Final thesis
History matters: impact of historical land-use on
butterfly diversity in clear-cuts in a boreal landscape
Mathias Ibbe
LitH-IFM-Ex—
Supervisor: Karl-Olof Bergman, Linköpings universitet
Examiner: Per Milberg, Linköpings universitet
Contents
1 Abstract ................................................................................................................................... 1
2 Introduction ............................................................................................................................. 1
3 Material and Methods .............................................................................................................. 2
3.1 Study area and selection of study sites ............................................................................. 2
3.2 Butterfly recordings .......................................................................................................... 3
3.3 Local habitat factors ......................................................................................................... 4
3.4 Statistical analyses ............................................................................................................ 4
4 Results ..................................................................................................................................... 5
5 Discussion ............................................................................................................................... 8
6 Acknowledgements ............................................................................................................... 11
7 References ............................................................................................................................. 11
1 Abstract
In boreal landscapes the loss of species rich semi-natural grasslands is a result of
abandonment of agricultural land-use, and large areas historically managed as meadows are
today coniferous forest. Historical land-has been shown to affect plant diversity in small
fragments of managed semi-natural grasslands, and this study focused on if this effect is still
present in clear-cuts with different land-use history. The butterfly fauna in clear-cuts with a
history as meadows that been abandoned long enough to allow a generation of conifers to
mature were compared with clear-cuts with a history as coniferous forest. The results showed
clear differences in butterfly diversity between the two types of clear-cuts, with 78% of the
individuals and 44 species (30 in forest clear-cuts) recorded in clear-cuts historically managed
as meadows. Several butterflies that are considered to be semi-natural grasslands specialists
were strongly associated with clear-cuts with a background as meadows. These results are
probably due to the larger availability of host plants. Several butterflies threatened in other
parts of Europe are present in the clear-cuts. This utilization of temporary resources in clearcuts with a history as meadows is, in line with recent studies on fragmented landscapes,
emphasizing the importance of the matrix for the persistence of populations at a landscape
level. However, the habitat quality of clear-cuts historically managed as meadows will
without proper management most probably wither with future forest generations, and in turn
affect the long-term persistence of butterfly populations.
Keywords: Butterflies, Clear-cuts, Conservation, Habitat quality, Historical impact,
Landscape ecology, Land-use, Matrix
2 Introduction
Agricultural rationalization during the last century has strongly influenced the land use
practices in Europe. Species rich habitats such as nutrient poor semi-natural grasslands, i.e.
pastures and meadows traditionally used for grazing and hay-making, have suffered major
declines (Oates 1995; Bernes & Lundgren 2009). As a consequence, widespread declines of
organisms inhabiting these habitats have been reported from Western Europe (Krebs et al.
1999; Maes & Van Dyck 2001; Robinson & Sutherland 2002; Van Swaay et al. 2006).
Regional specialization in agricultural production has lead to decreased amounts of seminatural grasslands in intensified areas, an effect of fertilization and expanding arable fields.
However, in other regions, the loss and fragmentation of semi-natural grasslands is instead
due to abandonment (Stoate et al. 2009). In boreal regions the decrease in the number of
farms has resulted in that large areas previously managed as meadows today are covered by
dense coniferous forest, creating a less heterogeneous landscape.
Multiple studies on butterfly diversity in grassland remnants in agricultural regions
dominated by arable fields highlight the effects of habitat isolation and patch size on the
persistence of populations (Feber & Smith 1995; Maes & Van Dyck 2001; Krauss et al. 2003;
Bergman et al. 2004). However, the same pattern does not seem to exist for grassland
remnants in the less studied boreal landscape, with no relationship between butterfly diversity
and the amount of semi-natural grasslands in the landscape (Bergman et al. 2008). In addition,
the boreal landscape of Sweden seems to be rich on butterfly species associated with seminatural grasslands, threatened in other parts of Europe (Bergman et al. 2008). The differences
in the effects of habitat fragmentation on populations between landscapes are, in line with
recent years of science, emphasizing the importance of the surrounding matrix (Phiter &
Taylor 1998; Rickets 2001; Erik & Priya 2003; Dennis & Hardy 2007; Jonasson et al. 2009).
