Trophic Hierarchy Research Articles

Aquatic invertebrates in riverine landscapes

1. Riverine systems consist of a mosaic of patches and habitats linked by diverse processes and supporting highly complex communities. Invertebrates show a high taxonomic and functional diversity in riverine systems and are in several ways important components of these systems. Their distribution patterns, movements and effects on ecological flows, testify to their importance in various landscape ecological processes. This paper reviews the invertebrate literature with respect to patterns and processes in the riverine landscape.2. The distribution of invertebrates in riverine habitats is governed by a number of factors that typically act at different scales. Hence, the local community structure can be seen as the result of a continuous sorting process through environmental filters ranging from regional or catchment‐wide processes, involving speciation, geological history and climate, to the small‐scale characteristics of individual patches, such as local predation risk, substratum porosity and current velocity.3. Dispersal is an important process driving invertebrate distribution, linking different ecological systems across boundaries. Dispersal occurs within the aquatic habitat as well as into the terrestrial surrounding, and also over land to other waterbodies. New genetic techniques have contributed significantly to the understanding of aquatic invertebrate dispersal and revealed the importance of factors such as physical barriers, synchrony of emergence and taxonomic affiliation.4. Invertebrates affect the cycling of nutrients and carbon by being a crucial intermediate link between primary producers, detritus pools or primary consumers, and predators higher up in the trophic hierarchy. Suspension feeders increase the retention of carbon. The subsidies of aquatic invertebrates to the terrestrial ecosystem have been shown to be important, as are reciprocal processes such as the supply of terrestrial invertebrates that fall into the water.5. Future studies are needed both to advance theoretical aspects of landscape ecology pertaining to the invertebrates in riverine systems and to intensify the experimental testing of hypotheses, for example with respect to the scaling of processes and to linkages between the terrestrial and aquatic systems. Another promising avenue is to take advantage of naturally steep environmental gradients, and of systems disturbed by humans, such as regulated rivers. By comparison with unimpaired reference sites, the mechanisms involved might be identified. The use of `natural' experiments, especially where environmental gradients are steep, is another technique with great potential.

Structure of tropical river food webs revealed by stable isotope ratios

Fish assemblages in tropical river food webs are characterized by high taxonomic diversity, diverse foraging modes, omnivory, and an abundance of detritivores. Feeding links are complex and modified by hydrologic seasonality and system productivity. These properties make it difficult to generalize about feeding relationships and to identify dominant linkages of energy flow. We analyzed the stable carbon and nitrogen isotope ratios of 276 fishes and other food web components living in four Venezuelan rivers that differed in basal food resources to determine 1) whether fish trophic guilds integrated food resources in a predictable fashion, thereby providing similar trophic resolution as individual species, 2) whether food chain length differed with system productivity, and 3) how omnivory and detritivory influenced trophic structure within these food webs. Fishes were grouped into four trophic guilds (herbivores, detritivores/algivores, omnivores, piscivores) based on literature reports and external morphological characteristics. Results of discriminant function analyses showed that isotope data were effective at reclassifying individual fish into their pre‐identified trophic category. Nutrient‐poor, black‐water rivers showed greater compartmentalization in isotope values than more productive rivers, leading to greater reclassification success. In three out of four food webs, omnivores were more often misclassified than other trophic groups, reflecting the diverse food sources they assimilated. When fish δ15N values were used to estimate species position in the trophic hierarchy, top piscivores in nutrient‐poor rivers had higher trophic positions than those in more productive rivers. This was in contrast to our expectation that productive systems would promote longer food chains. Although isotope ratios could not resolve species‐level feeding pathways, they did reveal how top consumers integrate isotopic variability occurring lower in the food web. Top piscivores, regardless of species, had carbon and nitrogen profiles less variable than other trophic groups.

Body size and food web structure: testing the equiprobability assumption of the cascade model.

