Intermediate Trophic Levels Research Articles

Defining Forage Species to Prevent a Management Dilemma

Forage species are often defined in scientific and popular literature using terms such as “small,” “schooling,” “short-lived,” pelagic fish found at intermediate trophic levels of marine food chain...

Effects of food web complexity on top-down control in tropical lakes

Top-down control in ecosystems is dependent on food web structure. In this study, we developed 126 models describing different trophic link combinations in order to assess the effects of food web structure on the top-down response of shallow tropical lakes. We evaluated the effects of the presence of invertebrate predators, large-bodied herbivorous zooplankton and the degree of omnivory. The results showed that the presence of invertebrate predators and large-bodied herbivorous zooplankton can invert the relation between planktivorous/omnivorous fish and producers (algae). The fact that large herbivores are absent in tropical lakes and invertebrate predators are present in large quantities results in a positive correlation between piscivorous fish and algae biomass, contradicting the classical top-down response described for temperate lakes. We show that omnivory should not be analyzed as a feeding strategy in itself. Omnivory affects many food web processes and its effects are dependent on the trophic level. In our models, omnivory in intermediate trophic levels dampened the top-down control by fish, but omnivory in top trophic levels has an opposite effect increasing the fish carrying capacity and also the strength of the top-down trophic cascade, while simultaneously decreasing the shortest chain length between fish and algae, thus reversing the relation between these two trophic levels.

Food Web Topology in High Mountain Lakes.

Although diversity and limnology of alpine lake systems are well studied, their food web structure and properties have rarely been addressed. Here, the topological food webs of three high mountain lakes in Central Spain were examined. We first addressed the pelagic networks of the lakes, and then we explored how food web topology changed when benthic biota was included to establish complete trophic networks. We conducted a literature search to compare our alpine lacustrine food webs and their structural metrics with those of 18 published lentic webs using a meta-analytic approach. The comparison revealed that the food webs in alpine lakes are relatively simple, in terms of structural network properties (linkage density and connectance), in comparison with lowland lakes, but no great differences were found among pelagic networks. The studied high mountain food webs were dominated by a high proportion of omnivores and species at intermediate trophic levels. Omnivores can exploit resources at multiple trophic levels, and this characteristic might reduce competition among interacting species. Accordingly, the trophic overlap, measured as trophic similarity, was very low in all three systems. Thus, these alpine networks are characterized by many omnivorous consumers with numerous prey species and few consumers with a single or few prey and with low competitive interactions among species. The present study emphasizes the ecological significance of omnivores in high mountain lakes as promoters of network stability and as central players in energy flow pathways via food partitioning and enabling energy mobility among trophic levels.

Effect of poly-β-hydroxybutyrate on growth and disease resistance of Nile tilapia Oreochromis niloticus juveniles

The growth promoting effect of the bacterial storage compound poly-β-hydroxybutyrate (PHB) has been studied for young fish of high trophic level (European sea bass) and intermediate trophic level (Siberian sturgeon). Here, the effect of PHB on growth, digestive enzyme activities, body composition and diseases resistance of juvenile Nile tilapia (Oreochromis niloticus) of low trophic level was investigated. Although dietary PHB supplementation (5, 25 and 50g PHBkg−1 formulated semi-purified diet) during 28 days resulted in a trend of increased weight gain, there was no significant difference in the mean final body weight (258–284mg) when compared to the fish from the control group (on average 218mg). Lipase activity increased significantly with about 20–40% by the supplementation of PHB in the diet, which may have led to the significant increase in total lipid content with about 10% in the PHB treatment groups. However, the profile of total (n-6) fatty acids (FAs), total monounsaturated FAs and total saturated FAs relative to the total lipid was similar among various PHB treatments. An additional challenge test on gnotobiotic Nile tilapia larvae using the pathogen Edwardsiella ictaluri gly09R showed that feeding challenged larvae with PHB-enriched Artemia nauplii resulted in a 20% higher survival as compared to the challenged control larvae. Overall, it is suggested that the trend of increased body weight gain resulted from intestinal lipid digestion, absorption and deposition and that PHB is effective as an antimicrobial agent for application in Nile tilapia larviculture.

