| 1. |
|
|
| 2. |
|
|
| 3. |
- Aunapuu, Maano, et al.
(författare)
-
Intraguild predation and interspecific co-existence between predatory endotherms
- 2010
-
Ingår i: Evolutionary Ecology Research. - Tucson : Evolutionary Ecology. - 1522-0613 .- 1937-3791. ; 12:2, s. 151-168
-
Tidskriftsartikel (refereegranskat)abstract
- Background: According to the current predominant view, intraguild predation leads to the replacement of intermediate predators from highly productive habitats, whereas top predators and intermediate predators can co-exist in habitats with intermediate primary productivity. These predictions are contradicted by the observed abundance of intermediate predators in productive environments. But the predictions are derived by modelling interactions in food chains where the top predator is primarily adapted to exploit intermediate predators but also has some capacity to exploit the resources of the intermediate predators. We call this 'food chain omnivory'. In contrast, 'genuine intraguild predation' is the case where the two predators have shared tactics of resource acquisition, resulting in broadly overlapping prey preferences that is, the interacting predators belong to the same guild as defined by Root (1967). Questions: What are the effects of productivity on genuine intraguild predation? Do the predictions for food chain omnivory apply also to genuine intraguild predation'? Methods: We modelled genuine intraguild predation by using parameter values such that the intermediate predator and the basal prey were equally valuable to the top predator. We assumed that the basal prey was a herbivore, with a carrying capacity directly proportional to primary productivity and a habitat-specific intrinsic rate of population growth that increases asymptotically in response to increasing primary productivity. Results: With the above premises, intermediate predators can prevail even in highly productive habitat. Also, a priority effect is possible. Predictable replacement of intermediate predators by top predators requires that intermediate predators are much easier to find than basal prey. Stable co-existence requires biologically implausible parameter values. Conclusions: Genuine intraguild predation is a destabilizing force in food webs. The dynamics of genuine intraguild predation systems differ from those in food chain omnivory systems where the intermediate and top predators have different feeding tactics and, therefore, different prey preferences.
|
|
| 4. |
- Aunapuu, Maano, et al.
(författare)
-
Spatial patterns and dynamic responses of arctic food webs corroborate the exploitation ecosystems hypothesis (EEH)
- 2008
-
Ingår i: American Naturalist. - : University of Chicago Press. - 0003-0147 .- 1537-5323. ; 171:2, s. 249-262
-
Tidskriftsartikel (refereegranskat)abstract
- According to the exploitation ecosystems hypothesis (EEH), productive terrestrial ecosystems are characterized by community‐level trophic cascades, whereas unproductive ecosystems harbor food‐limited grazers, which regulate community‐level plant biomass. We tested this hypothesis along arctic‐alpine productivity gradients at the Joatka field base, Finnmark, Norway. In unproductive habitats, mammalian predators were absent and plant biomass was constant, whereas herbivore biomass varied, reflecting the productivity of the habitat. In productive habitats, predatory mammals were persistently present and plant biomass varied in space, but herbivore biomass did not. Plant biomass of productive tundra scrublands declined by 40% when vegetation blocks were transferred to predation‐free islands. Corresponding transfer to herbivore‐free islands triggered an increase in plant biomass. Fertilization of an unproductive tundra heath resulted in a fourfold increase in rodent density and a corresponding increase in winter grazing activity, whereas the total aboveground plant biomass remained unchanged. These results corroborate the predictions of the EEH, implying that the endotherm community and the vegetation of the North European tundra behaves dynamically as if each trophic level consisted of a single population, in spite of local co‐occurrence of >20 plant species representing different major taxonomic groups, growth forms, and defensive strategies.
|
|
| 5. |
- Dahlgren, Jonas, et al.
(författare)
-
Plant defences to no avail? : Responses of plants of varying edibility to food web manipulations in a low arctic scrubland
- 2009
-
Ingår i: Evolutionary Ecology Research. - : Evolutionary ecology. - 1522-0613 .- 1937-3791. ; 11:8, s. 1189-1203
-
Tidskriftsartikel (refereegranskat)abstract
- Background: According to the Green World Hypothesis of Hairston, Smith, and Slobodkin, all plants are edible for some herbivores. Hence, the copious abundance of plant biomass, typical for terrestrial ecosystems, depends on the collective regulatory action of predators on the herbivore guild. According to the counterarguments of Polis and Strong, the defensive traits of terrestrial plants attenuate terrestrial trophic cascades to species-specific trickles, so elimination of predators might lead to increased abundance of inedible plants but will not influence community-level plant biomass. Question: Does the elimination of predators from a low arctic scrubland, with high-quality forage plants and poorly edible evergreen ericoids, lead to a reduction of community-level plant biomass or to an increased abundance of well-defended evergreen ericoids? Methods: In 1991, we introduced grey-sided voles (Myodes rufocanus) to islands, initially harbouring dense scrubland vegetation, and established permanent plots there. In 2000, we transplanted vegetation blocks from a large three-trophic-level island with voles and predators, to two-trophic-level islands with introduced voles but without resident predators, and also to vole-free one-trophic-level islands, and back to the three-trophic-level island. Vole densities were monitored by semi-annual live trapping. Vegetation was monitored by the point-frequency method. Results: In the absence of predators, vole densities increased 3.7-fold and the community-level plant biomass was decimated. The least palatable plant group, evergreen ericoids, suffered especially heavily, whereas palatable herbaceous plants increased in abundance. However, all three functional plant groups responded positively to the elimination of grey-sided voles. Conclusions: Our results corroborate the Green World Hypothesis, indicating that in the absence of predators, plant defences do not prevent runaway consumption of the vegetation. The fate of plants in predator-free systems with browsing vertebrates depends primarily on the accessibility of each plant during the limiting season. Evergreen ericoids then form the most sensitive functional group.
|
|
| 6. |
- Dahlgren, Jonas, et al.
(författare)
-
Plant defense at no cost? : The recovery of tundra scrubland following heavy grazing by grey-sided voles (Myodes rufocanus)
- 2009
-
Ingår i: Evolutionary Ecology Research. - : Evolutionary Ecology. - 1522-0613 .- 1937-3791. ; 11, s. 1205-1216
-
Tidskriftsartikel (refereegranskat)abstract
- Background: Evergreen ericaceous dwarf shrubs form a dominating component of low arctic and low alpine vegetation. They typically produce high contents of secondary chemicals such as phenolics. The primary function of these chemicals may be to defend the shrubs by making them less palatable to herbivores. Question: Does the production of secondary chemicals carry a fitness cost in terms of low growth rate and, therefore, low capacity to recover from past herbivory? Methods: In 2000, we constructed vole-proof exclosures on low arctic islands where vegetation had, since 1991, been heavily impacted by grey-sided voles. In 2000 and 2003,we surveyed the vegetation of the exclosures, of unfenced plots on the same islands, and of control plots on a vole-free island. We used the point-frequency method for vegetation surveys. Results: In the exclosures, the biomasses of most plant species increased, by and large, at the same pace. The two woody species, which increased most rapidly, were the maximally palatable bilberry (Vaccinium myrtillus) and the phenolics-laden, maximally unpalatable northern crowberry (Empetrum nigrum ssp. hermaprhoditum). The recovery rates of these species were similar. Conclusions: The high concentrations of phenolics typical for evergreen arctic dwarf shrubs do not carry any obvious cost in the form of reduced capacity for compensatory growth. The principle of trade-offs does not help to explain the variation in plant palatability.
|
|
| 7. |
- Dahlgren, Jonas, et al.
