Trophic Hypothesis Research Articles

Biomechanical analysis and new trophic hypothesis for Riojasuchus tenuisceps, a bizarre-snouted Late Triassic pseudosuchian from Argentina.

Biomechanical analysis and new trophic hypothesis for Riojasuchus tenuisceps, a bizarre-snouted Late Triassic pseudosuchian from Argentina.

Trophic structure of apex fish communities in closed versus leaky lakes of arctic Alaska.

Despite low species diversity and primary production, trophic structure (e.g., top predator species, predator size) is surprisingly variable among Arctic lakes. We investigated trophic structure in lakes of arctic Alaska containing arctic char Salvelinus alpinus using stomach contents and stable isotope ratios in two geographically-close but hydrologically-distinct lake clusters to investigate how these fish may interact and compete for limited food resources. Aside from different lake connectivity patterns ('leaky' versus 'closed'), differing fish communities (up to five versus only two species) between lake clusters allowed us to test trophic hypotheses including: (1) arctic char are more piscivorous, and thereby grow larger and obtain higher trophic positions, in the presence of other fish species; and, (2) between arctic char size classes, resource polymorphism is more prominent, and thereby trophic niches are narrower and overlap less, in the absence of other predators. Regardless of lake cluster, we observed little direct evidence of arctic char consuming other fishes, but char were larger (mean TL = 468 vs 264mm) and trophic position was higher (mean TP = 4.0 vs 3.8 for large char) in lakes with other fishes. Further, char demonstrated less intraspecific overlap when other predators were present whereas niche overlap was up to 100% in closed, char only lakes. As hydrologic characteristics (e.g., lake connectivity, water temperatures) will change across the Arctic owing to climate change, our results provide insight regarding potential concomitant changes to fish interactions and increase our understanding of lake trophic structure to guide management and conservation goals.

Species richness and functional structure of fish assemblages in three freshwater habitats: effects of environmental factors and management.

In this study, the inverted trophic hypothesis was tested in the freshwater fish communities of a reservoir. The distribution of fish species in three freshwater habitats in the Jurumirim Reservoir, Brazil, was examined using both species richness and the relative proportions of different trophic groups. These groups were used as a proxy for functional structure in an attempt to test the ability of these measures to assess fish diversity. Assemblage structures were first described using non-metric multidimensional scaling (NMDS). The influence of environmental conditions for multiple fish assemblage response variables (richness, total abundance and abundance per trophic group) was tested using generalised linear mixed models (GLMM). The metric typically employed to describe diversity; that is, species richness, was not related to environmental conditions. However, absolute species abundance was relatively well explained with up to 54% of the variation in the observed data accounted for. Differences in the dominance of trophic groups were most apparent in response to the presence of introduced fish species: the iliophagous and piscivorous trophic groups were positively associated, while detritivores and herbivores were negatively associated, with the alien species. This suggests that monitoring functional diversity might be more valuable than species diversity for assessing effects of disturbances and managements policies on the fish community.

The Response of Denervated Muscle to Long-Term Stimulation (1985, Revisited here in 2014).

In 1985, at a meeting in Abano, I presented results showing that direct stimulation of skeletal muscles with appropriate stimulus patterns prevents the effects of denervation on non-junctional properties of muscle fibers. Hence, it appeared unnecessary to postulate that unknown nerve-derived trophic factors control such properties, as posited by the (anterograde) neurotrophic hypothesis. Here I discuss this conclusion in the light of what we know today, particularly with respect to the many lines of evidence that were then taken to support the trophic hypothesis, but which today have alternative interpretations consistent with control by evoked impulse activity. Despite much effort, no one has yet identified any nerve-derived factor consistent with the neurotrophic hypothesis. Reports favoring the existence of neurotrophic factors were numerous before 2000. Now they have essentially disappeared from the literature, including original research papers, textbooks and handbooks, suggesting that the hypothesis is no longer arguable. Thus, the results that I presented in our paper in 1985 seem to have held up rather well.

Natal departure timing from spatially varying environments is dependent of individual ontogenetic status

Natal departure timing represents one of the first crucial decisions for juveniles born in spatially varying environments that ultimately disappear, but our knowledge on its determinants is limited. The present study aimed at understanding the determinants of juvenile natal departure by releasing individually tagged juvenile pike (Esox lucius L.) with variable body size and trophic position in a temporary flooded grassland. Specifically, we investigated whether natal departure depends on individual competitive status ('competition hypothesis'), physiological tolerance to environmental conditions ('physiological hypothesis') or individual trophic position and the spatial heterogeneity of trophic resources ('trophic hypothesis'). The results indicated that departure timing was negatively correlated with body size at release, showing that the dominance status among competing individuals was not the main trigger of juvenile departure. A positive correlation between departure timing and individual body size at departure was observed, suggesting that inter-individual variability in physiological tolerance did not explain departure patterns. While individual growth performances were similar irrespective of the timing of natal departure, stable isotope analyses revealed that juveniles with higher trophic position departed significantly earlier than individuals with lower trophic position. Therefore, the trade-off driving the use of spatially varying environments was most likely dependent upon the benefits associated with energetic returns than the costs associated with inter-individual competition or physiological stress. This result highlighted how ontogeny, and particularly ontogenetic niche shift, can play a central role in juvenile's decision to depart from natal habitats in a predatory species.

