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- Diehl, Sebastian
(author)
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Effects of habitat structure on resource availability, diet and growth of benthivorous perch, perca-fluviatilis
- 1993
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In: Oikos. - 0030-1299 .- 1600-0706. ; 67:3, s. 403-414
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Journal article (peer-reviewed)abstract
- I experimentally evaluated the impact of habitat structural complexity on the interactions between a generalist predator and a benthic macroinvertebrate prey assemblage in a freshwater pond. Benthivorous perch (Perca fluviatilis) were stocked over a range of natural densities (no fish, low, and high densities) into enclosures with or without dense submerged vegetation. The biomass of macroinvertebrate prey decreased over time in the presence of perch and was always higher in enclosures with vegetation present than in enclosures lacking vegetation. The increase in mass of perch was positively related to the abundance of macroinvertebrate prey and negatively related to perch density and the density of vegetation. In the treatments lacking vegetation, the proportion of zooplankton in the diet of perch increased, and the growth rate of perch decreased over time. In the vegetation treatments, the proportion of zooplankton in the diet was low throughout the experiment and the growth rate of perch was constant over time. As a consequence, initial increase in mass was considerably higher in the treatments lacking vegetation than in the vegetation treatments, whereas no such pattern was observed in the second half of the experiment. In the absence of vegetation, perch are apparently able to forage efficiently, but this may reduce the availability of macroinvertebrate prey to the extent that perch are forced to include less profitable zooplankton prey into their diet. In vegetated habitats, the foraging efficiency of perch is reduced, which possibly prevents over-exploitation of macroinvertebrate prey and consequently may allow for a moderate, but relatively constant, consumption of macroinvertebrates by perch. The density-dependence of growth rates in both vegetated and unvegetated habitats can only partly be explained by resource competition, which suggests the presence of an additional mechanism of density-dependence. In natural lake communities, efficient predation from benthivorous fish should keep the biomass of macroinvertebrate prey in structurally simple habitats below the high levels initially present in my experiment. In these communities, submerged vegetation may be an equally profitable habitat for juvenile perch as are open areas. Through its effects on the feeding efficiencies of juvenile perch and other benthivorous fish, submerged vegetation may affect individual growth rates and the size structure of perch populations, which may contribute to explain differences in fish community structure among lakes differing in submerged vegetation cover.
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| 2. |
- Diehl, Sebastian
(author)
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Relative consumer sizes and the strengths of direct and indirect interactions in omnivorous feeding relationships
- 1993
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In: Oikos. - 0030-1299 .- 1600-0706. ; 68:1, s. 151-157
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Journal article (peer-reviewed)abstract
- Omnivory (the consumption of resources from more than one trophic level) is widespread in nature and has the potential to produce a richness of indirect effects. Nevertheless, its effects on population dynamics have received very little attention. In its simplest case, omnivory involves a top consumer, an intermediate consumer, and a resource that is common to both consumers. Simple models predict that the intermediate consumer can only coexist with the top consumer if the former is more efficient in exploiting the common resource, which would imply a net positive effect of the top consumer on the equilibrium density of resources (compared to the situation where only the intermediate consumer is present). Among 22 experimental manipulations of omnivorous top consumers I found only 2 studies in which top consumers had significant positive effects on resources. This discrepancy between experimental results and model predictions is, at least partly, related to deviations of the experimental systems from model assumptions. However, considerations of relative body sizes of intermediate and top consumers suggest, that top consumers having negative net effects on the basic resource should be common in nature. I argue that in systems where intermediate consumers and basic resources are relatively similar in size, but both are much smaller than omnivorous top consumers (e.g. vertebrate omnivores feeding on benthos, soil invertebrates, terrestrial insects etc.), the direct negative effect of top consumers on basic resources should not be outweighed by indirect positive effects, and that other mechanisms (e.g. prey refuges) must be invoked to explain the persistence of intermediate consumers in many natural systems. I further argue that a better knowledge of the population dynamical consequences of omnivory and the role of relative consumer sizes is necessary to improve our understanding of the-trophic dynamics of different kinds of communities.
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| 3. |
- Persson, L, et al.
(author)
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Density dependent interactions in lake ecosystems : whole lake perturbation experiments
- 1993
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In: Oikos. - 0030-1299 .- 1600-0706. ; 66:2, s. 193-208
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Journal article (peer-reviewed)abstract
- Density dependent interactions between higher and lower trophic levels were studied in two consecutive whole lake experiments in a highly productive lake. In the first experiment, the zooplanktivorous fish species, roach (Rutilus rutilus) (primary carnivore), was reduced to 27% of its previous population size (53%) of biomass) by a selective rotenone treatment. In the second experiment, zander (Stizostedion lucioperca) (secondary carnivore), were stocked in the lake resulting in a reduction in the population size of roach to 51% of its previous population size (31% of biomass). In both experiments, seasonal average biomass of Daphnia cucullata) increased in the two years with the lowest roach biomasses. but no shift to larger zooplankton forms took place. In the years with increased Daphnia biomass, phytoplankton biomass and transparency showed an increased seasonal variation (measured as coefficient of variation). In contrast to manipulations in which planktivorous fish were totally removed, no effect on seasonal average phytoplankton biomass was observed. Due to the high reproductive capacity of roach, the system returned to previous conditions within a year or two after the perturbations. Under unperturbed conditions, the lake showed few indications of instability as suggested by the paradox of enrichment hypothesis. This lack of instability can. among other things, be related to high zooplanktivore predation pressure present in highly productive lakes preventing overexploitation of primary producers by grazers. In addition, the omnivorous feeding characteristics of roach (feeding on both zooplankton and algae/detritus) and interactions between the open water and the detritus nutrient pool are likely to increase the stability of the system. The two perturbation experiments plus a previous natural experiment (winter fish kill), which all were pulsed perturbations, provide no evidence for the presence of alternative stable states in highly productive lakes. Possibly, a sustained perturbation over several years may shift highly productive lakes from a dominance of phytoplankton production to a dominance of macrophyte production leading to an alternative state.
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