Journal of Environmental Management 92 (2011) 2539e2546
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The role of power line rights-of-way as an alternative habitat for declined mire butterflies Terhi Lensu a, Atte Komonen a, *, Outi Hiltula a, Jussi Päivinen b, Veli Saari a, Janne S. Kotiaho a, c a
Department of Biological and Environmental Science, P.O. Box 35, University of Jyväskylä, 40014 Jyväskylä, Finland Metsähallitus, Etelä-Suomen luontopalvelut, PL 36, 40101 Jyväskylä, Finland c Natural History Museum, P.O. Box 35, University of Jyväskylä, 40014 Jyväskylä, Finland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2008 Received in revised form 29 April 2011 Accepted 18 May 2011 Available online 12 June 2011
Habitat loss is one of the greatest threats for biodiversity. In Finland, two thirds of natural mires have been drained for silviculture, which transforms open wetlands into dense forests. However, vegetation management of power line rights-of-way (ROW) maintain the drained mires as open areas. The aim of this study was to determine the effect of the power line ROW vegetation management on butterfly abundance, species richness and community structure by comparing the managed power line ROWs to unmanaged drained control sites and to natural mires. The species richness or abundance of mire butterflies did not differ between the power line ROWs and natural mires. In contrast, both species richness and abundance of butterflies was low on the unmanaged control sites. Tree canopy cover had a negative effect on mire butterflies and this is most likely related to changes in microclimate. The results indicate that the active vegetation removal in the power line ROWs maintain alternative habitats for mire butterflies; yet, the power line ROWs cannot substitute the natural mires. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Lepidoptera Mires Vegetation management ROW
1. Introduction There is increasing evidence that the immediate cause of the worldwide biodiversity loss, the so called ‘sixth extinction crisis’, is a combination of anthropogenic disturbance, climate change and habitat loss (Wilson, 1985; Pimm et al., 1995; Chapin III et al., 2000, Thomas C. et al., 2004, Thomas J.A. et al., 2004). Of these, the most important and immediate threat is likely to be the habitat loss: human landuse has already transformed nearly 50% of natural biotopes worldwide (Fischer et al., 2007). One biotope that has been heavily affected by landuse is the peatlands. In Europe c. 60% of the area of all peatlands has been transformed: 50% of these for the agriculture, 30% for silviculture and 10% for peat industry (Vasander et al., 2003). In Finland, the percentage is even greater totalling c. two thirds of the original peatland area (Aapala and Lappalainen, 1998; Vasander, 1998; Virkkala et al., 2000; Heikkilä et al., 2002). Peatlands are drained for silvicultural purposes. As a consequence of drainage, subsidence of water level leads to changes in vegetation: characteristic mire vegetation declines, tree growth increases, and eventually open land area expires (Vanha-Majamaa
* Corresponding author. Tel.: þ358 (0) 14 260 2471. E-mail address: atte.komonen@jyu.fi (A. Komonen). 0301-4797/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2011.05.019
and Reinikainen, 2001). Growing tree stock increases evaporation, and consequently the water level subsides even further and the microclimate of mires changes (Päivänen and Paavilainen, 1998; Heikkilä et al., 2002). In addition, the gradually increasing overshadowing cause further changes to indigenous vegetation of mires still decades after drainage (Heikkilä et al., 2002). The 2010 Red List of Finnish Species identifies 420 species that are primarily or secondarily dependent on mires (Rassi et al., 2010). Of the red-listed species that are primarily dependent on mires, Lepidoptera is the largest group. Of the c. 100 butterfly species that are regularly found in Finland many occur in wetlands, but only 9 of these are specialised to live on mires (Marttila, 2005) (Table 1). Throughout southern Finland, mire butterflies have declined during the past two decades, and in places they have been driven to local extinction (Kontiokari, 1999; Marttila, 2005; Rassi et al., 2010). Mire butterflies are highly specialised on mire habitats and they are seldom found from other kinds of biotopes. It has been argued that mire butterflies have suffered especially from the changes of microclimate that result from drainage (Pöyry, 2001), and from the decline of the larval food plants due to increasing overshadowing of the tree canopy (de Becker et al., 1991; Aapala and Lappalainen, 1998; Pöyry, 2001). A few studies have examined the effects of drainage on mire butterflies in more detail. Pöyry (2001) suggested that some species, such as Pyrgus centaureae, Boloria freija, Boloria frigga and
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Table 1 Mire butterflies of this study. Mire species ¼ species dependent on mire; other species ¼ may occur on mire, but is not dependent on mire (based on Marttila, 2005). Butterfly/Scientific name
Butterfly/Common name
Host plant/Scientific name
Host plant/Common name
Pyrgus centaureae Colias palaeno Boloria eunomia Boloria freija Boloria frigga Boloria aquilonaris Coenonympha tullia Erebia embla Oeneis jutta
Northern grizzled skipper Arctic sulphur Bog fritillary Zig-zag fritillary Willow-bog fritillary Cranberry fritillary Large heath Lapland ringlet Baltic grayling
Rubus chamaemorus Vaccinium myrtillus, Vaccinium uliginosum Andromera polifolia, Vaccinium uliginosum Rubus chamaemorus, Vaccinium uliginosum Rubus chamaemorus Vaccinium oxycoccos, Andromeda polifolia Carex sp. Carex sp., Poaceae sp. Carex sp., Poaceae sp.
