Range shift of Pyrgus armoricanus

The future range shift of P. armoricanus is limited and the butterfly will not be able to expand north even when the temperature rises to more suitable levels. The butterfly is likely to increase in population size and area inhabited, even though it is possible that it will remain on a similar size as today or even go nationally extinct. The limited amount of habitat restricted the butterfly’s north expansion, which is evident since the butterflies with more general habitat requirements had an increased expansion. The only trait that effected the ability to expand north was an increased dispersal ability. One factor that suggests that P. armoricanus could have a higher dispersal ability than assumed is that individual butterflies of P. armoricanus has been found at shore meadows close to Simrishamn (Artportalen, 2016), situated approximately 25 – 30 km east of the populations surveyed. This indicates that the species either could spread longer distances than recorded or that there are stepping stones in the landscape that is not included in this study. The species managed to spread to habitat patches surrounding Simrishamn when their maximum dispersal distance was increased to 20 km. This was labelled as increased dispersal vagrancy, indicating that the butterfly could move and survive as an adult in a landscape lacking their host plant (Cook, et al., 2001; Stevens, et al., 2010). It is possible that P. armoricanus could have a higher dispersal vagrancy than recorded in the mark- and recapture survey. If so, then the butterfly would be able to inhabit more area and increase its distribution by roughly 15 km north of their original distribution. A previous description of the species defines it as loyal to its local habitat (Eliasson, et al., 2005), which point towards a low dispersal ability. This statement was confirmed with the data gathered from the meta- population and mark- and recapture survey, which together implies that the model captures the behaviour of the species.

The risk of not being able to move with the species range shift is that the species could go extinct. A threat of climate change is that climatic suitable habitat becomes too warm as the temperature increases, with the consequence of local extinction. The future distribution of climatic suitable habitat for P. armoricanus in Europe has been predicted by Settele et al. (2008). Habitat in the Mediterranean area, that sustain most stable populations of P. armoricanus, are predicted to become climatic unsuitable around the coast but sustain further into land. How much habitat that will disappear depends on the climate scenario. The more extreme scenario shows that almost all of the Mediterranean area will become unsuitable, and that their habitat will only persist in mountainous areas of central Europe (Settele, et al., 2008). It is common that the butterfly inhabits mountainous slopes, and it might be possible for them in some areas to expand upwards to find a more suitable climate. A second threat of climate change is those habitats that persist become more fragmented, isolated and small which increases the risk of extinction (Hanski, 1998; Gadgil, 1971). P. armoricanus may show a large distribution area in Europe, but the butterfly also shows isolated populations and could therefore be very sensitive to local extinctions.

Traits and range shift

This study recognized three main factors that affects species vulnerability to climate change; habitat specialization, dispersal ability and growth rate. The first factor is the species habitat requirements, which was identified as a major determinant in a species range shift success. The habitat availability showed to be extremely important for the butterfly’s range shift ability, where both the specialist and mid-specialist had a very limited northward expansion. The generalist habitat requirements made it possible to expand north regardless of the trait held by the butterfly, even if some traits enabled it to move faster than others. This confirms the theory that butterflies with general habitat requirements will be less vulnerable to climate change (Pöyry, et al., 2009; Warren, et al., 2001; Melero, et al., 2016), which could be expected since generalist butterflies are more successful in surviving in a disturbed habitat (Börschig, et al., 2013).

The second factor identified is the species ability to disperse, which will affect the chance to reach new habitats patches as they become climatically suitable. Dispersal ability has in previous studies been recognised as important to sustain in a changing climate since it and regulates its metapopulation dynamics (Hanski, 1998; Pulliam, 1988; Kuussaari, et al., 1998) and affects the species chance to follow its range shift (Pöyry, et al., 2009; Estrada, et al., 2015; Schloss, et al., 2012; Hellmann, et al., 2016). This study described three different dispersal behaviours, where both dispersal vagrancy and dispersal probability increased expansion relative to P. armoricanus. Dispersal vagrancy increased population size and area compared to P. armoricanus regardless of scenario, which shows the traits importance for a species possibility to expand. An increased long dispersal probability gave the highest increase in range shift ability, population size and inhabited area. In fact, only this trait made it possible for the butterfly to follow its range shift when the habitat availability of a generalist butterfly was assumed. Several studies has shown that high dispersal ability has a positive influence on a species range shift ability, even though some studies has not found any correlation between the two (Estrada, et al., 2016). This indicates that dispersal behaviour is important, but not the exclusive determinant for a species range shift ability.

The third factor is population growth. The growth rate of Pyrgus armoricanus was remarkably high compared to the growth rates found in other species and it was clearly a trait that had a positive influence on both its population size and distribution. A lower growth rate resulted in a smaller population and lower inhabited area regardless of landscape or climate scenario, but only a lower range shift ability if simulated on a landscape for a generalist butterfly. A high growth rate should aid a species to range shift since it affects its ability to persist in an unsuitable habitat, disperse and to establish in a new habitat (Hanski, 1998). The reproductive strategy of butterflies, which allows them to reproduce early in life and yield many offspring, has been observed to have a positive effect on range shift ability in several organism groups (Estrada, et al., 2016).

