In northern Utsjoki, mass outbreaks of autumnal moths have been documented for 1844, 1905, 1909, 1927, 1957, 1965 and 1966 (Holtmeier et al.). One herder recollected the following from the 1950s: "I remember this time, I was less than 20 years old... My late father told me that the area was eaten some 35 years ago, and there were only stumps of this height left; nothing had grown afterwards. This was around 1920-1930, and no new trees have grown back in this area" (M2). This story most likely refers to the moth outbreak of 1927.
The interviews allow us to evaluate construction of the niche, with its temporal and spatial dimensions. The main driver of the spring migration of reindeer is the need to have a peaceful, dry and mosquito-free calving site with good access to fresh green forage. If a destroyed birch forest was located at a calving site or was on the reindeer's route to the summer pasture, the herds still headed to these sites, led by mature females (Turi). The insect harassment made the reindeer head uphill on the fells to where there were fewer mosquitoes, and the herders then had an easier time managing the animals, which were said to be "softened by mosquitos" . The significance of permanent summer pasture areas comes out clearly in the herders' accounts: "There were no fences on the fell, just smokes, and reindeer came to sleep by the smokes during the day, when the mosquitoes were worst. In daytime people just walked around among reindeer and calves and cut earmarks. At night time the reindeer left; it was not hot, there were not that many mosquitoes and they went to eat all together" (M3).
A few years after a mass moth outbreak, wavy hair-grass (Deschampsia flexuosa) colonized the soil around the birches due to the combined impact of nutrients from moth larva faeces, increased light reaching the ground after the death of the birch canopy and decreased competition given that the larvae also destroy the dominant dwarf shrubs (Rybraten and Hovelsrud). As a herder from Kaldoaivi explained: "It went like this; first there was lichen, then autumnal moths ate the buds, then there was grass, this height; the grass killed the lichen. Our reindeer were really fat; then the grass ran out and mosses came, which meant no more nutrition (M6). As new forage appeared, the reindeer had no need to make any immediate changes. After 4 to 10 years, however, the disappearance of the grass affected the movements of the female reindeer and each year they had to enlarge their foraging areas. Changing the pasture rotation is a slow process, as seen in Abisko, Sweden, where the damaged pastures were abandoned slowly (Van Bogaert et al.). Both the moth outbreaks and border closures were more serious and permanent incidents than the occasional harsh winter weather. Negotiations were conducted and new arrangements made between the siidas, which were forced to change their nomadic routes. In the worst case, the herders started to lose their reindeer for good. Reindeer took on the role from the moths as key constructor of the shared niche and modified their own forage resources by depleting them. Moths first destroyed the primary grazing lands of reindeer, after which the reindeer, as flexible herbivores, carried on the destruction by browsing the damaged birch trees and later grazing on the new grass that appeared. Because parts of the niche were abandoned, the reindeer were forced to extend their pasture rotation to new lands. Here one can see a niche being constructed in a positive way by the action of an animal itself.
Until the 1950s reindeer were herded intensively in northern Finland (Kortesalmi). During disastrous years reindeer tended to expand their niche onto new pastures, as they had before. Tensions emerged between the members of different co-operatives. From the perspective of forest herders in the southern parts of the herding area, the big northern herds were a threat in forest areas, as the herders were afraid of losing their reindeer on the fells during late spring. Between 1960 and 1990 the present system of fences between co-operatives was introduced. The niches became "permanently" constructed, which mostly benefitted the small reindeer owners in forest areas. During hard years fences prevented reindeer looking for new forage or acting in enlarged niches. At the same time, extensive herding had replaced intensive herding because of "the 1960s snowmobile revolution" (Pelto et al.; Helander-Renvall; Nakkalajarvi). This meant that reindeer moved inside the fenced co-operatives without people herding them (Helle and Jaakkola; Vuojala-Magga et al.; Turunen and Vuojala-Magga). During this era of fenced and permanent herding co-operatives, diminished land use, severe winters and moth attacks, the negative ecological inheritance that emerged as a selection pressure favoured the fittest reindeers.