The cascade model successfuly predicts many patterns in reported food webs. A key assumption of this model is the existence of a predetermined trophic hierarchy; prey are always lower in the hierarchy than their predators. At least three studies have suggested that, in animal food webs, this hierarchy can be explained to a large extent by body size relationships. A second assumption of the standard cascade model is that trophic links not prohibited by the hierarchy occur with equal probability. Using nonparametric contingency table analyses, we tested this "equiprobability hypothesis" in 16 published animal food webs for which the adult body masses of the species had been estimated. We found that when the hierarchy was based on body size, the equiprobability hypothesis was rejected in favor of an alternative, "predator-dominance" hypothesis wherein the probability of a trophic link varies with the identity of the predator. Another alternative to equiprobabilty is that the probability of a trophic link depends upon the ratio of the body sizes of the two species. Using nonparametric regression and liklihood ratio tests, we show that a size-ratio based model represents a significant improvement over the cascade model. These results suggest that models with heterogeneous predation probabilities will fit food web data better than the homogeneous cascade model. They also suggest a new way to bridge the gap between static and dynamic food web models.

Resource partitioning by Lake Tana barbs predicted from fish morphometrics and prey characteristics

We develop a food-fish model (FFM), whichquantitatively relates properties of aquaticfood types (size, shape, escape velocity,habitat, mechanical properties and chemicalquality) to feeding structures of cyprinidfish. The model is based on functionalmorphology and experiments on search, capture,selection, and internal processing of food byfish. The FFM shows which food properties aremost critical in feeding and how fish canoptimise coping with them. Relative food sizeimposes the highest demands, followed by preyvelocity, food habitat and mechanicalproperties. These overrule taxonomic affinitiesof food types. Highly demanding food types(large, fast prey, suspensions of plankton,benthic prey and mechanically tough items)impose incompatible morphological requirementson fish. We apply the FFM to the endemic Barbus species flock of Lake Tana (Ethiopia),since the structural diversity of its 14species reflects recent adaptations to trophicniches. We predict their potentials inutilising different food types by quantitativecomparisons of 35 parameters, measured for eachspecies, with the values for each foodspecialist derived from the FFM. These dietpredictions are tested against gut contentsfrom 4,711 fish, sampled over seasons andhabitats. Gut contents and predictions show agood overall fit. The value of the model isshown by its resolution in predicting resourcepartitioning among the barbs. For the 14 barbsa trophic hierarchy with six major trophicgroups is reconstructed which closely matchesthe predictions. Trophic specialists (> 65%by volume of a single food type) are alsostructurally specialised, whereas less extremeanatomical structures characterise trophicgeneralists, allowing them to switch betweenfeeding modes. Trophic generalists are bestdefined by behavioral flexibility, sincefeeding modes integrate both fish and foodcharacters. The FFM is of practical use inevaluating the role of morphological diversityin an ecosystem and enables the analysis oftrophic interactions in fish communities and ofthe cascading effects by environmental change.Such an approach can be instrumental in thedevelopment of management strategies forfisheries and in conservation of biodiversity.

In Search of Operational Trophospecies in a Tropical Aquatic Food Web

Aggregations of biological species on the basis of trophic similarity (trophospecies) are the basic units of study in food web and ecosystem research, yet little attention has been devoted to articulating objective protocols for defining such aggregations. This study formulates several possible definitions based on alternative measures of similarity and hierarchical clustering. Twenty-four alternative methods were applied to a food web consisting of 116 original trophic entities (OTEs) from a tropical floodplain, for which the relative magnitude of each trophic link was estimated based on dietary data measured as volumetric proportions. The resulting 24 trophic hierarchies were compared based on cophenetic correlation and matrices of OTE pairwise similarity, and patterns ere interpreted based on additional ecological analyses for this system. Similarity measures based on topological food web (presence/absence) data yielded slightly greater cophenetic correlations than did measures based on dietary proportions (flow webs), but, overall, OTE pairwise correlations were not greater for one method relative to the other. The difference between these two approaches is driven by the treatment of weak feeding links; at least for the system considered here, distinctions among dietary generalists were obscured when weak links were weighted lightly. Additively combining the predator and resource aspects of each OTE's trophic role performed better than combining them multiplicatively, In general, there was little correspondence between OTE overlap in resource use and the extent to which predators were shared. Two measures of cluster similarity, one designed by us specifically for food webs (maximum linkage) and the other a standard method (average similarity between clusters), yielded more consistent and ecologically interpretable patterns of aggregation than other measures of cluster similarity considered. In deciding whether two trophospecies should be assigned a trophic link, the maximum linkage convention (in which a link is included if any pair of OTEs, one in each trophospecies, are linked) produced more aggregation than the minimum linkage convention (in which a link is included only if every pair of OTEs, pair members in each of the two trophospecies, is linked). The choice of similarity level for defining trophospecies remained an unresolved issue based on out analysis of this dataset. Perhaps the greatest challenge is posed by sampling bias within empirical datasets, and we ultimately conclude that it is difficult to identify trophospecies in this dataset by strictly objective criteria.