The matrix in the boreal landscape contains features suitable for butterflies, such as glades,
bogs and clear-cuts (Jonasson et al. 2009). Clear-cutting is a part of timber processing in
modern forestry and a considerable proportion, 4.7 %, of the productive forest land in Sweden
1
consists of clear-cuts (Fransson 2009). The degree of suitability for butterflies appears to shift
with the succession in vegetation at the clear-cuts, with a suggested top around three years
after cutting (Robertson et al. 1995; Wahlberg et al. 2002). Is it possible that butterfly species
associated with semi-natural grasslands utilize clear-cuts and thereby explain the lack of
effect of habitat fragmentation in boreal landscapes?
It is known that historical land-use 50-100 years back can explain plant species diversity in
small fragments of semi-natural grasslands (Lindborg & Eriksson 2004). In addition, remnant
populations of typical semi-natural grassland plant species have been found in previous
outland pastures which today are forest (Cousins & Eriksson 2002; Dahlström et al. 2006). Is
there an effect of historical land-use also in clear-cuts? It is unclear whether remnants of
typical grassland plant species are present in clear-cuts historically managed as meadows after
a generation of dense coniferous forest. It is also unknown if historical land-use in turn
influences habitat quality for butterflies and other phytophagous insects in areas that are
accessible again after a long period of unfavorable conditions. Considering the wide extension
of meadows in the historical landscape it is obvious that large areas which today are forested
have earlier been managed as meadows. It is possible that clear-cuts serve as sub-optimal
transient habitats for butterflies and in turn contributing to a higher persistence of populations
in the boreal landscape. However, further studies are necessary to investigate which factors
that determining clear-cuts suitability for butterflies.
The aim of this study was to investigate the impact of historical land-use on butterfly
assemblages in clear-cuts. We expected higher diversity of butterflies in clear-cuts historically
managed as meadows compared with clear-cuts with a long history as coniferous forest.
Further, we also expected that the presence of butterfly species would be correlated with the
amount of host plants and nectar sources.
3 Material and methods
3.1 Study area and selection of study sites
The study area is situated in the county of Östergötland (Fig. 1) southeast of Sweden. The
landscape consists mainly of coniferous forest with small patches of semi-natural grasslands
and arable fields. In this landscape a total of 24 clear-cuts, 12 historically managed as
meadows and 12 with a long history as coniferous forest, were selected for the study. All
clear-cuts were dominated by coniferous trees before logging (Tab. 1).
The historical background of the clear-cut sites was identified by using cadastral maps from
the second half of the 19th century. The cadastral maps show features of the historical
landscape such as coniferous forest, deciduous forest, meadows and arable fields. The sites
with a long history as coniferous forest were identified as sites located in coniferous forest
with no connection to meadows at the cadastral maps. The former meadow clear-cuts were
identified as sites that in part (varying between 21.4 and 84 %) consisted of meadows
according to the cadastral maps. The specific time since abandonment of the meadows is not
clear but all the clear-cuts historically managed as meadows have been abandoned long
enough to allow at least one generation of spruce dominated coniferous forest to become
mature until logging. Although, according to aerial photos some sites, of both types of clearcuts, had small patches not covered with forest in the beginning of the 1940s. The distance to
semi-natural grasslands today were considered in the selection of study sites because of the
potential influences from nearby butterfly and plant communities. All sites were located at a
distance > 300 m from closest semi-natural grassland and all but three > 500 m. All sites are
located in a landscape dominated by spruce forest at dry-mesic soil types. For practical
reasons and to avoid strong species-area relationships only clear-cuts with an area 2-6 ha were
included in the study. The age of the clear-cuts was 2-4 years, which represent a succession
stage in the vegetation that appear to be favorable for butterflies (Robertson et al. 1995,
2
Wahlberg et al. 2002). Northeastern slopes were rejected in the selection of study sites to
avoid variation caused by butterflies’ attraction to sunlit habitats
Fig. 1 Locations of the 24 studied sites in Östergötland, Sweden. (Triangles = clear-cuts
with a long history as forest, Squares = clear-cuts historically managed as meadows)
3.2 Butterfly recordings
Butterfly recordings were conducted 2 times on each site between the 22nd of June and 8th
of August 2009. The low number of visits was due to unfavorable weather conditions during
the butterfly season 2009, which limited the amount of recording days. The butterfly
recordings took place during daytime from 9:00 to 17:00 under predominantly sunny
conditions with a temperature > 17°C and with a maximum wind speed of 3 on the Beaufort
scale (only leaves and thin branches are moved by the wind).