Checking the pulse of Lake Ontario's microbial-planktonic communities: A trophic transfer hypothesis

The structure and function of the microbial food web of Lake Ontario was assessed at 15 stations distributed across 4 transects during the spring and summer of 2003. This was the first major binational study of Lake Ontario since the Lake Ontario Trophic Transfer initiative of 1990. The microbial loop (bacteria, autotrophic picoplankton, heterotrophic nanoflagellates (HNF) and ciliates) and phytoplankton, were enumerated microscopically in addition to measurements of chlorophyll a, size fractionated primary productivity (14C) and bacterial growth (3H). HNF dominated the total biomass in spring (≈300 mg m−3) and summer (≈1250 mg m−3). The size of the organic carbon pool increased from ≈90 mg C m−3 in spring to ≈270 mg C m−3 with HNF contributing 36% of the total organic carbon in the spring and 52% in the summer; however the net balance of the organic carbon pool shifted from autotrophic in the spring to heterotrophic in the summer. The available evidence suggests that HNF are a poor quality food resource for zooplankton and it is likely that the carbon sequestered by HNF is not available to higher trophic levels resulting in dietary stress for planktivores. The implications of high HNF for both organic carbon cycling and maintaining healthy fisheries needs further research. Independent observations show that oligotrophic conditions prevail as evidenced by low phosphorus, low chlorophyll a, low plankton and high water clarity. Such conditions have been generally regarded as the gold standard for managing healthy lakes. Lake Ontario is oligotrophic and healthy from a water quality perspective, but from a food web dynamics point of view, Lake Ontario appears to be unhealthy due to the dominance of HNF, low zooplankton and poor quality of food available to higher trophic levels. We hypothesize that the lake's poor health is attributable to inefficient energy transfer from lower to higher trophic levels. The traditional understanding of trophic state based mainly on water quality criteria needs to be broadened by the inclusion of food web and fisheries based metrics.

The trophic blockage hypothesis is not supported by the diets of fishes on Seine Seamount

AbstractSeveral hypotheses exist about the trophic mechanisms that support fish stocks at seamounts. This study investigated the diets of benthopelagic fish species on the summit plateau of Seine Seamount (NE Atlantic), testing the sound‐scattering layer interception hypothesis. A combined approach of gut content, stable isotopes and fatty‐acid biomarker analyses was employed. Fish species included zooplanktivores, benthivores, piscivores and species with mixed crustacean/cephalopod/fish diets. Trophic coupling between pelagic food sources and benthopelagic fish consumers was apparent based on three main lines of evidence: (i) dominance of pelagic prey in the guts of zooplanktivores, (ii) stable isotope enrichment of consumers indicative of a pelagic prey source, and (iii) high proportions of fatty acids that are typical of phytoplankton and zooplankton in the storage lipids of fishes and their similarity with fatty acid signatures of pelagic prey. Elevated levels of arachidonic acid in a benthivorous species suggested a minor dietary contribution of rhodophytes. The lack of larger taxa that undergo diel vertical migrations in the fish guts suggests that it is horizontal fluxes of non‐, or weakly migrating zooplankton that are the main food supply to the resident fish consumers. Overall, there was no unambiguous support for the trophic blockage hypothesis. Differences in gut contents, trophic position and storage lipid fatty acid signatures of zooplanktivorous fishes indicate some degree of resource partitioning with respect to feeding habitats, prey selection and ontogenetic diet shifts. Irrespective of body size and feeding mode, the benthopelagic fishes occupied intermediate trophic positions between the 3rd and 4th trophic level in a food web composed mainly of omnivorous species.