Indirect Energy Flows in Niche Model Food Webs: Effects of Size and Connectance.

Indirect interactions between species have long been of interest to ecologists. One such interaction type takes place when energy or materials flow via one or more intermediate species between two species with a direct predator-prey relationship. Previous work has shown that, although each such flow is small, their great number makes them important in ecosystems. A new network analysis method, dynamic environ approximation, was used to quantify the fraction of energy flowing from prey to predator over paths of length greater than 1 (flow indirectness or FI) in a commonly studied food web model. Web structure was created using the niche model and dynamics followed the Yodzis-Innes model. The effect of food web size (10 to 40 species) and connectance (0.1 to 0.48) on FI was examined. For each of 250 model realizations run for each pair of size and connectance values, the FI of every predator-prey interaction in the model was computed and then averaged over the whole network. A classification and regression tree (CART) analysis was then used to find the best predictors of FI. The mean FI of the model food webs is 0.092, with a standard deviation of 0.0279. It tends to increase with system size but peaks at intermediate connectance levels. Of 27 potential predictor variables, only five (mean path length, dominant eigenvalue of the adjacency matrix, connectance, mean trophic level and fraction of species belonging to intermediate trophic levels) were selected by the CART algorithm as best accounting for variation in the data; mean path length and the dominant eigenvalue of the adjacency matrix were dominant.

Regional benthic food web structure on the Alaska Beaufort Sea shelf

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 531:15-32 (2015) - DOI: https://doi.org/10.3354/meps11340 Regional benthic food web structure on the Alaska Beaufort Sea shelf Lauren M. Divine1,*, Katrin Iken1, Bodil A. Bluhm1,2 1School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, PO Box 757220, Fairbanks, Alaska 99775-7220, USA 2Department of Arctic and Marine Biology, UiT-The Arctic University of Norway, 9037 Tromsø, Norway *Corresponding author: lmdivine@alaska.edu ABSTRACT: Trophic structure and resilience of regional benthic food webs were studied on the Alaska Beaufort Sea shelf across 3 geographic regions (east, central, and west) and 2 depth ranges (14 to 90 m [shallow] and 100 to 220 m [deep]). Carbon (δ13C) and nitrogen (δ15N) isotope ratios were measured from water column and sediment particulate organic matter (POM) as markers of primary food sources and from common benthic invertebrate taxa. Isotopic niche space (δ13C-δ15N) occupied by benthic taxa provided measures of community-wide trophic redundancy and separation. δ13C values of the water column and sediment POM were patchy but generally indicative of terrestrial input across the shelf, especially near the mouth of the Colville River (< -25.5‰). Food web structures in the shallow and deep western Beaufort regions, however, reflected marine carbon inputs, with most consumers occupying intermediate trophic levels (TLs) and food webs of intermediate trophic redundancy and separation relative to the other regions. In the central shallow and east regions, the terrestrial carbon found in sediments was mirrored in consumer tissues. The central deep region contained the highest proportions of higher TL taxa, particularly within the predator feeding guild, possibly reflecting resource partitioning of a limited prey spectrum. The comparatively large isotopic niche space in the central deep region, with high trophic niche separation but low trophic redundancy, suggests that this region may be most vulnerable to perturbations. This study provides a valuable new understanding of benthic food web dynamics in the understudied Alaska Beaufort Sea region and represents a baseline for future comparison. KEY WORDS: Benthos · Carbon · Feeding guild · Nitrogen · Stable isotope · Trophic structure Full text in pdf format PreviousNextCite this article as: Divine LM, Iken K, Bluhm BA (2015) Regional benthic food web structure on the Alaska Beaufort Sea shelf. Mar Ecol Prog Ser 531:15-32. https://doi.org/10.3354/meps11340 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 531. Online publication date: July 02, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Organochlorine contaminants and maternal offloading in the lecithotrophic Pacific angel shark (Squatina californica) collected from southern California

Pacific angel sharks (Squatina californica) are a benthic elasmobranch that occupy intermediate trophic level positions in coastal food webs. Angel sharks’ life history characteristics make them susceptible to accumulating high amounts of contaminants. Four angel sharks were opportunistically captured in southern California and their liver and uterine contents were analyzed for PCBs, DDTs and other pesticides. High DDT:PCB ratios were found in the sharks indicating direct or indirect foraging near a local EPA Superfund Site. Organic contaminants were measured in ovulated eggs, indicating that females are able to maternally offload contaminants. Despite the potential mismatch between ovarian and uterine fecundity, we estimated females to offload approximately 13±5% of their total body load, which represents the upper limit of this capability. Although low in sample size, the initial findings from this study suggest that habitat use might play an important role in contaminant accumulation in this species.