The butterflies that were recorded included species from the superfamily Papilionoidea, the
families Hesperiidae and Zyganidae and two day flying species of Sphingidae (Hemaris tityus
and H. fuciformis). All these are referred to as “butterflies” in the following text. Species
names are according to Eliasson et al. (2005).
The transect line method was used for the butterfly recordings according to the method of
the Swedish Environmental Protection Agency (2003). Transects were located in straight lines
25 m apart, covering the whole of each site and oriented at right angles to the narrowest side
of the clear-cut. The transect line were walked at a steady pace (50 m min-1) and all
butterflies within 5 m of each side, up and in front of the surveyor were recorded. Butterflies
were caught in a hand net if identification could not be done immediately. If so, the transect
walk was stopped and resumed again after identification. Two pairs of butterfly species,
Plebeius argus/P.idas and Leptidea reali/ L. sinapsis, were treated together due to difficulties
in field identification.
Alternation between the two types of clear-cuts was done during the recordings to avoid
variation caused by differences in flying time between species. To minimize variation, roads
were avoided in the clear-cuts as road verges are known to be species rich habitats (Munguira
& Thomas 1992).
3
3.3 Local habitat factors
To estimate the availability of nectar the abundance of flowering plants was estimated in per
mille (‰) as a mean coverage for each transect according to the method of the National
Inventory of the Landscape in Sweden (Glimskär et al. 2008). In addition, the proportion of
the nectar sources that consisted of flower heads from the taxa Dipsacaceae and Cirsium in
each transect were recorded because they are important nectar sources for butterflies (Feber &
Smith 1995). However, due to the low amounts of nectar sources in the clear-cuts there was a
large number of transects with recordings < 1 ‰. Thus, the estimations of nectar availability
were replaced in the analysis by the abundance of nectar rich species in the genus Cirsium
from the plant survey.
Moreover, the abundance and composition of vascular plant species were recorded. Each site
was surveyed once between 3rd of September and 9th of October 2009. The transect lines
from the butterfly recordings were used for the plant recordings. For each clear-cut 100
circular sample plots with a radius of 1 m were evenly distributed over the total transect
length. The sample plots were positioned 2 m away from the transect lines in a right angle
towards next coming transect line, to avoid problems with damaged vegetation caused by the
butterfly recordings. In each sample plot the percentage coverage for all present vascular
plant species were recorded by eye, which is a widely used method (Bergstedt et al. 2009).
However, species covering < 1 percent were only recorded as present and no exact figure was
given. If identification to species failed the next possible taxonomic level was given. The data
from the plant survey were used to calculate the abundance of host plants and the amount of
nectar sources, which later were used in the analysis. The nomenclature follows Karlsson
(1997, 2002a, 2002b, 2002c, 2003). As in the butterfly recordings, road verges were excluded.
Species and basal area of stumps and living trees > 10 cm in diameter in an area of 100 m2
were recorded at every third sample plot in the plant survey, to get a measure of the density
and tree composition of the former forest and productivity of the land (Tab. 1).
For each sample plot in the plant survey the percentage coverage of exposed mineral soil,
bare rock and woody debris was also estimated (Tab. 1).
Table 1. Environmental variables of the 24 butterfly sites and p-values from the T-tests
History as meadow
History as forest
T-tests
Variable
Mean
Range
Mean Range
P-value
Size (ha)
4.2
2.0-5.8
3.1
2.0-5.9
0.058
Age of clear-cuts (y)
2.8
4-Feb
2.8
4-Feb
1
Area historically covered by meadow (%)
35
21.4-83.5
-
-
-
Basal area Picea abies (m2/ha)
34.2
11.1-66.3
25.2
9.7-38.4
0.082
Basal area Pinus sylvestris (m /ha)
6.4
0.3-20.4
9.3
2.9-15.6
0.167
Basal area decidous trees (m2/ha)
0.9
0.0-3.0
0.5
0.0-1.5
0.12
Woody debris (%)
6.5
3.3-12.3
6.3
1.9-15.3
0.889
Exposed mineral soil (%)
1.5
0.3-3.2
3.1
0.3-11.0
0.165
Bare rock (%)
4.3
1.2-8.4
4.4
2.0-7.4
0.984
2
3.4 Statistical analyses
T-tests were performed using the STATISTICA 8 software, comparing the size of the sites,
age of the clear-cuts, basal areas of spruce (Picea abies), pine (Pinus sylvestris) and deciduous
trees, amount of woody debris, exposed mineral soil and bare rock between the two types of
clear-cuts. This was done to investigate if there were any systematic differences between the
two types of clear-cuts.
