Food Web Responses Research Articles

Applications of nitrate and ammonium fertilizers alter soil nematode food webs in a continuous cucumber cropping system in Southwestern Sichuan, China

Nitrate (NO 3 - -N) and ammonium (NH 4 + -N) fertilizers are the main forms of chemical inorganic nitrogen fertilizers that are widely used in agro-ecosystem for high yield. However, the responses of soil nematode food web to different forms and rates of inorganic nitrogen fertilizers are not well understood. The objective of this study was to determine the responses of soil nematode food web to the applications of inorganic nitrogen fertilizers in a continuous cucumber ( Cucumis sativus L.) cropping system. Nitrate (NaNO 3 ) and ammonium (NH 4 HCO 3 ) fertilizers were applied to cucumber plants at the nitrogen (N) rate of 0, 67.5, 135.0 and 202.5 kg N hm - 2 before planting. It was conducted in a randomized complete block design with 4 replications at Huaizi village, Leshan district, Sichuan province, Southwestern China. The effects were analyzed at the stages of seedling, blooming and fruiting, respectively. The results indicated that the numbers of nematodes were significantly higher in soils with the addition of 67.5 kg N hm -2 than the control at the seedling and blooming stages. Nematode number strongly increased at the seedling stage and decreased at the blooming and fruiting stages in nitrate-treated soils compared to the ammonium-treated. The percentage of herbivores to total nematodes significantly decreased while that of bacterivores increased with a fertilizer rate less than 135 kg N hm - 2 at the seedling and fruiting stages. Nitrate significantly reduced the percentage of herbivores, and increased that of bacterivores to total nematodes by comparison with ammonium at the blooming and fruiting stages. The application of nitrate significantly increased nematode diversity and evenness, and decreased dominance at the blooming stage relative to ammonium. Nitrate significantly decreased the values of channel index at the blooming stage and maturity index at the seedling stage in comparison with ammonium, respectively. Enrichment index and structural index strongly increased at the seedling stage, and decreased at the blooming and fruiting stages under the treatment of nitrate relative to ammonium. The results suggested responses of nematode food web dependent on the rates and forms of inorganic nitrogen fertilizers and stages of cucumber growth.

Land–Sea Interactions and Consequences for Sub-Antarctic Marine Food Webs

Resource subsidies can influence the structure and dynamics of recipient food webs in a wide range of ecosystems through direct and indirect effects. On the sub-Antarctic Prince Edward Islands (PEIs), large populations of top predators import substantial amounts of nutrients from the oceans to the islands. Guano runoff from the islands increases the availability of nutrients in the nearshore marine environment, supporting phytoplankton blooms. We apply food web models to the PEIs marine ecosystem to investigate food web responses to variability in resource subsidies. Modelled scenarios of island-associated blooms indicated small effects on the ecosystem under both normal bloom conditions and when the input of blooms was removed entirely. Benthic communities benefited most strongly from blooms and a fivefold increase in bloom production had broadly positive impacts throughout the entire food web, including the pelagic food web. Land-based predators were shown to contribute significantly to nearshore ecosystem production via their contributions to ammonia runoff. Transfers of material across the land–sea interface were estimated to contribute 0.73 t N km−2 y−1 to the nearshore ammonium pool. The effects of removing terrestrial nutrient input were very small when 100% of nutrient runoff was converted to island-associated bloom phytoplankton while the macrophyte production rate was held constant. If, however, macrophyte production was also supported in part or wholly by terrestrial nutrient runoff, the ecosystem-level effects of runoff variability were much greater and more broadly distributed throughout the food web. Given the potential of habitat linkages to alter or structure communities, populations, and environments, it is important to incorporate land–sea linkages into ecological models.