A multifaceted trophic cascade in a detritus-based system: density-, trait-, or processing-chain-mediated effects?

We investigated three pathways by which predators on an intermediate trophic level may produce a trophic cascade in detritus-based systems. Predators may increase lower trophic levels (bacteria) by reducing density of bacteriovores, by altering behavior of bacteriovores, and by processing living bacteriovores into carcasses, feces, and dissolved nutrients that are substrates for bacteria. We tested these pathways in laboratory experiments with mosquitoes in water-filled containers. Larval Toxorhynchites rutilus prey on larval Aedes triseriatus, which feed on bacteria. Using containers stocked with oak leaf infusion as a bacterial substrate, we compared bacterial productivity at 7 and 14 days for: prey alone; prey with a predator; and prey with predation cues but no predator. Controls contained no larvae, either with predation cues or without cues. Predation cues in the control treatment increased bacterial abundance at 7 days, but this effect waned by 14 days. Aedes triseriatus larvae reduced bacterial abundance significantly at 14 days. Predator cues and real predation both eliminated the negative effect of A. triseriatus on bacterial abundance. Predation cues reduced survivorship of A. triseriatus larvae at 14 days, however this effect was smaller than the effect of real predation. We further tested effects of residues from predation as cues or as detritus in a second experiment in which A. triseriatus were killed at similar rates by: real predators; mechanical damage without the predator and carcasses left as detritus; or mechanical damage and carcasses removed. No prey larvae were killed in controls. Bacterial productivity was greater with real predation than in all other treatments and greater when prey larvae were killed or killed and removed, than in controls. Thus we find evidence that all three pathways contribute to the trophic cascade from T. rutilus to bacteria in tree hole systems.

Linkages between the biomass of Scomber japonicus and net primary production in the southern East China Sea

Fish biomass is a critical component of fishery stock assessment and management and it is often estimated from ocean primary production (OPP). However, the relationship between the biomass of a fish stock and OPP is always complicated due to a variety of trophic controls in the ecosystem. In this paper, we examine the quantitative relationship between the biomass of chub mackerel (Scomber japonicus) and net primary production (NPP) in the southern East China Sea (SECS), using catch and effort data from the Chinese mainland large light-purse seine fishery logbook and NPP derived from remote sensing. We further discuss the mechanisms of trophic control in regulating this relationship. The results show a significant non-linear relationship exists between standardized CPUE (Catch-Per-Unit-Effort) and NPP (P<0.05). This relationship can be described by a convex parabolic curve, where the biomass of chub mackerel increases with NPP to a maximum and then decreases when the NPP exceeds this point. The results imply that the ecosystem in the SECS is subject to complex trophic controls. We speculate that the change in abundance of key species at intermediate trophic levels and/or interspecific competition might contribute to this complex relationship.

Cascade effects of crop species richness on the diversity of pest insects and their natural enemies.

Understanding how plant species richness influences the diversity of herbivorous and predatory/parasitic arthropods is central to community ecology. We explore the effects of crop species richness on the diversity of pest insects and their natural enemies. Using data from a four-year experiment with five levels of crop species richness, we found that crop species richness significantly affected the pest species richness, but there were no significant effects on richness of the pests' natural enemies. In contrast, the species richness of pest insects significantly affected their natural enemies. These findings suggest a cascade effect where trophic interactions are strong between adjacent trophic levels, while the interactions between connected but nonadjacent trophic levels are weakened by the intermediate trophic level. High crop species richness resulted in a more stable arthropod community compared with communities in monoculture crops. Our results highlight the complicated cross-trophic interactions and the crucial role of crop diversity in the food webs of agro-ecosystems.

