Dominant Primary Consumers Research Articles

Elevated dissolved carbon dioxide and associated acidification delays maturation and decreases calcification and survival in the freshwater crustacean Daphnia magna

AbstractIncreasing pCO2 in freshwaters across the globe will likely be accompanied by acidification. Although this has been linked to reduced calcification, growth, and survival in several marine species, similar responses in freshwater organisms remain largely unexplored. Here, we investigated the direct effects of elevated pCO2 and associated acidification on the water flea Daphnia magna. As a highly efficient filter feeder, this crustacean is a dominant primary consumer in freshwater zooplankton assemblages, capable of controlling algal growth. It has a high calcium content compared to other zooplankton species, which might make it particularly sensitive since calcification may be impaired under elevated pCO2. We exposed newly hatched individuals of four clonal lineages to 27,000 ppm pCO2 corresponding to a pH of 6.7 in a controlled laboratory experiment. Both survival and calcium content were reduced and juveniles grew slower and matured later under elevated pCO2. Adult growth rate was not affected by pCO2 suggesting that surviving adults are less sensitive or may acclimate over time but at the cost of reduced offspring size. We hypothesize that the combination of elevated pCO2 and low pH interferes with the calcification process, reflected in slower growth, smaller body size and reduced calcification in D. magna. In turn, this might lead to less efficient control of phytoplankton as an ecosystem service and reduce their competitive advantage over other zooplankters with less calcified bodies.

Meta-Analysis of the Large Herbivores’ Trophic Spectra in Northern Asia Concerning Changes of Dominant Primary Consumers

The paper deals with influence of trophic competition on large herbivore community dynamics in Northern Asia related to landscape changes and extermination by humans. It was evaluated how the competitive asymmetry in trophic resource use corresponds to differences in geographic range or population size and in their trends and/or rates of change, which reflect directions of the dominant primary consumers change in the late Pleistocene and Holocene and at present. It was found that the directions mainly correspond to competitive asymmetry and were driven already during landscape changes after the Last Glacial Maximum, and later the human influence mainly enhanced and continues to enhance the competition influence. Regulating abundance relation of wild and domestic herbivore species humans become an element of the trophic competition regulation.

Saproxylic biodiversity and decomposition rate decrease with small-scale isolation of tree hollows

Biodiversity is fundamental for ecosystem functioning, but little is known about how function responds to biodiversity loss following habitat disturbance in natural systems. Due to the global decay of veteran trees, many associated saproxylic (i.e. deadwood-dependent) insects are considered threatened. Nevertheless, the role of habitat spatial configuration on saproxylic insect biodiversity and dead wood decomposition is poorly understood. We performed a six-year landscape-scale colonization experiment on saproxylic beetles inhabiting hollow oaks, using boxes filled with wood mould as standardized habitat patches. We placed boxes either on a hollow tree or on another tree 61–324 m from the hollows, thereby creating two habitat isolation levels. We quantified wood mould decay and biodiversity in the boxes, measuring species richness, total abundances and community-weighted mean of body mass (CWM) as an index of community functional composition. Isolation had a persistent negative effect on primary consumer biodiversity, but it only impaired decay at the beginning of the experiment. All effects were independent of landscape-level (500-m radius) habitat amount surrounding the boxes. Wood mould decay was mediated by CWM of primary consumers. Therefore function was driven by the body masses of the dominant primary consumer species but not by species numbers (richness) or individual numbers (abundance). Our experiment shows that small-scale habitat isolation leads to biodiversity loss and reduced function and indicates that habitats created by conservation efforts will be used by more saproxylic species if located within sites with a high density of veteran trees.

