Download Effects of herbivory on arctic and alpine vegetation Stockholm University

Effects of herbivory on arctic and
alpine vegetation
Åsa Lindgren
Stockholm University
©Åsa Lindgren, Stockholm 2007
Cover illustration: a curious reindeer in Svalbard
Photo: Åsa Lindgren
ISSN 91-7155-396-7 pp. 1-38
Printed in Sweden by Universitetsservice, US-AB, Stockholm 2007
Distributor: Department of Botany, Stockholm University
2
Till barnen
3
4
Doctoral dissertation
Åsa Lindgren
Department of Botany
Stockholm University
SE-10691 Stockholm
Sweden
Effects of herbivory on arctic and alpine vegetation
Abstract - The distribution of plant species and functional traits in
alpine and arctic environments are determined by abiotic conditions,
but also by biotic interactions. In this thesis, I investigate interactions
among plants and herbivory effects on plant community composition
and plant functional traits in three different regions: Swedish Lapland,
Beringia (USA/Russia) and Finnmark (Norway). Reindeer grazing
was found to be extensive in southern Lapland and had limited effects
on plant community composition and seedling germination. However,
reindeer presence was found to influence plant functional traits, particularly in the subalpine birch forest. Tall herbs were lower and had
lower SLA when reindeer were present, while small herbs showed an
opposite pattern. The contrasting effects on the two herb groups are
probably explained by a competitive release for small herbs when the
tall herbs are suppressed by reindeer. Rodents had the largest relative
impact on plant community composition in southern Lapland and this
is consistent with the study from Finnmark, where rodents heavily
affected dwarf shrubs on predator-free islands. With no predators present, vole densities increased profoundly and almost depleted some
dwarf shrub species. These results support the idea that small mammals in arctic and alpine tundra are controlled by predators (i.e. topdown). However, a decrease in the nutritional quality in a sedge after
defoliation gives support for the idea that small mammals are regulated by plant quality (i.e. bottom-up). In Beringia, small and large
herbivores differed in the relation to plant community composition,
since large herbivores were related to species richness and small herbivores were related to plant abundance. Plant functional traits were
related only to large herbivores and standing crop of vascular plants.
Keywords herbivory, reindeer, rodents, functional traits, plant species
composition, arctic, alpine, tundra, seed limitation, Carex bigelowii
5
6
List of papers
The thesis is based on five papers, which are referred to in the text by
their Roman numerals:
I
Lindgren, Å., O. Eriksson and J. Moen. The impact of
disturbance and seed availability on germination in alpine vegetation in the Scandinavian mountains.
Arctic, Antarctic and Alpine Vegetation. In press.
II
Lindgren, Å., J. Moen and O. Eriksson. The effect of
different herbivore groups on the vegetation in subalpine
birch forests and alpine heaths. Manuscript.
III
Lindgren, Å., J. Ehrlen, R. Bergström, K. Danell, G.
Ericsson and C. Skarpe. Herbivory and plant biodiversity
in an arctic environment. Manuscript.
IV
Lindgren, Å., J. Klint and J. Moen. Defense mechanisms
against grazing: a study of trypsin inhibitor responses to
simulated grazing by the sedge Carex bigelowii.
Accepted in Oikos.
V
Hambäck, P.A., L. Oksanen, P. Ekerholm, Å. Lindgren,
T. Oksanen and M. Schneider. 2004. Predators indirectly
protect tundra plants by reducing herbivore abundance.
Oikos 106: 85-92.
My contributions to the papers were: Planning, field work, data
analysis and writing in papers I-IV. For paper V, I was involved in the
data analysis, the writing of the paper and the fieldwork.
Previously printed and accepted papers are printed in this thesis with
the kind permission from the copyright holder.
7
Contents
Introduction...................................................................................................................... 9
Biotic interactions within a framework of alpine and arctic abiotic conditions ........... 9
Aim of the thesis....................................................................................................... 12
Methods......................................................................................................................... 13
Results and discussion.................................................................................................. 18
Concluding remarks ...................................................................................................... 22
Acknowledgement ......................................................................................................... 23
References .................................................................................................................... 23
Svensk sammanfattning ................................................................................................ 32
Tack! .............................................................................................................................. 36
Fieldwork recommendations ......................................................................................... 38
8
Introduction
Biotic interactions within a framework of alpine and
arctic abiotic conditions
Common characteristics for arctic and alpine habitats in northern regions are low temperatures, short growing seasons, and a relatively
low productivity (Grime 1977). Although the abiotic conditions are
very crucial, they are not necessarily overwhelming the importance of
biotic interactions. Biotic interactions include plant – plant interactions, such as competition and facilitation, and interactions between
herbivores and plants. The biotic interactions could either work directly or through abiotic factors (Mulder 1999), altering the physical
conditions of the environment (Hyvärinen et al. 2002). Interactions
among plants have been suggested to be dominated by facilitation
rather than competition in harsh environments (Callaway et al. 2002)
because of the amelioration of extreme conditions (Rixen and Mulder
2005). For example, Körner (1999) showed that cushion plants could
have a micro-climate profoundly differing from outside temperature,
wind velocity and relative humidity. However, Olofsson et al. (1999)
suggested that positive and negative interactions could work simultaneously even in harsh environments, and that experimental studies are
needed to assess the net effect of interactions.
Herbivory is generally negative for a grazed plant, but could be positive for plant species richness and plant community processes. Herbivory might affect plant interactions, e.g. prevent competitive exclusion or influence abiotic factors, e.g. increased soil temperatures due
to the reduction of the isolating moss layer (Brooker and van der Wal
2003; Olofsson et al. 2004a; van der Wal and Brooker 2004) and
changed nutrient cycling (Olofsson and Oksanen 2002; Stark et al.
2002). The net effect of herbivory on plant species richness and traits
9
depends on a combination of factors, e.g. the composition of the vegetation and the growth forms of plants (Chapin 1980; van der Wal et al.
2001) and the productivity (Moen and Oksanen 1998; Proulx and Mazumder 1998; Bardgett and Wardle 2003). Other factors of importance
are the grazing history of the area and the types of herbivores involved
(Virtanen et al. 1997; 2002; Hartley and Jones 2003; McIntire and Hik
2002; 2005; Olofsson et al. 2004b) and the grazing intensity (Olofsson
et al. 2001). For instance, reindeer have been found to cause vegetation shifts between different vegetation states in some studies (e.g.
from mosses to graminoids) (van der Wal and Brooker 2004; van der
Wal 2006; Eskelinen and Oksanen 2006). High grazing intensities is
required to change the vegetation from one state to another and this
change is not always reversible. Thus, abandoned summer grazing
areas do not always return to an un-grazed vegetation state, at least not
during a short time-scale (Olofsson 2006).
Herbivores moving over large distances can play an important role as
seed dispersers and thus, enhance the colonization of vascular plants,
but there is also indications of that arctic seed banks can be depleted
through intense grazing (Kuijper et al. 2006). Trampling and digging
by herbivores at intermediate intensities might also enhance colonisation and establishment of vascular plants by creating suitable microsites in closed vegetation, by trampling and digging (Cairns and
Moen 2004). Thus, herbivore activity can lead to increased vascular
plant species diversity in arctic and alpine areas, since many vascular
plants are limited by both seed and microsite availability (Eriksson
and Ehrlén 1992; Jakobsson and Eriksson 2000; Eskelinen and Virtanen 2005; Gough 2006).
The primary production is of importance determining the net effects of
interactions and processes in alpine and arctic tundra. In more productive environments, there is generally a hump-shaped relationship between productivity and species richness (Grime 1979; Grace 1999),
while this pattern is not always the case in alpine and arctic tundra
(Fox 1985; Waide et al. 1999; Grytnes and Birks 2003). Productivity
is not easily measured and a common method to estimate productivity
is by sampling biomass or standing crop (see e.g. Krebs et al. 2003).