Comparing the Effects of Aquatic Stressors on Model Temperate Freshwater Aquatic Communities

Identifying the relative importance of stressors is critical for effectively managing and conserving freshwater aquatic ecosystems. However, variability in natural ecosystems and the potential for multiple stressors make understanding the effects of stressors challenging in the field. To address these challenges, we assessed four common stressors in the northeastern USA including acidification (pH), climate change (water temperature), salinization (Na and Cl), and nutrient addition using laboratory mesocosms. Each stressor was evaluated independently, with ten mesocosms assigned across a gradient of concentrations for each stressor (total N = 40). We then monitored the effects of the stressors on a model community consisting of periphyton, zooplankton, Northern watermilfoil (Myriophyllum sibericum), American ribbed fluke snail (Pseudosuccinea columella), and larval American bullfrogs (Lithobates catesbeianus). Aquatic stressors varied in the strength of their effects on community structure: Nutrient addition was the least influential stressor, with no significant effects. Acidification influenced periphyton biomass, but not higher trophic levels. Water temperature influenced primary productivity and survival of amphibian larvae, but not intermediate trophic levels. Finally, road salt led to decreases in productivity for all trophic levels included in our model systems. Our results support the findings of prior research, although the effects of acidification and nutrient addition were less pronounced in our study. Importantly, we found that road salt had the most far-reaching effects on a model aquatic community. Given that road salt is the most easily managed of the stressors we compared, our results indicate that improving the condition of freshwater aquatic ecosystems in the northeastern USA may be a feasible objective.

Feeding comparisons of four batoids (Elasmobranchii) in coastal waters of southern Brazil

Food partitioning between species is a common strategy for avoiding resource competition which allows the coexistence of two or more species in the same place. In order to evaluate the feeding of four species of batoids regularly caught by artisanal fisheries in southern Brazil, the present study aimed to analyse and compare the diet of the four batoid species. The Chola guitarfish,Rhinobatos percellens, had a specialized diet, consisting predominantly of blue crabsCallinectessp., followed by teleost fish. The Lesser guitarfishZapteryx brevirostrisalso had a very specialized diet, consuming mainly on Polychaeta, followed by Caridea shrimp. The Cownose ray,Rhinoptera bonasus, fed mainly on Ophiuroidea, followed by Gastropoda and Bivalvia. Finally, the Rio skateRioraja agassizi, fed on teleost fish, Gammaridae, Caridea shrimp and Dendrobranchiata shrimp. An analysis of similarity showed significant differences among species in their diet. The trophic levels of the batoids in this study are &lt;4.0, placing them in intermediate trophic levels. The analysis of the diets indicates that feeding differs substantially among the four species, suggesting a partitioning of food resources available in the environment.

Effect of the Puyehue-Cordon Caulle volcanic complex eruption on crustacean zooplankton of Andean lakes

Ecología Austral, 24:75-82 (2014)We studied the effect of volcanic ash on the crustacean zooplankton (mainly cladocerans) in the Andean North-Patagonian lakes that were affected by the eruption of the Puyehue-Cordón Caulle complex. The study was carried out at three different scales: sub cellular, population and ecosystemic. Cladocerans are filter feeding organisms that cannot distinguish between food (algae less than 30 µm) and suspended ash, therefore when feeding they ingest this material. The ash affected negatively the individuals at a biochemical level, increasing the oxidative stress reacting enzymes like Glutation-S-transferase and the biomarker Acetyl cholinesterase. At the population level, we recorded a decrease in survival and fecundity, in particular, when ash concentration exceeded 8 mg/L, we observed that individuals did not reach adulthood. At the ecosystem level, we recorded a significant decrease of the intermediate trophic level (primary consumers) although producers were positively enhanced by the eruption. The absence of these primary consumers may affect upper trophic levels like fishes. Later, in the year following the eruption, we observed the resurgence of zooplankton populations suggesting a fast recovery from the initial conditions.

