| 1. |
- 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.
|
|
| 2. |
- 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.
|
|
| 3. |
- 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.
|
|
| 4. |
- 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.
|
|
| 5. |
- 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.
|
|
| 6. |
- 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.
|
|
| 7. |
- 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.
|
|
| 8. |
|
|