(författare)
-
Plant defenses to no avail? responses of plants with varying edibility to food web manipulations in a low arctic scrubland
- 2009
-
Ingår i: Evolutionary Ecology Research. - 1522-0613 .- 1937-3791. ; 11, s. 1189-1203
-
Tidskriftsartikel (refereegranskat)abstract
- Background: According to the Green World Hypothesis of Hairston, Smith, and Slobodkin, all plants are edible for some herbivores. Hence, the copious abundance of plant biomass, typical for terrestrial ecosystems, depends on the collective regulatory action of predators on the herbivore guild. According to the counterarguments of Polis and Strong, the defensive traits of terrestrial plants attenuate terrestrial trophic cascades to species-specific trickles, so elimination of predators might lead to increased abundance of inedible plants but will not influence community-level plant biomass.Question: Does the elimination of predators from a low arctic scrubland, with high-quality forage plants and poorly edible evergreen ericoids, lead to a reduction of community-level plant biomass or to an increased abundance of well-defended evergreen ericoids?Methods: In 1991, we introduced grey-sided voles (Myodes rufocanus) to islands, initially harbouring dense scrubland vegetation, and established permanent plots there. In 2000, we transplanted vegetation blocks from a large three-trophic-level island with voles and predators, to two-trophic-level islands with introduced voles but without resident predators, and also to vole-free one-trophic-level islands, and back to the three-trophic-level island. Vole densities were monitored by semi-annual live trapping. Vegetation was monitored by the point-frequency method.Results: In the absence of predators, vole densities increased 3.7-fold and the communitylevel plant biomass was decimated. The least palatable plant group, evergreen ericoids, suffered especially heavily, whereas palatable herbaceous plants increased in abundance. However, all three functional plant groups responded positively to the elimination of grey-sided voles.Conclusions: Our results corroborate Green World Hypothesis, indicating that in the absence of predators, plant defences do not prevent runaway consumption of the vegetation. The fate of plants in predator-free systems with browsing vertebrates depends primarily on the accessibility of each plant during the limiting season. Evergreen ericoids then form the most sensitive functional group.
|
|
| 8. |
|
|
| 9. |
|
|
| 10. |
- Ehrich, Dorothee, et al.
(författare)
-
Documenting lemming population change in the Arctic : Can we detect trends?
- 2020
-
Ingår i: Ambio. - : Springer Science and Business Media LLC. - 0044-7447 .- 1654-7209. ; 49:3, s. 786-800
-
Tidskriftsartikel (refereegranskat)abstract
- Lemmings are a key component of tundra food webs and changes in their dynamics can affect the whole ecosystem. We present a comprehensive overview of lemming monitoring and research activities, and assess recent trends in lemming abundance across the circumpolar Arctic. Since 2000, lemmings have been monitored at 49 sites of which 38 are still active. The sites were not evenly distributed with notably Russia and high Arctic Canada underrepresented. Abundance was monitored at all sites, but methods and levels of precision varied greatly. Other important attributes such as health, genetic diversity and potential drivers of population change, were often not monitored. There was no evidence that lemming populations were decreasing in general, although a negative trend was detected for low arctic populations sympatric with voles. To keep the pace of arctic change, we recommend maintaining long-term programmes while harmonizing methods, improving spatial coverage and integrating an ecosystem perspective.
|
|
| 11. |
|
|
| 12. |
- Ekerholm, Per, et al.
(författare)
-
The impact of short-term predator removal on vole dynamics in a arctic-alpine landscape
- 2004
-
Ingår i: Oikos. - : John Wiley & Sons. - 0030-1299 .- 1600-0706. ; 106:3, s. 457-468
-
Tidskriftsartikel (refereegranskat)abstract
- During 1991–95, mammalian predators (weasel, Mustela nivalis, stoat, M. erminea, mink, M. vison, and red fox, Vulpes vulpes) were excluded in late summers from a 2 ha piece of a north Norwegian mountain slope. The exclosure extended from an outpost of luxuriant sub-arctic birch forest to a typical arctic–alpine habitat complex, including productive willow scrublands, tundra heaths, dry ridges and snow-beds. The exclosure thus encompassed the entire range of habitat conditions encountered in a typical north Fennoscandian mountain and tundra landscapes. During 1991–95, the exclosure was predator-proof from late July to late September. In wintertime and in early summer, the exclosure was accessible to mammalian predators. Vole dynamics in the short-term exclosure were compared to dynamics in five reference areas with similar habitat conditions.In 1991, when vole densities were rising in the area, neither collective vole densities nor densities of individual vole species differed significantly between the exclosure and the replicated controls. Spring densities of voles were never significantly different between the exclosure and the controls. With respect to autumnal densities of voles, however, the exclosure was a statistical outlier in the peak year 1992 and throughout the gradual decline phase of 1993–95. In the peak year, the difference in collective vole densities was modest (30%), but increased to two-fold during the first two decline years and was almost four-fold in the crash year of 1995. The strongest response was displayed by field voles (Microtus agrestis), hypothesized to be the pivotal prey species of weasels, especially by females and young individuals, i.e. by the functional categories especially sensitive to mammalian predation. These results are consistent with the hypothesis that predation plays a pivotal role for the regulation of herbivorous mammals in relatively productive arctic–alpine habitats.
|
|
| 13. |
- Estes, James A., et al.
(författare)
-
Trophic Downgrading of Planet Earth
- 2011
-
Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 333:6040, s. 301-306
-
Forskningsöversikt (refereegranskat)abstract
- Until recently, large apex consumers were ubiquitous across the globe and had been for millions of years. The loss of these animals may be humankind's most pervasive influence on nature. Although such losses are widely viewed as an ethical and aesthetic problem, recent research reveals extensive cascading effects of their disappearance in marine, terrestrial, and freshwater ecosystems worldwide. This empirical work supports long-standing theory about the role of top-down forcing in ecosystems but also highlights the unanticipated impacts of trophic cascades on processes as diverse as the dynamics of disease, wildfire, carbon sequestration, invasive species, and biogeochemical cycles. These findings emphasize the urgent need for interdisciplinary research to forecast the effects of trophic downgrading on process, function, and resilience in global ecosystems.
|
|
| 14. |
- Gauthier, Gilles, et al.
(författare)
-
Taking the beat of the Arctic : are lemming population cycles changing due to winter climate?