Bottom-up responses of the lower oceanic food web are sensitive to copepod mortality and feeding behavior

The functional response, describing consumption rate as a function of food abundance, critically links consumer-resource dynamics. Yet, little is known about how its shape affects communities of multiple, dynamically linked species. We theoretically investigated how the functional response of copepods (saturating type 2 vs. sigmoidal type 3, both of which have been observed) mediates bottom-up and top-down influences on the lower oceanic food web as described by five compartments (“pentagon web”): a shared limiting nutrient, small and large algae, ciliates feeding on small algae, and copepods feeding on ciliates and large algae. We compare system dynamics in response to nutrient enrichment and copepod mortality, the latter being varied first directly and then indirectly through inclusion of planktivores (fish or jellyfish). Regardless of functional response type, when planktivores are absent, equilibrium densities of all members of the even food chain (nutrients-small algae-ciliates-copepods) typically increase with nutrient enrichment whereas large algae decrease. In contrast, and congruent with conventional wisdom, large algae increase with nutrient enrichment when copepods are controlled by planktivores. A type 2 response in copepods strongly destabilizes the pentagon web, whereas stable equilibria are possible when copepods have a type 3 response. High copepod mortality (e.g., caused by increasing planktivore pressure under nutrient enrichment) destabilizes such systems, however. Moreover, because community feedbacks produce a negative correlation between the copepod's alternative prey, type 3 switching behavior is amplified in the pentagon web. This prevents extinctions but can give rise to an alternative state with small algal dominance at high enrichment.

Indirect effect of environmental factors on interactions between microbial and classical food webs in freshwater ecosystems

The role of environmental factors in aquatic ecosystems results from basic lake characteristics, human disturbances (‘cultural eutrophication’) and climate-related trends in the physical and chemical components of lakes. Although the influence of environmental factors on the abundance of aquatic animals is fairly well documented, less has been done to research their influence on food web interactions. The aim of the study was to evaluate microbial and classical food webs in lakes, with special emphasis placed on the role of environmental factors as influencing strengths. Variation partitioning, based on redundancy analysis, revealed that environmental factors played the most important role in structuring aquatic communities by accounting for 87.5% of their variation. Among all the factors measured, total solids (TS), transparency (Secchi disc) and temperature were most closely related to the variation in trophic communities. The analyses of food web interactions under low and high levels of those factors revealed that they differently influenced strengths among food web components. The strongest relations among distinct trophic levels were found under conditions of low TS, the lowest number of relations was found under conditions of low temperature. Only in low TS did bacteria correlate significantly with biogenes. Under high TS, bacteria positively influenced plenty of higher trophic levels. Top-down control was observed under conditions of high temperature. Conditions of low and high transparency did not diversify food web interactions. The obtained results can broaden our knowledge of the response of food webs to environmental factors in advanced stages of global eutrophication of water bodies and in the early stage of projected trends of global climate change.

Pressure from top and bottom: Lower food web responses to changes in nutrient cycling and invasive species in western Lake Michigan

Abstract Over recent decades, Lake Michigan phytoplankton and zooplankton communities have experienced dramatic changes driven both from the bottom (e.g. P loading reduction) and top (e.g. predatory cladoceran invasions) of the food web. We used two data sets from nearly identical sampling at an offshore station (100 m depth) in western Lake Michigan (1988–92 and 2007–09), to test for bottom-up effects (i.e. declines in chlorophyll a (chl a ) or increases in particulate C:P, leading to declines in P-rich cladocerans versus copepods), and top-down effects of invasive predatory cladocerans (i.e. declines in native zooplankton from predation or competition). Between the two periods, total P and particulate C declined, while nitrate and silicate increased. While chl a in the largest cells (> 53 μm and 10–53 μm fractions) decreased, particulate C:P ratios were unchanged. Total zooplankton abundance and biomass declined significantly between sampling periods, notably cyclopoid copepods, but not Bosmina or Daphnia species, nor the invasive cladoceran, Bythotrephes longimanus . Bottom-up effects, usually associated with ‘benthification’ attributed to Dreissena grazing, are more consistent with the changes observed than are effects of invasive predatory cladocerans. Differences in observations from those in eastern Lake Michigan or other lake-wide surveys are difficult to reconcile but seem more likely due to temporal differences in sampling rather than spatial ones. Discerning the trajectory of Lake Michigan will require better accounting for zooplankton life histories, more sophisticated understanding of nutritional quality and diet for zooplankton, and clearer coupling of pelagic–benthic cycles of elements, including Si and N.