Biophysical Interactions Control the Size and Abundance of Large Phytoplankton Chains at the Ushant Tidal Front

Phytoplankton blooms are usually dominated by chain-forming diatom species that can alter food pathways from primary producers to predators by reducing the interactions between intermediate trophic levels. The food-web modifications are determined by the length of the chains; however, the estimation is biased because traditional sampling strategies damage the chains and, therefore, change the phytoplankton size structure. Sedimentological studies around oceanic fronts have shown high concentrations of giant diatom mats (>1 cm in length), suggesting that the size of diatom chains is underestimated in the pelagic realm. Here, we investigate the variability in size and abundance of phytoplankton chains at the Ushant tidal front (NW France) using the Video Fluorescence Analyzer (VFA), a novel and non-invasive system. CTD and Scanfish profiling characterized a strong temperature and chlorophyll front, separating mixed coastal waters from the oceanic-stratified domain. In order to elucidate spring-neap variations in the front, vertical microstructure profiler was used to estimate the turbulence and vertical nitrate flux. Key findings were: (1) the VFA system recorded large diatom chains up to 10.7 mm in length; (2) chains were mainly distributed in the frontal region, with maximum values above the pycnocline in coincidence with the maximum chlorophyll; (3) the diapycnal fluxes of nitrate enabled the maintenance of the bloom in the frontal area throughout the spring-neap tidal cycle; (4) from spring to neap tide the chains length was significantly reduced; (5) during neap tide, the less intense vertical diffusion of nutrients, as well as the lower turbulence around the chains, intensified nutrient-depleted conditions and, thus, very large chains became disadvantageous. To explain this pattern, we suggest that size plasticity is an important ecological trait driving phytoplankton species competition. Although this plasticity behavior is well known from experiments in the laboratory, it has never been reported from observations in the field.

Evidence for consumer regulation of biofilm–nutrient interactions among hardwater streams (Pennsylvania, USA)

Experimental studies evaluating the simultaneous effects of consumers, nutrients, and other biotic/abiotic factors on intact, natural food webs are rare, particularly among ecosystems of varying trophic conditions. We conducted a series of in situ studies that used nutrient-diffusing substrata with nitrogen (N) and phosphorus (P) concentrations in a full factorial design in three temperate, limestone streams in Pennsylvania across a trophic gradient (mesotrophic, eutrophic, and hypereutrophic streams). We assessed differences in algal and macroinvertebrate biomass, taxonomic composition, and functional groups relative to amended nutrients across the trophic gradient; as such, these results facilitated predictions about regulators of food web structure. All factors varied significantly among the streams (e.g., algal biomass P = 0.005, macroinvertebrate biomass P < 0.001, algal diversity P = 0.006, macroinvertebrate diversity P < 0.001, algal group P < 0.001, macroinvertebrate guilds P < 0.001); the streams, however, did not exhibit simple responses to nutrient amendment. Algal and macroinvertebrate biomass and diversity responded greatest in the mesotrophic stream while grazing seemed to be a strong factor preventing algal nutrient response in the eutrophic and hypereutrophic streams. Brillouin’s Evenness Index was most influenced by nutrient amendment (nutrient effect on algae and macroinvertebrates P = 0.021). As such, we concluded that biomass and diversity were mediated by complexity within intermediate trophic levels.

Effect of mesozooplankton feeding selectivity on the dynamics of algae in the presence of intermediate grazers—a laboratory simulation