However, biomass or standing crop is not always easily interpreted
into productivity, particularly not in habitats with only plants and herbivores (Oksanen 1983; Oksanen et al. 1992; Oksanen and Oksanen
2000). In systems, like tundra heaths, with no, or only transient preda10
tors, herbivores are assumed to be resource-limited. Thus, plants are
kept below their carrying capacity and standing crop is not correlated
to productivity or to herbivore numbers (Rosenzweig 1971, but see
van de Koppel et al. 1996; Wegener and Odasz-Albrigtsen 1998).
However, biomass or standing crop might still be a qualitative estimate of the competitive situation experienced by the plants.
The composition of plant functional traits may determine the competitive ability of a plant individual, the ability to grow in extreme conditions and the ability to cope with herbivory. High plant height can be
an important competitive character, but prostrate growth can be an
advantage both to avoid herbivory and to adapt to a harsh climate
(Diaz et al. 2001; 2007; Oksanen 1990; Körner 1999). Nutrient availability will be of great importance for the functional traits developed
by a plant since there is a positive correlation between productivity,
specific leaf area, leaf palatability and digestibility (Williams and
Rastetter 1999; Lavorel and Garnier 2002; Cornelissen et al. 2006).
Plants growing in low productive areas are presumed to have low N
contents due to a limited nutrient availability, while carbon is abundant, resulting in low palatability and digestibility. Nitrogen-based
defence substances, working more specifically (e.g. trypsin inhibitors;
Seldal et al. 1994), are suggested to be more common in productive
habitats (Mattson 1980; Bryant et al. 1983; 1989). There are studies
indicating that grazing leads to increased levels of nitrogen (Beaulieu
et al. 1996; Olofsson et al. 2004a) in arctic and alpine environments.
Increased levels of nitrogen imply an improved litter quality (Olofsson
and Oksanen 2002), further enhancing the rate of nutrient cycling as a
result of herbivory.
In this thesis, three different regions are investigated and there are
regional differences in the compositions of plant communities. The
arctic tundra around Beringia is characterized by more herb species
than the tundra in Lapland and Finnmark in the Fennoscandian mountain region. During the last Pleistocene, Beringia was largely unglaciated (Brubaker et al. 2005; Gualtieri et al. 2005), while other parts of
Arctic were covered by ice, and those differences have had a major
impact on the recent evolutionary history of Arctic species (Flagstad
and Roed 2003). The herbivorous fauna (both present and historical;
Zimov et al. 1995) differ between the regions with more species both
in terms of large and small herbivores in Beringia.
11
Aim of the thesis
In this thesis, I investigated effects of herbivory on arctic and alpine
tundra vegetation and interactions among plants, with a focus on effects on plant species richness, abundance, biomass production and
plant functional traits related to competition and herbivory. More specifically, I addressed the following questions:
•
Are vascular plant distributions in alpine and subalpine environments limited by seed availability and/or suitable microsites, and could the presence of reindeer influence germination rates by creating microsites? (paper I)
•
How do reindeer, rodents and insects affect plant species
composition and biomass production in alpine heaths and
subalpine birch forests? Are vascular plant functional traits in
the heath and in the birch forest related to reindeer grazing?
(paper II)
•
Are densities of small and large herbivores and vascular plant
standing crop in Beringia correlated to plant species richness,
abundance and functional traits? (paper III)
•
Do food quality and trypsin inhibitors in the common sedge
Carex bigelowii vary with different grazing intensities, and do
the response to defoliation vary over time and with productivity of the habitat? (paper IV)
•
Do predators regulate the population dynamics of rodents, and
thus the standing crop of plants via a trophic cascade?
(paper V)
12
Methods
The field work of this thesis was performed in three different study
areas shown in Figure 1 and 2: Ammarnäs in southern Lapland (Sweden), Joatkanjavrit research area in Finnmark (Norway) and the regions around Bering Strait (Russia/USA).
Ammarnäs - The study sites in
paper I, II and IV were situated in
the vicinity of Ammarnäs, Southern Lappland, Sweden. The sites
in the alpine heath were located
above the tree line at an altitude
of 800 – 1100 m.a.s.l., and the
sites in the subalpine birch forest
were located at an altitude of
500-650 m.a.s.l. The heath sites
were dominated by mosses, lichens, dwarf shrubs, grasses and
sedges. The sites in the birch forest were situated in humid slopes
where tall herbs dominated.
Mosses, liverworts, Vaccinum
myrtillus, graminoids and small
herbs were also significant constituents of the field layer. In one
birch forest site, the vegetation
consisted of patches of more dry
heath-like birch forest with
grasses, herbs and mosses. All
sites were inhabited by free ranging reindeer (Rangifer tarandus)
during the summer season.
Finnmark - The study area in paper V was situated in the Joatkanjávrit research area on Finnmarksvidda in northernmost Norway.
Three islands in the large tundra lake Iesjaure were used as predatorfree areas and two sites at mainland approximately 10 km from the
lake were used as reference areas. The vegetation consisted of produc-
13
tive, low arctic scrublands or barren lowland tundra heaths with
patches of scrubland.
Beringia - The fieldwork in paper III was conducted during a polar
expedition to Beringia in 2005. The areas visited were Kamtjatka,
Chukotka and Wrangel Island in Russia and Alaska in the United
States. The 9 sites were of a drier type of tundra on alpine slopes with
a vegetation cover consisting of “Dwarf Shrub Types” described by
Hansson (1953). Herbivore composition differed between the sites and
was divided into small and large herbivores.
Chukotka sites
Alaska sites
Kamchatka sites
Wrangel Island site
Figure 2. Nine sites were investigated in the Beringia region:
Esso, Karaginski and Mysnotski in Kamchatka, Penkigney Bay, Yan
Rakinot and Lavrentiya in Chukotka, Wrangel Island and Salmon
lake and Tisuk river in Alaska (map from www.polar.se).
14
Paper I: impact of seed limitation and seed availability on germination
Six species were selected based on their different functional traits, e.g.
life span, seed size, dispersal mode, and occurrence. Inside reindeer
exclosures (1000m2) and in adjacent control areas, a factorial experiment with four different treatments was performed: seeds added/not
added combined with plots disturbed/not disturbed. The disturbance
treatment consisted of the removal of all aboveground biomass. Forty
20 × 20 cm-plots (ten replicates of each treatment) were used in each
reindeer exclosure and control area with two replicates in the alpine
heath and two replicates in the subalpine birch forest. Seedlings
emerging in the plots were recorded at the end of July for three years
(2000 – 2002).
Paper II: effects of different herbivore groups (reindeer, rodents and
insects)
A split-plot design was used to investigate the effects of different herbivore groups (reindeer, rodents and insects) on the vegetation. The
experiment was based on reindeer exclosures (1000 m2), rodent exclosures (16 m2) and insecticide treatments (16 m2) with three replicates
in the alpine heath and three replicates in the subalpine birch forest.
Estimations of reindeer densities were done by sampling all reindeer
faeces along a transect (100 m × 1 m) in each control area in both
habitats. Rodent densities were estimated by the sampling of rodent
faeces from ten 1 m2 plots in reindeer exclosure and control area at
each site. Additionally, rodents were also monitored by trap lines with
snap traps twice a year along a transect situated approximately in the
middle of the study area. Insect densities were recorded by traps consisting of four pit-fall traps in each treatment. All insects were examined but only herbivorous insects were used in the analyses. Plant species abundance was sampled using a frame (50 × 50 cm) divided into
smaller squares (5 × 5 cm in the alpine heath and 10 × 10 cm in the
birch forest), where all species of vascular plants were recorded.
