European regional assessment: Near Threatened (NT)
EU 27 regional assessment: Near Threatened (NT)

Aeshna caerulea is widespread and common in a large part of its northern range but has a fragmented occurrence in the Alps and in some other Central European mountain ranges, north of the Alps, where it is generally uncommon. It has for a long time decreased in Central Europe and this trend has continued. A. caerulea is threatened in most of Central Europe in Austria, the Czech Republic, France, Germany, Poland, Slovakia, Switzerland, as well as the UK. In its main distribution in the north, there are indications of a decline in southern Sweden, where the species has disappeared from some localities. The species is adversely affected by climate change, which may result in rapid decline and local extinction in parts of its range. This is particularly true of the central European populations which, however, only constitute a very small part of its European range. Should the species disappear from here, however, its European extent of occurrence (EOO) would be drastically reduced. It is expected that the species in Central Europe and locally elsewhere will become even more restricted to higher altitudes. The decline in southern Sweden indicates that the species can also be at risk in the northern lowlands, and it can therefore be expected that its distribution will gradually shift in a northerly direction. Local occurrences in the southern parts of the Fennoscandian peninsula will most probably be increasingly threatened and isolated and this development, as well as the populations in the scattered Central European range, must be followed closely. The decline is estimated to have been 20% in the past ten years and is expected to be more than 20% in the next decade. The species is thus assessed as Near Threatened (NT A2c+3c) in Europe as well as in the EU 27.

A. caerulea has a very large distribution that mainly includes the boreal zone from Scotland in the west to the Kamchatka Peninsula and the Bering Strait in the east. In Europe, it is a boreal-alpine species that is one of the most common dragonflies throughout the boreal zone in Fennoscandia, the European part of Russia, Estonia, and the northern part of Latvia. In the northern part of its distribution area, the species occurs both in lowlands and up to at least 750 metres above sea level. Outside this core area, it has only isolated outposts in Central Europe. Here it is a distinct high-altitude species from 1,000 meters above sea level but with the main occurrence much higher. In the Alps it is found mainly in Austria and Switzerland with only few or very few populations in France, Italy, and Germany. In the Sudetenland and the Bohemian Forest, few sites are known from Poland and the Czech Republic. It has not been seen in Slovenia since the end of the 20th century and is believed to have disappeared from Baden-Württemberg in Germany. There are also some records from Belarus, Romania and Slovakia that have not been confirmed and are considered doubtful (Kalkman 2010, Kalkman and Monnerat 2015, Kitel 2022).

A. caerulea is perhaps the most common Anisopteran in the northern parts of its lowland range in Finland, Norway, Sweden, parts of Estonia and Latvia and presumably European Russia. It is also found in Scotland. In Sweden, there is some lack of knowledge regarding its distribution, but it should be found in all provinces north of Småland. This is indicated by new provincial discoveries in recent years in three regions. It is however uncertain whether some old data from the most southern Swedish regions (Skåne and Öland) and the so far only records from Belarus are correct, and no observations have been made despite extensive pursuits. Suitable habitats in the south of Sweden are subject to drought and the species already seems to be gone from some previously known sites despite these being in protected areas and well monitored (Billqvist et al. 2019, Kitel 2022).

Further south, the species is limited to high elevations and occurs very fragmented, and it has been declining here for a long time. Most of the occurrences in Central Europe are small and isolated, and although some may consist of more individual-rich populations, it is vulnerable to local extinction. It occurs mainly in Switzerland and Austria, with a much more uneven occurrence in Poland and the Czech Republic. In France, it is only known from four locations in Haute-Savoie where it occurs between 1,700 and 2,200 meters above sea level. In Italy, it has been found at 31 sites, but some of these need to be confirmed (Assandri et al. 2022). In Germany, the species is limited to a few localities in southern Bavaria, while the only one known in Baden-Württemberg is probably extinct. It is also probably extinct in Slovenia, where it has only been encountered once (Kiauta and Kiauta 1992). Previous claims from Romania and Slovakia are today considered either dubious or incorrect (Kalkman and Monnerat 2015).

A. caerulea is a cold-adapted species and therefore mainly found in areas with long winters and short summers with low air temperatures. In warmer regions, competition with other dragonflies is greater. It is a generalist in the most northern part of its range where it occurs in many different types of habitats. Further south it is mainly found in nutrient-poor aquatic environments on open to semi-open bogs in or near forests. The habitats can be small or large, but they are usually lined with Sphagnum and have a lot of bottom detritus. More rarely, it is also found along slow-flowing watercourses. In the southern part of Sweden, it occurs almost entirely on larger, open bogs bordered by forest or with forested islands. Here, the populations might be very small and occurrences difficult to ascertain. In Fennoscandia, the species occurs both in lowlands and up to at least 750 meters altitude, while its habitat in Scotland is at a maximum of 550 meters above sea level. In Central Europe, the species occurs almost exclusively in stagnant water between 1,000 to 2,600 meters above sea level, with the main reproduction habitats at 1,400 to 2,200 meters. The habitat consists of small, permanent, and very shallow wetlands (usually 20 cm or less) which are frozen for a large part of the year. The vegetation consists of Sphagnum, Carex and Eriophorum (Billqvist et al. 2019, Kalkman and Monnerat 2015).