- 2024
-
Ingår i: Proceedings of the Royal Society of London. Biological Sciences. - : Royal Society. - 0962-8452 .- 1471-2954. ; 291:2016
-
Tidskriftsartikel (refereegranskat)abstract
- Reports of fading vole and lemming population cycles and persisting low populations in some parts of the Arctic have raised concerns about the spread of these fundamental changes to tundra food web dynamics. By compiling 24 unique time series of lemming population fluctuations across the circumpolar region, we show that virtually all populations displayed alternating periods of cyclic/non-cyclic fluctuations over the past four decades. Cyclic patterns were detected 55% of the time (n = 649 years pooled across sites) with a median periodicity of 3.7 years, and non-cyclic periods were not more frequent in recent years. Overall, there was an indication for a negative effect of warm spells occurring during the snow onset period of the preceding year on lemming abundance. However, winter duration or early winter climatic conditions did not differ on average between cyclic and non-cyclic periods. Analysis of the time series shows that there is presently no Arctic-wide collapse of lemming cycles, even though cycles have been sporadic at most sites during the last decades. Although non-stationary dynamics appears a common feature of lemming populations also in the past, continued warming in early winter may decrease the frequency of periodic irruptions with negative consequences for tundra ecosystems.
|
|
| 15. |
|
|
| 16. |
- Hoset, Katrine S., et al.
(författare)
-
Spatial variation in vegetation damage relative to primary productivity, small rodent abundance and predation
- 2014
-
Ingår i: Ecography. - : Wiley. - 0906-7590 .- 1600-0587. ; 37:9, s. 894-901
-
Tidskriftsartikel (refereegranskat)abstract
- The relative importance of top-down and bottom-up mechanisms in shaping community structure is still a highly controversial topic in ecology. Predatory top-down control of herbivores is thought to relax herbivore impact on the vegetation through trophic cascades. However, trophic cascades may be weak in terrestrial systems as the complexity of food webs makes responses harder to predict. Alternatively, top-down control prevails, but the top-level (predator or herbivore) changes according to productivity levels. Here we show how spatial variation in the occurrence of herbivores (lemmings and voles) and their predators (mustelids and foxes) relates with grazing damage in landscapes with different net primary productivity, generating two and three trophic level communities, during the 2007 rodent peak in northern Norway. Lemmings were most abundant on the unproductive high-altitude tundra, where few predators were present and the impact of herbivores on vegetation was strong. Voles were most common on a productive, south facing slope, where numerous predators were present, and the impacts of herbivores on vegetation were weak. The impact of herbivores on the vegetation was strong only when predators were not present, and this cannot be explained by between-habitat differences in the abundance of plant functional groups. We thus conclude that predators influence the plant community via a trophic cascade in a spatial pattern that support the exploitation ecosystems hypothesis. The responses to grazing also differed between plant functional groups, with implications for short and long-term consequences for plant communities.
|
|
| 17. |
- KORPIMÄKI, ERKKI, et al.
(författare)
-
Vole cycles and predation in temperate and boreal zones of Europe
- 2005
-
Ingår i: Journal of Animal Ecology. - : Wiley. - 0021-8790 .- 1365-2656. ; 74:6, s. 1150 -1159
-
Tidskriftsartikel (refereegranskat)abstract
- 1.Graham & Lambin (2002) have reported on a weasel-reduction experiment, concluding that the impact of weasel predation on field vole survival was neither sufficient nor necessary to initiate and drive the cyclic decline of field vole populations in Kielder Forest, northern England. They also stated that their findings contradict conclusively the specialist predator hypothesis put forward to explain population cycles of voles in North Europe. 2.Straightforward inferences from Kielder Forest to the northern boreal zone are misleading, because the population cycles of voles in Kielder Forest differ essentially from North European vole cycles. The low amplitude of the vole cycles in Kielder Forest, their restricted spatial synchrony in comparison to northern Europe and the virtual lack of interspecific synchrony in Kielder Forest suggest that there are essential differences between the mechanisms responsible for the two types of cyclic fluctuations of voles. 3.The weasel-reduction experiment may provide a misleading picture on the role of predators, even in the Kielder Forest cycle. The experimental reduction of weasels alone may not stop the population decline of voles, because competing larger predators are expected to increase their hunting in the weasel-reduction areas. The small spatial scale of the experiment, which produced only slight, short-term differences in weasel densities between reduction and control areas, also suggests that other predators could have compensated easily for the weasels that were removed. 4.We propose a new version of the predation hypothesis to explain low-amplitude population cycles of voles in temperate Europe, including the Kielder Forest. The interaction between generalist predators and vole populations might account for these cycles because generalists can have a functional response that is destabilizing in the neighbourhood of the equilibrium point. As most generalists are orders of magnitude larger than weasels, and thus need much more food for survival, generalist-driven cycles should be characterized by high prey minima, as observed in Kielder Forest.
|
|
| 18. |
- Oksanen, Lauri, et al.
(författare)
-
Islands as tests of the green world hypothesis.
- 2010
-
Ingår i: Trophic Cascades – Predators, Prey, and the changing dynamics of Nature. - Washihgton DC : Island Press. - 9781597264860 ; , s. 163-178, s. 163-178
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)
|
|
| 19. |
|
|
| 20. |
- Oksanen, Tarja, et al.
(författare)
-
Exploitation ecosystems in heterogeneous habitat complexes II : Impact of small-scale heterogeneity on predator-prey dynamics
- 1992
-
Ingår i: Evolutionary Ecology. - 0269-7653 .- 1573-8477. ; 6:5, s. 383-398
-
Tidskriftsartikel (refereegranskat)abstract
- The model of exploitation ecosystems was re-analysed, assuming that habitat patches are so small that they form only parts of the home range of an individual predator. For habitat complexes where productive patches abound, the results suggested that predation will strongly spill over from productive patches, which set the tune for population dynamics within the whole landscape, to barren ones. This result conforms to the one obtained by T. Oksanen by assuming despotic habitat choice and essentially larger patch sizes. For habitat complexes heavily dominated by the barren habitat, spillover predation was predicted to be weak, as was the case in her large patch model. Unlike in her analysis, however, predation pressure was substantially reduced also within the productive habitat. In habitat complexes where patches are so small that they are exploited in a fine-grained manner, predation pressure was always found to be more intense in the barren habitat, contrary to the predictions of the original model of exploitation ecosystems. This analysis thus suggests that their model is applicable mainly on the landscape level. On the level of individual habitats, the applicability of their results depends on the habitat configuration (at its best for the prevailing habitat of the landscape and for moderate-sized patches of an essentially more productive habitat) and generally decreases with decreasing patch sizes.
|
|
| 21. |
|
|
| 22. |
- Oksanen, Tarja, et al.
