Rising Treelines in the European Alps

Rising Treelines in the European Alps and their Role in Alpine Lepidoptera Population Decline

Will Treeline Elevational Shifts of 300 m cause decline of Alpine Lepidoptera?

Changing climate, as a result of greenhouse gas emissions, caused global temperatures to increase by 0.8°C in the 20th Century [1]. This acted as a driver for increasing altitude of treelines, as it modified alpine zones to be successfully invaded [2]. Local winter warming is particularly important as a driver of these elevational shifts [3]. In the European Alps, an acceleration in the rate that the treeline is shifting altitude has been observed, although this effect is expected to vary throughout the mountain range depending on what species dominates the treeline [4,5]. In some parts of the Alps, Larix decidua, a species that can disperse rapidly, dominates the treeline and will facilitate a strong response to climate change [4]. Other parts are dominated by Pinus mugo, which disperses poorly and will respond to climate slowly [5]. Regardless of dispersal abilities, treeline shifts will lag behind changes in climate, as seen by the disequilibrium of current distributions in the Alps and climate predictions [4]. This means even if warming did not continue, treelines would continue to rise. By the year 2100, temperatures are modelled to increase by a further 2-3°C, and treelines could move up by 300 m [1,6]. As a consequence we expect at least 25% of alpine habitat in the European Alps to be lost by 2050 [7].

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Alpine habitats in the European Alps host an estimated 5,500 species of Lepidoptera, many of which are endemic and restricted to small populations [8]. Lepidopterans, the insect order including moths and butterflies, often have specific habitat requirements during the larval stage and will not be able to adjust to changing habitat [8]. This means they will be particularly affected by increasing altitudes of treelines. If these species are disrupted, it will bear consequences within alpine ecosystems such as decreased pollination services and grazing intensity [9,10]. This review will explore how a treeline elevation increase of 300m in the European Alps by 2100 would contribute to population decline in Lepidopterans. In particular, attention will be given to habitat loss and habitat fragmentation as causes of decline, what the consequences of these will be both to insects and the wider ecosystems, and how land-use could be utilised to modify treelines.

Habitat Fragmentation Threat with Upwards Shifts in Treelines

Habitat fragmentation can cause decline in butterfly and moth populations [11]. The treeline in the European Alps is shifting up into alpine grassland [4]. Due to geomorphological features of the Alps, treeline shifts will be uneven [7]. This will lead to pockets of alpine grassland being separated by forest. Some of these will act as a connection between other grasslands. As the treeline rises, patches of alpine grassland will be lost entirely, and the connections between remaining habitats will be lost [11].  When habitats are fragmented, it can reduce the ability for individuals to migrate between populations and share genes [11]. Populations that were isolated because of invading forest are at an increased risk of local extinction, but it could lower the risk of large scale extinctions [11]. This is because when stochastic events cause decline or extinction in one population, the effects will be isolated from other populations. The risk of extinction will be spread between multiple small populations, instead of a single large population [12]. When local extinctions occur in isolated populations, it is possible for the habitat to be recolonized through dispersal from nearby populations [11]. Parnassius smintheus in the Rocky Mountains had five local extinctions between 1995 and 2005, but dispersal consistently allowed recolonization within two generations [11]. This dispersal ability is lost if the amount of forest between grassland is increased, with population linkage effectively lost after a distance greater than 1 km [11]. However, it is also suggested that fragmenting populations can increase the likelihood of a migrating individual encountering a suitable patch of habitat, thus increasing functional connectivity [12]. This suggests that as forest continues to divide grassland, the ability for Lepidoptera populations to disperse and recolonize will be lost. This will also make inbreeding depression and local extinctions more common [11].

It is possible that climate change may help butterflies and moths overcome this. Alpine Lepidoptera utilise solar radiation to increase body temperature and remain active longer [13]. As alpine zones become warmer, they may be able to increase their activity and ability to migrate [1,14]. The United Kingdom has seen an influx of new species as warming allows migratory Lepidoptera to travel greater distances [14]. This increased capacity for migration may help alpine Lepidoptera to counter the difficulties of dispersal brought by expanding forest.