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 486:47-58 (2013) - DOI: https://doi.org/10.3354/meps10393 Effect of mesozooplankton feeding selectivity on the dynamics of algae in the presence of intermediate grazers—a laboratory simulation Mianrun Chen1,2, Hongbin Liu1,*, Hoitung Li1 1Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR 2South China Sea Marine Engineering and Environment Institute, South China Sea Branch, SOA, Guangzhou 510300, PR China *Corresponding author. Email: liuhb@ust.hk ABSTRACT: An indirect increase in algal abundance that is induced by mesozooplankton predation on intermediate trophic level grazers (microzooplankton) is among the factors that shape the structure of a marine planktonic food web (copepods/cladocerans-ciliates-algae). Marine mesozooplankton include species with diverse feeding strategies, and hence they play different roles in trophic interactions. In this study, we simulated a simple pelagic food web in the laboratory using 3 copepod species and 1 cladoceran as top predators to test the hypothesis that different species with different feeding behaviors will cause different impacts on prey communities. Our results showed that among the 3 crustacean mesozooplankton species, Parvocalanus crassirostris was the most carnivorous species that caused the strongest cascading effect, which led to an increase in algal density as the rate of the cascading effect exceeded the direct consumption of algal prey by intermediate consumers. In contrast, the marine cladoceran Penilia avirostris generally caused a decline in algal density because it created no indirect positive effect on algae since it was incapable of capturing the intermediate grazers. Temora turbinata fed on ciliates and algal prey at similar rates so that the direct consumption of the algal prey was balanced by the indirect trophic cascade effect. The strength of the cascade effect induced by Acartia erythraea was significantly enhanced by increasing the densities of ciliates. The mechanism was due to a switch in preying behavior from suspension feeding to ambush feeding. Our results imply that mesozooplankton omnivory is important in maintaining the stability of the community structure of microplankton because the effects of direct consumption and the cascading effect balanced each other due to the broad feeding strategies of predators. KEY WORDS: Parvocalanus · Temora · Acartia · Marine cladocerans · Grazing · Trophic cascade Full text in pdf format PreviousNextCite this article as: Chen M, Liu H, Li H (2013) Effect of mesozooplankton feeding selectivity on the dynamics of algae in the presence of intermediate grazers—a laboratory simulation. Mar Ecol Prog Ser 486:47-58. https://doi.org/10.3354/meps10393 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 486. Online publication date: July 12, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Complex wasp-waist regulation of pelagic ecosystems in the Pacific Ocean

‘Wasp-waist’ control of marine ecosystems is driven by a combination of top-down and bottom-up forcing by a few abundant short-lived species occupying intermediate trophic levels that form a narrow ‘waist’ through which energy flow from low to high trophic levels is controlled. It has been assumed that wasp-waist control occurs primarily in highly productive and species-poor systems (e.g. upwelling regions). Two large, species-rich, pelagic ecosystems in the relatively oligotrophic eastern and western Pacific Ocean also show wasp-waist-like structure, in that short-lived and fast-growing cephalopods and fishes at intermediate trophic levels comprise the vast majority of the biomass. Possible forcing dynamics of these systems were examined using ecosystem models by altering the biomass of phytoplankton (bottom-up forcing), large pelagic predators (top-down forcing), and intermediate ‘wasp-waist’ functional groups independently and observing how these changes propagated throughout the ecosystem. The largest effects were seen when altering the biomass of mid trophic-level epipelagic and mesopelagic fishes, where dramatic trophic cascades occurred both upward and downward in the system. We conclude that the high productivity and standing biomass of animals at intermediate trophic levels has a strong top-down influence on the abundance of primary producers. Furthermore, their importance as prey for large predators results in bottom-up controls on populations at higher trophic levels. We show that these tropical pelagic ecosystems possess a complex structure whereby several waist groups and alternate trophic pathways from primary producers to apex predators can cause unpredictable effects when the biomasses of particular functional groups are altered. Such models highlight the possible structuring mechanisms in pelagic systems, which have implications for fisheries that exploit these wasp-waist groups, such as squid fisheries, as well as for fisheries of top predators such as tunas and billfishes that prey upon wasp-waist species.

The Ecological Role of the Vaquita, Phocoena sinus, in the Ecosystem of the Northern Gulf of California

From an ecosystem management perspective, analysis of the functional roles of species is a challenge. It is valuable to determine which species are irreplaceable within a given community based on their contribution to the system’s organization. This study relates the emergent functional and structural indices of biological groups estimated from a trophic model of the Northern Gulf of California to identify the roles of these groups in the ecosystem context, with a particular focus on the role of the vaquita, an endemic porpoise in critically endangered status. The simulation of removing each group allowed the analysis of the removal’s functional effect on the ecosystem’s global attributes and organization (based on Ulanowicz’s ascendency concept). Groups from lower trophic levels (TL) were more related to complexity indicators, suggesting their contribution to the organization and structure of energy flows in the food web. Groups from intermediate TL had higher values of structural indexes, indicating their function in the control of flows throughout the network. The vaquita along with other marine mammals, aquatic birds, and some species of fish with a high TL contribute in a similar way to the order (for example, ascendency/capacity-of-development ratio) of the system, showing a relatively high value of ascendency (contribution of the group to the organization inherent to the ecosystem) and the change in ecosystem ascendency when they were removed. The vaquita, like marine and coastal birds, plays a small role in the ecosystem. But like them, it does contribute substantially to ecosystem organization. This study thus provides information potentially useful for management in understanding the species’ role and in reducing uncertainty in decision-making.