Biodiversity changes in a shallow lake ecosystem: a multi‐proxy palaeolimnological analysis

SummaryBiodiversity is a key measure of environmental quality in lake ecosystems. Lake biodiversity can be assessed using modern survey data, but typically these data only provide a ‘snap‐shot’ measure and in most cases it is not possible to reconstruct temporal trends in biodiversity, so that human impacts can be detected. Palaeoecological techniques offer an alternative means of identifying changes in biodiversity over the period of historical records and far beyond, but there are problems associated with this approach. This is because only a select set of organisms leave a trace in the sediment record such that it is not usually possible to make reliable assessments of diversity changes within an entire taxonomic order (e.g. the algae). Moreover these organisms are typically from the lower levels of the trophic hierarchy (i.e. plants and insects).The problems of identifying changes in biodiversity from the palaeolimnological record are addressed with reference to Groby Pool, a shallow, eutrophic, medieval lake in the English Midlands, which has been subjected to eutrophication over the last 150 years. 210Pb and 137Cs‐dated sediment cores have been used to estimate short‐term alterations in the composition and diversity of three groups of indicators, representing different levels in the trophic cascade, namely diatoms, aquatic pollen and chironomids. By exploring relationships, both between these indicators and with archival macrophyte records, an assessment is made of eutrophication‐related changes in overall habitat diversity at the ecosystem level. These data suggest that the lake has undergone considerable nutrient enrichment, resulting in the loss of a diverse, mesotrophic macrophyte flora from at least the turn of the century onwards and its replacement by a few highly competitive species tolerant of high nutrient concentrations. Reductions in macrophyte diversity seem to be reflected palaeoecologically by a decline in the diversity of fossil chironomid assemblages, related to the breakdown of particular host‐plant relationships amongst the phytophagic species. However, diatom assemblages generally exhibit the opposite trend, which may be related to increases in macrophyte cover and increasing opportunities for the colonization of diverse epiphyte communities. The different fossil indicators have different limitations and merits, and for this reason a ‘multi‐proxy’ approach is essential if meaningful inferences are to be made of changes in lake biodiversity using palaeoecological data.

Nitrogen isotope variations in the subarctic northeast Pacific: relationships to nitrate utilization and trophic structure