4
Multivariate statistical analyses were performed with CANOCO 4.5 software (ter Braak &
Smilauer 1998). Due to relatively low beta-diversity multivariate methods based on linear
assumptions were used.
The butterfly data was summarized with Principal Component Analyzes (PCA). The sample
scores along Principal Component 1 (PC1) and PC2 were correlated with the historical
background and the local habitat factors to show their relative importance for the main
structure in the butterfly data. A partial Redundancy Analyses (pRDA) were conducted with
local habitat factors that contributed to differences in the butterfly data between the clear-cut
types used as co-variables to give information about the remaining variance caused by the
historical background. With a Monte Carlo test (9,999 permutations) a P-value were
calculated. The butterfly data were square-root transformed in the multivariate analyzes to
minimize the influence of a small number of species which were highly abundant.
Correlations of selected butterfly species with nectar richness and host plant abundance were
analyzed with generalized linear models (GLZ) using STATISTICA 8 software. Poisson
distribution and link function Log were used in the GLZ.
4 Results
A total of 2,653 observations of 44 butterfly species were individually recorded in the study.
Number of individuals and species observed per site varied between 18 and 339 and 5 and 26,
respectively. There were clear differences in the butterfly diversity between the two types of
clear-cuts, with a total of 44 butterfly species recorded in clear-cuts historically managed as
meadows and 30 species in clear-cuts with a long history as forest. In addition, a majority,
78%, of the total number of individuals were recorded in clear-cuts historically managed as
meadows.
In the PCA, PC1 explained 43.3 % and PC2 15.8 % of the variance in the butterfly data
(Fig.1). The PCA showed clear patterns that a majority of the species were positively
associated with clear-cuts historically managed as meadows. Moreover, the basal areas of
pine, spruce and deciduous trees as well as the size of the sites and the amount of exposed
mineral soil seem to have been the most important variables influencing the butterfly data and
contributing to the variation between the two types of clear-cuts. The age of the clear-cuts,
despite the narrow range 2-4 years, also explained some of the variation in the butterfly data
in the PCA. There was a clear trend that highly mobile butterfly species (Bink 1992) were
associated with young clear-cuts (Fig 1d). The association between the amount of woody
debris and younger clear-cuts is probably explained by left-overs from the cutting process,
and is not likely to influence the butterfly assemblages. Moreover, the amount of bare rock
seems to have had very little influence on the butterfly data.
Several butterfly species closely associated with semi-natural grasslands, such as Argynnis
adippe, A. aglaja and Boloria selene, (Kuussaari et al. 2007) were recorded in the study.
According to the PCA the majority of them were strongly associated with a historical land-use
as meadows (Fig. 1).
The main aim of this study was to investigate the impact of the historical land-use on present
butterfly assemblages and therefore a pRDA was performed contrasting the two land-use
types, using the basal areas of spruce, pine and deciduous species, the size of the sites and the
amount of exposed mineral soil as covariables. The Monte Carlo test of significance
performed in the pRDA showed that the historical background of the clear-cuts alone
significantly explained 7.3 % of the variation in the butterfly data (9,999 permutations, Pvalue = 0.0297). The amount of variation in individual species showed that a clear majority of
species, of them several associated with semi-natural grasslands such as Argynnis adippe and
Boloria selene, were positively associated with a historical land-use as meadows, also without
influences from local habitat factors (Table 1). Moreover, a number species were positively
5
associated with a long history as forest, while the presence of some species were not, or very
little, associated with the historical background.
Fig. 2 (a) PCA showing all of the recorded species in the 30 clear-cuts in Östergötland, (b) the 29
butterfly species that had the highest impact on the model, with species closely associated with
semi-natural grasslands (Kuussaari et al. 2007) underlined, (c) the environmental variables and
(d) figures showing the mobility index according to Bink et al. for each individual butterfly species
classified.