Cloudberry Bilberry, Bog bilberry Bog rosemary, Bog bilberry Cloudberry, Bog bilberry Cloudberry Small cranberry, Bog rosemary Sedges Sedges, grasses Sedges, grasses
Erebia embla, will disappear soon after drainage, whereas Boloria eunomia survives until the drained mire turns into peat heath. Kontiokari (1999) determined the changes in butterfly communities of two drained mires in South-Ostrobothnia over twenty years and concluded that the populations of most mire species decreased strongly or disappeared completely. Similar conclusions about the decline of mire butterflies following drainage were also reached by Pöyry (2001) and Uusitalo et al. (2006). The conclusion from these studies is that drainage is a serious threat to specialist mire butterflies. Over 20 m wide power line rights-of-ways (ROW) cover almost 500 km2 in Finland (A. Levula, personal communication). In general, the construction and maintenance of power line ROWs have negative ecological effects such as fragmentation of continuous forest habitats (Rich et al., 1994; Nellemann et al., 2001, 2003; Vistnes et al., 2004) and related edge effects, which favour habitat generalist species at the expense of forest interior species (Rich et al., 1994). Power line ROWs may also filter animal movement (Willey and Marion, 1980; Doucet and Brown, 1997; Vistnes et al., 2004), cause bird mortality due to collisions with overhead wires (e.g. Henderson et al., 1996; Bevanger, 1998; Bevanger and Brøseth, 2004), and expose species to electromagnetic fields (e.g. Fernie and Bird, 1999; Fernie et al., 2000). However, vegetation management on power line ROWs is essential to ensure secure and uninterrupted transfer of electricity. Corridors are maintained by mechanical cutting on an average of six year rotation. Clearing of vegetation maintains open, mainly treeless areas in various successional stages (Vuorinen, 2001). Evaporation and shading decrease, while moisture, light intensity and openness increase, which may to some extent counterbalance the effect of drainage on mire microclimate. Thus, the creation of open habitat on power line ROWs may be beneficial to species that thrive in early successional habitats (e.g. Confer and Pascoe, 2003; Askins, 1994; Lanham and Nichols, 2002; Yahner et al., 2001; Smallidge et al., 1996). Some studies indicate that ROWs can serve as an alternative habitat for threatened plant, grassland butterfly and bee species (de Becker et al., 1991; Smallidge et al., 1996; Piirainen, 1997; Sheridan et al., 1997; Kyläkorpi and Grusell, 2001; Kuussaari et al., 2003; Forrester et al., 2005; Russel et al., 2005). Kuussaari et al. (2003) conducted a large scale study on power line ROWs in Southern Finland to determine whether they might function as an alternative habitat for day-active Macrolepidoptera species living in grasslands. They assessed the effects of different environmental variables on species abundance and composition on power line ROWs and compared the results to those of valuable traditional seminatural grasslands. Their results indicate that power line ROWs may have a great significance to the butterfly species living in grasslands. The number of species and individuals of butterflies encountered on power line ROWs suggested that the conditions were suitable for grassland species to establish populations and persist in the area. Nevertheless, it was evident that the species composition of butterflies on power line ROWs differed significantly
from that of traditional grasslands. Studies carried out by Kyläkorpi and Grusell (2001) show that also in Sweden several grassland butterfly species have been observed to utilise power line ROWs. In fact, in some areas power line ROWs are vitally important alternative habitat for populations of Euphydryas aurinia (Kyläkorpi and Grusell, 2001), a species enshrined by EU habitat directive. Our aim in this study is to determine whether declining mire butterflies use power line ROWs as an alternative habitat. This was achieved by comparing the abundance, species richness and community structure of butterflies on power line ROWs to the ones at unmanaged drained mire control sites and at natural mires. 2. Materials and methods 2.1. Study sites This comparative study was carried out during 2004 and 2006. Study sites were located in Central Finland (62 C, 26 E) in middle boreal vegetation zone. The 220 kV power line ROW is 65 m wide and runs about 70 km from south to north from Uurainen to Karstula. We established butterfly monitoring transects on habitats that had experienced three different management regimes: drained mires with vegetation management (on power line ROWs) (n ¼ 15), drained mires without the management (adjacent to power line ROWs) (n ¼ 15), and natural mires without drainage or management (adjacent to or near power line ROWs) (n ¼ 5) (Fig. 1). Number of natural mires we could find from the area adjacent to or near the power line ROWs was only 5 and this reflects the fact that nearly all mires are effectively drained in the study area. On each of the drained mires we established two transects: one on the power line ROW and another on c. 70 m outside the power
Fig. 1. Schematic diagram of the study set up.