This study also investigated the importance of emigration and establishment probability on the range shift ability of a butterfly. A high emigration probability had a small negative effect on population size and area compared to Pyrgus armoricanus. The negative effect could be explained by the risks connected to dispersal, since this model assumes that the butterfly could not reproduce if it were not able to establish in its new patch. Another explanation is that an increase emigration probability leads to more butterflies disperse to sink habitats. A metapopulation is an assemblage of source and sink subpopulations, where the source subpopulation has a strong reproduction whereas the sink subpopulation has a reproduction that is insufficient to balance the mortality. Sink populations can be locally maintained by the continued immigration from a nearby source population, and thereby causing a reduction in population size (Pulliam, 1988). Site selection by butterflies dispersing could avoid this consequence, but recent science has shown that the benefit of careful site selection is evened out by harmful indirect effects (Nurmi, et al., 2017). It should be noted that the difference in population size, inhabited area and expansion between P. armoricanus and a butterfly with increased emigration probability was very small. The importance of emigration probability to range expansion has also been difficult to find in other organisms (Estrada, et al., 2016) and the trait could not be concluded as important to a species range shift capacity. The establishment probability had a positive, but surprisingly small, effect on range shift ability. The low effect could be due to the highly variating carrying capacity assumed in this study. The carrying capacity of a habitat patch will decide the success of the establishment, and one could suspect that a highly variating carrying capacity decreases the significance of the establishment probability. This study suggests that establishment probability is of less importance, at least when one assumes a high growth rate and a strong and varying of environmental stochasticity.

Habitat generalisation, growth rate and dispersal ability was identified as important for a butterflies northward expansion. In fact, only the combination of the three factors made it possible for the butterfly to closely follow its range shift limit. The butterfly expanded with a speed up to 6.08 km yr^-1 at climate scenario RCP8.5 and 3.54 km yr^-1 at RCP4.5 (maximum expansion / 50 years) assuming a high habitat generalisation, growth rate and dispersal probability. Studies who observed butterflies expansion north the last decades has detected a range shift velocity between 6-30 km yr^-1 (48 species) (Pöyry, et al., 2009) and 3.5 – 24 km yr^-1 (22 species) (Parmesan, et al., 1999). The range shift ability seems to differ greatly between organism groups, where butterflies are generally considered to follow their range shift better than other groups. A meta-analysis of terrestrial organisms showed a median velocity of 1.69 km yr^{-1 (Chen, et al., 2011)} while another showed a mean of 0.61 km yr^-1 (Parmesan & Yoha, 2003). A recent study over species that expands north in Great Britain showed a mean northward expansion over several taxonomic groups of 1.8 and 2.3 km yr^-1 (depending on time period), where Lepidoptera had an increased expansion rate during the last decades (Mason, et al., 2015). A butterfly in this study with traits aiding range shift ability had a low northward expansion relative to other observed butterflies but high relative to other terrestrial organisms.

Vulnerability and conservation

The question of which species that are more vulnerable than others of climate change is very difficult to answer, and one can expect it to vary among habitats and species groups. This study suggests that specialized butterflies with low growth rate and dispersal ability are less likely to be able to follow their range shift, leading to more isolate populations and thereby becoming prone to extinction (Gadgil, 1971). The consequence of biome range shift is a biodiversity impoverishment, where the butterfly community will become dominated by generalist species. As a result, species with high mobility and generalized habitat demands could be expected to immigrate to more northern countries in the future. In fact, species of both butterflies and moths have been seen to expand north in several countries the last decades (Warren, et al., 2001; Parmesan, et al., 1999; Mason, et al., 2015; Pöyry, et al., 2009; Sgardeli, et al., 2016; Kwon, et al., 2014; Lewthwaite, et al., 2017), including Sweden (Pettersson & Franzén, 2009). Scania, the south most province in Sweden, has had 37 new butterfly and moth species between 1973 and 2009, where of 16 of these has established reproducing populations (Pettersson & Franzén, 2009). 33 of the species are moths and 22 of them belong to Noctuidae, which is not surprising in consideration of that they in general are more mobile than butterflies (Chapman , et al., 2010). One could explain the resent range expansion of some butterflies to ecological drift rather than environmental stress, but Sgardeli et al. (2016) found that a community turnover was greater than what was expected to be natural and thereby indicating an effect of environmental change. This was confirmed in a study by Lewthwaite et al. (2017), who showed that butterfly community turnover responded both predictably and proportionally to temperature change. Warren et al. (2001) showed that only 25 % of the generalized butterflies with high dispersal ability that were predicted to respond positively to an increased temperature did so. The other butterflies were unable to spread north because of habitat loss, which is the main reason of the fast extinction rate of today (Steffen, et al., 2011). The combined effect of habitat loss and climate warming has shown diverging response in different butterflies, where a successful range expansion is not necessarily equal to an increased distribution area or abundance (Mair, et al., 2012).

Pyrgus armoricanus, as many butterflies, are dependent on well managed meadows, a habitat that is predicted to decline in Sweden due to land use change. The most important conservation act is to maintain and restore meadows high natural values so that they in the future could hold a population. It would enhance dispersal if the restoration would be planned according the landscape and their spatial distribution, especially in the south parts of Götaland where meadows are scarce. A suitable complement to restoration is to preserve and create a variating landscape that could counteract isolation of habitat and thereby enable both survival and dispersal (Öckinger, et al., 2012). Another conservation act could be to translocation, which has been proved successful for other butterfly species (Kuussaari, et al., 2015). A translocation of individual Pyrgus armoricanus to more northern locations could help them get passed the south Götaland, that has very few habitat patches, so that they possibly could establish and spread further north.