Mosses, liverworts and lichens were determined to species level if
possible, otherwise to genus. During 1999-2002, the vegetation
change in biomass due to reindeer presence was monitored each year
by biomass samples from 0.25 m2. In 2003, biomass samples were
taken from 4 dm2 within each of the different treatments. All aboveground biomass was sampled, sorted into species or genus, dried and
weighed. The amount of litter was measured by 5 samples of 1 dm2 of
15
litter from each treatment that were collected in the end of August
2003, dried and weighed. Vegetation height was measured by 9 points
in each treatment, where the highest photosynthetic plant part was
measured in the field layer (i.e. not trees). Plant functional traits were
sampled from ten individuals of each species within reindeer exclosures and in control areas in both habitats using a standardized method
by Cornelissen et al. (2003). The sampled traits were plant height,
specific leaf area (SLA), and the amount of nitrogen in the plant.
Paper III: effects of herbivores and standing crop on plant species and
trait composition
The data on herbivore activities at each site was collected from triangle transects (Lindén and Helle 1996), where each triangle leg was
1000 meters (with a few exceptions). When walking along the transect, the vegetation type and every contact with mammals, some birds
and visible insect attacks were noted. A contact could be trampling,
faeces, carcass remains, nests, grazed plants and visible animals or
warning calls. The signs of herbivore activities were divided into two
classes related to small and large herbivores, respectively. The group
of large herbivores included muskoxen, reindeer, moose and bears,
and the group of small herbivores included pikas, ground squirrels,
lemmings, voles, ptarmigan, grouse, geese, and caterpillars. Standing
crop of vascular plants was sampled in five 10 × 10 cm-square per
site, dried and weighed. Plant species diversity and abundance were
sampled using five frames (50 × 50 cm) per site. Each frame consisted
of twenty five 10 × 10 cm – squares, and all plants present in the small
squares were recorded. Vascular plants were examined to the species
level, in a few cases only to family, while graminoids, mosses and
lichens were generally grouped into genera or family. The abundance
of a plant group, e.g. herbs, was then calculated as the sum of all the
abundance of all occurring herb species within the frame. Plant functional traits were measured for ten samples of all herb species present
in the frames. We investigated six traits; maximum photosynthetic
tissue height, mean photosynthetic tissue height, height of flowers,
specific leaf area (SLA; leaf area per unit leaf mass), C/N-ratio and
individual mass, using methods developed by Cornelissen et al.
(2003).
Paper IV: nutritional quality of a common sedge after defoliation
The nutritional quality of a common sedge Carex bigelowii was investigated in three habitats with differing productivity in the alpine heath.
16
Different intensities of grazing were simulated by clipping at three
different levels in the end of July 2003. In order to study the kinetics
of the inhibitor response, ramets were harvested at three different
times after the clipping experiment. The response to simulated grazing
by both vegetative and reproducing ramets of C. bigelowii was measured by the activity of trypsin inhibitors (TIA), the amount of total
soluble protein in the ramet (SPP), and the ratio TIA/SPP.
Paper V: predators regulate herbivore densities and thus, indirectly
protect plants
Grey-sided voles were introduced to predator-free islands in a large
tundra lake at densities representing the densities on mainland areas
(35 voles per ha) in 1991. Thereafter, vole densities were monitored
both in mainland areas and in the islands by live-trapping two times
each year, early July and late August. Plant cover was estimated in
open plots and in vole exclosures (approximately 1 m2) using a modified intercept method. Shoot mortality due to herbivory was assessed
by marking of bilberry shoots. 240 shoots were marked in 12 plots in
each site and checked every spring and autumn.
17
Results and discussion
Vascular plants in alpine and subalpine vegetation seemed not only to
be limited by abiotic factors, but also by seed availability (paper I). To
some extent, plants were also favoured by removal of aboveground
biomass in both habitats, while the effect of reindeer presence was
negligible except for negative effects on germination of Trollius europeus in the birch forest. All of the investigated herbivore groups
(reindeer, rodents and insects) had some effects on the vegetation in
the exclosure experiment (paper II), and if evaluating the relative importance, rodents have the largest impact on the vegetation in this
study. In the subalpine birch forest, the biomass of herb species was
higher inside rodent exclosures, while in the alpine heath, biomass of
dwarf shrub species was favoured by the exclusion of rodents. In a
similar exclusion study of reindeer and rodents, Olofsson et al.
(2004b) found the largest relative effects on the abundance of some
common plant species, both in the forest and in the open heath. Large
local effects of rodent herbivory was also found in the predator-free
islands in Iesjaure, where grey-sided voles had strong negative effects
on some plants, especially dwarf shrubs (paper V). In contrast, small
herbivores in Beringia were positively related to the abundance of
dwarf shrubs in alpine tundra (paper III). This positive relationship
could be due to a negative relationship between the density of small
herbivores and the abundances of graminoids and mosses. A decrease
in mosses might benefit vascular plants (Virtanen et al. 1997; van der
Wal and Brooker 2004). The relationship between the density of small
herbivore activities and different plant groups could also be explained
by small-scale habitat selection by the rodents, i.e. the rodents in this
study avoid areas rich in graminoids and mosses, while preferring
dwarf shrub heaths. The different effects on the vegetation by rodents
among the regions, i.e. Beringia and Fennoscandia and between habitats in Ammarnäs, might be explained by a different rodent (and plant)
species composition and thus, different food preferences. The most
common rodent species in the study areas are listed in Table 1 and
there are differences in food preferences, resulting in different effects
on the vegetation.
Reindeer had limited effects on plant community composition in the
exclosure experiment, but had significant effects on plant functional
18
19
field vole
root vole
wood lemming
Microtus agrestis
Microtus oeconomus
Myopus schisticolor
skogslämmel
mellansork
åkersork
brun lämmel
brown lemming
Lemmus trimucronatus
mosses, graminoids
grasses, herbs (dwarf shrubs)
grasses, herbs, dwarf shrubs
graminoids, mosses
mosses, graminoids
fjällämmel
Norwegian lemming
Lemmus lemmus
herbs, dwarf shrubs
herbs, seeds, insects
Food preferences
dwarf shrubs, (graminoids)
collared lemming
Dicrostonyx groenlandicus
gråsiding
skogssork
Swedish name
halsbandslämmel
grey-sided vole
bank vole
Chlethrionomys glareolus
Chlethrionomys rufocanus
Common name
Species
Ammarnäs
Beringia
Ammarnäs
Beringia
Ammarnäs
Beringia
Ammarnäs, Finnmark
Ammarnäs
Presence in the
study regions
Table 1. The vole and lemming species found in the three study areas and their primary food preferences
(Watson 1956; Siivonen 1976; Angerbjörn et al. 2005)
traits (paper II). In order to investigate more detailed effects of reindeer (in paper II) and of small and large herbivores (in paper III) on
the vegetation, some selected plant functional traits were examined:
plant height, specific leaf area (SLA) and nitrogen content. The data
on plant functional traits are used in two ways in paper II. Firstly, a
site specific estimation of the value of a special trait was obtained using the species abundances of vascular plants in each frame as weights
and then calculating a weighted average for each of the three traits.
The results indicated that plant nitrogen content was higher where
reindeer were present in the alpine heath (paper II), which is in concordance with other studies indicating that grazing leads to increased
levels of nitrogen (Beaulieu et al. 1996; Olofsson et al. 2004a). This
site specific approach was also used in Beringia (paper III) and similarly, there was a negative relation between the density of large herbivores and the C/N-ratio, indicating higher nitrogen concentrations at
high densities of large herbivores. The density of large herbivores in
Beringia was also negatively related to mean green plant height,
flower height and SLA. The SLA is related to the re-growth capacity
of the plant, leaf palatability and digestibility (Diaz et al. 2001; Williams and Rastetter 1999; Lavorel and Garnier 2002; Hoffmann et al.