Aquatic fauna as indicators for Everglades restoration: Applying dynamic targets in assessments

Abstract A major goal of the Comprehensive Everglades Restoration Plan (CERP) is to recover historical (pre-drainage) wading bird rookeries and reverse marked decreases in wading bird nesting success in Everglades National Park. To assess efforts to restore wading birds, a trophic hypothesis was developed that proposes seasonal concentrations of small-fish and crustaceans (i.e., wading bird prey) were a key factor to historical wading bird success. Drainage of the Everglades has diminished these seasonal concentrations, leading to a decline in wading bird nesting and displacing them from their historical nesting locations. The trophic hypothesis predicts that restoring historical hydrological patterns to pre-drainage conditions will recover the timing and location of seasonally concentrated prey, ultimately restoring wading bird nesting and foraging to the southern Everglades. We identified a set of indicators using small-fish and crustaceans that can be predicted from hydrological targets and used to assess management success in regaining suitable wading bird foraging habitat. Small-fish and crustaceans are key components of the Everglades food web and are sensitive to hydrological management, track hydrological history with little time lag, and can be studied at the landscape scale. The seasonal hydrological variation of the Everglades that creates prey concentrations presents a challenge to interpreting monitoring data. To account for the variable hydrology of the Everglades in our assessment, we developed dynamic hydrological targets that respond to changes in prevailing regional rainfall. We also derived statistical relationships between density and hydrological drivers for species representing four different life-history responses to drought. Finally, we use these statistical relationships and hydrological targets to set restoration targets for prey density. We also describe a report-card methodology to communicate the results of model-based assessments for communication to a broad audience.

Seasonal and interannual variability in the taxonomic composition and production dynamics of phytoplankton assemblages in Crater Lake, Oregon

Taxonomic composition and production dynamics of phytoplankton assemblages in Crater Lake, Oregon, were examined during time periods between 1984 and 2000. The objectives of the study were (1) to investigate spatial and temporal patterns in species composition, chlorophyll concentration, and primary productivity relative to seasonal patterns of water circulation; (2) to explore relationships between water column chemistry and the taxonomic composition of the phytoplankton; and (3) to determine effects of primary and secondary consumers on the phytoplankton assemblage. An analysis of 690 samples obtained on 50 sampling dates from 14 depths in the water column found a total of 163 phytoplankton taxa, 134 of which were identified to genus and 101 were identified to the species or variety level of classification. Dominant species by density or biovolume included Nitzschia gracilis, Stephanodiscus hantzschii, Ankistrodesmus spiralis, Mougeotia parvula, Dinobryon sertularia, Tribonema affine, Aphanocapsa delicatissima, Synechocystis sp., Gymnodinium inversum, and Peridinium inconspicuum. When the lake was thermally stratified in late summer, some of these species exhibited a stratified vertical distribution in the water column. A cluster analysis of these data also revealed a vertical stratification of the flora from the middle of the summer through the early fall. Multivariate test statistics indicated that there was a significant relationship between the species composition of the phytoplankton and a corresponding set of chemical variables measured for samples from the water column. In this case, concentrations of total phosphorus, ammonia, total Kjeldahl nitrogen, and alkalinity were associated with interannual changes in the flora; whereas pH and concentrations of dissolved oxygen, orthophosphate, nitrate, and silicon were more closely related to spatial variation and thermal stratification. The maximum chlorophyll concentration when the lake was thermally stratified in August and September was usually between depths of 100 m and 120 m. In comparison, the depth of maximum primary production ranged from 60 m to 80 m at this time of year. Regression analysis detected a weak negative relationship between chlorophyll concentration and Secchi disk depth, a measure of lake transparency. However, interannual changes in chlorophyll concentration and the species composition of the phytoplankton could not be explained by the removal of the septic field near Rim Village or by patterns of upwelling from the deep lake. An alternative trophic hypothesis proposes that the productivity of Crater Lake is controlled primarily by long-term patterns of climatic change that regulate the supply of allochthonous nutrients.

The Ecological Effects of Fishing and Implications for Coastal Management in San Miguel Bay, the Philippines

This article uses ecological theory to explore the interactions between fishing and the ecosystem and to examine the implications for fisheries assessment and management in San Miguel Bay, the Philippines. The fishery is modeled using a trophodynamic multispecies model, Ecopath with Ecosim. The impacts of fishing by a multisector fishery on a multispecies resource were dynamically explored under top-down and bottom-up trophic hypotheses. The results demonstrated that the interplay of fishing mortality, species interactions and flow dynamics can have profound implications for fisheries assessment and management. Top-down control is more precautionary than bottom-up control. The uncertainties concerning the resource dynamics were explored using an adaptive management approach. Four models of the San Miguel Bay were analyzed: top-down, bottom-up, immigration plus top-down and immigration plus bottom-up. Results showed that there was no value in learning more about the uncertainty or distinguishing between the different resource models. It was concluded that although an active experimental adaptive management was not worthwhile, adaptive management, using feedback information from the response of the resource to management actions, was recommended.

The geomorphic—trophic hypothesis for arctic lake food webs

The geomorphic—trophic hypothesis for arctic lake food webs

Are fetal brain tissue grafts necessary for the treatment of brain damage?

