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| 2. |
- Cichon, M, et al.
(author)
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Mass-dependent mass loss in breeding birds : getting the null hypothesis right
- 1999
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In: Oikos. - : JSTOR. - 0030-1299 .- 1600-0706. ; 87:1, s. 191-194
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Journal article (peer-reviewed)abstract
- An assumption central to many tests of statistical association between two variables is the null expectation of zero association. Here, we draw attention to the fact that in many published tests of mass-dependent mass loss in breeding birds, this assumption has been violated. We show that a correct null hypothesis can be derived by using resampling methods, and analyse three data sets (two previously published) from passerine birds to illustrate the approach. Our results show, that under a correct null hypothesis, the biological interpretation of the previously published results is reversed-initially heavy birds do actually lose less mass (relative to their weight) than the initially light birds.
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| 3. |
- D'Hertefeldt, Tina, Universitetslektor, 1965-, et al.
(author)
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Physiological integration of the clonal plant Carex arenaria and its response to soil-borne pathogens
- 1998
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In: Oikos. - Chichester : Wiley-Blackwell. - 0030-1299 .- 1600-0706. ; 81:2, s. 229-237
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Journal article (peer-reviewed)abstract
- We test the hypothesis that the expansive horizontal clonal growth of Carex arenaria may provide a method of escape from soil-borne pathogens (fungi and nematodes) by growing away from the site of infection. Plants were grown in non-sterilized or sterilized dune sand, i.e., with or without soil-borne pathogens. The effects of soil-borne pathogens were studied on the whole genet, on the mother alone, and on the first primary rhizome. Genets with the mother plant infected produced less total biomass and had less biomass allocated to roots than genets with uninfected mothers. Infected genets had fewer primary rhizomes and lower total rhizome length, but rhizome specific weight or the distance between shoots did not decrease in infected plants. In C. arenaria, uninfected mothers with an infected first primary rhizome produced shorter and fewer rhizomes than uninfected genets. The infected first rhizome continued to grow at the same speed as uninfected rhizomes, probably by support from the uninfected mother plant. However, secondary rhizome branching was affected only by direct exposure to soil pathogens and not by the status of the mother plant. The results provide evidence that clonal growth may facilitate escape from soil-borne pathogens. The rhizome explores a patchy environment by supporting the growth of young tillers when passing pathogenic patches.
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| 4. |
- 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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| 5. |
- 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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| 6. |
- Dinnétz, Patrik, et al.
(author)
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Spatial distribution of male sterility in Plantago maritima
- 1998
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In: Oikos. - : JSTOR. - 0030-1299 .- 1600-0706. ; 81, s. 255-265
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Journal article (peer-reviewed)abstract
- Sexual polymorphism in angiosperms can be explained both by the functional responses of male and female function to autogamy and geitonogamy, and by the conflict between the nuclear and cytoplasmic genomes. In predominantly hermaphrodite species, cytoplasmically determined male sterility may persist in a population because of maternal inheritance, i.e, the loss of male function does not change the fitness of the cytoplasmic genome. However, in populations with cytoplasmic male sterility, male fertility is often restored by nuclear genes. Therefore, in populations with genetical substructure, the frequencies of the different sex-morphs will fluctuate depending on the presence of both the male sterile cytoplasms, and of their specific nuclear restorer genes. In Plantagomaritima, we showed that the frequencies of male sterility were highest in regions with the highest population turnover rates and that male sterile individuals were more frequently found in the lower, less dense parts of the meadows. This indicates that male sterile cytoplasms have their highest probabilities to escape their nuclear restorer genes during recolonisation in disturbed regions within populations. We also found that male sterile individuals dispersed their seeds a little bit further than did the hermaphrodites. This can be interpreted as an adaptive response to the local occurrence of nuclear restorer genes.
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| 10. |
- Frost, Ingela, et al.
(author)
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Effects of competition, grazing and cotyledon nutrient supply on growth of Quercus robur seedlings
- 1997
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In: Oikos. - 0030-1299 .- 1600-0706. ; 79:1, s. 53-58
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Journal article (peer-reviewed)abstract
- In this greenhouse experiment we examined how competition and herbivory affects the growth of Quercus robur seedlings and if the cotyledon nutrient reserve is of importance for survival and growth during unfavourable conditions. We planted oak seedlings with or without a strong competitor (grass turf) and subjected them to factorial grazing and cotyledon removal, in a split-plot design. After one growing season (20 weeks) we found large negative main effects from competition, grazing and cotyledon removal on all biomass components of the seedling. Seedling mortality was also significantly increased by competition. We observed an additional effect of cotyledon removal if the seedlings were also grazed or were growing in competition with grass. This gives some support to the hypothesis that cotyledon nutrient reserves are used under unfavourable conditions, but the effect was often relatively small and not detectable in the growth of all plant parts.
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