6
Table 2. Butterfly species with their explained variance in the pRDA, association with
historical land-use as meadow, abundance and frequency in 24 clear-cuts in Östergötland,
Sweden
Species
Lasiommata maera
Boloria euphrosyne
Argynnis adippe
Coenonympha arcania
Boloria selene
Inachis io
Thymelicus lineola
Polyommatus amandus
Ochlodes sylvanus
Gonepteryx rhamni
Aglais urticae
Aphantopus hyperantus
Polygonia c-album
Vanessa atalanta
Zygaena lonicerae
Cynthia cardui
Zygaena osterodensis
Argynnis aglaja
Brenthis ino
Pieris napi
Maniola jurtina
Pieris rapae
Melitaea athalia
Leptidea sinapsis/reali
Polyommatus semiargus
Argynnis paphia
Erebia ligea
Issoria lathonia
Aporia crataegi
Colias palaeno
Plebejus argus/idas
Callophrys rubi
Hipparchia semele
Pyrgus malvae
Lycaena virgaureae
Celastrina argiolus
Nymphalis antiopa
Hesperia comma
Lycaena phlaeas
Aricia artaxerxes
Coenonympha pamphilus
Zygaena viciae
Adscita statices
Pieris brassicae
Explained (%)
21.4
15.4
14.8
14.2
12.5
11.6
7.6
7.2
6.6
6.6
6.1
5.7
5.5
3.7
3.3
3.1
3.0
2.4
2.1
1.4
1.3
1.0
0.7
0.6
0.5
0.5
0.0
0.1
0.2
3.5
7.4
13.2
17.3
Association with
historical landuse as meadow
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
Number of occurrences
Number of individuals in in meadow/forest clearmeadow/forest clear-cuts cuts
39/13
98/10
139/36
355/140
132/13
49/3
141/24
5/0
19/4
135/14
48/9
346/51
4/0
3/1
8/0
42/7
6/0
119/5
39/12
42/3
11/0
6/0
51/18
8 /2
6/0
54/14
12/7
4 /1
13/4
1 /3
96/159
1 /2
8 /25
1/0
0/1
0/1
0/1
3/0
1 /1
1/0
1/0
9/0
2/0
4/0
10 /3
11 /4
12 /7
12 /12
10 /3
9 /2
11 /5
3/0
9 /3
10 /6
10 /7
12 /7
3/0
3 /1
3/0
7/3
3/0
10 /3
9 /3
8 /3
3/0
3/0
11 /7
2/2
3/0
8 /1
2 /3
3 /1
3 /3
1 /2
8 /12
1 /2
2 /5
1/0
0/1
0/1
0/1
2/0
1 /1
1/0
1/0
2/0
1/0
2/0
Explained variance in the pRDA and association with historical land-use are shown for
species with >2 occurrences.
7
To further elucidate butterflies association with clear-cuts historically managed as meadows,
and the pattern with highly mobile species associated with young clear-cuts, species
correlations with nectar sources and host plants were analyzed with GLZ (Poisson distribution
and Log as link-function). Species found in > 5 sites and that are specialized on <5 different
host plant species or a certain plant genus were included in the GLZ. An exception was made
for the species Pieris napi which can use > 4 host plant species but is restricted to the plant
family Brassicaceae. However, since none of its host plants were present in the plant survey a
simple regression with nectar sources was carried out. The butterflies host plant preferences
are according to Eliasson et al. (2005) and Janz (2005). As a measure of nectar richness the
amount of thistles from the genus Cirsium were used since this group is an important source
of nectar for the investigated butterfly species according to Eliasson et al (2005) and were the
dominating nectar sources found in the clear-cuts.
The results from the GLZ showed that the presence of four out of seven sedentary
butterflies, mobility index ≤ 4, was significantly correlated with the abundance of their host
plants and not with nectar richness. In contrast, four out of five highly mobile species
(mobility index ≥ 5) were significantly correlated with nectar richness and not to host plant
abundance, with Gonepteryx rhamni as an exception who was also significantly correlated
with host-plant abundance. (Table2). In, addition Aglais urticae was close to significantly
correlated with nectar sources.
