European regional assessment: Endangered (EN)
EU 27 regional assessment: Endangered (EN)

Previously it was thought that the acidification and eutrophication, which impacted many species of bogs and heath lakes, did not lead to a decline of Sympetrum danae. However, the decline has worsened and populations throughout the lowlands are threatened, and climate change is likely making those sites unsuitable for the species in the near future. It can be surmised that the species will be increasingly dependent on sites at higher altitudes. The decline of S. danae is clearly visible in distribution trends, such as the in Czech Republic (-82%), Finland (-39%), France (-36%), as well as several regions of Germany, the Netherlands, Sweden, and the UK. It is Regionally Extinct in Croatia and threatened in Belgium, France, Slovenia, and Switzerland.

S. danae faces threats due to a combination of factors that are amplified by ongoing climate change. These factors involve eutrophication through nitrogen deposition, recurrent droughts, increased rate of afforestation and scrub encroachment, lower oxygen availability through higher water temperatures, increased intra-guild predation from thermophilic species, lack of or poor management plans and removal of tree cover essential for the species to escape hot summer temperatures. Based on occupancy modelling and distribution trends the species have been inferred to have declined by 30–55% during the past 10 years. The threats and trends are not likely to cease during the next 10 years. S. danae is hence considered to be Endangered (A2c+3c+4c) in Europe as well as in the EU 27.

Sympetrum danae is a Holarctic, mainly boreal species, occurring in Europe from the Pyrenees and the Alps in the south to above the Arctic Circle in Fennoscandia and the European part of Russia. In the west it reaches Ireland and in the east the Ural Mountains. In the southern range, it is mainly restricted to occurrences at higher altitudes (up to at least 2,040 m asl) in the Alps, the Massif Central, the Carpathians and in the Pyrenees (Kalkman et al. 2015). In other parts of Europe, it also occurs in the lowlands, but for some time it has been declining, like other species associated with oligotrophic habitats, and is becoming increasingly restricted to higher altitudes. Claims from Albania in some sources are incorrect and the species does not occur there.

S. danae is common in parts of its boreal distribution, but sparser in the far north as well as in the lowlands overall. It has seen a general and significant decline over the past 50 years and is still declining, historically probably due to fragmentation and degradation of suitable habitats. S. danae is Regionally Extinct in Croatia and threatened in Belgium-Flanders, Belgium-Wallonia, France, Slovenia, and Switzerland. The distribution trends as well as its occupancy is severe almost throughout its European distribution with dramatic population losses in Flanders and Wallonia, Germany, the Netherlands, France, and the UK. There is no proof of the species' presence in Slovenia in this millennium. In two of the species' real strongholds, Finland and Sweden, it is also declining. Only in Denmark and Lithuania the situation seems to be positive. It is likely that the species will be increasingly dependent on sites at higher altitudes.

S. danae is mainly associated with quagmires consisting of Sphagnum, which means clean, nutrient-poor environments on heaths, mires, and peat bogs. Slightly acidic conditions are favourable because most predatory fish do not thrive in these environments. It is also found in other habitats, but then mainly in areas where there are strong populations in the vicinity of the preferred environments. It can also be found in smaller, mainly fish-free waters and ditches in agricultural landscapes as well as in newly created small ponds, sometimes also along slowly flowing watercourses. These substandard habitats tend to be smaller and more short-lived (sink populations). To the south, the species gradually becomes more and more dependent on higher-altitude habitats. S. danae has a rather jerky and bouncy flight. The male in particular shows great dispersal ability and is often seen far from aquatic environments suitable for the species. It rests directly on the ground or in low vegetation in open fields or on viewpoints along edge zones. Early in the morning, they warm up by pressing themselves against solar-heated surfaces, such as rocks. The eggs are laid mainly in Sphagnum. From the time the eggs hatch in spring, it takes two months for the larva to leave the water. The exuviae are found in low vegetation close to the breeding habitats (Billqvist et al. 2019).