Modeling spatial patterns of limits to production of deposit-feeders and ectothermic predators in the northern Bering Sea

Network models can help generate testable predictions and more accurate projections of food web responses to environmental change. Such models depend on predator–prey interactions throughout the network. When a predator currently consumes all of its prey's production, the prey's biomass may change substantially with loss of the predator or invasion by others. Conversely, if production of deposit-feeding prey is limited by organic matter inputs, system response may be predictable from models of primary production. For sea floor communities of shallow Arctic seas, increased temperature could lead to invasion or loss of predators, while reduced sea ice or change in wind-driven currents could alter organic matter inputs. Based on field data and models for three different sectors of the northern Bering Sea, we found a number of cases where all of a prey's production was consumed but the taxa involved varied among sectors. These differences appeared not to result from numerical responses of predators to abundance of preferred prey. Rather, they appeared driven by stochastic variations in relative biomass among taxa, due largely to abiotic conditions that affect colonization and early post-larval survival. Oscillatory tendencies of top-down versus bottom-up interactions may augment these variations. Required inputs of settling microalgae exceeded existing estimates of annual primary production by 50%; thus, assessing limits to bottom-up control depends on better corrections of satellite estimates to account for production throughout the water column. Our results suggest that in this Arctic system, stochastic abiotic conditions outweigh deterministic species interactions in food web responses to a varying environment.

Phosphorus resource heterogeneity in microbial food webs

Food webs are often regulated by the bottom-up effects of resource supply rate. However, heterogeneity within a resource pool may also affect the structure and function of com- munities. To test this hypothesis, we measured the responses of aquatic microbial food webs in experimental mesocosms to the addition of 4 different phosphorus (P) sources: orthophosphate (PO4 3� ), 2-aminoethylphosphonate (AEP), adenosine triphosphate (ATP), and phytic acid (PA). Based on 16S rRNA gene sequencing, we found that P resource heterogeneity altered community assembly for bacteria and eukaryotic algae, suggesting that these microbial functional groups may be comprised of P-specialists. In contrast, cyanobacteria were relatively unaffected by our treatments, suggesting that these microorganisms may adopt a more generalist strategy for P- acquisition. Furthermore, our results revealed that P resource heterogeneity affected food web and ecosystem attributes such as nutrient concentrations, bacterial productivity, algal biomass, and ecosystem respiration. Lastly, we found no evidence for non-additive effects of resource het- erogeneity based on a treatment where a set of mesocosms received all 4 sources of P. Instead, our results support the view that there may be non-substitutable classes of P in aquatic ecosystems. Specifically, microbial food webs were more sensitive to P-containing biomolecules (PO4 3� and ATP) than P-containing structural or storage molecules (AEP and PA). Our results demonstrate that not all P resources are the same; although historically overlooked, P resource heterogeneity may have important implications for understanding and predicting the structure and function of aquatic communities.

Direct and indirect responses of a freshwater food web to a potent synthetic oestrogen.