Alpine Habitat Losses with Rising Treelines

Shrinking alpine habitat will cause a decline in species richness of alpine Lepidoptera [15]. Forest plant species have been moving uphill an average of 29.4 ± 10.9m per decade [2]. Although alpine plant species have been moving up at a similar rate (27.8 ± 14.6m per decade) [2], because of the conical shape of mountains and the presence of alpine plateaus, the expected treeline rise of 300 m will cause a pronounced decrease in available habitat for alpine Lepidoptera [7]. If the treeline rises by only 100 m, 41% of alpine habitat is expected to be lost [6]. Changes in habitat that have already occurred will continue to affect alpine insects, as there is a time lag between habitat change and population dynamics [16]. The area of alpine habitat available correlates to the number of species that inhabit it [17]. This is because a smaller area contains less resources, has a smaller carrying capacity and can support fewer individuals. Less individuals in a population poses a threat through the risk of inbreeding depressions [18]. The alpine butterfly Erebia epiphron silesiana demonstrates that a small population has less genetic diversity than a large one, with the smaller population having a lower proportion of heterozygosity and fewer rare alleles [18]. Individuals showed no increased risk of deformity, but did have an increased incidence of albinism [18]. This is indirect evidence for inbreeding, as albinism in alpine insects can prevent camouflage and reduces energy absorbed to increase body temperature, making it an unfavourable trait [13]. This trait should be selected against and is evidence of reduced viability of the population due to inbreeding [13,18].

The larval stages of moths and butterflies require very specialised habitats [19]. As alpine habitat is lost, the risk of extinction of alpine species correlates to the ability to find new habitat [15]. With very specific habitat requirements, and increasing difficulty in dispersal from habitat fragmentation, it will become difficult for populations to establish in new places [20]. Human intervention to relocate butterfly and moth populations is a possibility to ensure the survival of a species with habitat loss. Relocation of Erebia epiphron silesiana, with 50 females,to a similar habitat outside of natural dispersal range has been successful [18]. There were no significant differences in genetic diversity found between the original population and the relocated population [18]. This offers a possible salvation for alpine insect species, with only a small proportion of the population needed [18]. Relocation may offer a temporary solution to habitat loss caused by the rising treeline, but as the alpine zone continues to shrink it is likely that chosen refuges will be lost too. The potential risks of relocation, such as introduction of diseases or competition with established species [14], may outweigh any hope of saving a species.

Mitigation of Treeline Shifts by Land-Use as Summer Pasture

If the treeline does experience an upward shift of 300 m it will undoubtedly damage populations of alpine Lepidoptera, through both habitat loss and fragmentation [11,18]. However, treelines can be difficult to model and often do not account for all contributing factors [4]. Different dominant tree species, such as Pinus mugo and Larix decidua, have different dispersal models and can move up hills at different rates [4,5]. Geomorphological features, including unconsolidated soil, can also influence the ability of treelines to invade higher altitudes [4]. Perhaps most significant, and most often unaccounted for, is the conflicting impact of land-use for summer pasture [4,15]. Grazing by livestock can prevent trees from establishing, thus slowing forest expansion and providing refuges for alpine systems [7,15]. It may be possible to prevent the invasion of alpine zones through the establishment and maintenance of summer pasture [7,15]. Despite these potential benefits, use of summer pasture has declined and is expected to continue declining [4,21].