The isotopic composition of dissolved nitrate, size-fractionated suspended particulate organic matter (SPOM) and zooplankton was determined on a transect (Line P) between a coastal upwwelling domain and Station Papa in the subarctic northeast Pacific. Station Papa lies in one of the three extensive high-nutrient low-chlorophyll (HNLC) domains of the open ocean, where plankton standing stocks are seasonally uniform and production is lower than the potential production based on available nutrients. In spring 1993, surface water δ 15NO 3 − decreased from east to west along the transect, from ca 12%. in the coastal domain to 8%. at Station Papa, while nitrate concentration ([N0 3 −) increased from 3 to 12 μM in the same direction. Concurrently, δ 15NO 3 − at 400 m depth showed a much smaller change, from 5.3 to 3.1%., indicating a larger δ 15N difference between deep and surface waters at the coastal end of the transect. The isotopic trend for SPOM was similar, from approximately 11 to 3%. for bulk SPOM, 8.5 to 3%. for the < 5 μm fraction, 10 to 4%. for the 50–253 μm fraction, and 12 to 5%. for the < 253 μm fraction. Zooplankton had δ 15N values ranging from 12%. off the coast to 8%. at Station Papa. The fractionation factor for the nitrogen isotopes from these field observations was estimated to be 5%. from the change in δ 15NO 3 − between surface and deep water and from the difference between δ 15N SPOM and δ 15NO 3 − and assuming the SPOM represents an instantaneous product. Seasonal transects of δ 15N SPOM and [N0 3 −] show that there are different relationships between these two parameters depending on the availability and relative utilization of N0 3 −; δ 15N SPOM is more or less constant in the open ocean where [NO 3 −] > 7 μM, it increases rapidly where nitrate utilization is high (that is, where biological uptake is greater than physical supply), and it decreases during upwelling events when physical supply of nutrients overwhelms the biological uptake rate. Finally, the nitrogen isotopes show the expected trophic enrichment 15N, with bulk zooplankton being isotopically heavier than SPOM by 3.9%. at Station Papa and 2.2%. in the coastal domain. This difference possibly reflects the existence of a relatively short food chain in the coastal domain and a longer food chain, involving microzooplanktonic grazers, at Station Papa. The range of δ 15N among seven zooplankton groups was 3.7%., the data suggesting an increasing trophic hierarchy: pteropods, salps, euphausiids, copepods-medusaeamphipods, chaetognaths.

Action Principles as Determinants of Ecosystem Structure: The Autonomous Lake as a Reference System

Action in theoretical physics is an abstract quantity describing the overall motion of a system; action has the dimensions of energy x time (in joule—seconds). The Principle of Least Action states that physical systems follow that path for which the action has the least value. By analogy, we hypothesize that, if the principle of least action is universal, ecosystems must be structured on interaction between this principle of least action and an overriding Principle of Most Action. Most action implies energy acquisition, concentration, and conservation. In an autonomous ecosystem a trophic hierarchy is formed in which, for stability, the action increases at each hierarchical level. We predicted that energy density (in joules per gram), being one of the components of most action, must increase at each level along the food chain. This prediction was tested in Keyhole Lake, a small Arctic lake in the Canadian Northwest Territories, and found to be substantiated. The major non—conformity with the hypothesis was found in the zooplankton, which at all times showed the highest energy density. We concluded that the plankton forms an essentially separate compartment of the ecosystem. Total biomass energy in each hierarchal level confirmed the existence of an energy pyramid, in which the relationship between levels exhibited the characteristics of an inverse cooling curve. An autonomous lake, such as Keyhole Lake, may be regarded as the nearest approach in the natural world to a reference system, against which other systems may be evaluated.

Predator-prey relationships and trophic level reconstruction in a fossil fish community

All living species occupy an ecological niche, and are positioned within a trophic hierarchy. Extinct organisms presumably held similar behavioral and coevolutionary characteristics in the past, and were susceptible to the same kinds of natural ecological pressures operating today. Paleoecological investigations are limited by the incompleteness of the fossil record, and particularly by a lack of behavioral data that are so fundamental to ecological studies of living communities and habitats. Opportunities to examine the coevolutionary structure of ancient communities from empirical data are extremely rare. One such opportunity is provided by the Lower Cretaceous Santana Formation of north-eastern Brazil, a series of richly fossiliferous strata approximately 110 million years old. Many fossil fishes from the Santana Formation contain identifiable prey, including decapod crustaceans and fishes. A trophic hierarchy of these organisms is reconstructed here, and their ecological relationships are discussed. Comparison is made with a similar fish fauna from the Upper Jurassic Solnhofen Limestone of Germany. Low-level, intermediate and high-level predators are identified in each fauna. Predator-prey relationships in the Santana fauna are strongly hierarchical, and are more focussed at the intermediate predator level than in Solnhofen. Comparison with a model of predator-prey relationships between fishes and benthic fauna of the Baltic Sea (which like the Araripe Basin represents a semi-enclosed environment) suggests that heavy predation on teleosts such asRhacolepis, occupying an intermediate trophic level, may have permitted benthic decapods to proliferate and exclude other benthic organisms. Less intense predation on fishes at the intermediate trophic level would allow their numbers to increase, thereby increasing the intensity of predation on the benthos at the base of the trophic hierarchy.