B
8 6 4 2 0
Number of mire butterfly species
A
Number of mire butterfly species
T. Lensu et al. / Journal of Environmental Management 92 (2011) 2539e2546
8 6 4 2 0 Power line Control site Natural mire (n=15) (n=15) (n=5)
D
10 8 6 4 2 0 Power line (n=15)
Number of other butterfly species
Number of other butterfly species
Power line Control site Natural mire (n=15) (n=15) (n=5)
C
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10 8 6 4 2 0
Power line Natural mire (n=15) (n=5)
Power line (n=15)
Control site Natural mire (n=15) (n=5)
Fig. 2. Species richness (mean per 250 m transect SE) of (A) mire butterflies in 2004, (B) mire butterflies in 2006, (C) other butterflies in 2004 and (D) other butterflies in 2006.
line ROW. Distance between the separate drained mires were on average 1.5 km, and range between 500 m and 18 km. Distances between the natural mires were 500 m, 1 km, 7 km and 35 km. Natural mires were always 500 m - 2 km from the drained mires. The area of the drained mires was 1.5e4 ha and that of natural mires 2e6 ha. We included only sites that could be classified to be, or in the case of drained mires to have been, a pine mire: they were wet, grew Sphagnum moss and other characteristic pine mire vegetation and the thickness of the peat layer was over 50 cm. Natural control areas were sparse pine mires, in which the tree stock was small. Because of the small number of natural mires in the study area, mire was defined as being in a natural stage, if the distance between the mire and the nearest drain was more than 100 m. The management treatment was on the power line ROW that has been kept open for about 50 years and the time from preceding vegetation management varied from zero to eight years. There were 1e2 ditches on the managed power line ROWs, and these had
been dug after the power line ROW construction. Unmanaged control areas were drained more effectively, as well as successfully, since these areas had grown into dense forest. Vegetation management on power line ROWs maintains study areas open and delays succession towards peat heath. Based on vegetation study on the same study sites (Hiltula et al., 2005) we classified mires on the power line ROWs as transitional mires. The number and coverage of mire species did not differ between the drained ROWs and natural mires, but the number and coverage of forest species was larger on the ROWs than on the natural mires. The number of forest species did not differ between the ROWs and the control sites, but the coverage of forest species was smaller on the ROWs than on the controls. The number of mire species did not differ between the ROWs and the control sites, but the coverage of mire species was larger on the ROWs than the control sites. The number of Sphagnum moss species did not differ between treatments, but the coverage of Sphagnum was larger on the ROWs than on the control sites, but smaller than on the natural mires.
Table 2 Tukey post-hoc comparisons of the species richness and abundance of mire butterflies and other butterflies between the treatments. MD ¼ mean difference, SE ¼ standard error of the MD, P ¼ probability level. Variable
Species
Year
Richness
Mire
2004 2006 2004 2006 2004 2006 2004 2006
Other Abundance
Mire Other
ROW vs. control site
ROW vs. natural mire
Control vs. natural mire
MD
SE
P
MD
SE
P
MD
SE
P
2.80 2.33 3.53 5.13 1.40 2.57 1.83 1.95
0.37 0.15 0.60 0.60 0.21 0.32 0.33 0.26