Moth outbreaks in the 1960s destroyed about two-thirds of the mountain birch forests in northernmost Finland (Tenow). For example, in the year 1964-65 in Utsjoki, autumnal moths defoliated 1350 km2. One of the herders described the situation as follows: "They just ate everything, the ground vegetation, too. It's just open, from here all the way to the north, to Pestausselka 10 km...[] There is nothing but open, flat (landscape) once you drive some 20 km to Inari. There used to be birches before. These green caterpillars, 2 or 3 on each leaf - just count how many there were in each tree. It was just ... you should just listen to it." (M6). According to herders, "the oddest thing was the green leaves in the autumns; after the moth outbreak if the weather is warm enough, green leaves will sprout." (M7, F1). These late-summer leaves attracted reindeer to browse again. Wavy hair-grass, which reindeer grazed on (Rybraten and Hovelsrud:323), grew well for many years before it disappeared and the landscape changed into open land with black soil, like "the surface of the moon". (M1). As one of the herders noted: "The foliage destroyed by autumnal moth outbreaks in the 1960s has not yet recovered; what we see is a biological death; it has been a poison (M8). What had been forests became open tundra (Lehtonen and Yli-Rekola; Haukioja et al.; Klemola et al.:354). Still today rotten birch stumps or peaty hummocks signal former birch habitats (Holtmeier et al.). The birch defoliation in 1960 was most probably connected to the combined response to mass moth outbreaks, reindeer browsing, rotting roots and cold summers (Lehtonen and Heikkinen; Holtmeier et al.; Holtmeier and Broll; Holtmeier:7; Huttunen et al.).
There has been discussion regarding the role of reindeer in determining the location of the birch tree line. Whether reindeer grazing should be considered a disturbance for fell areas is a complex issue (Kallio and Lehtonen; Helle and Kojola; Lehtonen and Heikkinen; Oksanen et al.; Helle et al.; Helle, den Herder and Niemela; Holtmeier et al.; Holtmeier and Broll; Riipi et al.; Holtmeier). What is more, there is not much scientific evidence for the recovery of forests after the moth outbreaks in the 1960s, because high reindeer numbers caused considerable grazing pressure on birch forests. In Kaldoaivi and Paistunturi, reindeer numbers rose rapidly from their minimum in 1974, reaching their maximum in the period 1987-1988 (RHA) (Figure 2).
Since the beginning of the 1990s, herding in Kaldoaivi and Paistunturi has faced new changes. Summer round-ups, in which calves were earmarked, have been discontinued and this has protected lands and increased the well-being of calves. The composition of herds has changed as well. The number of adult males has decreased, because meat sales are based on the slaughter of male calves; this protects the winter pasture lands (Heikkinen). Finally, in the 1990s herders changed the free-ranging system to one of spring herding with hay. Owing to this, the herds are more resistant to harsh winters and the rest of the delicate lichen cover can be protected (Vuojala-Magga et al.). Since the 1960s, humans have become indirect key constructors by establishing permanent niches - fenced-in co-operatives - on a smaller scale than ever before. People had no experience-based knowledge of seasonal pasture rotation inside the fences. This system relied on the reindeer to find forage in any conditions. Access to a temporally extended niche was not possible. Moths and reindeer became key constructors one after another during the years of mass moth outbreaks. As there was no old solution for overcoming the hard times, herders had to apply the traditional systems of herding and knowledge of the herd's behaviour/social system in new ways with new technology. This can be seen as an adaptive process in response to new ecological inheritance in a specific, restricted environment. The herders gradually took on the role of niche constructors in tandem with their animals. The co-operation between herders and reindeer can be seen as increased domestication of reindeer (Helle and Jaakkola; Vuojala-Magga et al.; Turunen and Vuojala-Magga).
