Changes In Trophic Structure Research Articles

The distribution and abundance of animal populations in a climate of uncertainty

Current predictions regarding the ecological consequences of climate change on animal populations are generally autecological and species‐specific, and/or non‐mechanistic extrapolations of recent short‐term patterns. To better understand and predict the effects of climate change on the distribution of species and the abundance of populations we offer a novel, broad theoretical framework. Climate‐induced changes in trophic structure may actually be more predictable than effects on individual species. The logic is that there are general differences in climatic sensitivity among trophic levels – specifically, that as one moves up trophic levels, there is an increase in the temperature sensitivity of vital rates. More precisely, we provide: (1) a formal mathematical definition of distribution limits that is both operational and conceptual, introducing the concept DL50, defined as the geographic and climatic isoline representing an equilibrium occupancy of half of the suitable habitats; (2) a matrix of the possible changes in trophic structure from climate change and the general theoretical consequences; and (3) a new idea that predicts broad effects of climatic warming on trophic systems. Our intention is to help meet the challenge of developing and testing general theoretical models that can predict which species will be winners and losers in ecological time, which evolutionary traits will be favoured or selected against, and what will be consequences for ecosystem structure and function.

Producer Nutritional Quality Controls Ecosystem Trophic Structure

Trophic structure, or the distribution of biomass among producers and consumers, determines key ecosystem values, such as the abundance of infectious, harvestable or conservation target species, and the storage and cycling of carbon and nutrients. There has been much debate on what controls ecosystem trophic structure, yet the answer is still elusive. Here we show that the nutritional quality of primary producers controls the trophic structure of ecosystems. By increasing the efficiency of trophic transfer, higher producer nutritional quality results in steeper ecosystem trophic structure, and those changes are more pronounced in terrestrial than in aquatic ecosystems probably due to the more stringent nutritional limitation of terrestrial herbivores. These results explain why ecosystems composed of highly nutritional primary producers feature high consumer productivity, fast energy recycling, and reduced carbon accumulation. Anthropogenic changes in producer nutritional quality, via changes in trophic structure, may alter the values and functions of ecosystems, and those alterations may be more important in terrestrial ecosystems.

Direct versus indirect effects of habitat reduction on the loss of avian species from tropical forest fragments

AbstractTropical forest fragments typically decrease in avian diversity at rates inversely related to area. However, the mechanisms by which area reduction drives avian species loss remain poorly understood. Changes in habitat area may directly lead to species loss through stochastic fluctuations of reduced populations. Alternatively, area‐dependent changes in top‐down and bottom‐up processes associated with fragmentation may indirectly lead to accelerated avian extinctions. For example, on land‐bridge islands in Lago Guri, Venezuela, fragmentation has resulted in complex changes in the biotic environments through altered abundances of nest predators and generalist herbivores. Using path analysis, we quantified the relative importance of these indirect versus direct effects of area reduction on the rates of avian species loss from 11 fragments during the period 1993–2003. Area reduction had a direct effect on species loss but this was relatively minor compared with indirect effects, especially those mediated through changes in herbivore abundances: species loss was slowed on islands occupied by hyperdense howler monkeys and accelerated on islands with leaf‐cutter ants but lacking howlers. The effects of herbivores on bird species loss are likely indirect and resulting from bottom‐up processes. The primacy of indirect effects at Lago Guri suggests that the loss of species from forest fragments may be driven by active biotic processes (i.e. changes in trophic structure) and is not only a passive response to reduction in habitat areaper se. These findings have important implications for the design and management of reserves aimed at protecting birds and other threatened species.

The Water Framework Directive: Setting the phosphorus loading target for a deep lake in Denmark using the 1D lake ecosystem model DYRESM–CAEDYM