Stable isotope ratios indicate differential omnivory among syntopic rocky shore suspension-feeders

We utilised stable isotope ratios to assess differences in diet among three indigenous and syntopic rocky shore suspension-feeders (mussels Perna perna, barnacles Tetraclita serrata, and polychaetes Gunnarea gaimardi). We also determined the spatial and temporal variability in the suspension-feeder diets by collecting specimens on two occasions from two regions adjacent to hydrologically distinct river mouths (i.e. one with larger annual freshwater throughput than the other). The results showed that the isotopic niches (used as proxies for trophic niches) of the three species did not overlap and that the barnacles occupied a trophic position (3.4) well above those of the mussels (2) and polychaetes (2.6). We ascribed the interspecific differences primarily to the disparate feeding mechanisms used by the species. Large-scale regional (~50 km) differences in suspension-feeder diets were apparent, but not small-scale (up to a few km north and south of each estuary mouth). The regional differences in diet resulted from the increased availability of estuarine-origin suspended particulate matter (SPM) and zooplankton in the region adjacent to the river with relatively larger freshwater output, although overall incorporation of zooplankton versus mixed SPM into consumer diets was relatively consistent between regions and through time. Temporal shifts in suspension-feeder diets were apparent from stable carbon isotope ratios in the consumers. Our results showcase the measurable effects of regional-scale processes that can alter the food sources for dominant primary consumers in the rocky intertidal, thus potentially affecting entire food webs through bottom-up processes. The clear evidence for trophic niche partitioning offers valuable insights into how potentially strong competitors can coexist.

Nutrient enrichment differentially affects body sizes of primary consumers and predators in a detritus‐based stream

We assessed how a 5‐yr nutrient enrichment affected the responses of different size classes of primary consumers and predators in a detritus‐based headwater stream. We hypothesized that alterations in detritus availability because of enrichment would decrease the abundance and biomass of large‐bodied consumers. In contrast, we found that 2 yr of enrichment increased the biomass and abundance of all consumers regardless of body size. Furthermore, during the fourth and fifth year of enrichment, the abundance and biomass of largebodied primary consumers continued to increase, while small‐bodied primary consumers returned to pretreatment levels. The size structure of a dominant primary consumer (Pycnopsyche spp.) also shifted during the 5‐yr enrichment: its average and maximum individual body size increased in the treatment stream compared with the reference stream. Positive enrichment effects also occurred on small‐bodied predators, but not on large‐bodied predators. Thus, enrichment increased prey body size, but these positive effects on large prey did not propagate up to higher trophic levels to affect large predators. Because consumer body size can be an important species‐specific trait determining population dynamics and ecosystem processes, these observed shifts in consumer size distributions suggest a potentially important pathway for global increases in nutrient enrichment to alter stream structure and function.

Carbon sources for lake food webs in the Canadian High Arctic and other regions of Arctic North America

We investigated the role of autochthonous and terrestrial carbon in supporting aquatic food webs in the Canadian High Arctic by determining the diet of the dominant primary consumer, aquatic chironomids. These organisms were studied in fresh waters on 3 islands of the Arctic Archipelago (*74-76N) including barren polar desert watersheds and a polar oasis with lush meadows. Stomach content analysis of 578 larvae indicated that chironomids primarily ingested diatoms and sediment detritus with little variation among most genera. Carbon and nitrogen stable isotope mixing models applied to 2 lakes indicated that benthic algae contributed 68-95% to chironomid diet at a polar desert site and 70-78% at a polar oasis site. Detritus, originating from either phytoplankton or terrestrial sources, also contributed minor amounts to chironomid diet (0-32%). Radiocarbon measurements for the 2 lakes showed that old terrestrial carbon did not sup- port chironomid production. Carbon stable isotope ratios of chironomids in other High Arctic lakes provided further dietary evidence that was consistent with mixing model results. These findings indicate that, in the Canadian High Arctic, chironomids (and fish that consume them) are supported primarily by benthic algae in both polar desert and oasis lakes. In contrast, our review of carbon flow studies for lakes in other Arctic regions of North America shows that terrestrial carbon and phytoplankton can be important energy sources for consumers. This study pro- vides a baseline to detect future climate-related impacts on carbon pathways in High Arctic lakes.

