaminated carbonate build‐ups that are formed via the metabolic activities of complex microbial mat communities. The thrombolitic mats of Highborne Cay, Bahamas develop in close proximity (1–2 m) to accreting laminated stromatolites, providing an ideal opportunity for biogeochemical and molecular comparisons of these two distinctive microbialite ecosystems. In this study, we provide the first comprehensive characterization of the biogeochemical activities and microbial diversity of the Highborne Cay thrombolitic mats. Morphological and molecular analyses reveal two dominant mat types associated with the thrombolite deposits, both of which are dominated by bacteria from the taxa Cyanobacteria and Alphaproteobacteria. Diel cycling of dissolved oxygen (DO) and dissolved inorganic carbon (DIC) were measured in all thrombolitic mat types. DO production varied between thrombolitic types and one morphotype, referred to in this study as ‘button mats’, produced the highest levels among all mat types, including the adjacent stromatolites. Characterization of thrombolite bacterial communities revealed a high bacterial diversity, roughly equivalent to that of the nearby stromatolites, and a low eukaryotic diversity. Extensive phylogenetic overlap between thrombolitic and stromatolitic microbial communities was observed, although thrombolite‐specific cyanobacterial populations were detected. In particular, the button mats were dominated by a calcified, filamentous cyanobacterium identified via morphology and 16S rRNA gene sequencing as Dichothrix sp. The distinctive microbial communities and chemical cycling patterns within the thrombolitic mats provide novel insight into the biogeochemical processes related to the lithifying mats in this system, and provide data relevant to understanding microbially induced carbonate biomineralization.