We used the one-dimensional DYRESM–CAEDYM model to elucidate and quantify the influence of the external phosphorus loading on ecosystem dynamics in moderately deep Lake Ravn, which is situated in an agricultural landscape in Denmark. Model simulations were used to quantify the extent to which the external phosphorus loading must be reduced to meet upcoming lake ecological quality requirements according to the European Union Water Framework Directive (WFD). The model generally showed good agreement with observed data for temperature and oxygen from the epilimnion and hypolimnion during the calibration period (7 years) as well as the validation period (5 years); although peaks of oxygen concentrations in epilimnion during late spring often were underestimated. Phosphate and total phosphorus (TP) concentrations were generally well reproduced in both the epilimnion and hypolimnion, though hypolimnetic phosphorus was occasionally underestimated in late summer. There was also good agreement between monitored data and modelled biomass of diatoms and dinoflagellates as well as the zooplankton biomass of cladocerans and calanoid copepods, although the timing of biomass peaks occasionally deviated from observations. Root-mean-square-errors (RMSE), used to quantify the model error, were overall similar for the calibration and the validation period. Simulations of scenarios with a reduced external TP loading suggest that a substantial reduction (40–50%) of the TP loading is required if phytoplankton biomass is to drop to a level sufficiently low to meet the proposed WFD requirements (summer average <6.5 μg chlorophyll a l −1). The predicted outcomes of considerable loading reductions should, however, be treated with some caution, as the conceptual model in this study could not fully account for the changes in trophic structure occurring at radically reduced TP loading. This particularly applies to changes in the fish stock, which may have extensive cascading effects via increased zooplankton grazing on the phytoplankton when external TP loading is reduced. This would most likely lead to higher transparency than that predicted by the model. To further improve the reliability of the predicted outcomes of various model scenarios, future work should include a series of test simulations also including fish predation. Today, fish predation is available in the DYRESM–CAEDYM model as well as other dynamic lake models; however, only sparse data is available on fish stock dynamics in Lake Ravn (as for most other lakes). An applicable test of the fish algorithms in today’s dynamic models will require reliable estimates of fish stock biomass in the study lakes during both the model calibration and validation period.

Asymmetrical food web responses in trophic-level richness, biomass, and function following lake acidification

We tested for disproportional changes in annual and seasonal species richness and biomass among five trophic levels (phytoplankton, herbivorous, omnivorous, and carnivorous zooplankton, and fish) as well as altered trophic structure and ecosystem function following the 5-year experimental acidification of Little Rock Lake (Wisconsin, USA) from pH 6.1 to 4.7. Abiotic and biotic controls of trophic level response during acidification were also identified. Asymmetric reductions of species richness among trophic levels, separated by life stage and feeding type, were evident and changes in trophic structure were most pronounced by the end of the acidification period. Relative declines in richness of fish and zooplankton were greater than phytoplankton, which were generally unaffected, leading to a reduction of upper trophic level diversity. Each of the lower four trophic levels responded to a distinct combination of abiotic and biotic variables during acidification. pH was identified as a direct driver of change for only carnivorous zooplankton, while all other trophic levels were affected more by indirect interactions caused by acidification. Fluctuations in ecosystem function (zooplankton biomass and primary production) were also evident, with losses at all trophic levels only detected during the last year of acidification. The acidified basin displayed a tendency for greater variation in biomass for upper trophic levels relative to reference conditions implying greater unpredictability in ecosystem function. Together, these results suggest that trophic asymmetry may be an important and recurring feature of ecosystem response to anthropogenic stress.

Predictable changes in trophic community structure along a spatial disturbance gradient in streams

Summary1. We assessed insect and resource standing stocks along a spatial gradient of flood disturbance in 19 sub‐alpine Swedish streams to test the prediction that change in trophic structure arises from the joint action of disturbance, which affect basal resources, and resource‐control, which ties the response of the consumers to the response of the resources.2. Trophic structure, quantified as scores of non‐metric multidimensional scaling based on the biomass of insect trophic groups, changed predictably along the disturbance gradient. In early summer, predators and algae feeders decreased relative to suspension feeders with increasing disturbance; in autumn, algae feeders decreased relative to leaf feeders with increasing disturbance.3. Across the disturbance gradient, the biomass of algae‐, deposit‐ and leaf‐feeders was principally controlled by the availability of the respective resource (algae, fine detritus and coarse detritus), while disturbance only had subsidiary effects on algae feeders in early summer.4. Overall, patterns in trophic‐group biomass along the disturbance gradient were more likely to reflect indirect effects of disturbance via impact on the resources, which reverberated to the consumers because of resource‐control, rather than direct effects. In contrast with the view that stream communities are the result of stochastic colonization following disturbance events, in the study streams the trophic structure of insect assemblages is predictable and partly organized by resource‐control across a broad range of disturbance conditions.