(författare)
-
The impact of thermal seasonality on terrestrial endotherm food web dynamics : a revision of the Exploitation Ecosystem Hypothesis
- 2020
-
Ingår i: Ecography. - : John Wiley & Sons. - 0906-7590 .- 1600-0587. ; 43:12, s. 1859-1877
-
Tidskriftsartikel (refereegranskat)abstract
- Many terrestrial endotherm food webs constitute three trophic level cascades. Others have two trophic level dynamics (food limited herbivores; plants adapted to tackle intense herbivory) or one trophic level dynamic (herbivorous endotherms absent, thus plants compete for the few places where they can survive and grow). According to the Exploitation Ecosystems Hypothesis (EEH), these contrasting dynamics are consequences of differences in primary productivity. The productivity thresholds for changing food web dynamics were assumed to be global constants. We challenged this assumption and found that several model parameters are sensitive to the contrast between persistently warm and seasonally cold climates. In persistently warm environments, three trophic level dynamics can be expected to prevail almost everywhere, save the most extreme deserts. We revised EEH accordingly and tested it by compiling direct evidence of three and two trophic level dynamics and by studying the global distribution of felids. In seasonally cold environments, we found evidence for three trophic level dynamics only in productive ecosystems, while evidence for two trophic level dynamics appeared in ecosystems with low primary productivity. In persistently warm environments, we found evidence for three trophic level dynamics in all types of ecosystems. The distribution of felids corroborated these results. The empirical evidence thus indicates that two trophic level dynamics, as defined by EEH, are restricted to seasonally cold biomes with low primary productivity, such as the artic-alpine tundra and the temperate steppe.
|
|
| 23. |
- Olofsson, Johan, et al.
(författare)
-
Long-Term Experiments Reveal Strong Interactions Between Lemmings and Plants in the Fennoscandian Highland Tundra
- 2014
-
Ingår i: Ecosystems (New York. Print). - : Springer Science and Business Media LLC. - 1432-9840 .- 1435-0629. ; 17:4, s. 606-615
-
Tidskriftsartikel (refereegranskat)abstract
- Both the theory and the observations suggest that, there are strong links between herbivores and plants in terrestrial ecosystems; although, the effect of herbivores on plant community biomass is often attributed to variations in plant palatability. The existence of a strong link is commonly tested by constructing exclosures that exclude herbivores during a period of time. We here present data from two long-term (9 and 20 years, respectively) herbivore exclosure studies in lemming habitats on arctic tundra in northernmost Norway. The exclusion of all mammalian herbivores triggered strong increases in community level plant biomass and substantial changes in plant community composition. Palatable plants like graminoids and large bryophytes, as well as unpalatable plants like evergreen ericoids, deciduous shrubs, and lichens were all favored by excluding lemmings. These results reveal that a substantial increase in community biomass which occurs only when plant species capable of accumulating biomass are present, and palatability is a poor predictor of long-term responses of plants to excluding herbivores.
|
|
| 24. |
- Rammul, Üllar, et al.
(författare)
-
Vole–vegetation interactions in an experimental, enemy free taiga floor system
- 2007
-
Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 116:9, s. 1501-1513
-
Tidskriftsartikel (refereegranskat)abstract
- Vole–vegetation interactions in a predation-free taiga environment of northern Fennoscandia were studied by transferring vegetation from natural Microtus habitats into a greenhouse, where three habitat islands of about 30 m2 were created. The ‘islands’ were subjected to simulated summer conditions and a paired female field vole, Microtusagrestis, was introduced to each ‘island’. The development of the female and her young was followed by recurrent live trapping. The development of the vegetation was followed by recurrent marking and censusing of plant shoots at intervals of five days. In the next growing season, two ‘islands’ were subjected to a new grazing treatment to study the impacts of repeated grazing on the vegetation and on the growth and reproduction of voles. Plant biomasses were harvested at the end of each trial. In all trials, the biomasses of graminoids and non-toxic herbs other than ferns, fireweeds and rosaceous plants were profoundly decimated. Even the biomass of a toxic herb Aconitum lycoctonum decreased largely at pace with the palatable herbs. The least preferred plant categories maintained their biomasses at control levels. Their neutral collective response was created by opposite species-level trends. Species typical for moist and nutrient-rich forests suffered from vole grazing, whereas the biomass of species adapted to disturbed habitats increased.In spite of the dramatic changes in the vegetation, the introduced female voles survived throughout the trials and reproduced normally. The young of their first litters survived well and reached the final weights typical for individuals starting to winter as immatures. We conclude that most of the plant biomass found on productive boreal forest floors is potential food for field voles and remains palatable for them even when subjected to recurrent, severe grazing. If nothing else than summer resources were limiting the growth of the field vole populations, the plants currently dominating moist and nutrient-rich taiga floors could not survive in this habitat.
|
|
| 25. |
- Turchin, Peter, et al.
(författare)
-
Are lemmings prey or predators?
- 2000
-
Ingår i: Nature. - 0028-0836. ; 405, s. 562-565
-
Tidskriftsartikel (refereegranskat)
|
|
| 26. |
- Virtanen, Risto, et al.
(författare)
-
Where do the treeless tundra areas of northern highlands fit in the global biome system : toward an ecologically natural subdivision of the tundra biome
- 2016
-
Ingår i: Ecology and Evolution. - : Wiley. - 2045-7758. ; 6:1, s. 143-158
-
Tidskriftsartikel (refereegranskat)abstract
- According to some treatises, arctic and alpine sub-biomes are ecologically similar, whereas others find them highly dissimilar. Most peculiarly, large areas of northern tundra highlands fall outside of the two recent subdivisions of the tundra biome. We seek an ecologically natural resolution to this long-standing and far-reaching problem. We studied broad-scale patterns in climate and vegetation along the gradient from Siberian tundra via northernmost Fennoscandia to the alpine habitats of European middle-latitude mountains, as well as explored those patterns within Fennoscandian tundra based on climate-vegetation patterns obtained from a fine-scale vegetation map. Our analyses reveal that ecologically meaningful January-February snow and thermal conditions differ between different types of tundra. High precipitation and mild winter temperatures prevail on middle-latitude mountains, low precipitation and usually cold winters prevail on high-latitude tundra, and Scandinavian mountains show intermediate conditions. Similarly, heath-like plant communities differ clearly between middle latitude mountains (alpine) and high-latitude tundra vegetation, including its altitudinal extension on Scandinavian mountains. Conversely, high abundance of snowbeds and large differences in the composition of dwarf shrub heaths distinguish the Scandinavian mountain tundra from its counterparts in Russia and the north Fennoscandian inland. The European tundra areas fall into three ecologically rather homogeneous categories: the arctic tundra, the oroarctic tundra of northern heights and mountains, and the genuinely alpine tundra of middlelatitude mountains. Attempts to divide the tundra into two sub-biomes have resulted in major discrepancies and confusions, as the oroarctic areas are included in the arctic tundra in some biogeographic maps and in the alpine tundra in others. Our analyses based on climate and vegetation criteria thus seem to resolve the long-standing biome delimitation problem, help in consistent characterization of research sites, and create a basis for further biogeographic and ecological research in global tundra environments.
|
|
| 27. |
- Vuorinen, Katariina E. M., et al.