Table 3. Species with mobility index and p-values for the
correlation with nectar sources and host plants from the GLZ
Species
Mobility index
Nectar sources
Host plants
Brenthis ino
2
0.417
0.000***
Argynnis aglaja
3
0.209
0.000***
Boloria selene
3
0.387
0.920
Boloria euphrosyne
3
0.086
0.000***
Melitaea athalia
3
0.625
0.633
Argynnis adippe
4
0.114
0.040*
Argynnis paphia
4
0.196
0.341
Pieris napi
5
0.002**
-
Inachis io
6
0.024*
0.801
Aglais urticae
6
0.051
0.880
Gonepteryx rhamni
Cynthia cardui
6
8
0.000***
0.031*
0.005**
0.907
5 Discussion
The results from this study emphasize the impact of historical land-use on present butterfly
assemblages. Historical land-use as meadows significantly explained variance in the butterfly
data recorded in clear-cuts even though the meadows has been abandoned long enough to
allow at least one generation of coniferous forest to mature. The results showed clear
differences in butterfly diversity between the two types of clear-cuts, with a clear majority,
78%, of the total number of individuals and a higher total number of species, 44 in contrast to
30, recorded in clear-cuts historically managed as meadows. Several butterflies that are
considered to be semi-natural grasslands specialists (Kuussaari et al. 2007) were strongly
associated with a background as meadows, which implying that also more specialized species
are able to utilize the temporary resources in these areas.
8
It is known that historical management regimes more than 200 years ago (Gustavsson et al.
2007) and connectivity 50-100 years ago (Lindborg & Eriksson 2004; Eriksson et al. 2002)
influences the diversity of plants in grassland remnants. Moreover, long lived plants with lifehistory traits such as clonal propagation or extensive seed banks are able to persist during
phases of unfavorable conditions of successional development and build up remnant
population systems (Eriksson 1996). Time lags in the extinction process following habitat loss
are also known from studies of primates (Cowlishaw 1999), forest birds (MacHunter et al.
2006), saproxylic beetles (Laaksonen et al. 2008) and butterflies (Polus et al. 2007). However,
short lived relatively mobile animals, such as butterflies, are in general considered to respond
quickly to landscape changes (Kuussaari et al. 2009). The results from this study for the first
time, to our knowledge, demonstrates that a memory of historical land-use, in form of
remnant plant communities, in turn could affect butterfly diversity in areas which are
accessible again after a long time of unfavorable conditions. This is not suggesting a time lag
for butterfly populations per se, but it is implying that the historical land-use can play a role
for population dynamics even for species considered to respond quickly to habitat
fragmentation.
The differences in habitat preferences found between groups of butterflies in this study seem
to be explained by the distribution of resources in the landscape and the species capacity to
utilize them. Butterflies have three essential demands to be able to persist in a landscape,
possibility to mate, enough of adult food, mostly in form of nectar sources, and access to host
plants (Wiklund & Åhrberg 1978). A potential conflict between searching for adult resources
and for larval food plants is presented by Janz (2005) and Janz et al. (2005). According to our
results the outcome in this conflict is dependent on life-history traits of the butterfly species.
Four out of seven species with a mobility index ≤ 4 were positively correlated with the
abundance of host plants and uncorrelated with nectar richness. In contrast, four out of five
highly mobile species, with a mobility index ≥ 5, were strongly correlated with nectar
richness but not with host plant abundance. Janz (2005) draws similar conclusions from an
experiment on the highly mobile butterfly Cynthia cardui in large outdoor cages, where the
resource search strategy is suggested to be to search for host plants in nectar rich patches. On
the other hand, in a study of the threatened butterfly Euphydryas aurinia, who has weak
dispersal ability, the host plant density was second to patch area the most important variable
explaining the presence of the species (Wahlberg et al. 2002). Consequently, butterflies with
high dispersal ability and widely distributed host plants are capable of searching for and
utilizing nectar rich patches in the landscape. In contrast, low mobile habitat specialists appear
to be more restricted to host plant distribution and are not able to use nectar sources with large
spatial segregation from their host plant. In our result these differences in landscape
utilization between groups of butterflies is also reflected by the clear shift in the butterfly
assemblages correlated with the age of the clear-cuts. A group of highly mobile species, for
example Aglais urticae and Inachis io, (Bink 1992) were associated with young clear-cuts,
this despite the narrow range of 2-4 years. This pattern appear to be explained by higher
abundances of high quality nectar sources in young clear-cuts consisting of ruderal plants,
such as several species of thistles in the genus Cirsium, which due to life-history traits
flourish in newly disturbed areas but are outcompeted later in the succession.