Diel trophic structuring of seagrass bed fish assemblages in the Wakatobi Marine National Park, Indonesia

The faunal communities of seagrass beds throughout SE Asia are highly threatened by continued overexploitation, yet their ecology is poorly understood. Developing a greater understanding of the faunal linkages between seagrass beds and associated coastal habitats can facilitate more informed ecosystem level management. The present study used beach seine netting to sample seagrass bed fish assemblages in the Wakatobi Marine National Park, Indonesia, to investigate diel migrations of fish into and out of seagrass beds. These fish assemblages were found to be diverse relative to other studies within the region, with many species being economically important to local subsistence fisheries. The abundance, species richness and trophic structure of these fish assemblages changed with time of day indicating that fish populations are in a dynamic state. Mean fish abundance increased by ≈45% from day to night (Day: 8.61 ± 0.13 fish 100 m −2; Night: 15.6 ± 1.4 fish 100 m −2) while mean species richness increased from 6.6 ± 1.9 per seine haul to 11.4 ± 0.2. Increasing abundance and diversity of fish at night suggests migration onto these habitats from nearby habitats such as reefs, mangroves or deep water; and/or increased activity of those fish resident within seagrass habitats. Division of species into trophic categories enabled the trophic structure of changing fish assemblages to be examined. Assemblages were dominated during both the day and night by invertebrate and fish feeders; however, a major diel change in trophic structure occurred in the abundance of omnivores. During the day omnivores were abundant, but they were replaced at night by exclusive invertebrate feeders. We therefore propose that diel changes in seagrass fish assemblages are predominantly structured by food availability, although other factors such as increased night-time shelter provision were also found to be important albeit to a much lesser extent.

Inference of past changes in zooplankton community structure and planktivorous fish abundance from sedimentary subfossils - a study of a coastal lake subjected to major fish kill incidents during the past century

To describe past changes in trophic structure, analyses were conducted of zooplankton (Cladocera, Rotifera) assemblages recovered in a 210 Pb dated sediment core from a shallow brackish lake exposed to several fish kill episodes during the 20 th century. A weighted-averaging zooplankton-based transfer function was applied to infer planktivorous fish densities (PL-CPUE) and to elucidate the potential of identify- ing fish kill incidents using sedimentary subfossils. The stratigraphical record showed a shift during the 20 th century. The cladoceran community structure changed from high abundance of macrophyte-associated taxa and chydorids, characteristic of clear water conditions, towards a community composed mainly by sediment-associated taxa, small-sized pelagic taxa as well as rotifers and chydorids, characteristic of eutrophic turbid conditions. Coinciding with these changes, the mean size of Daphnia ephippia decreased while inferred PL-CPUE increased. The results point towards progressive eutrophication with additional impact of salinity. Reduced temporal resolution of the sediment record made identification of fish kill incidents difficult. The indications of high fish planktivory and a decline in macrophyte abundance are consistent with anec- dotal observations of past fish investigations and contemporary data. The study dem- onstrates the usefulness of applying zooplankton subfossils to detect long-term changes in the trophic structure of brackish lakes in the past. In addition, it draws at- tention to the difficulties of using sediment records to highlight short-term events, such as fish kills, in shallow wind-impacted lakes.