(författare)
-
Open tundra persist, but arctic features decline-Vegetation changes in the warming Fennoscandian tundra
- 2017
-
Ingår i: Global Change Biology. - : WILEY. - 1354-1013 .- 1365-2486. ; 23:9, s. 3794-3807
-
Tidskriftsartikel (refereegranskat)abstract
- In the forest-tundra ecotone of the North Fennoscandian inland, summer and winter temperatures have increased by two to three centigrades since 1965, which is expected to result in major vegetation changes. To document the expected expansion of woodlands and scrublands and its impact on the arctic vegetation, we repeated a vegetation transect study conducted in 1976 in the Darju, spanning from woodland to a summit, 200 m above the tree line. Contrary to our expectations, tree line movement was not detected, and there was no increase in willows or shrubby mountain birches, either. Nevertheless, the stability of tundra was apparent. Small-sized, poorly competing arctic species had declined, lichen cover had decreased, and vascular plants, especially evergreen ericoid dwarf shrubs, had gained ground. The novel climate seems to favour competitive clonal species and species thriving in closed vegetation, creating a community hostile for seedling establishment, but equally hostile for many arctic species, too. Preventing trees and shrubs from invading the tundra is thus not sufficient for conserving arctic biota in the changing climate. The only dependable cure is to stop the global warming.
|
|
| 28. |
- Bognounou, Fidele, et al.
(författare)
-
Role of climate and herbivory on native and alien conifer seedling recruitment at and above the Fennoscandian tree line
- 2018
-
Ingår i: Journal of Vegetation Science. - : Wiley-Blackwell. - 1100-9233 .- 1654-1103. ; 29:4, s. 573-584
-
Tidskriftsartikel (refereegranskat)abstract
- Questions: We investigated the importance of climate and herbivory on native and alien conifer colonization of the birch-dominated Fennoscandian tree line by addressing the following questions: (a) are tree line and tundra habitats similarly suitable for conifer seedling recruitment; (b) do ungulate and rodent herbivores differentially impact seedling recruitment; and (c) how does the role of habitat and herbivory on seedling recruitment vary across a marked climate gradient?Location: Northern Fennoscandia, Sweden (Vassijaure and Paddus), and Norway (Joatka and Seiland).Methods: We conducted an experiment to assess the emergence rate, survival probability and height development of Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and Siberian larch (Larix sibirica) seedlings. Three experimental plots (i.e., open control, reindeer exclosure and complete vertebrate exclosure) were established in both tree line and tundra habitats at each of the four locations. Seeds of the three conifer species were sown in each plot in June 1999 during three consecutive years. The surviving seedlings were counted in August to September 1999, 2000, 2001, 2002 and 2007. The height of all seedlings was measured in 2007.Results: Our study reveals that Norway spruce, Scots pine and Siberian larch can regenerate from seed at and above the current tree line in northern Fennoscandia. Their performance was generally higher above tree line in tundra than at tree line, but depended on species identity, climate aridity and mammal herbivory, particularly by rodents. These results suggest that the species composition and latitudinal limit of the tree line in the future might depend not only on direct effects of the future climate on the current tree line species, but also on the intensity of alien and native conifer introductions, as well as changes in herbivore populations.Conclusion: If sufficient seeds of Norway spruce, Scots pine and Siberian larch should reach the current tree line, their performances will increase with a warmer and wetter climate, and this effect will be markedly modulated by herbivores (particularly rodents). Further work is required to extend these results to determine the ability of these conifers to become tree line-forming species in the future.
|
|
| 29. |
- Bruun, Hans Henrik, et al.
(författare)
-
Effects of altitude and topography on species richness of vascular plants, bryophytes and lichens in alpine communities
- 2006
-
Ingår i: Journal of Vegetation Science. - 1100-9233 .- 1654-1103. ; 17:1, s. 37-46
-
Tidskriftsartikel (refereegranskat)abstract
- Question: What is the relationship between species richness of vascular plants, bryophytes and macrolichens, and two important gradients in the alpine environment, altitude and local topography? Location: Northernmost Fennoscandia, 250-152 m a.s.l. corresponding to the range between timberline and mountain top. Methods: The vegetation was sampled in six mountain areas. For each 25 vertical metres, the local topographic gradient from wind-blown ridge to snowbed was sampled in quadrats of 0.8 m x 0.8 m. Patterns in species richness were explored using Poisson regression (Generalized Linear Models). Functional groups of species, i.e. evergreen and deciduous dwarf-shrubs, forbs, graminoids, mosses, hepatics and lichens were investigated separately. Results: Functional groups showed markedly different patterns with respect to both altitude and topography. Species richness of all vascular plants showed a unimodal relationship with altitude. The same was true for graminoids, forbs and lichens analysed separately, but forb richness peaked at Much higher altitudes than total richness. The richness of dwarf-shrubs decreased monotonically with altitude, whereas richness of mosses and liverworts showed an increasing trend. Significant interactions between altitude and local topography were present for several groups. The unimodal pattern for total plant species richness was interpreted in terms of local productivity, physical disturbance, trophic interactions, and in terms of species pool effects. Conclusions: Patterns in local species richness result from the action of two opposing forces: declining species pool and decreasing intensity of competition with altitude.
|
|
| 30. |
- Burman, Erik, et al.
(författare)
-
Primal-Dual Mixed Finite Element Methods for the Elliptic Cauchy Problem
- 2018
-
Ingår i: SIAM Journal on Numerical Analysis. - : SIAM Publications. - 0036-1429 .- 1095-7170. ; 56:6, s. 3480-3509
-
Tidskriftsartikel (refereegranskat)abstract
- consider primal-dual mixed finite element methods for the solution of the elliptic Cauchy problem, or other related data assimilation problems. The method has a local conservation property. We derive a priori error estimates using known conditional stability estimates and determine the minimal amount of weakly consistent stabilization and Tikhonov regularization that yields optimal convergence for smooth exact solutions. The effect of perturbations in data is also accounted for. A reduced version of the method, obtained by choosing a special stabilization of the dual variable, can be viewed as a variant of the least squares mixed finite element method introduced by Darde, Hannukainen, and Hyvonen in [SIAM T. Numer. Anal., 51 (2013), pp. 2123-2148]. The main difference is that our choice of regularization does not depend on auxiliary parameters, the mesh size being the only asymptotic parameter. Finally, we show that the reduced method can be used for defect correction iteration to determine the solution of the full method. The theory is illustrated by some numerical examples.