2005; Cornelissen et al. 2006). Thus, low plant heights and low SLA
might be advantageous in order to avoid defoliation. Secondly, the
data in paper II was used to assess intraspecific variations in herbs due
to reindeer presence. We sampled ten individuals of all herbs present
both inside and outside reindeer exclosures. Although other studies
have shown that at least some of the traits are relatively consistent
within species, both temporarily and spatially (Garnier et al. 2001,
Tolvanen et al. 2004), we found intraspecific variation in plant height
and specific leaf area. Tall, dominating herbs in the subalpine birch
forest were taller and had higher SLA inside reindeer exclosures.
However, the pattern was the opposite for low herbs, which could be
explained by a competitive release when the dominant tall herbs were
suppressed by reindeer.
Rodents are shown to have relatively large impacts on plant community composition in this thesis (paper II, III and V). Simultaneously,
they constitute a crucial food resource for many predators (Ims and
Fuglei 2005). Thus, their population dynamics have been in focus and
several studies have tried to evaluate both causes and consequences of
the dynamics of small mammals in arctic and alpine ecosystems (e.g.
Elton 1924; Boonstra et al. 1998; Turchin et al. 2000; Butet and Spitz
20
2001; Kent et al. 2005). Some studies suggest that the population dynamics in small mammals are structured by the nutritional quality of
their food resources (e.g. Seldal et al. 1994; Jensen and Doncaster
1999; Bråthen et al. 2004, but see Oksanen et al. 1987; Klemola et al.
2000). Other studies suggest that small mammals in arctic and alpine
tundra are regulated by predators (Korpimäki and Norrdahl 1998;
Ekerholm et al. 2004; Lima et al. 2006).
In contrast to studies by e.g. Seldal et al. (1994) and Bråthen et al.
(2004), an investigation of trypsin inhibitors in the common sedge
Carex bigelowii did not support the idea of an induced defense in response of defoliation (paper IV). However, we did find a significant
decrease of soluble plant protein (SPP) with an increased intensity of
defoliation. This may have nutritional consequences for herbivores
and thus might be of importance for an assumed bottom-up-regulation
of rodents in this ecosystem. We also found support for a top-downregulation of voles in the study of predator-free islands in Finnmarksvidda (paper V). On islands without predators, the vegetation (dwarf
shrubs in particular) was heavily grazed due to high vole densities,
compared to mainland reference areas. The question whether the
world is green (see Hairston et al. 1960) because of low nutritional
quality of plants or because of predators reducing herbivore numbers
may still need further investigations, but my suggestion is that a combination of factors is needed for a full explanation.
21
Concluding remarks
It is of great importance to understand the processes going on in an
ecosystem, when constructing management plans for a sustainable use
of natural resources and when making plans for conserving biological
diversity. The management of arctic and alpine areas might be a complicated assignment, since there are two strong parties with differing
perspectives. On one side, there are opinions favouring the idea of
“the last wilderness area”, primarily by people not living there. On the
other side, there are opinions that arctic and alpine areas are cultural
landscapes managed, for example by reindeer herding. In addition,
there is a growing interest among tourists for the mountain areas,
which may lead to increased damage of the vegetation along hiking
paths during summer and increased snow mobile transports during
winter, which could cause disturbances for both reindeer and other
wildlife. In this context, all information we could get about the processes and mechanisms working in the alpine and arctic tundra, is of
great importance.
Small mammals are keystone species in the arctic and alpine tundra,
and thus, knowledge gained about causes and consequences of their
population dynamics is of importance. Small mammals are a crucial
food resource for most predators in the arctic and alpine tundra, and
some predator species are not even breeding during years with low
rodent densities (e.g. polar fox and snowy owl). Small mammals may
have a large impact on the vegetation, at least on a local scale, and the
vegetation may also have an impact on small mammals through
changes in nutritional quality or quantity. There have been discussions
about whether the tundra ecosystem is driven by bottom-up processes
or top-down processes. If the ecosystem is driven by bottom-up processes, the herbivores are regulated by their food resources, which
mean that the population densities of herbivores decrease when available food plants or their nutritional quality decrease. If the ecosystem
is driven by top-down processes, the predators play a crucial role in
suppressing the densities of herbivores and changes in the predator
fauna will indirectly have consequences for the vegetation through so
called trophic cascades. We still lack information for fully understanding the mechanisms driving the population dynamics of as well plants
as of herbivores and predators in arctic and alpine regions.
22
To summarize, the results suggest that small mammals are regulated
by a combination of predators and plant nutritional quality (paper III
and V). Small herbivores have relatively large impact on plant community composition, measured by biomass (paper II) and by plant
group abundance (paper III). Reindeer do not have any profound effect on the species composition but do affect plant functional traits
significantly (paper II). Large herbivores, however, are negatively
related to dwarf shrub species richness in Beringia, while small herbivores are positively related to dwarf shrub abundance, further pointing
out that the type of herbivore investigated is of great importance for
the vegetation responses (paper III). An additional conclusion from
this thesis is that plant species distributions in alpine tundra and
subalpine birch forests are not only regulated by abiotic factors but
also by a lack of suitable microsites and seed limitation.
Acknowledgement
I am grateful to Ove Eriksson, Jon Moen and Regina Lindborg for
valuable comments on this manuscript.
References
Angerbjörn, A., Dalén, L., Dalerum, F., Henttonen, H., Fedorov, V. B.
and Abramson, N. 2005. Evolutionary consequences of the Pleistocene glacial cycles. In: Rickberg, S. (ed.) Yearbook 2005, Swedish
Polar Research Secretariat.
Bardgett, R. D. and D. A. Wardle. 2003. Herbivore-mediated linkages
between aboveground and belowground communities. Ecology 84:
2258-2268.
Beaulieu, J., Gauthier, G., and Rochefort, L. 1996. The growth response of graminoid plants to goose grazing in a High Arctic environment. Journal of Ecology 84: 905- 914.
Boonstra, R., Krebs, C. J. and Stenseth, N. C. 1998. Population cycles
in small mammals: The problem of explaining the low phase.
Ecology 79: 1479-1488.
23
Brooker, R. and van der Wal, R. 2003. Can soil temperature direct the
composition of high arctic plant communities? Journal of Vegetation
Science 14: 535-542.
Brubaker, L. B., Anderson, P. M., Edwards, M. E. and Lozhkin, A. V.
2005. Beringia as a glacial refugium for boreal trees and shrubs: new
perspectives from mapped pollen data. Journal of Biogeography 32:
833-848.
Bryant, J. P., Chapin III, F. S. and Klein, D. 1983. Carbon/nutrient
balance of boreal plants in relation to vertebrate herbivory. Oikos 40:
357-368.
Bryant, J. P., Kuropat, P. J., Cooper, S. M., Frisby, K. and OwenSmith, N. 1989. Resource availability hypothesis of plant antiherbivore defence tested in a South African savanna ecosystem. Nature
340: 227-229.
Butet, A. and Spitz, F. 2001. Cyclic fluctuations of microtine populations: half a century of research. - Revue D Ecologie – la Terre et la
Vie 56: 353-372.
Bråthen, K. A., Agrell, J., Berteaux, D. and Jònsdòttir, I. S. 2004. Intraclonal varation in defence substances and palatability: a study on
Carex and lemmings. - Oikos 105:461-470.
Cairns, D. M. and Moen, J. 2004. Herbivory influences tree lines.