INFLUENCE OF HABITAT FRAGMENTATION AND CROP SYSTEM ON COLUMBIA BASIN SHRUBSTEPPE COMMUNITIES

Crop production in much of the western United States has caused extensive fragmentation of shrubsteppe and other grasslands and has increased regional levels of disturbance through repeated tillage of crop fields and applications of agrochemicals. Although there have been numerous studies of the effects of fragmentation on arthropods in forests, old fields, and cropland, there have been very few studies of fragmentation effects on grassland arthropods. I conducted a two-year study in the Columbia Basin region of the U.S. Pacific Northwest to determine the effects of fragmentation and regional disturbance on shrubsteppe communities. I sampled plants and arthropods in 20 shrubsteppe fragments associated with annual crops, 20 fragments associated with perennial crops, and 20 large sections of shrubsteppe distant from crop fields. Fragments were corners of crop irrigation circles, and only sites that were undisturbed, with an apparent high coverage of shrubs, were included in the study. Results indicated no significant differences in coverages of shrubs, perennial grasses, total perennial forbs, total annual/biennial forbs, and species richness of grasses and shrubs between fragments and large sections of shrubsteppe. Mean percent coverage of the annual grass Bromus tectorum was 18.6% greater in small fragments than in large sections of shrubsteppe distant from crop fields, and mean coverage of bare ground was 22.9% higher and coverage of biological soil crusts was 41.5% lower in the fragments than in large shrubsteppe sections. Shrubsteppe fragments had significantly lower abundances of obligate herbivores, omnivores, total herbivores, obligate predators, total predators, plant detritivores, total detritivores, and butterflies, as well as fewer individuals of numerous other taxa within these trophic groups. Fragmentation had very little effect on species richness within trophic groups, probably because fragments were not that isolated from each other and large sections of shrubsteppe. Shrubsteppe communities that were next to highly disturbed annual crops had greater coverages of total annual/biennial forbs, and lower coverages of total perennial forbs and biological soil crusts when compared to shrubsteppe near perennial crops. Abundances of obligate herbivores, omnivores, total predators, plant detritivores, vertebrate-detritus feeders, and bees were similar in shrubsteppe near annual crops and near perennial crops. However, within these trophic groups, shrubsteppe near annual crops supported fewer obligate shrub-feeding grasshoppers, herbivorous ground beetles, harvestmen, antlions, darkling beetles, and butterflies than did shrubsteppe near perennial crops. The results suggest that a reduction in resources within fragments and regional disturbances caused by the production of annual crops were responsible for changes in arthropod trophic structure in fragments. Changes in trophic structure were probably not due to reductions in habitat heterogeneity or habitat isolation because plant community structure was similar in fragmented and unfragmented shrubsteppe, and fragments were not isolated from large sections of shrubsteppe.

Spatial and temporal dynamics of nematode populations under Zygophyllum dumosum in arid environments

Soil nematode activity in arid ecosystems is determined by well-known limiting factors such as soil moisture and organic matter. The main aim of our investigation was to determine the composition and distribution of the nematode community in response to soil water and organic content in the rhizosphere of the perennial plant Zygophyllum dumosum. The results demonstrated that soil moisture, organic matter, total nematode population and trophic groups differed under the plants and in the control samples. Significant differences in soil moisture, total nematode population and trophic groups were observed. Bacterial-feeders were found to be the dominant nematode trophic group at the research sites. Bacterial- and fungi-feeders were present in the upper 0–10 cm layer during most of the year. A negative correlation was found between nematodes and soil moisture and a positive correlation was found between nematodes and organic matter. Moreover, trophic structure changes and quantity changes in the nematode community were observed within 1 month after quantitative changes occurred in the organic matter. Generally accepted nematode fauna indices such as Wasilewska index (WI), F/B, T, H′, λ, species richness (SR), PPI, maturity index (MI), and modified maturity index (MMI), reflect the temporal changes that occurred in the nematode community during the research period. This study illustrates the integrated effect of soil moisture and organic matter on the activity and structure of the nematode communities existing under plants.

Stable isotope ratios in archived striped bass scales suggest changes in trophic structure

Stable carbon isotope ratios were measured in archived striped bass, Morone saxatilis (Walbaum), scales to identify changes in the feeding behaviour of this species over time. Striped bass tissue and scale samples were collected from Rhode Island coastal waters during 1996 and archived scale samples (1982–1997) were obtained from Chesapeake Bay. Known striped bass prey items were also collected from Chesapeake Bay and analysed for δ13C. A significant correlation was observed between carbon isotope ratios in striped bass scales and muscle tissue (r2 = 0.52; P < 0.05). Carbon isotope ratios were enriched (less negative) in scales relative to muscle tissue by about 3‰. Carbon isotope ratios in archived striped bass scales from Chesapeake Bay increased significantly from −16.7 ± 0.2‰ in 1982 to −15.1 ± 0.3‰ in 1997. Benthic species, especially invertebrates, were isotopically enriched relative to pelagic fish species collected from the main-stem of Chesapeake Bay. Prey samples collected from riverine locations within Chesapeake Bay were isotopically depleted relative to those collected in the open portion of the Bay. The changes in the carbon isotope ratios of the striped bass scales could be related to changes in the relative proportions of pelagic and benthic food items in the diet of striped bass or to changes in the feeding locations of this species. In either case, there have been changes in the feeding behaviour and/or relationships of the striped bass between 1982 and 1997. Such changes may be related to changing ecological conditions within the estuary, which could influence the health of Chesapeake Bay striped bass.