|
|
| 31. |
- Dahlgren, Jonas, 1969-
(författare)
-
Interactions between gray-sided voles (Clethrionomys rufocanus) and vegetation in the Fennoscandian tundra
- 2006
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- I have, in this thesis, studied the interactions between gray-sided voles (Clethrionomys rufocanus) and tundra vegetation, on islands in, and mainland sites close to the lake Iešjávri, in northern Norway. As isolated islands are virtually free of predation, I have been able to compare plant-herbivore interactions in the presence and absence of predators. I transplanted vegetation from an island with predators and voles, to predator-free islands with and with out voles. The results reveal the existence of a terrestrial trophic cascade as voles had a severe impact on the transplanted vegetation on the predator-free islands, but only minor effects on the mainland where predators are present. Moreover, this study shows that plant defence was only a successful strategy when predators were present. Voles reduced the abundance of all available plants during winter on the predator-free islands. The results imply that cascading effects of predation are most important for well-defended plants with grazing-sensitive morphology as these plants escape herbivore impacts in the presence of predators but are vulnerable in their absenceI studied the recovery of intensively grazed vegetation by building exclosures on islands that have been heavily grazed by voles for almost a decade.This study shows that the collective biomass of vascular plants recovered completely on three years, when voles were excluded. Although most species that are dominating the ungrazed vegetation recovered rapidly in the exclosures, the vegetation did not simply return to its ungrazed state. Herbaceous plants increased and there were pronounced differences in response among evergreen dwarf-shrub species. The semi-prostrate and tannin rich crowberry (Empetrum nigrum), showed the strongest recovery of all species, while the erect lingonberry (Vaccinium vitis-idaea) only showed weak signs of recovery. Thus, growth form determined the potential of plants to recover, whereas the trade-off between defensive investments and capacity to recover was weak or absent.I studied the interaction between gray-sided voles and their main winter food plant, bilberry (Vaccinium myrtillus) on islands in and mainland sites close to the lake Iešjávri. I compared the abundance, population structure and palatability of bilberry ramets between vole-free islands, islands with voles but no predators and mainland sites with both voles and predators. Voles decreased the abundance of bilberry on the mainland, but the effect was much stronger on predator-free islands. Bilberry was fairly tolerant to grazing as it partially compensated for the lost tissue by producing more new ramets. Moreover, a cafeteria experiment showed that voles preferred the ramets from predator-free islands, which is inconsistent with conjectures emphasizing inducible plant defenses. The vole-bilberry interaction lacks features of delayed density dependence that could explain the vole cycles. I conducted a clipping and fertilization experiment to further investigate the effects of herbivory on palatability of bilberry shoots. Fertilization decreased the concentration of condensed tannins in shoots of bilberry and voles preferred fertilized and clipped shoots. I found no indication of induced defense that could reduce the palatability of bilberry twigs in response to herbivory.The relationships between gray-sided vole densities, levels of invertebrate herbivory and chemical quality of leaves of Northern willow (Salix glauca) were studied on islands and mainland sites with contrasting vole densities. I found a positive correlation between level of invertebrate herbivory and vole density. The number of leaves per shoot, leaf size and leaf nitrogen content were also positively correlated with vole densities, while leaf C/N ratios were negatively correlated with vole densities. The positive correlation between vole densities and level of invertebrate herbivory is probably due to a facilitative effect of voles on invertebrate herbivores, mediated through changes in plant chemistry.
|
|
| 32. |
- Dahlgren, Jonas, et al.
(författare)
-
Interactions between gray-sided voles (Clethrionomys rufucanus) and bilberry (Vaccinium myrtillus), their main winter food plant
- 2007
-
Ingår i: Oecologia. - : Springer. - 0029-8549 .- 1432-1939. ; 152:3, s. 525-532
-
Tidskriftsartikel (refereegranskat)abstract
- We compared the abundance, population structure and palatability of bilberry ramets on vole-free islands, islands with voles but no predators (predator-free islands) and mainland sites with both voles and predators. As expected, bilberry biomass was strongly correlated with the herbivory pressure exerted by the voles, since it was significantly lower on the mainland, and much (>80%) lower on the predator-free islands, than on the vole-free islands. However, another finding, which conflicts with hypotheses postulating that herbivory generally induces plant defenses, was that voles preferred ramets from predator-free islands. Bilberry plants were fairly tolerant to grazing since they compensated for some of the lost tissue by producing more new ramets. This response should promote stability in the plant–herbivore interaction by reducing the impact of past grazing on current food production and thus minimizing time delays in the interactions that could potentially generate population cycles.
|
|
| 33. |
|
|
| 34. |
|
|
| 35. |
- Ekerholm, Per, 1959-
(författare)
-
Population dynamics of tundra-living grey-sided voles
- 2003
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with the dynamics of tundra living voles with emphasis on the most common one, the grey-sided vole (Clethrionomys rufocanus). The tundra area chosen for the study was Finnmarksvidda, a vast flatland in northernmost Norway. All small mammal herbivores in the area showed dramatic fluctuations, and field experiment were conducted in order to elucidate these density fluctuations. The specific subjects addressed included: 1/ Temporal and spatial appearance of density fluctuations of voles and lemmings in the area, 2/ The generality of the density patterns observed, 3/ The impact of predation by vole predators during summertime, 4/ The impact of grey-sided vole grazing on food plants of different preference in a predator free environment, in the presence and absence of extra food, and 5/ The impact of food availability on density and demography of grey-sided voles in a predator free environment.The results achieved showed that voles in the slope and lowland had cyclic density fluctuations with 5 years duration. The cycles consisted of four phases: an increase phase, a peak phase, a decline phase and a crash phase. In the unproductive lowland and on the moderately productive slope, small pockets of productive habitats seemed to work as “triggers” for the cycles. The lemming fluctuations in the upper plateau (separated from the slope by a steep zone of boulders) differed markedly from the vole patterns in the lowland.Only two lemming peaks were recorded in twenty years. Both peaks had very short increase phases, a knife-sharppeak phase and no decline phase before the crash. A comparison between our results and lemming and vole populations from two other areas in Fennoscandia revealed the same difference in fluctuation pattern between lemmings and voles as seen in our area. This results suggests that lemmings in barren tundra highlands and voles in slightly more productive tundra lowlands are regulated by different mechanisms.The exclusion of vole predators from vole populations during summertime led to increase in overall vole density. Densities of the clumsy field vole (Microtus agrestis) and juveniles of all species showed the strongest positive effects of the exclusion.An experiment analysing the effects of food availability was conducted in islands in a large lake where grey-sided voles were introduced to predator free islands . Supplemental food was given to the voles in two unproductive, and two productive islands. Two unproductive and two productive islands were used as reference islands. The density of voles and the vole weight were higher in both the islands with supplemental food and those with high natural productivity. Increased vole density did not significantly increase grazing damage to plants. The cyclic density pattern of the voles in the nearby mainland (that harboured resident vole specialist predators as stoat and weasel) showed little resemblance to the seasonal fluctuations found in the islands (devoid of resident vole specialist predators). This result suggested that predation by stoat and weasel on grey-sided vole populations may cause the cyclic vole fluctuations seen in the area.