A. caerulea is quite easy to find even in less favourable weather and males can be astonishingly daring. Sometimes they fly into outstretched nets or perch on the viewer, or fully exposed far from wetlands, such as on tree trunks and rocks. The female is often more cautious and easily flushed. On hot days, the male is very active and patrols low over open ground or adjacent to wetlands while searching for females. A. caerulea can during midnight sun be seen hunting almost all hours of the day. The species is also active when it is overcast and can even be seen in light rain. Then they fly low over and through vegetation to flush prey. The eggs are laid in small, open water sections in wet Sphagnum or other soft material. The larval development lasts two to four years, but usually three. The larvae live in very shallow water, but often deep in bottom material and can thus be difficult to find. The exuviae are left low on upright straws or something similar in or along water edges (Billqvist et al. 2019).

A. caerulea is threatened in Austria, the Czech Republic, France, Germany, Poland, Switzerland, and the UK. It has shown a long-term decline in Central Europe, but these populations only represent a very small part of its European range and the decline in this region does not affect its conservation status on a European scale. Climate change is likely making some of these sites unsuitable for the species and it is, therefore, likely that it will become increasingly dependent on sites at even higher altitudes. Most of the populations in the Central European mountains are found in areas with relatively little human impact. Sites located in agricultural landscapes or in tourist areas are, however, threatened by the effects of cattle trampling the vegetation, eutrophication, and tourist activities (introduction of fish, boats, and winter sports management). The populations in the western parts of the Tatra and Sudetes are threatened because there are few populations, and most are located at altitudes that are already on the verge of the species’ climatological limit (Kalkman 2010, Kalkman and Monnerat 2015).

Also worrying from a European perspective is the downward trend in Sweden as this forms part of the species' core area, and this could be an indication of declines of a more severe nature. The decline here is probably largely linked to the isolated occurrences in the southern part where some populations in recent years have disappeared. Here A. caerulea is almost entirely dependent upon large, open bogs bordered by forest or with forested islands. With a high probability, it is mainly recurrent drought that is behind the decline. In the past, the species has been able to re-migrate to places that have dried out in some years, but as droughts recur more frequently and more severely, there are fewer suitable places to re-migrate to and from.

A. caerulea is cold-adapted and tied to nutrient-poor environments, so it is likely that, like other species with similar requirements, it is also negatively affected by several further factors. The habitats are becoming less nutrient-poor, which causes the larvae to suffer from increased intra-guild predation from thermophilic species that were previously rare or absent in these environments, such as Anax imperator. Higher water temperatures lead to a lower oxygen supply for the larvae, which leads to higher mortality and altered larval development and flight period. With more nutrients and lower water levels, there is also an accelerated and ongoing reforestation that takes place on a large scale, at least in Scandinavia. The excavation of bogs to acquire more land for forestry, as well as old ditches that drain the bogs long after peat mining has been abandoned, turn the previously open bogs into forests. When the water level is lowered, it paves the way for the establishment of trees and shrubs, which in turn is increased by nitrogen deposition and climate change. The trees and bushes that become established also take up water, which further dries out the soil and accelerates afforestation (Billqvist et al. 2019, Rova and Paulsson 2015). The Species Temperature Index (STI) shows that a decrease or expansion can be explained by a change in a species' preferred temperature (Termaat et al. 2019). For A. caerulea, this means that higher average temperatures alone can explain a rapid decline. Further north the species is mainly found in areas with low environmental pressure and is not under any current threat.

There is no trade or use of this species.

A. caerulea still thrives in the upper northern part of its distribution and locally also on higher elevations in its Central European range, but the impact of future climate change and nitrogen deposition is difficult to estimate. The isolated occurrences in Central Europe and Southern Sweden require immediate attention while there is no need for conservation measures for the populations further north. The sites of the Central European populations should be protected from the impact of grazing by cattle and from tourist activities.

There is a great need to raise awareness of the dragonfly species associated with nutrient-poor habitats. Fieldwork and studies are needed on cold-adapted species in general to conclude the exact threats and if they are reversible or not. They are often missed and lacking as indicators in peatland restoration projects. These projects often focus on birds and hence management plans and actions taken can, at least in Central Europe, be averse to the measures needed to restore habitats for dragonflies. Climate-adaptive management plans for oligotrophic habitats are imperative. A review and analysis of experiences from different restoration projects from different countries is needed. Perhaps more knowledge of the exact threats can lead to more appropriate management plans. It should entail keeping scrubs and trees adjacent to sites in open habitats in areas where hot summer days are a threat to the species, but to keep afforestation and scrub encroachment to a minimum in smaller, more shaded sites on higher elevations and in the northern distribution range. It is conceivable that natural fires played a role in the past in keeping the bogs from turning into forests. Perhaps controlled fires can be a tool in keeping habitats open. It is crucial that water levels are stable over time. If the level is too low for longer periods, vascular plants, encroaching trees, and shrubs can establish themselves.