Journal of Ecology 92: 1019-1024.
Callaway, R. M., Brooker, R. W., Choler, P., Kikvidze, Z., Lortie, C.
J., Michalet, R., Paolini, L., Pugnaire, F. I., Newingham, B., Aschehoug, E. T., Armas, C., Kikodze, D. and Cook, B. J. 2002. Positive
interactions among alpine plants increase with stress. Nature 417: 844848.
Chapin, F. S. III. 1980. Nutrient allocation and responses to defoliation in tundra plants. Arctic and Alpine Research 12: 553-563.
Cornelissen, J. H. C., Lavorel, S., Garnier, E., Diaz, S., Buchmann, N.,
Gurvich, D. E., Reich, P. B., ter Steege, H., Morgan, H. D., van der
24
Heijden, M. G. A., Pausas, J. G. and Poorter, H. 2003. A handbook of
protocols for standardised and easy measurement of plant functional
traits worldwide. Australian Journal of Botany 51: 335 – 380.
Cornelissen, J. H. C., Quested, H. M., van Logtestijn, R. S. P., PerezHarguindeguy, N., Gwynn-Jones, D., Diaz, S., Callaghan, T. V.,
Press, M. C. and Aerts, R. 2006. Foliar pH as a new trait: can it explain variation in foliar chemistry and carbon cycling processes
among subarctic plant species and types? Oecologia 147: 315-326.
Diaz, S., Noy-Meir, I. and Cabido, M. 2001. Can grazing response of
herbaceous plants be predicted from simple vegetative traits? Journal
of Applied Ecology 38: 497-508.
Diaz, S., Lavorel, S., McIntyre, S., Falczuk, V., Casanoves, F., Milchunas, D. G., Skarpe, C., Rusch, G., Sternberg, M., Noy-Meir, I.,
Landsberg, J., Zhang, W., Clark, H. and Campbell, B.D. 2007. Plant
trait responses to grazing – a global synthesis. Global Change Biology
13: 313-341.
Ekerholm, P., Oksanen, L., Oksanen, T. and Schneider, M. 2004. The
impact of short-term predator removal on vole dynamics in an arcticalpine landscape. - Oikos 106: 457-468.
Elton, C. S. 1924. Periodic fluctuations in the number of animals: their
causes and effects. - British Journal of Experimental Biology 2: 119163.
Eriksson, O. and Ehrlen, J. 1992. Seed and microsite limitation of recruitment in plant populations. Oecologia 91:360-364.
Eskelinen, A. and Virtanen, R. 2005. Local and regional processes in
low-productive mountain plant communities: the roles of seed and
microsite limitation in relation to grazing. Oikos 110: 360-368.
Eskelinen, A. and Oksanen, J. 2006. Changes in the abundance, composition and species richness of mountain vegetation in relation to
summer grazing by reindeer. Journal of Vegetation Science 17: 245254.
25
Flagstad, O. and Roed, R. H. 2003. Refugial origins of reindeer
(Rangifer tarandus L.) inferred from mitochondrial DNA sequences.
Evolution 57: 658-670.
Fox, J. F.1985. Plant diversity in relation to plant production and disturbance by voles in Alaskan tundra communities. Arctic and Alpine
Research 17: 199-204.
Garnier, E., Laurent, G., Bellmann, A., Debain, S., Berthelier, P., Ducout, B., Roumet, C. and Navas, M.-L. 2001. Consistency of species
ranking based on functional leaf traits. New Phytologist 152: 69-83.
Gough, L. 2006. Neighbour effects on germination, survival, and
growth in two arctic tundra plant communities. Ecography 29: 44-56.
Grace, J. B. 1999. The factors controlling species density in herbaceous plant communities: an assessment. Perspectives in Plant Ecology, Evolution and Systematics 2: 1-28.
Grime, J. P. 1977. Evidence for the existence of three primary strategies in plants and its relevance to ecological and evolutionary theory.
American Naturalist 111: 1169-1194.
Grime, J. P. 1979. Plant Strategies and Vegetation Processes. John
Wiley and Sons., New York, NY.
Grytnes, J. A. and Birks, H. J. B. 2003. The influence of scale and
species pool on the relationship between vascular plant species richness and cover in an alpine area in Norway. Plant Ecology 169: 273284.
Gualtieri, L., Vartayan, S. L., Brigham-Grette, J. and Anderson, P. M.
2005. Evidence for an ice-free Wrangel Island, northeast Siberia during the Last Glacial Maximum. Boreas 34: 264-273.
Hairston, N. G., Smith, F. E. and Slobodkin, L. B. 1960. Community
structure, population control, and competition. American Naturalist
149: 421-425.
26
Hansson, H. C. 1953. Vegetation Types in Northwestern Alaska and
Comparisons with Communities in Other Arctic Regions. Ecology 34:
111-140.
Hartley, S. E. and T. H. Jones. 2003. Plant diversity and insect herbivores: effects of environmental change in contrasting model systems.
Oikos 101: 6-17.
Hoffman, W. A., Franco, A. C., Moreira, M. Z. and Haridasan, M.
2005. Specific leaf area explains differences in leaf traits between
congeneric savanna and forest trees. Functional Ecology 19: 932-940.
Hyvärinen, M., Walter, B. and Koopmann, R. 2002. Secondary metabolites in Cladina stellaris in relation to reindeer grazing and thallus
nutrient content. Oikos 96: 273-280.
Ims, R. A. and Fuglei, E. 2005. Trophic interaction cycles in tundra
ecosystems and the impact of climate change. - Bioscience 55: 311322.
Jakobsson, A. and Eriksson, O. 2000. A comparative study of seed
number, seed size, seedling size and recruitment in grassland plants.
Oikos 88: 494:502.
Kent, A., Jensen, S. P. and Doncaster, C. P. 2005. Model of microtine
cycles caused by lethal toxins in non-preferred food plants. - Journal
of Theoretical Biology 234: 593-604.
Klemola, T., Norrdahl, K. and Korpimäki, E. 2000. Do delayed effects
of overgrazing explain population cycles in voles? - Oikos 90: 509516.
Van de Koppel, J., Huisman, J., van der Wal, R. and Olff, H. 1996.
Patterns of Herbivory Along a Productivity Gradient: An Empirical
and Theoretical Investigation. Ecology 77: 736-745.
Korpimäki, E. and Norrdahl, K. 1998. Experimental reduction of
predators reverses the crash phase of small-rodent cycles. - Ecology
79: 2448-2455.
Krebs, C. J., Danell, K., Angerbjörn, A., Agrell, J., Berteaux, D.,
Bråthen, K. A., Danell, Ö., Erlinge, J., Fedorov, V., Fredga, K.,
27
Hjältén, J., Högstedt, G., Jónsdóttir, I. S., Kenney, A. J., Kjellén, N.,
Nordin, T., Roininen, H., Svensson, M., Tannerfeldt, M. and Wiklund,
C. 2003. Terrestrial trophic dynamis in th Canadian Arctic. Canadian
Journal of Zoology 81: 827-843.
Kuijper, D. P. J., Bakker, J. P., Cooper, E. J., Ubels, R., Jonsdottir, I.
S. and Loonen, M. J. J. E. 2006. Intensive grazing by Barnacle geese
depletes High Arctic seed bank. Canadian Journal of Botany 84: 9951004.
Körner, C. 1999. Alpine Plant Life. Functional Plant Ecology of High
Mountain Ecosystems. Springer-Verlag Berlin.
Lavorel, S. and Garnier, E. 2002. Predicting changes in community
composition and ecosystem functioning from plant traits: revisiting
the Holy Grail. Functional Ecology 16: 545-556.