AbstractDespite some clinical promise, using fetal transplants for degenerative and traumatic brain injury remains controversial and a number of issues need further attention. This response reexamines a number of questions. Issues addressed include: temporal factors relating to neural grafting, the role of behavioral experience in graft outcome, and the relationship of rebuilding of neural circuitry to functional recovery. Also discussed are organization and type of transplanted tissue, the “trophic hypothesis” of transplant viability, and whether transplants are really needed to obtain functional recovery after brain damage.

Trophic effect of collicular proteoglycan on neonatal rat retinal ganglion cells in situ

Naturally occuring neuronal death is widespread in the central nervous system of mammals. To date, the causes and mechanisms of such death are poorly understood. A major hypothesis is that developing neurons compete for limited amounts of trophic factor(s) released from their target centres as in the case of the peripheral nervous system and nerve growth factor. The present study aims to test this ‘trophic hypothesis’ in the mammalian central nervous system. In the rat, more than 50% of retinal ganglion cells die in the early post-natal period. Schulz and coworkers [57] purified a potential trophic agent from their major target, the superior colliculus, which was identified as a 480 kDa chondroitin sulfate proteoglycan. This proteoglycan or control solutions were injected into the eyes of rat pups during the post-natal part of the period of naturally occurring ganglion cell death. It was found that the collicular proteoglycan prevented the death of a significant number of the ganglion cells that would normally have been lost over a post-injection of one or two days. The effect of the proteoglycan was dose- and time-dependent. These results support the notion that trophic interactions are a determining factor in the survival of retinal ganglion cells during the period of naturally occurring cell death. It is also the first time that a proteoglycan has been shown to possess neurotrophic properties in situ.

Changes in Plasma Membrane Properties of Peach Tree Buds and Stands during Dormancy

Summary Fluidity changes between PM from buds and stands, isolated from dormant, non-dormant and nondormant with an additive cold period peach trees, were monitored by steady-state fluorescence polarization using the lipophilic probe 1,6-diphenyl 1,3,5-hexatriene. The break of dormancy, induced by low temperatures, appeared to be correlated with an increase of the PM fluidity from buds, which became higher than from stands. The corresponding apparent energies of activation, calculated from the Arrhenius plots, were modified with the break of dormancy. Indeed, energies of activation were different between the two territories from dormant material but appeared very close in non-dormant material. A superimposed cold treatment on non-dormant peach tree triggered an enhancement of the fluidity differences between PM from buds and stands. However, the apparent energies of activation between the two territories were not modified. These results were discused according to the trophic hypothesis.

Growth and elimination of nerve terminals at synaptic sites during polyneuronal innervation of muscle cells: a trophic hypothesis

This paper examines the possibility that the elimination of synapses from cells arises from a competition between the nerve terminals for trophic molecules made available by the cells. This idea is applied to the elimination of synapses that occurs during the polyneuronal innervation of muscle cells which accompanies both the development and reinnervation of muscles. In the proposed model, each motorneuron makes the same amount of receptor in its soma for a trophic molecule provided in limited quantities by each muscle cell; this receptor is then distributed to the collateral terminals of the motorneuron in concentrations proportional to the amount of receptor made in the soma by the motorneuron; the more collateral terminals initially possessed by a motorneuron the less will be their concentration of receptor. The receptors in the several collateral terminals on a muscle cell then compete for the trophic molecule provided by the muscle, and terminal growth is proportional to the number of receptor-trophic-molecule bonds formed. An autocatalytic effect has been introduced whereby the increase in size of a terminal accelerates the rate by which the trophic molecule is made available to that terminal for bonding with its receptors. In addition, the affinity between nerve terminal receptors and muscle molecules can be varied in the model. Finally, motorneuron cell death has been analysed as the elimination of neurons that have insufficient terminal area to take up a growth factor in amounts that will allow for the survival of the neuron.

Impact of Ecological Succession on the Parasite Fauna in Centrarchids from Oligotrophic and Eutrophic Ecosystems

Approximately 30 species of larval and adult parasites were collected from 549 centrarchid fish and 7718 gastropod molluscs from an oligotrophic and two eutrophic lakes in southwestern Michigan. The distribution pattern indicates that centrarchids from the oligotrophic lake harbor a wide range of species of adult parasites and a comparatively smaller number of larval forms, many of which complete their life cycles in predatory fish. On the other hand, bass and sunfish from the two eutrophic lakes harbor a proportionately larger number of larval parasites, most of which culminate their life cycles in fish-eating birds and mammals. A trophic hypothesis, based on the nature of predatorprey relationships in each of the two types of ecosystem, is proposed to explain the distribution patterns of parasites.