Endocrine-disrupting chemicals (EDCs) in municipal effluents directly affect the sexual development and reproductive success of fishes, but indirect effects on invertebrate prey or fish predators through reduced predation or prey availability, respectively, are unknown. At the Experimental Lakes Area in northwestern Ontario, Canada, a long-term, whole-lake experiment was conducted using a before-after-control-impact design to determine both direct and indirect effects of the synthetic oestrogen used in the birth control pill, 17α-ethynyloestradiol (EE2). Algal, microbial, zooplankton and benthic invertebrate communities showed no declines in abundance during three summers of EE2 additions (5-6 ng l(-1)), indicating no direct toxic effects. Recruitment of fathead minnow (Pimephales promelas) failed, leading to a near-extirpation of this species both 2 years during (young-of-year, YOY) and 2 years following (adults and YOY) EE2 additions. Body condition of male lake trout (Salvelinus namaycush) and male and female white sucker (Catostomus commersonii) declined before changes in prey abundance, suggesting direct effects of EE2 on this endpoint. Evidence of indirect effects of EE2 was also observed. Increases in zooplankton, Chaoborus, and emerging insects were observed after 2 or 3 years of EE2 additions, strongly suggesting indirect effects mediated through the reduced abundance of several small-bodied fishes. Biomass of top predator lake trout declined by 23-42% during and after EE2 additions, most probably an indirect effect from the loss of its prey species, the fathead minnow and slimy sculpin (Cottus cognatus). Our results demonstrate that small-scale studies focusing solely on direct effects are likely to underestimate the true environmental impacts of oestrogens in municipal wastewaters and provide further evidence of the value of whole-ecosystem experiments for understanding indirect effects of EDCs and other aquatic stressors.

Lake Ontario zooplankton in 2003 and 2008: Community changes and vertical redistribution

Lake-wide zooplankton surveys are critical for documenting and understanding food web responses to ecosystem change. Surveys in 2003 and 2008 during the binational intensive field year in Lake Ontario found that offshore epilimnetic crustacean zooplankton declined by a factor of 12 (density) and factor of 5 (biomass) in the summer with smaller declines in the fall. These declines coincided with an increase in abundance of Bythotrephes and are likely the result of direct predation by, or behavioral responses to this invasive invertebrate predator. Whole water column zooplankton density also declined from 2003 to 2008 in the summer and fall (factor of 4), but biomass only declined in the fall (factor of 2). The decline in biomass was less than the decline in density because the average size of individual zooplankton increased. This was due to changes in the zooplankton community composition from a cyclopoid/bosminid dominated community in 2003 to a calanoid dominated community in 2008. The increase in calanoid copepods was primarily due to the larger species Limnocalanus macrurus and Leptodiaptomus sicilis. These cold water species were found in and below the thermocline associated with a deep chlorophyll layer. In 2008, most of the zooplankton biomass resided in or below the thermocline during the day. Increased importance of copepods in deeper, colder water may favor Cisco and Rainbow Smelt over Alewife because these species are better adapted to cold temperatures than Alewife.

How precautionary is the policy governing the Ross Sea Antarctic toothfish (Dissostichus mawsoni) fishery? - ERRATUM

This article reviews the adequacy of data and models currently being used to estimate the present and future population sizes of the Antarctic toothfish (Dissostichus mawsoni Norman) in the Ross Sea regional ecosystem. The current tagging programme is unlikely to provide an accurate picture of total population size, and estimates of both the pre-exploitation spawning stock biomass and the ratio of current to pre-exploitation biomass are unreliable. Many parameters necessary for estimating future population growth or decline have not been measured, and the current objective of a 50% reduction in biomass relative to unexploited biomass may easily fail to prevent a much larger reduction from taking place. The need to guess values of important parameters makes it impossible to set bounds on the potential errors of population forecasts. Current scientific knowledge is far from what is needed to predict the likely effects of food-web responses to harvesting of toothfish in the Ross Sea, or to predict the feedback effects of those food-web changes on toothfish populations.

Differential responses of size‐based functional groups to bottom–up and top–down perturbations in pelagic food webs: a meta‐analysis