|
|
| 36. |
- Grellmann, Doris, 1961-
(författare)
-
Top-down and bottom-up effects in a Fennoscandian tundra community
- 2001
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The objective of this thesis was to investigate the effects of mammalian grazers, such as microtine rodents and reindeer, (top-down effects) and nutrient availability (bottom- up effects) on the plant community of a tundra heath.I conducted a large-scale fertilization experiment and studied the impact of grazers using exclosures. I measured the effects of fertilization and grazing on soil microbial activity and nutrient cycling. I investigated the responses to fertilization of the invertebrate community, I studied the effects on the quality of bilberry as food for mammalian herbivores, and I looked at how concentrations of nutrients and carbon-based secondary defences against herbivory fluctuated between seasons in unfertilized and fertilized treatments.The results of my thesis show that the plant community investigated is exposed to a strong top-down control by mammalian herbivores. On the fertilized and grazed areas the aboveground biomass of the vascular plant community did not increase compared to unfertilized areas. However, the productivity of the plant community was clearly nutrient- limited. During the eight years of the experiment, on the fertilized areas plant biomass was significantly increased inside the herbivore exclosuresIn my study mammalian herbivores at comparatively low densities and grazing outside the growing season were sufficient to control the biomass of a heterogeneous plant community. Microtine rodents (Norwegian lemmings and grey-sided voles) preferred the fertilized areas for overwintering. The food plant quality of bilberry for grey-sided voles was improved on the fertilized areas throughout the year. Grazing decreased the nitrogen storage in the aboveground plant biomass. Reindeer and rodents had also important indirect effects on the plant community by decelerating soil nutrient cycling and soil microbial activity. This effect may be accelerated by the impact of herbivore on plant species composition. Graminoids, which contained the highest nitrogen concentrations in their tissues, increased rapidly on the fertilized areas, but their abundance was significantly lower on grazed fertilized areas.The invertebrate community was detritus-based and received their energy indirectly from the litter via soil microbes and detritivores. Fertilization increased the biomass of invertebrate carnivores, but had no effect on the biomass of invertebrate herbivores. Apparent competition between detritivores and invertebrate herbivores, mediated by carnivorous invertebrates predating on both of them, is supposed to keep the densities and grazing pressure of invertebrate herbivores low. Grazing damage by invertebrates was very low and only 0.021 % of the total vascular plant biomass was removed.
|
|
| 37. |
- Kivisaari, Pyry, et al.
(författare)
-
Diffusion-driven current transport to near-surface nanostructures
- 2015
-
Ingår i: 15th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2015. - 9781479983797 ; , s. 117-118
-
Konferensbidrag (refereegranskat)abstract
- Diffusion-driven current transport (DDCT) has recently been proposed as a new way to organize the current injection in nanoscale optoelectronic devices. The very recent first proof-of-principle experiments have also shown that DDCT works as predicted theoretically. In this work we perform simulations on DDCT-based III-Nitride devices and demonstrate how the optimization of DDCT differs significantly from the optimization of conventional double heterostructure based devices.
|
|
| 38. |
- Oksanen, Lauri, et al.
(författare)
-
On the indices of plant–plant competition and their pitfalls
- 2006
-
Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 112:1, s. 149-155
-
Tidskriftsartikel (refereegranskat)abstract
- The index of relative competition intensity (RCI) has serious built-in biases, due to its asymptotic behavior when competition intensity is high and its tendency to obtain very low values when plants with neighbors intact perform better than neighbor removal plants. These biases have been partially corrected in the index of relative neighbor effect (RNE), but the existence of fixed upper and lower bounds (−1≤RNE≤+1) still creates problems and biases in communities where the average intensity of competition or facilitation is high and plant performance pronouncedly varies in space. The third commonly used index, the logarithm of response ratio (lnRR), is mathematically and statistically sound, but when computed from pair-wise comparisons between neighbor removal and control plants, this index reflects the geometric mean of the treatment effect. Moreover, linear patterns in lnRR reflect exponential patterns in the intensity of competition. As the interest of ecologists usually focuses on arithmetic means, we propose a corrected index of relative competition intensity, CRCI=arc sin (RNE). This index is fairly linear within the observed ranges of competition and facilitation, and for the range of competition intensities where RNE behaves reasonably, the two indices obtain almost identical values. We compared the performance of the four indices, using both imagined and real data, the latter from systems where the responses of plants to neighbor removal ranged from weak to moderate, so that RNE and CRCI were expected to behave similarly. The indices were computed both from pooled data for each community and as averages of pair-wise comparisons. lnRR and CRCI were found to behave in a consistent and bias-free manner, yielding similar results regardless of method of computation. This was, by and large, the case with RNE, too, but as the values of indices grew, the values from pair-wise comparisons became increasingly smaller than values computed from pooled data. RCI yielded grossly aberrant results in computations based on pair-wise comparisons. Therefore, the further use of RCI is unadvisable and studies where RCI has been derived from pair-wise comparisons should be excluded from meta-analyses.
|
|
| 39. |
- Oksanen, Lauri, et al.
(författare)
-
Vertebrate Herbivory and Its Ecosystem Consequences
- 2009
-
Ingår i: Encyclopedia of Life Sciences. - Chichester : John Wiley & Sons, Inc.. - 9780470015902
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Herbivory is a process where animals obtain energy and nutrients from vegetative plant parts (leaves, stems, etc.). Herbivory is called grazing, browsing or folivory, depending on the size of the herbivore and the type of plant tissue consumed. The consumption is actually performed by microbes in the digestive system. The role of the animal is to prepare the material and to provide a good environment for the microbes. This requires specializations, making herbivores a distinct animal group. This article focuses on consequences of herbivory by vertebrates. Vertebrate herbivory can change forests to heathlands or grasslands and influence the composition and species richness of the herbaceous vegetation.
|
|
| 40. |
- Olofsson, Johan, et al.
(författare)
-
Effects of herbivory on competition intensity in two arctic-alpine tundra communities with different productivity
- 2002
-
Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 96:2, s. 265-272
-
Tidskriftsartikel (refereegranskat)abstract
- The effects of long-term (I I yr) exclusion of vertebrate herbivores on competition intensity and plant community structure ere studied using manipulative field experiment,; in two arctic-alpine plant communities with contrasting productivity : an unproductive snowbed and a considerably more productive tall herb meadow. In the snowbed. the exclusion of herbivorous mammals resulted in a significant increase in the biomasses of vascular plants and cryptogams, whereas no corresponding response Lis observed on the tall herb meadow. The intensity of competition. measured with a neighbour removal experiment, did not differ significantly between three of the four habitat x treatment combinations - snowbed exposures. meadow exclosures and open meadow plots but as significantly loader on open snowbed plots, Our results thud, suggest that the low competition intensity in the unproductive snow bed is caused by herbivorous mammals, which tend to depress plant biomass in relatively unproductive habitats When herbivorous mammals have been excluded for a sufficiently long time to allots the build-up of plant biomass even in Unproductive habitat,. between-habitat differences in competition intensity disappear.
|
|
| 41. |
- Olofsson, Johan, et al.