S. danae has for a long time shown a decline, but as it is widespread and still common in large parts of northern Europe, the decline has not been perceived as serious on a European scale. Previously it was thought that the acidification and eutrophication, which impacted many species of bogs and heath lakes, did not lead to a decline of this species (Kalkman 2010), but the decline has accelerated. Nitrogen deposition in combination with climate change has meant that the habitats have crossed a threshold, where cascade effects have dramatically affected the sites negatively. In areas where this threshold has been passed, it has resulted in strong declines or extinction of S. danae. The habitats are becoming less nutrient-poor, which has meant that the larvae suffer from increased intra-guild predation from thermophilic species that previously were rare or did not occur in these environments. Higher water temperatures lead to a lower oxygen availability for the larvae which leads to higher mortality and changes in larval development and flight period. In the past, the species has been able to re-immigrate to places that have dried out during some years, but as droughts return more and more often and more severely, there are fewer suitable sites to disperse to and from. The drought also fundamentally affects the wetland's plant communities since, for example, the crucial Sphagnum is disappearing. When a bog is trenched and dewatered, the peat is oxygenated, the pH levels change and metals and nutrients that have previously been bound in the peat are released into the water (Billqvist et al. 2019, Rova and Paulsson 2015, Taylor et al. 2022).

With more nutrients and lower water levels there is also an accelerated and ongoing afforestation that is occurring on a large scale. Trenching of bogs to acquire more land for forestry as well as old trenches that drain the bogs long after peat extraction has been abandoned is turning the former open mires into forest. When water levels are 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 shrubs that establish themselves also absorb water, which dries out the soil further and accelerates the afforestation (Rova and Paulsson 2015). In the southern part of the species' distribution, the opposite can be a problem, when tree cover essential for the species to escape hot summer temperatures is removed. This is often due to a lack of management plans or plans that are either poor or not designed for cold-adapted species (De Knijf et al. 2021). The Species Temperature Index (STI) shows that a decline or expansion can be explained by a change in a species' preferred temperature (Termaat et al. 2019). For S. danae this implies that higher mean temperatures alone can explain a rapid decline.

Populations in lowlands and especially those along the southern fringes of the distribution range are most severely threatened and climate change is likely making those sites unsuitable for the species in the near future. It is likely that the species will be increasingly dependent on sites at higher altitudes. Most worrying however is that based on distribution trends the decline of S. danae is also clearly visible in countries situated in the core of its range, such as Finland, France, Germany, Sweden, and the UK. In the UK the species has seen a general and significant decline over the past 50 years, particularly in England and Northern Ireland (Taylor et al. 2022). Previously a common dragonfly in the Netherlands it has now lost large parts of its distribution (van Grunsven 2020). In Flanders, it used to be a common species of which it was easy to see in the hundreds, but in recent years it has decreased by approx. 60% in abundance, and in population size as well as the number of sites, and S. danae has gone from being considered Least Concern to Endangered (De Knijf et al. 2021).

It is plausible that the decline is further accelerated by the presence of non-native invasive fish species such as Pumpkinseed sunfish (Lepomis gibbosus) in an aquatic environment that is naturally fishless. Those invasive species do not only predate on the larvae, but their presence leads to changes in behaviour (e.g. less active foraging) and finally to a longer larval development increasing the mortality rate. In some regions, such as the Netherlands, the Pumpkinseed sunfish is particularly abundant in moorland pools i.e., habitats for oligotrophic dragonfly species. Studies show that the average macroinvertebrate abundance in ponds with Pumpkinseed sunfish was 83% lower than in ponds without the fish, making Pumpkinseed sunfish predation a plausible explanation for the apparent decline of these taxa in the presence of the alien fish (van Kleef et al. 2008).

There is no trade or use of this species.

S. danae still thrives in the northern parts 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 diminishing fringe populations and isolated occurrences throughout Europe requires immediate attention. To ensure that S. danae and other cold-adapted species survive in the long run, restoration projects might have to be undertaken even in areas where they presently have strong populations. When choosing sites for conservation planning, it is wise to choose basins that are least susceptible to the introduction of alien invasive species. Conservation practices are best done at distances greater than 250 m from human settlement and 100 m from roads and trails to minimise the chances of introductions of alien species like the invasive species Pumpkinseed sunfish (Lepomis gibbosus) (van Kleef et al. 2008, Lettevall 2022).

There is a great need to raise the 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 seldom included as indicators in peatland restoration projects. These projects often focus on birds and hence management plans and actions taken can be averse to the measures needed to restore habitats for dragonflies. Climate-adaptive management plans of mires is imperative. A review and analysis of experiences from different restoration projects from different countries is needed. Perhaps more knowledge on 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 at 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 can establish themselves and out-compete the Sphagnum. Likewise, a too-high water level drowns the Sphagnum. A stable water table also makes it difficult for encroaching trees and shrubs to survive.

A monitoring program across European countries is needed to obtain an accurate overview of the current distribution and population trends for Sympetrum danae and to elaborate conservation plans.