Written by Maureen Nieuwschepen
This article is the second in a two-part, scientifically-based series on Parnassius apollo.
Worldwide climate change effects – changing weather patterns and shifting temperature ranges
Climate change, caused by increased levels of greenhouse gasses, is leading to changing weather patterns and an increase in extreme weather events worldwide (Scott, 2016), with an increase in daily temperature and precipitation extremes especially. For example, there has been an increase in daily record-high temperatures in Europe compared to daily record-low temperatures and this ratio is projected to increase in the future (Ummenhofer & Meehl, 2017). As air temperature increases, air water holding capacity will also change, influencing precipitation patterns. Heavy rainfall events and the duration of dry periods are increasing and are expected to increase in intensity in the future (Scott, 2016), thereby negatively affecting terrestrial ecosystem production across biomes (Zhang et al., 2013). Other climate change effects significantly impacting terrestrial ecosystems are, for example, increased numbers of heat waves and wildfires (Ummenhofer & Meehl, 2017).
For Europe specifically, climate change has led to an earlier onset of summer, with a change of ~10 days between 1960 and 2000 (Cassou & Cattiaux, 2016). The projected effects of climate change on terrestrial Europe are looking grim. Not only is Europe subjected to worldwide trends in climate change-induced weather effects, such as increases in precipitation extremes and severity of droughts, but Europe also faces unique challenges according to climate prediction models (Carvalho et al., 2021). Mean temperatures have increased almost double compared to the global average rate (Harris et al., 2014). This trend is predicted to persist in the future, with the highest relative temperature increase in Iberia, the Mediterranean, the Alps, Scandinavia, and Eastern and Northern Europe (IPCC, 2018).
Climate change effects specifically for central Europe and P. apollo habitats
Parnassius apollo (Linnaeus, 1758) habitats are mainly located in central European highlands. Climate change disproportionately affects mountainous areas, with more severe temperature rises than other ecosystems (Nogués-Bravo et al., 2007). Also, mountains are unique in their gradient of microhabitats along an altitudinal scale, which makes them harder to place into generalizable patterns. An upward shift in the distribution of plant and animal species has already been detected in European mountain areas (Lenoir et al., 2008), as temperatures are generally lower at higher altitudes. For plants, it is already established that the projected habitat loss is more significant for species found at higher elevations. 36-55% of alpine species, 31-51% of subalpine species, and 19-46% of montane species can lose over 80% of their suitable habitat by 2070-2100 (Engler et al., 2011).
Effect on P. apollo
Temperature increases
As P. apollo habitats are situated in mountain areas, they have been and are subjected to climate change to a severe extent. Firstly, increasing temperatures drive butterflies northwards. During the last few decades, P. apollo retracted northwards along both the northern and southern boundaries of its range (Parmesan et al., 1999). Another response to the rising temperatures might be the earlier onset of larval hatching.
In the French Brançon region of the Alps, populations exhibited earlier larval hatching, along with a one-month shift in the emergence of flying adults in biotopes above 1900 a.s.l. (Descimon et al., 2005).
Weather anomalies
Weather anomalies caused by climate change might have catastrophic effects on P. apollo populations. Several events have been documented that caused big declines in population sizes or caused bottlenecks. The events have been documented before the year 2000 but do show the vulnerability of Apollo populations to weather anomalies.
In the Pieniny mountains in 1957, following an early and warm spring, a prolonged period of cold and rainy weather accompanied by snowfall in July caused a bottleneck for the regional P. apollo populations (Żukowski 1959). As males emerge from pupae earlier than females, those emerging in June could not mate due to the absence of females. Subsequently, when females did appear after the cold weather, only a limited number were fertilized as only a few males survived.
A ‘false spring’ event in winter, i.e., a warm period followed by a return of the cold, in the late 1980s caused the decline of P. apollo populations in the southern part of the Central Massif in France (Descimon et al., 2005). A repetition of the event ten years later caused the complete extinction of these populations.
P. apollo larvae are adapted to low ambient temperatures, including temperatures below 0°C. The dark pigmentation of their cuticle enables rapid warming in sunlight for feeding. This trait is considered crucial in mountain habitats, where the maximum daily temperature rarely exceeds 15°C during the larval development stage (Richarz et al., 1989). However, larvae are highly susceptible to humidity. On cold and rainy days, larvae stop feeding and significantly reduce their locomotion. Consequently, extended periods of heavy rainfall, especially when combined with low ambient temperatures, decrease larval development and increase mortality rates (Descimon et al., 2005). However, temperatures above 40°C may also significantly increase larval mortality rates as they become more prone to developing opportunistic diseases, i.e., infections (Descimon et al., 2005).
Natural forest expansion
Across Europe, forests are common climax ecosystems, especially in the central and northern regions of the continent. The progression of forest succession significantly challenges P. apollo populations, leading to the fragmentation of habitats and reducing the food plant availability for both larvae and adults (Nakonieczny et al., 2007). So far, this process has mostly affected lowland areas. Consequently, natural succession of forests has been mostly threatening ‘telephiophagous’ forms, i.e., feeding on S. telephium, of P. apollo, rather than the forms feeding on S. album.
However, the alpine grasslands above the treeline inhabited by P. apollo are also severely threatened by climate change due to upward forest expansion driven by increasing temperatures (Hülber et al., 2020). This means that the albophagous forms are also threatened, especially when taking the predictions for temperature increase at higher altitude into consideration.
Conclusion
Climate change affects both P. apollo populations, the availability of host plants for caterpillars and habitat persistence. Small and isolated populations are more susceptible to extreme weather conditions, which can lead to a bottleneck effect or complete extinction of the local population. Efficient conservation strategies are essential for the survival of the species, and will enhance habitat conditions for other species thriving in similar environments. Projects like LIFE Apollo2020 are crucial in developing and implementing these strategies, playing a vital role in the conservation of P. apollo.
Bibliography
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