Ecosystem effects of bluefin tuna Thunnus thynnus thynnus aquaculture in the NW Mediterranean Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 456:215-231 (2012) - DOI: https://doi.org/10.3354/meps09700 Ecosystem effects of bluefin tuna Thunnus thynnus thynnus aquaculture in the NW Mediterranean Sea F. Forrestal1,*, M. Coll2,3,4, D. J. Die1, V. Christensen3,4 1Rosenstiel School of Marine and Atmospheric Science, 4600 Rickenbacker Csy, Miami, Florida 33133, USA 2Institut de Ciènces del Mar, ICM-CSIC, Passeig Marítim de la Barceloneta, 37−49, 08003 Barcelona, Spain 3Ecopath International Initiative Association, Spain 4Fisheries Centre, University of British Columbia, 2202 Main Mall, Vancouver, V6T 1Z4 British Columbia, Canada *Email: fforrestal@rsmas.miami.edu ABSTRACT: In recent decades the eastern stock of Atlantic bluefin tuna Thunnus thynnus thynnus has declined, driven by excessive catches partially mediated by the growth of the capture-based aquaculture of bluefin in the Mediterranean. This study addresses the potential direct and indirect food-web effects on trophic linkages in the ecosystem through the removal of both small pelagic fish species and wild bluefin tuna for aquaculture operations in the Mediterranean. A mass-balanced model representing the southern Catalan Sea during the 1990s was modified to include a bluefin tuna farm partially supplied from fish captured in the area modeled. Six scenarios were developed to simulate possible changes to the capture-based aquaculture operations and possible impacts to the ecosystem. The addition of one bluefin tuna farm in the Catalan Sea did not produce substantial effects in the ecosystem. Simulation scenarios that include the level of production already present in a similar ecosystem in the Murcia region resulted in large fluctuations of both biomass and yield for bluefin tuna, as well as for many species in the modeled ecosystem. Increases in biomass of lower trophic level functional groups were observed with reductions in biomass from higher trophic level predators. These outcomes demonstrate that removal of biomass at top and intermediate trophic levels can have direct and indirect outcomes on the structure of the ecosystem due to the complexity of the food web. Our results suggest, in the case of the Western Mediterranean Sea, increasing bluefin tuna farming activities will likely contribute towards further degradation of an already highly exploited ecosystem. KEY WORDS: Capture-based aquaculture · Bluefin tuna · Food-web model · Ecopath with Ecosim · Mediterranean Sea Full text in pdf format PreviousNextCite this article as: Forrestal F, Coll M, Die DJ, Christensen V (2012) Ecosystem effects of bluefin tuna Thunnus thynnus thynnus aquaculture in the NW Mediterranean Sea. Mar Ecol Prog Ser 456:215-231. https://doi.org/10.3354/meps09700 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 456. Online publication date: June 07, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Food web structure in Mediterranean streams: exploring stabilizing forces in these ecosystems

We constructed the food webs of six Mediterranean streams in order to determine ecological generalities derived from analysis of their structure and to explore stabilizing forces within these ecosystems. Fish, macroinvertebrates, primary producers and detritus are the components of the studied food webs. Analysis focused on a suite of food web properties that describe species’ trophic habits, linkage complexity and food chains. A great structural similarity was found in analyzed food webs; we therefore suggest average values for the structural properties of Mediterranean stream food webs. Percentage of omnivorous species was positively correlated with connectance, and there was a predominance of intermediate trophic level species that had established simple links with detritus. In short, our results suggest that omnivory and the weak interactions of detritivores have a stabilizing role in these food webs.