We performed a meta‐analysis of 31 lake mesocosm experiments to investigate differences in the responses of pelagic food chains and food webs to nutrient enrichment and fish presence. Trophic levels were divided into size‐based functional groups (phytoplankton into highly edible and poorly edible algae, and zooplankton into small herbivores, large herbivores and omnivorous zooplankton) in the food webs. Our meta‐analysis shows that 1) nutrient enrichment has a positive effect on phytoplankton and zooplankton, while fish presence has a positive effect on phytoplankton and a negative effect on zooplankton in the food chains; 2) nutrient enrichment has a positive effect on highly edible algae and small herbivores, but no effect on poorly edible algae, large herbivores and omnivorous zooplankton in the food webs. Planktivorous fish have a positive effect on highly edible algae and small herbivores, a negative effect on large herbivores and omnivorous zooplankton, and no effect on poorly edible algae. Our meta‐analysis confirms that nutrient enrichment and planktivorous fish affect functional groups differentially within trophic levels, revealing important changes in the functioning of food webs. The analysis of fish effects shows the well‐described trophic cascade in the food chain and reveals two trophic cascades in the food web: one transmitted by large herbivores that benefit highly edible phytoplankton, and one transmitted by omnivorous zooplankton that benefit small herbivores. Comparison between the responses of food webs and simple food chains also shows consistent biomass compensation between functional groups within trophic levels.

Piscivore enhancement effects on food webs depend on planktivore body size and species composition in replicated whole lake experiments

Recent syntheses of trophic cascade and biomanipulation research have suggested that the effects of piscivores on planktivorous fish populations are reduced, when planktivores are capable of outgrowing predator gape limitation and in systems with complex food web interactions. These hypotheses, however, have not been tested in long-term, whole-lake, experiments where processes such as fish recruitment and compensatory food web responses may be important. We conducted a replicated whole-lake experiment to test for the effects of supplemental piscivore introductions on food webs of eutrophic lakes dominated by deep-bodied planktivores. Responses to piscivore enhancement were compared between lakes differing in food web structure due to the presence of omnivorous gizzard shad (Dorosoma cepedianum). A significant decrease in the relative abundance of juvenile planktivorous fish, and an increase in total benthic macroinvertebrate density was observed in lakes containing mainly bluegills (Lepomus machrochirus). In contrast, lakes containing gizzard shad exhibited no significant responses to piscivore manipulation. Our results support the hypothesis that food webs in lakes dominated by deep-bodied planktivorous fish species respond weakly to piscivore enhancement. In addition, our findings support the hypothesis that cascading trophic interactions are weaker in lake ecosystems with more complex food web interactions such as those containing gizzard shad.

Effects of experimental nitrogen and/or phosphorus additions on soil nematode communities in a secondary tropical forest

In tropical forest ecosystems, highly weathered soils are often considered as relatively nitrogen-rich but phosphorus-poor. Nutrient availability greatly regulates ecosystem processes and functions of tropical forests. However, little is known about how nitrogen and/or phosphorus additions affect the conditions of soil food web which is an important component of belowground ecosystems. In the present study, soil nematode communities were monitored and served as indicators of soil food web conditions under experimental nitrogen and phosphorus additions in a secondary forest in tropical China. The principal response curves of soil nematode community structure revealed same tendency of changes under nitrogen and/or phosphorus additions compared with control, in terms of nematode functional guild compositions: apparent successions from communities dominated by He3 and Ba1 to communities dominated by Ba2 and Fu2 occurred after nitrogen and/or phosphorus additions. The diversity of soil nematode genera was not sensitive to either nitrogen or phosphorus addition. Phosphorus addition significantly suppressed total nematode density, density of omnivore-predators, and four nematode faunal indices (i.e. MI25, EI, SI, and SFI), but increased two faunal indices (i.e. CI and BaI). However, nitrogen addition did not induce remarkable changes of these variables in the present study. Our results suggest that nitrogen and/or phosphorus additions suppress soil nematodes in tropical secondary forests, which was inconsistent with our expectation that nitrogen addition was detrimental to and phosphorus addition was conducive to soil nematodes in this nitrogen-rich but phosphorus-poor soil. Furthermore, the effects of phosphorus addition are more powerful than the effects of nitrogen addition. Moreover, phosphorus addition degrades the structure and trophic links of the soil food web, reduces the carbon utilization of soil nematodes, and leads to a more fungal dominated decomposition pathway. The alterations of soil food web conditions might result in altered ecosystem functioning. Our findings could provide a better understanding of the responses of soil food web to nitrogen and phosphorus additions in nitrogen-rich but phosphorus-poor soils.