δ13C‐δ15N values as indicators of trophic position and competitive overlap for Pacific salmon (Oncorhynchus spp.)

ABSTRACTAnalysis of δ13C and δ15N isotope values for five species of Pacific salmon indicate that they form a trophic hierarchy on the high seas. On the basis of an analysis of these stable isotope ratios, chinook salmon feed at the upper end of the food chain and pink salmon at the lower end, in the sequence pink → sockeye → coho → chinook, with chinook and pink salmon separated by ca. 0.8 of a trophic level. Chum salmon occupy a peculiar position, with low δ13C values and high δ15N values, possibly reflecting an unusual diet that includes large amounts of gelatinous zooplankton (salps, ctenophoi;s, and medusae). Chum appear to occupy a unique trophic position, at a trophic level nearly as high as that of chinook but at the end of a different branch of the food chain. If true, the potential for trophodynamic competition with other salmon species is small. Our results suggest that the greatest potential for trophodynamic competition occurs within the pink‐sockeye‐coho grouping. A similar analysis, restricted to five stocks of North American sockeye, shows that four widely separated stocks have similar heavy isotope compositions but that the separation between these stocks is still statistically significant. The Chilko stock is strikingly different, however, suggesting that it is located in the central Gulf of Alaska, a region of intense upwelling. Circumstantial evidence supporting the possibility that the Chilko and other sockeye stocks are geographically separated within the Gulf of Alaska is reviewed.

Mapping Ecologists’ Ecologies of Knowledge

Ecologists, particularly those who consider socially generated effects in the environment, grapple with complex, changing situations. Historians, sociologists and philosophers studying the construction of science likewise attempt to account for (or discount) a wide variety of influences, which make up what historian Charles Rosenberg has called “ecologies of knowledge” (Rosenberg 1988). This paper introduces a graphic methodology, mapping, designed to assist researchers at both levels—in science and in science studies—to work with the complexity of their material. By analyzing the implications and limitations of mapping, I aim to contribute to an ecological approach to the philosophy of science. Let me start with two diagrams to open up the territory I will be exploring.Dansereau’s diagram (Fig. 1) conveys a dynamic equilibrium of energy and resource import, export, flows up a trophic hierarchy, and down again to exert control over lower levels in an ecosystem (Dansereau 1973).

Invertebrate predator-prey body size relationships: an explanation for upper triangular food webs and patterns in food web structure?

It has been suggested by Cohen and Newman (1985) that many of the patterns in published food webs can be derived from a stochastic model in which the species are arranged in a trophic hierarchy (the 'cascade model'). We suggest that, if predators are larger than their prey, a trophic hierarchy can be generated on the basis of body size Empirical evidence from the literature shows that there is a positive relationship between predator and prey size for a range of invertebrates and that predators are usually larger than their prey. Using experimental data on an aquatic food web we show that body size can lead to the type of trophic hierarchy used in the cascade model, suggesting that many food web patterns may be a product of body size. This conclusion is discussed with respect to the limitations of the food web data and the relationship between 'static' and 'dynamic' models of web structure.

Trace metal enhancement in the biotic and abiotic components of an estuarine tidal front

A frontal system of lower Delaware Bay was investigated to illustrate the role that such tidal fronts can have in concentrating trace metals and serving as a means for introducing trace metals into the trophic hierarchy of estuaries. This frontal system typically develops on ebb tide and can extend at least 32 km parallel to the central axis of Delaware Bay. In relation to samples collected on either side of the front, chlorophyll a concentrations and concentrations of selected trace metals in zooplankton, particulates, and the dissolved fraction were significantly higher in samples collected at the front. Seasonally, concentrations of cadmium and copper in all fractions were highest during the summer or fall. Differences in taxonomic composition of zooplankton assemblages, as well as differences in total numbers of organisms across the front, suggested advective transport by convergent water masses.