Use of thermodynamic indices as ecological indicators of the development state of lake ecosystems: 2. Exergy and specific exergy indices

The effectiveness of exergy and specific exergy indices as ecological indicators of the trophic state of lake ecosystems is here tested on a small homogeneous set of shallow lakes which, in spite of their similar nutrient concentrations, morphology and hydrology, show a different trophic state and structure, species composition and abundance. The findings reveal that exergy and specific exergy indices have good negative correlation with phytoplankton biomass and Carlson's trophic state index (TSI) and strong positive correlation to water transparency (the relationship between exergy and eutrophication is clearer if the exergy refers to surface units, rather than volume units) and, hence, that they may be used as ecological indicators of the trophic state of lake ecosystems. The relationship between the responses of the thermodynamic approach and other conventional trophic classification methods (Vollenweider's eutrophication model based on phosphorus loading, the Hillbrich-Ilkowska method and the Vollenweider–OECD classification criterion) previously applied to Lake Trasimeno, was also investigated. The decreasing trend of exergy and specific exergy indices with eutrophication increase appears to be essentially due to the change in species composition and trophic structure, rather than to a different trophic potentiality of the ecosystems investigated. Concerning the identification of the environmental factors responsible for exergy and specific exergy trends, the coherence of the correlation structure between water depth, TSI, exergy and specific exergy indices, suggests that the lake's mean water depth plays a significant role in determining the changes in trophic structure and state (and consequently in exergetic indices) within the set of lakes examined and emphasises the importance of lake morphology in the development and ageing of lake ecosystems.

The feeding behaviour of some demersal fish species in response to artificial discarding

In an attempt to detect changes in the diet and the feeding intensity of demersal fish caused by fishery discarding, two hauls were carried out 22 h apart at a depth of 90 m in an area of the Cantabrian Sea closed to trawling. Before the second haul, dead fauna that were not common in the sampling area under natural conditions, such as Micromesistius poutassou and squat lobster (Munida spp.), which are both usually discarded by the fleet and mainly found in deeper water, were thrown overboard. A total of 319 stomach contents from 12 demersal fish species caught in both hauls were analysed. The percentage emptiness decreased in the second haul (32%) with respect to the first (43%), and at the same time the feeding intensity of scavenger species such as Trachinus draco, Pagellus acarne, Scyliorhinus canicula, Raja montagui and Leucoraja naevus increased. Discards of M. poutassou was actually found in the stomach of a typically planktophagous species, Pagellus bogaraveo. Observed changes in trophic structure of the species analysed as a response to discards are discussed.

Long‐term changes in the trophic level of the Celtic Sea fish community and fish market price distribution

Summary The intensive exploitation of fish communities often leads to substantial reductions in the abundance of target species, with ramifications for the structure and stability of the ecosystem as a whole. We explored changes in the mean trophic level of the Celtic Sea (ICES divisions VII f–j) fish community using commercial landings, survey data and estimates of trophic level derived from the analysis of nitrogen stable isotopes. Our analyses showed that there has been a significant decline in the mean trophic level of survey catches from 1982 to 2000 and a decline in the trophic level of landings from 1946 to 1998. The decline in mean trophic level through time resulted from a reduction in the abundance of large piscivorous fishes and an increase in smaller pelagic species which feed at a lower trophic level. Similar patterns of decline in the trophic level of both catches and landings imply that there have been substantial changes in the underlying structure of the Celtic Sea fish community and not simply a change in fishery preferences. We suggest that the reported changes in trophic structure result from reductions in the spawning stock biomass of traditional target species associated with intensive fishing, together with long‐term climate variability. The relative distribution of fish market prices has changed significantly over the past 22 years, with high trophic level species experiencing greater price rises than lower trophic level species. Although decreased abundance of high trophic level species will ultimately have negative economic consequences, the reduction in mean trophic level of the fish community as a whole may allow the system to sustain higher fishery yields. Management objectives in this fishery will depend on the relative values that society attaches to economic profit and protein production.