Evidence for the role of methane‐derived carbon in a free‐flowing, lowland river food web

We measured the δ13C values of dominant primary consumers and their potential food sources in a groundwater‐fed lowland river. The δ13C of most consumers, such as Gammarus and Simulium, reflected that of the dominant forms of photosynthetic production, whereas the cased larvae of two caddis flies (Agapetus and Silo) were consistently 13C depleted (mean δ13C: ‐41.2% and ‐40.4%, respectively) throughout the year. The river water was supersaturated (approximately 50 times atmospheric) with methane, reflecting both supersaturation in the groundwater and local production in fine sediments. We measured appreciable rates of methane oxidation, relative to water only controls, in the biofilms on gravel, on the caddis fly cases, and on the bottom of larger rocks. In addition, there was a marked difference in the ratio of methane‐oxidizing potential to chlorophyll across those substrata. This ratio was below detection in the biofilm (i.e., no methane oxidation) on the tops of rocks, greater on the bottom of rocks, and maximal for the gravels and the caddis cases. If the caddis larvae acquire most of their carbon by grazing the tops of such rocks (where they are normally found), then they must acquire their depleted δ13C values by occasionally grazing biofilm where the ratio of methane oxidation to chlorophyll was much greater, and the most likely candidate is from their own or conspecific cases. Grazing methane‐oxidizing bacteria could provide the caddis larvae with up to 30% of their carbon, which could represent a true subsidy from an ancient groundwater source.

Feeding of the chaetognath Sagitta crassa Tokioka in heavily eutrophicated Tokyo Bay, Japan

Sagitta crassa is the only autochthonous chaetognath in Tokyo Bay, central Japan. We analyzed the gut contents of S. crassa at a fixed station in May and September 1997. The species fed on prey organisms whose body widths were in a range between 1.4-6.0% of the length of S. crassa. Prey with body widths less than 2% of the length of S. crassa were minor components of the gut contents. Although the cyclopoid copepod Oithona davisae was abun- dant in the water column, the prey selectivity of S. crassa on O. davisae was low. Our results suggest that S. crassa con- sumes catchable prey organisms that are larger and easier to detect. As planktonic species diversity in the Tokyo Bay ecosystem is low, it has been assumed that a simple trophic pathway from the most dominant primary consumer O. davisae to the secondary consumer S. crassa exists. However, such a simple assumption is inadequate to understand the prey-predator relationships and food-web dynamics in Tokyo Bay.

Fast growth and turnover of chironomid assemblages in response to stream phosphorus levels in a tropical lowland landscape

We studied the response of a dominant primary consumer, larval Chironomidae (Diptera), to phosphorus (via a microbial response) across eight detritus-based streams spanning a gradient in soluble reactive phosphorus (range 0.2‐8.6 mmol L 21 ) in a tropical landscape in Central America. We predicted that phosphorus would enhance food resources for larval Chironomidae and positively affect their production. A low-phosphorus stream was also experimentally enriched to isolate effects of phosphorus from other solutes present. We found considerable variation in daily growth rates (range 0.09‐0.57 mg mg 21 d 21 ) and annual biomass turnover rates (range 66‐126 mg mg 21 yr 21 ) among streams, both positively related to phosphorus levels. Larval biomass was replaced every 5 d under low-phosphorus and every 3.5 d under high-phosphorus conditions (including the experimentally phosphorus-enriched stream). Annual secondary production (range 793‐9,346 mg m 22 yr 21 ) was variable among streams and negatively related to benthic organic matter, potentially because of low dissolved oxygen. The increase in larval biomass turnover along the phosphorus gradient indicates that a gradient of increasing energy flow exists from detrital-based resources (e.g., fungi) to primary consumers. Larval chironomids matured more rapidly under high phosphorus conditions; this strategy results in (1) similar larval biomass among streams and (2) a potential gradient of increasing energy flow from primary consumers to upper trophic levels as phosphorus increases. Nutrientenhanced turnover rate of primary consumers can occur without increases in secondary production in stream ecosystems.