Land-use is reported to provide refuges for alpine species and maintain biodiversity [7,15,21]. It offers economic benefits to livestock owners, as it provides more land for summer grazing and thus allows more livestock to be kept [21]. It also makes grazing animals more robust [21]. Without alpine grazing, numbers of livestock kept must be reduced, and treelines will face less resistance to their invasion of the alpine zone [15,21]/ Although summer pasture prevents the treelines from shifting upward, it may still force alpine species to higher altitudes [22]. Use of land by pasture caused an upwards shift in the range of Zygaena anthyllidis by 60 ± 74 m per decade [22]. In addition, grazing may modify habitat and alter competitive dynamics [19]. Live-stock grazing in England resulted in shorter shrubs, which favour the butterfly Polyommatus bellargus, a species that had been in decline, over Thelymicus acteon [19]. The subsequent recovery of P. bellargus and decline of T. acteon demonstrates how changes in grazing intensity have rapid and varying consequences for Lepidoptera populations [19]. We must then take caution when establishing pasture in hopes to mitigate land use, and in abandoning pasture in hopes to restore alpine insects’ range, as both actions could bear negative consequences for Lepidoptera populations [19,21,22]. While mitigation of treeline advances through land-use offers an interesting solution, climate change is the primary driver of treeline shifts and should be the focus of any preventative efforts.

Significance of Alpine Lepidoptera Decline in the European Alps

The loss or reduction of butterflies and moths in the alpine zone will impact other organisms in the ecosystem. Lepidopterans can contribute to ecosystems as both pollinators and herbivores [9,10]. Moths are significant nocturnal pollinators for a range of species [10]. Pollination is a critical ecosystem service and allows increased dispersal and gene flow of plants [10]. Disruption of this pollination relationship will negatively influence the reproductive success of the plant [10]. If generalist insect pollinators, such as alpine moths, are lost, a number of plant species may also experience decline. Lepidopterans also contribute as herbivores. Grazing by moths and butterflies can prevent one plant species from dominating and help maintain plant species composition [9]. For example, the Australian alpine Lepidopterans, Lomera caespitosae and Oncopera alpina, are associated with extensive grass death, allowing shrubs and other plants to survive the alpine zone [9]. If herbivores are lost, competitive dynamics of plants will be disrupted and a previously controlled species may thrive and outcompete other flora [9]. Through pollination and herbivory, alpine insects contribute to plant diversity.

The short life cycle of Lepidopterans also makes them useful as indicators [19]. Butterflies can be used as a proxy to model declines in habitat [23]. They are especially useful as they are conspicuous and there are large amounts of historical data surrounding them [20]. Butterflies and moths have short life cycles so respond quickly to environmental changes [19]. This makes them a valuable tool for modelling populations of other alpine species, and indeed any other species undergoing habitat loss. Plants and animals with longer life cycles will have populations that lag behind habitat changes [16]. This can result in species going extinct generations after a habitat is modified [16]. As butterflies have short life cycles, there will only be a short time lag between habitat changes and any potential extinction [16]. This will make them useful for modelling the responses of other species to climate change and to upward shifts in treeline.

Concluding Remarks and Future Perspectives

Upwards shifts of the treeline in the European Alps in the next century will cause decline and extinction in populations of alpine Lepidoptera. While a number of proposed models for treeline rise and alpine contraction exist [4-7], an average rise of 300 m in the next century is a reasonable prediction that would result in extensive loss of the alpine zone and fragment habitats for moths and butterflies [9,11]. In future, more refined modelling surrounding the European Alps, and the heterogeneity in regards to plant composition and geomorphological features, would be useful for predicting the response of many endemic insects it houses.  The rising treeline will cause populations to experience inbreeding depressions and lose viability [11]. As butterflies and moths are lost, the loss of key services they provide such as pollination and herbivory will cause decline in alpine plant species [9,10]. Population dynamics of butterflies and moths under changing habitat can be used to model responses of longer-lived alpine species [16,19].

The effects of changing land-use may counter upwards shifts in the treeline, but will have their own consequences for alpine ecosystems. At present, there are conflicting views as to whether land-use for summer pasture is helping or harming alpine Lepidoptera [7,15,21,22]. More research should be done to assess whether this is a viable option to help manage Lepidopteran populations. The most important actions that should be taken are ones that reduce emissions of greenhouse gases and mitigate climate change.

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