Increases in disturbance and reductions in habitat size interact to suppress predator body size

Food webs are strongly size-structured so will be vulnerable to changes in environmental factors that affect large predators. However, mechanistic understanding of environmental controls of top predator size is poorly developed. We used streams to investigate how predator body size is altered by three fundamental climate change stressors: reductions in habitat size, increases in disturbance and warmer temperatures. Using new survey data from 74 streams, we showed that habitat size and disturbance were the most important stressors influencing predator body size. A synergistic interaction between that habitat size and disturbance due to flooding meant the sizes of predatory fishes peaked in large, benign habitats and their body size decreased as habitats became either smaller or harsher. These patterns were supported by experiments indicating that habitat-size reductions and increased flood disturbance decreased both the abundance and biomass of large predators. This research indicates that interacting climate change stressors can influence predator body size, resulting in smaller predators than would be predicted from examining an environmental factor in isolation. Thus, climate-induced changes to key interacting environmental factors are likely to have synergistic impacts on predator body size which, because of their influence on the strength of biological interactions, will have far-reaching effects on food-web responses to global environmental change.

Paleodietary changes by penguins and seals in association with Antarctic climate and sea ice extent

Positioned near the top of the food web, the dietary composition of Antarctic penguins and seals can be an excellent indicator of the regional food web and thus the status of the marine ecosystem. The dietary composition of modern penguins and seals has been well investigated; a long-term time series of data on penguin and seal diets, however, are rare. Such data, especially any predating the initiation of human harvesting of fish, whales and seals in Antarctica, are crucial for understanding and predicting responses of regional marine food webs to natural climate changes. Here we review recent progress on research of paleodietary change in Antarctic penguins and seals, specifically the Adelie penguin (Pygoscelis adeliae) and Antarctic fur seal (Arctocephalus gazella). These studies indicate that the dietary changes of penguins correspond quite well with fluctuations in climate and sea ice extent during the Holocene. The depleted δ 15N ratios found in modern Adelie penguins support the “krill surplus hypothesis” in relation to historic human depletion of krill-eating fish, seals and whales.

Integrating ecosystem engineering and food webs

Ecosystem engineering, the physical modification of the environment by organisms, is a common and often influential process whose significance to food web structure and dynamics is largely unknown. In the light of recent calls to expand food web studies to include non‐trophic interactions, we explore how we might best integrate ecosystem engineering and food webs. We provide rationales justifying their integration and present a provisional framework identifying how ecosystem engineering can affect the nodes and links of food webs and overall organization; how trophic interactions with the engineer can affect the engineering; and how feedbacks between engineering and trophic interactions can affect food web structure and dynamics. We use a simple integrative food chain model to illustrate how feedbacks between the engineer and the food web can alter 1) engineering effects on food web dynamics, and 2) food web responses to extrinsic environmental perturbations. We identify four general challenges to integration that we argue can readily be met, and call for studies that can achieve this integration and help pave the way to a more general understanding of interaction webs in nature.SynthesisAll species are affected by their physical environment. Because ecosystem engineering species modify the physical environment and belong to food webs, such species are potentially one of the most important bridges between the trophic and non‐trophic. We examine how to integrate the so far, largely independent research areas of ecosystem engineering and food webs. We present a conceptual framework for understanding how engineering can affect food webs and vice versa, and how feedbacks between the two alter ecosystem dynamics. With appropriate empirical studies and models, integration is achievable, paving the way to a more general understanding of interaction webs in nature.