Factors influencing the distribution and feeding of the larvae of Chironomus riparius

AbstractChironomids are the most widely distributed group of insect larvae in freshwater environments. They are frequently the dominant primary consumers and show competitive interactions due to their requirements for foraging space in suitable substrates. In order to investigate the factors involved in substrate selection a simple choice chamber has been constructed that enables larvae of Chironomus riparius Meigen (Diptera, Chironomidae) to select wells, in which they can establish tubes, according to a number of chemical and physical parameters. The statistical probabilities of them aggregating randomly in this way were calculated for a variety of occupancies. The larvae developed distinct patterns of non‐random aggregation in the wells. Subsequent experiments showed that the presence of food at sites of burrowing affected settling but physical irregularities were necessary in order to facilitate burrowing and establish random patterns. Once the animals had selected their habitat the rate of feeding on the sediment was relatively constant with a gut‐clearance time of about 12 h although starving the animals before or after feeding affected the rate of passage of sediment through the anterior of the gut.

Variation in the Vulnerability of Prey to Different Predators: Community‐Level Consequences

Midge larvae (Diptera, Chironomidae) that weave filamentous algae into retreats of tufts, are dominant primary consumers in a river food web. In a previous study, densities of tuft—weaving midges increased in the presence of large fish. In the absence of large fish, midges decreased as densities of predatory invertebrates built up, and higher standing crops of algae were maintained. To examine the mechanisms underlying these dynamics, we compared the vulnerability of tuft—weaving midges (naked or in algal tufts) to fish and predatory invertebrates, in field and laboratory experiments. When midges were exposed for 1 h in the river to fish, 15 out of 15 midges in tufts survived, while 15 of 15 naked midges were consumed. Tufts afforded only partial protection to midges exposed to invertebrate predators, however. After 1 h, enhancement of survivorship by tufts was moderately significant for midges exposed to aeshnids, and insignificant for midges exposed to lestids and naucorids. We suggest that the vulnerability of tuft—weaving midges to invertebrate predators, and their relative invulnerability to fish, sets the stage for trophic cascades observed in the system. Fish, by consuming small predators, release midges, which graze down algae. The strong effects of fish as fourth—level consumers would not be predicted from their diets, in which algivorous mayflies dominate (>60% of the insect biomass found in each of the two most common fish species). Nevertheless, fish in this food web act as fourth—level, rather than third—level, consumers because of the differential vulnerability of one guild of primary consumer, which, when released from predation, can suppress plants.

Patterns of evolution in the Dicynodontia, with special reference to austral taxa

Abstract Dicynodonts were the dominant primary consumers of the latest Palaeozoic and early Mesozoic ecosystems. Of the 66 currently accepted genera of dicynodonts, three (possibly four) of these occur in the Fremouw Formation of Antarctica (i.e. Kingoria, Lystrosaurus, Myosaurus and ? Kannemeyeria ). Modifications to the basic shearing mechanism of the jaws as well as postcranial morphology presumably reflect differences in the physical nature of the food consumed and foraging techniques used by dicynodonts. To date, five dicynodont feeding types have been recognized: invertebrate specialists, grubbers, browsers, forest litter foragers and flexible foragers. Three out of the five ‘central’ dicynodont feeding types are represented among the members of the early Mesozoic Antarctic palaeofauna. In order to more accurately assess systematic and ecomorphic relationships among dicynodonts, comprehensive generic diagnoses (that incorporate postcranial characters) are warranted; suggested revisions to the generic diagnoses of Antarctic dicynodonts are provided. Dicynodont distributions are evaluated in light of ecomorphic considerations, tectonic-orogenic events, sea-level fluctuations, palaeoclimates and floral changes. Available evidence, pertaining to austral taxa, suggests that: (1) Antarctica served as a refugium for kingoriamorphs in the Early Triassic, and (2) the informal members of the Fremouw Formation are correlated with more than one African biozone.

The role of methylmercury production in the transfer of mercury in a salt marsh ecosystem

A salt marsh estuary which has received large discharges of inorganic mercury for several years was studied to evaluate the occurrence and production of methylmercury. The methyl form was not detected (<1 ng/g) in marsh sediments or marsh vegetation ( Spartina alterniflora). Significant concentrations were detected in the dominant primary consumers ( Littorina irrorata and Uca sp.). Assuming these organisms accumulated a significant percentage of the methylmercury produced in the salt marsh ecosystem, its annual production there is estimated at about 50 μg/g of total mercury in the upper 5 cm of the sediment column.