According to the IPCC, the increase in the global mean surface temperature for the period 2081-2100 relative to 1986-2005 is projected to be in the range of 0.3 to 4.8 C. The Arctic region will warm more rapidly than the global mean. Due to warming, both coniferous and mountain birch forests are expanding northwards and upwards on the fell slopes. Fast-growing graminoids, herbs and shrubs may replace slowly growing lichens (Kullman and Oberg; Turunen et al.; CAFF). Similar observations have been made by herders: "There has been enormous growth of birches. The trees and leaves are so dense on the riversides that you can no longer see the rivers (M1, M8). Those areas (destroyed) in 1964 have been regenerated and are now dense with birch trees (M1).
It is predicted that the combined occurrence of mass outbreaks of autumnal moths and a new invasive species of winter moth will be more frequent, because milder winters will increase the survival rate of moth eggs (Neuvonen et al.; Ayres and Lombardero; Logan et al.; den Herder et al.; Klemola; Ammunet, Ammunet et al.). For example, at Kevo, in northernmost Finland, the number of days with a minimum temperature below -36 C (critical for egg survival) decreased during the period 1962-2013 (FMI) (Figure 3). The winter moth has already now expanded its range to continental areas of northern Finland (Jepsen et al.; Klemola; Karlsen et al.). In the years 2006-2008 (2009) the destruction of birch forest in the Nuorgam and Polmak areas of Utsjoki was caused by winter moths. It was a new experience for herders: "I went to catch whitefish with my cousin. I was driving in front of him, and I stopped, and asked, 'Can you see ... that the ground and soil are alive?' There were these balls the size of a human fist - worms stuck together - awful balls full of life; they were greenish worms and just full of life. They had just eaten all the birches, and it was (as late as) September (M2).
Studies on the effects of warming and reindeer browsing on mountain birch growth have produced conflicting results (Kallio and Lehtonen; Lehtonen and Heikkinen; Oksanen et al.; Helle et al.; Suominen and Olofsson; Helle, den Herder and Niemela;Cairns and Moen). Helle claims that where regeneration of mountain birch is inhibited by herbivory, the forest will gradually change into tundra. Where there is intensive browsing by reindeer, birches exhibit a typical apple tree-like shape, with no lower branches (e.g. Kallio and Lehtonen; Oksanen et al.; Helle et al.; Suominen and Olofsson:240; Holtmeier and Broll). A hypothesis regarding the removal of the "lichen barrier effect" maintains that heavy reindeer grazing could lead to the decline of the lichen cover, which would then make upward migration of the tree line possible (Helle and Aspi; Tommervik et al.). This occurs because lichens can inhibit seed germination and growth of seedlings and sprouts through chemical and physical impacts (Brown and Mikola). On the other hand, reindeer herbivory could act as a buffer countering the rise of the tree line due to warming (Helle and Aspi; Virtanen et al.; Tommervik et al.; Bond; den Herder et al.; Kaarlejarvi). A recent study indicates that large mammalian grazers may slow down vegetation changes and that moderate grazing pressure could protect small tundra forbs from being outcompeted by taller plants under climatic warming (Kaarlejarvi). The connection between reindeer grazing and moth outbreaks is a delicate issue for herders. The ways in which this topic is addressed depend on the nature of the livelihood. Herders who earn a substantial proportion of their income from other sources, for example tourism, might attribute the signs of erosion in fell areas to high numbers of reindeer: "We only need to have a couple of severe years...before desertification starts" (M9).
When discussing future birch destruction, herders recalled their past experiences. They know that reindeer are able to use 200 to 300 plant species for nutrition (Warenberg et al.) and that, in addition to their mountain birch forests, the regions of Kaldoaivi and Paistunturi herding co-operatives have vast wetlands: "There will be always green somewhere, like willows, and birch trees among willow areas are always undamaged; there is grass in those areas. Once damage occurs, the reindeer seek new green areas where they can eat a lot of different types of green plants - like blueberry and whatever" (M1). "If there is pressure on the soil, the number of reindeer will decline and then small reindeer owners will be out first" (F2). This means that with higher reindeer numbers there is a greater possibility to sustain the livelihood. "We might well face a hard situation and, indeed, we are getting ready for this - nature has taught us.... We have been prepared for this type of situation; we will use various means to overcome and cope and get through the hardships. Those who do not bother to work will drop out (of reindeer herding) (M1).