Changes in trophic structure of soil arthropods after the application of vermicomposts

The trophic structure of soil arthropods in earthworm casts or vermicomposts produced by Eisenia fetida (Savigny) from paper waste, food waste and cow manure, and in field trials was studied during summer 1999. The numbers of soil arthropods in the soil were counted before treatment (as a control), in the vermicomposts, and after application of vermicomposts to soil in the field. The vermicomposts were applied to soil in rows of tomatoes and peppers, at rates of 4.5 ton ha –1, and the numbers of soil arthropods in trophic groups were compared with those in soil receiving conventional composts and inorganic fertilizers. All treatments received the same total amounts of nutrients. Most of the vermicomposts were rich in microbial biomass-N. There was a tendency for the application of inorganic fertilizers, and conventional compost, to tomato and pepper plots to decrease the numbers of trophic groups of soil arthropods. The applications of vermicomposts increased the number of trophic groups of soil arthropods.

Sensitivity of ecosystem functioning to changes in trophic structure, functional group composition and species diversity in belowground food webs

The objective of the present paper, using decomposer food webs as a tool, is to explore the levels of the ecological hierarchy (trophic groups, feeding guilds, species populations) at which reduction in complexity brings about significant changes in ecosystem performance. A review is given of various mini‐ecosystem studies that have recently been conducted at the University of Jyväskylä. It is hypothesized that the typical features of soils as a habitat, and the peculiarities of belowground food webs, such as the commonness of indirect interactions (mediated through abiotic resources) among the biota, together with the high frequency of polyphagy/omnivory among soil organisms, produce a diversity‐ecosystem functioning relationship that is likely to differ from those of aquatic and aboveground food webs. Experiments showed that alterations in ‘trophic levels’ were reflected in significant changes in decomposition processes, which, in turn, had substantial impact on primary production. Similarly, heterogeneity within ‘trophic levels’ was shown to be associated with increased growth of birch and pine seedlings. In contrast, species diversity within a feeding guild had little or no influence on ecosystem‐level processes. However, the species‐specific properties of individual taxa were shown to be more influential in affecting plant growth than species number per se. For example, the presence of an omnivorous enchytraeid species in the mini‐ecosystems was observed to consistently be associated with high biomass production of tree seedlings. It is concluded that the so‐called trophic dynamic models based on direct feeding interactions are of limited value in predicting the outcomes of interactions taking place belowground.

Changes in the abundance of planktivorous fish in Lake Skanderborg during the past two centuries—a palaeoecological approach

Fish have a profound effect on food-web structure and water quality in lakes. Quantitative historical data for fish are however scarce, which render it difficult to interpret palaeoecological records in the sediment. To reconstruct the historical changes of the fish community structure and abundance of Lake Skanderborg during the past 150 yr we conducted a stratigraphic analysis of selected zooplankton remains in the lake sediment. From these data we inferred the abundance of planktivorous (expressed as catch net −l night −l in multi mesh-sized gillnets, CPUE) and percentage piscivores using a previously developed transfer functions for Danish lakes. Inferred CPUE was high throughout the study period, being highest before 1900 after which it declined following introduction of pikeperch ( Stizostedion lucioperca) in 1903–1904. Further reductions occurred after a major reduction in external P-loading in 1974 and fish removal in 1983–1985. The inferred CPUE in the surface sediment is in accordance with recent monitoring data. Good correspondence was also obtained between measured zooplankton:phytoplankton biomass ratio and Daphnia mean body weight versus that calculated from inferred CPUE using equations established earlier from contemporary data on Danish lakes. At present, the fish stock is numerically dominated by roach ( Rutilus rutilus) bream ( Abramis brama) and small perch ( Perca fluviatilis), while pikeperch ( Stizostedion lucioperca) is the most important piscivore. In accordance with the inferred data, earlier fish investigations and angler descriptions dated as far back as to 1927 indicate little changes in fish community structure. The early dominance of plankti-benthivorous fish most likely reflects an early eutrophication of the lake due to early urban development in the lake catchment. The development of a transfer function for planktivores fish have improved the possibilities of discovering changes in trophic structure and ecological state of lakes in the past.