Prey Vulnerability Limits Top-Down Control and Alters Reciprocal Feedbacks in a Subsidized Model Food Web

Resource subsidies increase the productivity of recipient food webs and can affect ecosystem dynamics. Subsidies of prey often support elevated predator biomass which may intensify top-down control and reduce the flow of reciprocal subsidies into adjacent ecosystems. However, top-down control in subsidized food webs may be limited if primary consumers posses morphological or behavioral traits that limit vulnerability to predation. In forested streams, terrestrial prey support high predator biomass creating the potential for strong top-down control, however armored primary consumers often dominate the invertebrate assemblage. Using empirically based simulation models, we tested the response of stream food webs to variations in subsidy magnitude, prey vulnerability, and the presence of two top predators. While terrestrial prey inputs increased predator biomass (+12%), the presence of armored primary consumers inhibited top-down control, and diverted most aquatic energy (∼75%) into the riparian forest through aquatic insect emergence. Food webs without armored invertebrates experienced strong trophic cascades, resulting in higher algal (∼50%) and detrital (∼1600%) biomass, and reduced insect emergence (−90%). These results suggest prey vulnerability can mediate food web responses to subsidies, and that top-down control can be arrested even when predator-invulnerable consumers are uncommon (20%) regardless of the level of subsidy.

How to predict community responses to perturbations in the face of imperfect knowledge and network complexity.

Recent attempts to predict the response of large food webs to perturbations have revealed that in larger systems increasingly precise information on the elements of the system is required. Thus, the effort needed for good predictions grows quickly with the system's complexity. Here, we show that not all elements need to be measured equally well, suggesting that a more efficient allocation of effort is possible. We develop an iterative technique for determining an efficient measurement strategy. In model food webs, we find that it is most important to precisely measure the mortality and predation rates of long-lived, generalist, top predators. Prioritizing the study of such species will make it easier to understand the response of complex food webs to perturbations.

How precautionary is the policy governing the Ross Sea Antarctic toothfish (Dissostichus mawsoni) fishery?

AbstractThis article reviews the adequacy of data and models currently being used to estimate the present and future population sizes of the Antarctic toothfish (Dissostichus mawsoniNorman) in the Ross Sea regional ecosystem. The current tagging programme is unlikely to provide an accurate picture of total population size, and estimates of both the pre-exploitation spawning stock biomass and the ratio of current to pre-exploitation biomass are unreliable. Many parameters necessary for estimating future population growth or decline have not been measured, and the current objective of a 50% reduction in biomass relative to unexploited biomass may easily fail to prevent a much larger reduction from taking place. The need to guess values of important parameters makes it impossible to set bounds on the potential errors of population forecasts. Current scientific knowledge is far from what is needed to predict the likely effects of food-web responses to harvesting of toothfish in the Ross Sea, or to predict the feedback effects of those food-web changes on toothfish populations.

Effects of global environmental changes on parasitoid–host food webs and biological control

Global environmental changes threaten biodiversity and the interactions between species, and food-web approaches are being used increasingly to measure their community-wide impacts. Here we review how parasitoid–host food webs affect biological control, and how their structure responds to environmental change. We find that land-use intensification tends to produce webs with low complexity and uneven interaction strengths. Dispersal, spatial arrangement of habitats, the species pool and community differences across habitats have all been found to determine how webs respond to landscape structure, though clear effects of landscape complexity on web structure remain elusive. The invasibility of web structures and response of food webs to invasion have been the subject of theoretical and empirical work respectively, and nutrient enrichment has been widely studied in the food-web literature, potentially driving dynamic instability and altering biomass ratios of different trophic levels. Combined with food-web changes observed under climate change, these responses of food webs could signal changes to biological control, though there have been surprisingly few studies linking food-web structure to pest control, and these have produced mixed results. However, there is strong potential for food-web approaches to add value to biological control research, as parasitoid–host webs have been used to predict indirect effects among hosts that share enemies, to study non-target effects of biological control agents and to quantify the use of alternative prey resources by enemies. Future work is needed to link food-web interactions with evolutionary responses to the environment and predator–prey interactions, while incorporating recent advances in predator biodiversity research. This holistic understanding of agroecosystem responses and functioning, made possible by food-web approaches, may hold the key to better management of biological control in changing environments.