Lima, M., Berryman, A. A. and Stenseth, N. C. 2006. Feedback structures of northern small rodent populations. - Oikos 112: 555-564.
Mattson, W. J. Jr. 1980. Herbivory in relation to plant nitrogen content. Annual Reviews of Ecology and Systematics 11:119-161.
McIntire, E. J. B. and Hik, D. S. 2002. Grazing history versus current
grazing: leaf demography and compensatory growth of three alpine
plants in response to a native herbivore (Ochotona collaris). Journal
of Ecology 90: 348-359.
McIntire, E. J. B. and Hik, D. S. 2005. Influences of chronic and current season grazing by collared pikas on above-ground biomass and
species richness in subarctic alpine meadows. Oecologia 145: 288297.
Moen, J. and Oksanen, L. 1998. Long-term exclusion of folivorous
mammals in two arctic-alpine plant communities: a test of the hypothesis of exploitation ecosystems. Oikos 82: 333-346.
Mulder, C. P. H. 1999. Vertebrate herbivores and plants in the arctic
and subarctic: effects on individuals, populations, communities and
ecosystems. Perspectives in Plant Ecology, Evolution and Systematics
2: 29-55.
28
Oksanen, L. 1983. Trophic exploitation and Arctic phytomass patterns. American Naturalist 122: 45-52.
Oksanen, L., Oksanen, T., Lukkari, A. and Síren, S. 1987. The role of
phenol-based inducible defense in the interaction between tundra
populations of the vole Clethrionomys rufocanus and the dwarf shrub
Vaccinium myrtillus. - Oikos 50: 371-380.
Oksanen, L. 1990. Predation, herbivory and plant strategies along gradients of primary productivity. In: D. Tilman and Grace, J., eds. Perspectives on plant competition. Academic Press, New York. Pp. 445473.
Oksanen, L., Moen, J., and Lundberg, P. A. 1992. The time-scale
problem in exploiter-victim models: does the solution lie in ratiodependent exploitation? American Naturalist 140: 938-960.
Oksanen, L. and Oksanen, T. 2000. The logic and realism of the hypothesis of exploitation ecosystems. American Naturalist 155: 703723.
Olofsson, J., Kitti, H., Rautiainen, P., Stark, S. and Oksanen, L. 2001.
Effects of summer grazing by reindeer on composition of vegetation,
productivity and nitrogen cycling. Ecography 24: 13-24.
Olofsson, J. and Oksanen, L. 2002. Role of litter decomposition for
the increased primary production in areas heavily grazed by reindeer:
a litterbag experiment. Oikos 96: 507-515.
Olofsson, J., Stark, S. and Oksanen, L. 2004a. Reindeer influence on
ecosystem processes in the tundra. Oikos 105: 386-396.
Olofsson, J., Hulme, P. E., Oksanen, L. and Suominen O. 2004b. Importance of large and small mammalian herbivores for the plant community structure in the forest ecotone. Oikos 106: 324-334.
Olofsson, J. 2006. Short- and long-term effects of changes in reindeer
grazing pressure on tundra heath vegetation. Journal of Ecology 94:
431-440.
29
Proulx, M. and Mazumder, A. 1998. Reversal of grazing impact on
plant species richness in nutrient-poor vs. nutrient-rich ecosystems.
Ecology: 79: 2581-2592.
Rixen, C. and Mulder, C. P. H. 2005. Improved water retention links
higher species richness with increased productivity in arctic tundra
moss communities. Oecologia 146: 287-299.
Rosenzweig, M. L. 1971. Paradox of enrichment: destabilization of
exploitation ecosystems in ecological time. Science 171: 385-387.
Seldal, T., Andersen, K. J. and Högstedt, G. 1994. Grazing-induced
proteinase-inhibitors – a possible cause for lemming populationcycles. Oikos 70: 3-11.
Siivonen, L. 1976. Nordeuropas däggdjur (Mammals of northern Europe). Norstedts, Stockholm, (in Swedish).
Stark, S., Strömmer, R. and Tuomi, J. 2002. Reindeer grazing and soil
microbial processes in two suboceanicand two subcontinental tundra
heaths. Oikos 97: 69-78.
Tolvanen, A., Alatalo, J. M. and Henry, G. H. R. 2004. Resource allocation patterns in a forb and a sedge in two arctic environments –
short-term response to herbivory. Nordic Journal of Botany 22: 741747.
Turchin, P., Oksanen, L., Ekerholm, P., Oksanen, T. and Henttonen,
H. 2000. Are lemmings prey or predators? - Nature 405: 562-565.
Waide, R. B., Willig, M. R., Mittelbach, G., Steiner, C., Gough, L.,
Dodson, S. I., Juday, G. P. and Parmenter, R. 1999. The relationship
between primary productivity and species richness. Annual Review of
Ecology and Systematics 30: 257-300.
Van der Wal, R., Brooker, R., Cooper, E. and Langvatn, R. 2001. Differential effect of reindeer on high Arctic lichens. Journal of Vegetation Science 12: 705-710.
30
Van der Wal, R. and Brooker, R. W. 2004. Mosses mediate grazer
impact on grass abundance in arctic ecosystems. Functional Ecology
18: 77-86.
Van der Wal, R. 2006. Do herbivores cause habitat degradation or
vegetation state transition? Evidence from the tundra. Oikos 114: 177186.
Watson, A. 1956. Ecological Notes on the Lemmings Lemmus trimucronatus and Dicrostonyx groenlandicus in Baffin Island. The
Journal of Animal Ecology 25: 289-302.
Wegener, C. and Odasz-Albrigtsen, A. M. 1998. Do Svalbard reindeer
regulate standing crop in the absence of predators? A test of the “exploitation ecosystems” model. Oecologia 116: 202-206.
Williams, M. and Rastetter, E. B. 1999. Vegetation characteristics and
primary productivity along an arctic transect: implications for scalingup. Journal of Ecology 87: 885-898.
Virtanen, R., Henttonen, H. and Laine, K. 1997. Lemming grazing and
structure of a snowbed plant community – A long-term experiment at
Kilpisjärvi, Finnish Lapland. Oikos 79: 155-166.
Virtanen, R., Parviainen, J. and Henttonen, H. 2002. Winter grazing
by the Norwegian lemming (Lemmus lemmus) at Kilpisjarvi (NW
Finnish Lapland) during a moderate population peak. Annales Zoologici Fennici 39: 335-341.
Zimov, S., Chuprynin, V. I., Oreshko, F. S., Chapin III, F. S., Reynolds, J. F. and Chapin, M. C. 1995. Steppe-tundra transition: a herbivore-driven biome shift at the end of the Pleistocene. American Naturalist 146: 765-794.
31
Svensk sammanfattning
Arktiska och alpina miljöer kännetecknas generellt av abiotiska miljöfaktorer som låga temperaturer, korta växtsäsonger och relativt låg
produktivitet. Dessa faktorer är grundläggande för växters utbredning
och funktionella egenskaper, men behöver inte överskugga betydelsen
av andra faktorer, till exempel biotiska interaktioner. Biotiska interaktioner inkluderar dels interaktioner mellan olika växter (som kan vara
både positiva och negativa) men även interaktioner mellan växter och
djur. Fokus i den här avhandlingen är på effekter av betesdjur på
sammansättningen av växtarter och växters funktionella egenskaper på
den alpina och arktiska tundran.