Spatial and dietary overlap in the Georges Bank groundfish community

Patterns in spatial and trophic resource partitioning in the fish community of the Georges Bank region are identified, accounting for size-based changes in diets. During autumn and spring, this community is divided into geographic assemblages of species that have high spatial overlap. Similarity in spatial distribution is primarily related to similarity in depth preferences, and seasonal differences in species composition within assemblages are related to migrations. There is also important trophic structure within the Georges Bank community separating predators based upon prey size and location in the water column. Ontogenetic changes in diets are an important feature of the trophic structure in this system, particularly in the major piscivores. Seasonal changes in trophic structure reflect both predator and prey migrations. Dietary overlap among predator types is independent of either spatial overlap or depth preferences. There is spatial segregation within trophic guilds, and this spatial partitioning reduces the potential for exploitative competition within this community. Given the observed spatial patterns in trophic structure, the geographic assemblages on Georges Bank may be considered ecologically distinct functional units within this ecosystem.

Benthic microbial loop and meiofaunal response to oil-induced disturbance in coastal sediments: a review

This review of benthic microbial loop and meiofaunal response to oil-induced disturbance describes examples from field investigations on some of the major oil spills of the past 20 years. It focuses on the microbial and meiofaunal compartments that, being characterized by short generation time, life cycle, sensitivity to environmental disturbance, are expected to respond rapidly to environmental changes and to provide accurate indications on the oil spill impact. The results of these researches demonstrate that increased sedimentary oil contents have generally limited effects on microphytobenthic assemblages, but stimulates benthic bacterial response (in terms of biomass and activity). Benthic protozoa appear to be initially sensitive to oil contamination but are afterwards stimulated. Such increased organic levels (i.e. oil input) might change the structure and functioning of the benthic microbial loop, with important consequences on the higher trophic levels. Generally, oil toxicity induces only initially strong meiofaunal mortality (particularly on copepods, turbellarians and ostracods, but in certain cases also on nematodes) with consequences in terms of diversity, richness and k-dominance curves. Oil disturbance has generally only temporary effects, but changes in trophic structure of meiofaunal assemblages suggest an adaptation of these communities to changes in the benthic microbial loop structure. The use of different microbial and meiofaunal indicators of oil disturbance are discussed and proposed for future monitoring studies.

Trophic structure stability and extinction dynamics of beetles (Coleoptera) in tropical forest fragments

A first analysis of the stability of trophic structure following tropical forest fragmentation was performed in an experimentally fragmented tropical forest landscape in Central Amazonia. A taxonomically and trophically diverse assemblage of 993 species of beetles was sampled from 920 m 2 of leaf litter at 46 sites varying in distance from forest edge and fragment area. Beetle density increased significantly towards the forest edge and showed non-linear changes with fragment area, due to the influx of numerous disturbed-area species into 10 ha and 1 ha fragments. There was a marked change in species composition with both decreasing distance from forest edge and decreasing fragment area, but surprisingly this change in composition was not accompanied by a change in species richness. Rarefied species richness did not vary significantly across any of the sites, indicating that local extinctions of deep forest species were balanced by equivalent colonization rates of disturbed-area species. The change in species composition with fragmentation was non-random across trophic groups. Proportions of predator species and xylophage species changed significantly with distance from forest edge, but no area-dependent changes in proportions of species in trophic groups were observed. Trophic structure was also analysed with respect to proportions of abundance in six trophic groups. Proportions of abundance of all trophic groups except xylomycetophages changed markedly with respect to both distance from forest edge and fragment area. Local extinction probabilities calculated for individual beetle species supported theoretical predictions of the differential susceptibility of higher trophic levels to extinction, and of changes in trophic structure following forest fragmentation. To reduce random effects due to sampling error, only abundant species ( n ≥ 46) were analysed for extinction probabilities, as defined by absence from samples. Of these common species, 27% had significantly higher probabilities of local extinction following fragmentation. The majority of these species were predators; 42% of all abundant predator species were significantly more likely to be absent from samples in forest fragments than in undisturbed forest. These figures are regarded as minimum estimates for the entire beetle assemblage because rarer species will inevitably have higher extinction probabilities. Absolute loss of biodiversity will affect ecosystem process rates, but the differential loss of species from trophic groups will have an even greater destabilizing effect on food web structure and ecosystem function.