Butterflies as a group are closely associated with semi-natural grasslands but, in line with
our results, other habitats in the landscape are suggested to be utilized (Wahlberg et al. 2002;
Jules & Shahani 2003; Jonason et al. 2009), and in turn neutralizing the effect of isolation on
populations in habitat patches (Bergman et al. 2008). Thus, the traditional view of
metapopulations as patches of habitat located in unavailable matrix, like islands in the sea, is
now often complemented with attention for the matrix quality. Bergman et al. (2008) found
no correlation between species diversity and habitat isolation in a study of the butterfly
9
diversity in semi-natural grasslands located in a boreal landscape. The absent pattern is, as
suggested by the authors, probably explained by the impact of habitats other than semi-natural
grasslands, such as clear-cuts, bogs and glades, which have been overlooked. Considering the
extension of clear-cuts in the boreal landscape today (Swedish University of Agricultural
Science 2009) it may look like large areas of potential butterfly habitat are available.
However, our results suggest that clear-cuts differ much in their composition of butterflies.
The suitability of clear- cuts for butterflies is, in contrast to more static habitat such as
managed meadows and pastures, limited in time (Wahlberg et al. 2002). Our results imply that
historical land-use can influence the population dynamics of present butterfly assemblages by
enabling higher species richness in sites historically managed as meadows. It is likely that
other groups of phytophagous insects such as bumblebees and hoverflies are affected in a
similar way, and species higher in the food chain may in turn also be influenced by the
historical land-use. However, further studies are needed to confirm this.
In addition to managed semi-natural grasslands, the boreal landscape harbor potential high
quality habitat for butterflies in form of transient clear-cuts with a history as meadows.
Species dependent on transient habitats have a higher risk of extinction compared to those
living in more static habitats, a result of the dependence of creation of new suitable patches in
a sufficiently fast rotation and accessible distance (Wahlberg et al. 2002). In data simulations
of a meta-population of E. aurinia in Finland for which the habitat is a mix of transient and
static habitats, similar to the current boreal landscape, the latter is suggested to be essential for
the persistence of the investigated population (Wahlberg et al. 2002). Nevertheless, the role of
transient habitats for the persistence of populations at a landscape level should not be
underestimated. A significant amount of transient habitat facilitates migration (Roland et al.
2000; Rickets 2001) and enables recolonization of vacant habitat patches (Brown & KodricBrown 1977). Moreover, a larger total amount of potential butterfly habitat enables larger
populations and in turn stronger resistance to extinctions.
Several of the recorded butterflies, for example the Viola-feeding fritillaries, are threatened
in other parts of Europe such as Flanders (Maes & Van Dyck 2001), Germany (Wenzel et al.
2006) and The UK (Fox et al. 2007). In addition, in the Netherlands, a country stronger
affected by urbanization than Sweden, there is a severe decline in distribution and abundance
even for species earlier considered being common and widespread, such as Gonepteryx
rhamni (Van Dyck et al 2008). Thus, our results in line with Bergman et al. (2008) indicate
that the boreal landscape of Sweden may be a stronghold for the butterflies of northern
Europe. However, the effect of former land-use on the habitat quality of clear-cuts will most
probably wither with time and future forest generations. This makes it hard to predict the
status of the boreal butterfly fauna, but loss and degradation of habitats and consequently
smaller and more isolated populations tend to increase the risk of extinctions.
With proper management there may be possibilities to maintain the quality of clear-cuts
historically managed as meadows. Grazing by livestock would be optimal to prevent
succession and preserve a rich flora and in turn good butterfly habitat. However, this strategy
has economical, e.g. compensation to land-owners, and practical, e.g. hard logistics,
disadvantages. Another option would be to involve cadastral maps in forest management to
identify former meadows. Replanting the areas with deciduous trees or allow self generation,
optimally together with temporary preservation of openness in some areas, are better
alternatives than conifer plantations to preserve the biological values. Then it is possible to
avoid dense even-aged coniferous forest and maintain an herb rich ground flora, and in turn
keep the historical natural values. In addition, the amounts of deciduous forest is scarce in the
boreal landscape dominated by production forest and an increase would be beneficial for a
large number of organisms both those associated with semi-natural grasslands and forest.
10
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