Beteseffekter är i regel negativa för den betade växten, men kan ha
positiva effekter på växtsamhället, till exempel i form av ökad näringsomsättning. Aktiviteter av betande däggdjur kan även leda till att
artdiversiteten bland växter ökar genom att djuren öppnar upp luckor i
vegetationen. Detta kan till exempel vara av stor betydelse på fjällheden där markskiktet ofta är helt täckt av mossor och halvbuskar. Genom att större djur, till exempel renar, trampar och bökar öppnas det
upp fläckar med bar jord där frön från kärlväxter kan gro. Även mindre djur, som gnagare, kan störa vegetationen och skapa så kallade microsites där groningsmöjligheterna för frön ökar. I alpina och arktiska
tundramiljöer är många kärlväxter fröbegränsade eftersom det är svårt
att upprätthålla en stabil fröproduktion i större omfattning under rådande miljöförhållanden och därför kan det vara av extra stor betydelse att befintliga frön lyckas etablera sig för att diversiteten bland kärlväxter ska bibehållas.
Effekterna av bete på växters utbredning och egenskaper är beroende
av flera faktorer. Bland annat spelar det stor roll vilken typ av herbivor det är som betar och vilken betesintensitet det är. Olika herbivorer
föredrar olika födoväxter och beroende på vilka typer av växter det är
som blir betade kan konsekvenserna skilja sig åt. Om det är dominanta
arter som blir betade kan artdiversiteten öka genom att mindre dominanta växter får ökat utrymme (”competitive release”). Det är även
skillnader i respons efter bete både mellan olika växtarter och inom en
växtart, beroende på vilka egenskaper en individuell växt har och i
detta sammanhang kan områdets produktivitet vara en viktig faktor.
Näringstillgången kan påverka hur en växt kan återväxa efter att ha
32
blivit betad och även hur sannolikt det är att en växt blir betad. Egenskaperna hos en växt kan också vara avgörande för hur konkurrenskraftig växten är samt hur en växt klarar av att växa i extrema miljöer.
En sådan funktionell egenskap kan till exempel vara höjden hos en
växt. I områden med hög produktivitet och konkurrens om ljus är det
bra att vara högre än sina grannar, men det är också en större risk för
höga växter att bli betade. Genom ett krypande växtsätt kan en växt
undvika att bli betad. Ett krypande växtsätt är även en bra egenskap
för att klara miljöförhållandena (framförallt hård vind) på den öppna
tundraheden.
I relation till bete är även kapaciteten hos en växt att återväxa av stor
betydelse för hur man tolererar att bli betad och denna kapacitet är till
stor del beroende av näringstillgången. Tillväxt, och därmed även
återväxtkapacitet, går att uppskatta genom att mäta en kvot mellan yta
och torrvikt hos bladen (specific leaf area, SLA). SLA påverkar även
risken att bli betad eftersom det finns en koppling mellan SLA, bladets
smaklighet och nedbrytbarhet. Växter som växer i lågproduktiva områden kännetecknas generellt av låg SLA och låga kvävehalter. Eftersom ljustillgången inte är någon begränsande faktor på tundraheden,
leder detta till höga kol/kväve-kvoter hos växterna, vilket resulterar i
låg smaklighet och nedbrytbarhet.
En strategi för att undvika bete är att producera försvarsubstanser och
i områden med låg näringstillgång dominerar kolbaserade försvarsämnen som verkar genom att sänka smakligheten generellt hos växten.
De kolbaserade försvarssubstanserna skiljer sig således från kvävebaserade försvar, där effekten ofta är specifik som till exempel de trypsin
inhibitorer man har funnit i vissa halvgräs. Det finns studier som indikerar att försvarssubstanser och den näringsmässiga kvaliteten, tillsammans med mängden tillgänglig föda, kan vara av avgörande betydelse för hur populationsdynamiken hos de mindre däggdjuren styrs.
Andra studier tyder på att de mindre däggdjuren, till exempel lämlar
och sorkar, i första hand styrs av predatorer. Om predatorerna reglerar
tätheterna av smågnagare, kan de indirekt styra växtligheten genom så
kallade kaskadeffekter.
För att få en något mer detaljerad bild av de biotiska interaktionerna i
alpina miljöer och på den arktiska tundran genomfördes fyra olika
studier där syftet var att dels kartlägga vilken betydelse olika herbivorgrupper har för växtsamhällets sammansättning av arter och egen33
skaper (paper II och III) och dels undersöka vilka faktorer som kan
styra populationsdynamiken hos små däggdjur (paper IV och V).
Dessutom genomfördes en studie som syftade till att se om kärlväxter
är begränsade av frötillgång och/eller tillgång på bra groningsplatser,
samt om renar har någon betydelse i detta sammanhang (paper I).
I den första studien som genomfördes i fjällnära björkskog och på
fjällhed nära Ammarnäs i södra Lappland, Sverige fann vi att de kärlväxter vi undersökte var fröbegränsade. Renar förekom i alltför
slumpmässig och begränsad omfattning för att åstadkomma luckor i
vegetationen, men de artificiella luckor vi gjorde genom att avlägsna
all biomassa på små fläckar ledde till ökad groning för vissa arter men
inte för andra (paper I). I den andra studien, också i Ammarnästrakten,
stängde vi ute olika herbivorer och fann att gnagare hade den största
effekten på samhällsförändringar i vegetationen genom förändringar i
biomassa. I björkskogen fanns mer biomassa av örter när gnagare var
utestängda och på fjällheden fanns mer biomassa av halvbuskar i de
rutor där det inte fanns gnagare. Insekter påverkade artsammansättningen i vegetationen i liten omfattning, liksom även renar. Renar
hade en större effekt på funktionella egenskaper hos örter, dels då
egenskaper var viktade mot abundans för att få ett värde på egenskaper typiska för en viss ruta och dels då variationer i egenskaper undersöktes inom arter. Generellt ökade kvävehalten i bladen i vegetationen
när renar var närvarande. Variationer inom arter visade på skillnader
beroende på vilken typ av växt som undersöktes. Höga, dominanta
örter i björkskogen var lägre och hade lägre SLA när renar var närvarande, medan lågvuxna örter visade ett motsatt mönster. Lågvuxna
örter var högre och hade högre SLA när renar var närvarande, vilket
kan tolkas som att småvuxna örter får bättre tillväxtmöjligheter när
konkurrensen från de större arterna minskar (paper II).
Under en polarexpedition till Beringia i den tredje studien jämförde vi
nio olika platser med olika artsammansättning både med avseende på
herbivorer och på växter. Aktivitet av stora herbivorer var negativt
relaterat till antalet arter av halvbuskar, vilket kan tolkas antingen som
en negativ påverkan av stora herbivorer på artrikedom bland halvbuskar eller som ett val av stora herbivorer att uppehålla sig i vegetation
som har många arter av halvbuskar. Aktivitet av små herbivorer var
positivt relaterat till abundans av halvbuskar, men negativt relaterat till
abundanser av graminoider och mossor. Detta samband kan bero på
att små herbivorer väljer habitat med mycket halvbuskar, men också
34
på att halvbuskar gynnas om de små herbivorerna framförallt lever av
mossor och graminoider. I de studerade områdena finns flera olika
typer av gnagare och eftersom de har olika födopreferenser kan detta
leda till stora skillnader i påverkan på vegetationen. Till skillnad från
det positiva sambandet mellan små herbivorer och abundans av halvbuskar i Beringia, fanns det ett tydligt negativt samband mellan gråsidingar och halvbuskar i den femte studien från predatorfria öar i
Nordnorge. På öar utan predatorer ökade sorktätheterna betydligt och
beteseffekterna blev stora framförallt på halvbuskar. Detta tyder på att
predatorer kan spela en stor roll i att reglera tätheterna av mindre
däggdjur.