(författare)
-
Effects of summer grazing by reindeer on composition of vegetation, productivity and nitrogen cycling
- 2001
-
Ingår i: Ecography. - : Wiley. - 0906-7590 .- 1600-0587. ; 24:1, s. 13-24
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, we investigated the effect of reindeer grazing on tundra heath vegetation in northern Norway. Fences. erected 30 yr ago, allowed us to compare winter grazed, lightly summer grazed and heavily summer grazed vegetation at four different sites. At two sites, graminoids dominated the heavily grazed zone completely, while ericoid dwarf shrubs had almost disappeared. In the other two areas, the increase of graminoids was almost significant. At one of the sites where graminoids dominated the heavily grazed area. we also measured plant biomass, primary production and nitrogen cycling. In this site: heavy grazing increased primary production and rate of nitrogen cycling, while moderate grazing decreased primary production. These results were inconsistent with the view that the highest productivity is found at intermediate grazing pressure. These results rather support the hypothesis that intensive grazing can promote a transition of moss-rich heath tundra into productive, graminoid-dominated steppe-like tundra vegetation. More over the results suggests that intermittent intensive reindeer grazing can enhance productivity of summer ranges.
|
|
| 42. |
- Olofsson, Johan, et al.
(författare)
-
On the balance between positive and negative plant interactions in harsh environments
- 1999
-
Ingår i: Oikos. - 0030-1299 .- 1600-0706. ; 86:3, s. 539-543
-
Tidskriftsartikel (refereegranskat)abstract
- Positive interactions between plants typically occur where the presence of a species ameliorates the abiotic environment for another. However, there is also a potential for resource competition to act at the same time, which creates a situation where the net outcome is a balance between positive and negative interactions. We present data from a nine-year study in two extreme high alpine habitats that was designed to test whether the effects of established Ranunculus glacialis individuals on germination and growth of Oxyria, digyna are primarily positive or negative at the altitudinal limit of vascular plants. We show net effects ranging from neutral to negative, but no positive effects were detected. We also argue that close associations between plants in these harsh environments may both ameliorate and deteriorate the abiotic environment, and that experimental manipulations are necessary to tell the difference.
|
|
| 43. |
- Olofsson, Johan, et al.
(författare)
-
Role of litter decomposition for the increased primary production in areas heavily grazed by reindeer : a litterbag experiment
- 2002
-
Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 96:3, s. 507-515
-
Tidskriftsartikel (refereegranskat)abstract
- Heavy grazing and trampling by reindeer increase nutrient cycling and primary production in areas where grasslands hake replaced shrub and moss tundra. One way in which herbivores can affect nutrient cycling is through changing the litter decomposition processes. We studied the effect of herbivory on litter decomposition rate by reciprocal transplantation of litter between lightly grazed and heavily grazed areas. using the litterbag technique, We used litter from two of the most common species on the lightly grazed side, Betula nana and Empetrum nigrum, and two of the most common species on the heavily grazed side. Carex bigelowii and Deschampsia flexuosa, We found that herbivory improved litter quality by favouring species with easily decomposed litter. However. herbivore also improved litter quality by increasing the nitrogen content and lowering the C/N ratio of each species. Decomposition rates even correlated with the abundance of the plant category in question, Shrub litter decomposed faster in the lightly grazed area where shrubs were common, and graminoid litter decomposed faster in the heavily grazed area where graminoids were common. These results indicate that the decomposer micro-organisms are adapted to the most common litter types. This studs shoals that detailed information about the effect of herbivore on litter quality is important to understand differences between the short-term and long-term effects of herbivory on nutrient cycling and primary production.
|
|
| 44. |
- Sammul, Marek, et al.
(författare)
-
Regional effects on competition-productivity relationship: a set of field experiments in two distant regions
- 2006
-
Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 112:1, s. 138-148
-
Tidskriftsartikel (refereegranskat)abstract
- We studied the effect of productivity on competition intensity and the relationship between competition intensity and community species richness, using a removal experiment with the perennial plant Solidago virgaurea. The experiment was conducted in 16 different communities from two geographically distant areas (western Estonia and northern Norway). The results were compared with the results of previous experiments with Anthoxanthum odoratum from the same areas. Removal of neighbors had a positive effect on the biomass of both Solidago and Anthoxanthum, and this response was stronger in communities with higher productivity. Thus, the corrected index of relative competition intensity, CRCI, increased with increasing community productivity. Species richness was negatively correlated with CRCI in Estonia but not in Norway and not in the case of the pooled material. The results suggest that competitive exclusion operates at least in these communities which species pool is large. Our results indicate that the relationship between competition intensity and productivity is non-linear. In our data, competition prevails in communities where living plant biomass exceeds 200 g m−2, whereas in less productive communities, competition remains undetected and direct plant–plant relationships might at times be even mutualistic. Moreover, we found that the relationship between competition intensity and productivity is strongly dependent on regional differences and is intimately connected to a concordant variation in the intensity of grazing. The least productive communities both in Estonia and in Norway are characterized by intensive grazing, which reduces importance of competition. Hence, the contrasting results corroborates the predictions of the hypothesis of exploitation ecosystems, predicting that trophic dynamics account for the relationship between competition intensity and primary productivity.
|
|
| 45. |
- Ylänne, Henni, et al.
(författare)
-
Consequences of grazer-induced vegetation transitions on ecosystem carbon storage in the tundra
- 2018
-
Ingår i: Functional Ecology. - : John Wiley & Sons. - 0269-8463 .- 1365-2435. ; 32:4, s. 1091-1102
-
Tidskriftsartikel (refereegranskat)abstract
- 1. Large herbivores can control plant community composition and, under certain conditions, even induce vegetation shifts to alternative ecosystem states. As different plant assemblages maintain contrasting carbon (C) cycling patterns, herbivores have the potential to alter C sequestration at regional scales. Their influence is of particular interest in the Arctic tundra, where a large share of the world's soil C reservoir is stored.2. We assessed the influence of grazing mammals on tundra vegetation and C stocks by resampling two sites located along pasture rotation fences in northern Norway. These fences have separated lightly grazed areas from heavily grazed areas (in close proximity to the fences) and moderately grazed areas (further away from the fences) for the past 50years. Fourteen years earlier, the lightly and moderately grazed areas were dominated by dwarf shrubs, whereas heavy grazing had promoted the establishment of graminoid-dominated vegetation. Since then, both reindeer densities and temperatures have increased, and more time has passed for transient dynamics to be expressed. We expected that the vegetation and C stocks would have changed under all grazing intensities, but not necessarily in the same way.3. At the site where relative reindeer numbers and trampling intensity had increased the most, graminoid-dominated vegetation was now also found in the moderately grazed area. At the other site, the dominant vegetation types under all grazing intensities were the same as 14 years earlier.4. We show that the heavily grazed, graminoid-dominated areas stored less C above-ground than the lightly grazed, shrub-dominated areas. Yet, the below-ground consequences of grazing-induced grassification varied between the sites: Grazing did not alter organic soil C stocks at the site where both evergreen and deciduous shrubs were abundant in the lightly grazed area, whereas heavy grazing increased organic soil C stocks at the site where the deciduous shrub Betula nana was dominant.5. Our results indicate that, despite the negative impacts of grazers on above-ground C storage, their impact on below-ground C may even be positive. We suggest that the site-specific responses of organic soil C stocks to grazing could be explained by the differences in vegetation under light grazing. This would imply that the replacement of deciduous shrubs by graminoids, as a consequence of grazing could be beneficial for C sequestration in tundra soils.
|
|