I den fjärde studien undersöktes om styvstarr, som är en vanlig växt på
tundran och prefererad av lämlar som födoväxt, kan reagera på bete
genom att producera försvarssubstanser i form av trypsininhibitorer
och sänka sin näringskvalitet. Vi fann inga tecken på att trypsininhibitorer producerades, oberoende av graden av skada, tiden som gick
efter skadan eller näringstillgång. Däremot fann vi ett negativt samband mellan skadegrad och näringskvalitet (mängden lösliga växtproteiner) . Ett sådant negativt samband kan leda till att en ramet som
betats får sämre näringsvärde och därigenom skulle populationsdynamiken hos lämlar kunna påverkas. Slutsatsen blir således att det tycks
finnas stöd för att smågnagare styrs både av högre nivåer i näringsväven (predatorer) och av lägre nivåer (kvalitet och kvantitet hos födoväxter).
35
Tack!
Allra först vill jag tacka mina handledare, framförallt Ove Eriksson
och Jon Moen. Tack Ove för din förmåga att få förvirrade doktorander
att komma ut ur ditt rum som fokuserade och konstruktiva artikelförfattare! Ett jättetack också till Jon Moen för att du har haft så mycket
bra synpunkter och uppmuntrat i precis rätt ögonblick, för att du sakligt och förnuftigt rett ut begrepp, Lauri-teorier och annat. Du har
verkligen varit en stor tillgång i författandet av denna avhandling!
Tack även till Anders Angerbjörn och Tommy Lennartsson. Under
åren som har gått är det många som har passerat på botan och jag är
rädd att glömma någon och därför riktar jag ett gemensamt tack till
alla ni som gör botan till en trevlig arbetsplats (jag måste bara få ge
Sonja ett extra tack för alla tips och råd i slutspurten, och till Johan K
och Johan E för bra samarbete och medförfattarskap)! Ett stort tack till
alla jag har varit i fält med under de här åren! Många har timmarna
varit som vi har legat på knä och stirrat i rutramarna, men det har varit
tillräckligt att lyfta blicken för att man ska inse att man har haft världens bästa arbetsmiljö! Tack alla som har hjälpt till i fält, speciellt
Anna Lagerström som kommit tillbaka år efter år! Tack för alla grötfrukostar, alla timmar i fält, och för att du aldrig skulle komma på tanken att klaga över tung packning, regn eller mygg! Tack också till
Bodil Elmhagen i Fjällrävsprojektet för många och långa pratstunder
och peppningar när vi var inne på forskningsstationen samtidigt mellan fältperioderna. Tack till Lauri Oksanen, för att fältarbetet i dina
projekt fört mig till fantastiska platser (öarna utanför Nordnorge, Iesjaure, Joatka och vårvintern i Padjelanta), för att du öppnade den fascinerande världen av populationsdynamik och för att man aldrig någonsin har tråkigt i din närhet! Boris, Boris!
Once upon a time in Kola peninsula, there was a confused journalist, a
confused anthropologist and a plant ecologist (never confused ☺)
joining a PhD-course about reindeer. Thanks for a sustainable friendship, Terje and Dessislav! Thanks Dess for unforgettable visits in
lovely mountainous Bulgaria and for all future expeditions (what’s
next after dogsledging?)! Thanks Terje for help with the layout of this
thesis, för trevliga luncher och för att du och jag är så bra på att reda
ut det mesta i livet genom att analysera! Thanks also to Barbara in
Slovenia for a nice company during the Eureco 2005 in Turkey!
36
Ammarnäs kommer alltid att vara en speciell plats för mig och min
familj, denna vackra lilla by vid vägens ände! Tack till Lasse Strömgren för alla trevliga pratstunder på forskningsstationen, tack Ludmilla
för att det har varit ett trevligt avbrott att komma ner till byn och prata
med dig efter att ha varit på fjället en längre tid och ett stort tack till
allas vår Curt, som alltid hämtat oss i din röda taxi med dansbandsmusiken på högsta volym, vilken tid på dygnet vi än har ringt, hur skitiga
vi än har varit och hur mycket döda lämlar eller renskit vi än har haft i
packningen! Tack till Polarforskningssekretariatet för att Beringia
2005 verkligen blev av och ett speciellt tack till Per-Olof Edvinsson
för att vi hade så skoj tillsammans, för att du hjälpte mig i fält, gjorde
goda pannkakor (☺), för att du förstod den där konstiga längtan till
bergen som är så svår att förklara (blä för platta tråkiga Barrow, hurra
för alla härligt bergiga ställen vi var på!), och för en vänskap som jag
hoppas håller i sig! Utanför arbetet med den här avhandlingen vill jag
tacka några speciellt viktiga personer som har gett mig motivationen,
nämligen: Tack till alla Junimammor för att ni förgyllde min första
mammaledighet! MP-gänget är inte riktigt detsamma så länge Erika
håller sig kvar i USA, men hur som helst så vill jag tacka Erika, Cilla
och Jessica för alla våra måndagsmiddagar vecka ut och vecka in, år ut
och år in. Jag hoppas att vi fortsätter fast vi sprids, blir vuxna (☺) och
får familjer, det får väl bara bli lite annorlunda ett tag! Mina gamla
kursare från Umeå, speciellt Anna G, Ida och Kicki, inte minst för alla
trevliga fjällturer vi gjorde! Tack också till gamla kompisarna Eva och
Camilla! Tack till mamma och pappa för att ni alltid stöder mig och
för att ni har blivit en så viktig del i barnens liv! Källstugan är en viktig del av världen, inte minst på grund av Bysjön, hästarna och
smultronbackarna! Tack till farfar Gunnar för att du var ÅGIF’s mest
hetlevrade fotbollssupporter genom tiderna och alltid skjutsade mig!
Tack till Ola för att du alltid, alltid står på min sida i livet och peppar
mig! Du är den stora tryggheten och realisten i mitt liv, men är ändå
aldrig främmande för att prova på nya saker! Tack för alla härliga turer i fjällen (sommar som vinter), alla skridskoturer, kanotturer, alla
resor och för att du bara sa upp dig från jobbet och kom ner till mig i
Afrika när vi ganska nyligen hade träffats och jag gjorde praktik på
UNEP, tack för att jag fick dela de fantastiska ögonblicken hos bergsgorillorna med dig! Slutligen, tack till våra barn, ni ger livet mening
och framtidstro! Denna avhandling vill jag framförallt tillägna Saga
och Birk, ni två är nog de som mest påtagligt drabbats av mammas
avhandling. Nu har vi återigen tid att leka, busa, bada, klättra i berg,
hitta på äventyr och resa jorden runt, ni är det bästa som finns!
37
Fieldwork recommendations
Brief recommendations for field researchers working in polar bear
habitats. (specially adapted for field work on Wrangel island, Expedition Beringia 2005)
Polar bears may take a mistake in identifying an object. Such situations may occur when humans behave incorrectly within polar bear’s
view. A motionless human, particularly a human sitting or laying in
low profile, looks to a polar bear like an image of the bear’s common
prey. A dark-colored, low-profiled body looks like a seal or a walrus
on the ice or the beach, or like a reindeer carcass in tundra. Any darkcolored, low-profiled subject is an image of prey for the polar bear; as
a predator, the bear’s goal is to quickly reach the prey and take it. If a
bear detects prey (or what looks like prey to him) at some distance and
can’t detect the scent of human, he will start hunting. All his attention
will be focused on that one goal. The polar bear is rather inertial and
persistent in his behaviour. If he switch on to a hunting mode, strong
stimulation is needed to switch him into another mode. Unfortunately,
this may not happen before he takes the body, especially if the person
is paralyzed by fear and does not strongly resist. Even if the prey does
resist, the polar bear’s initial success usually stimulates further efforts.
So, it is better not to lead the situation to this stage. But if that has
happened, the victim should fight as strongly as possible. This is the
rule for anyone who happens to become a prey.
By Nikita Ovsyanikov
38