European regional assessment: Vulnerable (VU)
EU 27 regional assessment: Vulnerable (VU)

The range of Leucorrhinia dubia has become more fragmented with isolated occurrences mainly on higher elevations. In England the species has declined significantly since 1995 and for ten years up to 2020. In the Netherlands, it declined by 3% in distribution per year (Taylor et al. 2022, van Grunsven 2020). In Flanders, it has long been in decline, and this has continued with more than 28% during the period 2010–2020 (De Knijf et al. 2021). It has disappeared regionally from southern England and Luxembourg and has been assessed as threatened in Austria, Belgium, Bulgaria, the Czech Republic, France, Germany, the Netherlands, Slovakia, Slovenia, Switzerland, and the UK. In the north, it still thrives in parts of the boreal zone, as well as in rather isolated populations in the Alps and the Massif Central. A decline is however also clearly visible in the distribution trends for countries situated in the core of its range, such as Denmark, Czechia, Finland, Lithuania, and Sweden. L. dubia faces threats due to a combination of factors that are amplified by ongoing climate change. These factors involve eutrophication through nitrogen deposition, reoccurring droughts, increased rate of afforestation and scrub encroachment, lower oxygen availability through higher water temperatures, increased intra-guild predation by thermophilic species, invasive fish species, lack of or poor management plans and removal of tree cover essential for the species to escape hot summer temperatures. These threats and trends are not likely to cease in the next 10 years. Based on occupancy modelling and distribution trends the species has been inferred to have declined by 15–35% during the past 10 years and this decline is suspected to continue. L. dubia is hence considered to be Vulnerable (A2c+3c) in Europe as well as in the EU27.

L. dubia is a northern Palearctic species and the Leucorrhinia species with the widest range, from Scotland in the west to Japan and Kamchatka in the east. In Europe it is found in a more or less continuous wide belt from the Alps in France and towards the northeast. In its most southern, southeastern, and western European distribution L. dubia has a fragmented occurrence and is confined to foothills or mountains such as the Massif Central, and the Pyrenees. There are only a handful known populations in the Balkans outside the Alps, from the Carpathians and the mountains on the Balkan Peninsula (Kalkman et al. 2015).

L. dubia is rather common in suitable habitats in Finland, Norway, Sweden, and probably also in Russia as well as in the Vosges mountains, the Jura Plateau, parts of the Alps and the Massif Central (Kalkman et al. 2015). It has a more scattered distribution and is declining throughout most of the rest of its European distribution, especially along the fringes of its range. It has disappeared locally such as in southern England and Luxembourg and has declined in many more countries and is therefore assessed as threatened in Austria, Belgium, Bulgaria, the Czech Republic, France, Germany, the Netherlands, Slovakia, Slovenia, Switzerland, and the UK.

L. dubia is found mainly in nutrient-poor habitats such as bogs or open fens with forest adjacent to the water bodies. The habitats are small, often fish-free, at least 50 cm deep, edged with Sphagnum and the pH is usually between 4.5 and 6.0. Moderately acidic conditions are favourable because most predatory fish do not thrive in these conditions. It shuns open agricultural areas and nutrient-rich habitats, but males in particular show good dispersal abilities and can sometimes be found in less suitable environments far (at least 30 km) from appropriate breeding habitats. Sometimes more short-lived and smaller populations (sink populations) can be found in less favourable habitats close to areas with strong populations. The eggs are laid within floating Sphagnum where the conditions for rapid larval growth are optimal. It is important that the water level is deep enough to allow for a “lift effect” on the Sphagnum and to avoid drought in summer and freezing in winter as these are badly tolerated. L. dubia is most easily found by scanning aquatic environments for the male that holds territory from vantage points, such as protruding straws or branches across the water. After emergence, the tenerals immediately leave for nearby clearings, edge zones, forest roads etc. The exuviae are found low adjacent to the water (Termaat and Groenendijk 2006, Billqvist et al. 2019).

L. dubia has for a long time shown a decline, but as it is widespread and still common in large parts of Northern and Central Europe, the decline has not been perceived as serious on a global or European scale. This decline has however recently been accelerated. In the past, the degradation and destruction of its habitats (such as peat extraction or conversion into farmland) and their deterioration through eutrophication have constituted the biggest threats. These threats have been perceived as reversible in the long run (Kalkman 2010) but nitrogen deposition in combination with climate change (especially rainfall deficit and longer periods of ponds drying up) has meant that the habitats have crossed a threshold, where cascade effects have drastically affected the sites negatively. This has been going on for a long time but slowly and when it really took hold, it had serious consequences for the species that are linked to nutrient-poor environments. In areas where this threshold has been passed, it has resulted in strong declines or losses of L. dubia. The habitats are less and 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, such as Crocothemis erythraea. Higher water temperatures lead to a lower oxygen availability for the larvae which leads to higher mortality, change in larval development and flight period. In the past, the species has been able to re-immigrate to places that have dried out during certain 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 levels of pH changes and as a consequence various substances such as metals and nutrients that have previously been bound in the peat is released into the water (Rova and Paulsson 2015, Billqvist et al. 2019, Suling and Suhling 2013, Taylor et al. 2022).

With more nutrients and lower water levels, there is also an accelerated and ongoing afforestation which is occurring on a large scale, at least in Scandinavia. 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 to 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 or bad weather conditions 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 L. dubia this implies that higher mean temperatures alone can explain a rapid decline. 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). L. dubia has been shown to be able to develop defences against at least moderately rich predatory fish populations by the larvae developing longer spines (Johansson and Samuelsson 1994). However, L. gibbosus also affects the habitat directly, for example by the males building nests that stir up sediment.

Populations in lowlands and especially those along the 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. Most worrying however is that based on occupancy modelling and distribution trends the decline of the species is also clearly visible in countries situated in the core of its range, such as Denmark, Germany, Finland, Lithuania, and Sweden. In England, the species has declined significantly since 1995 and was lost in Surrey in the late 1990s and from Cheshire in 2003 (Taylor et al. 2022). For ten years up to 2020 L. dubia in the Netherlands declined with 3% in distribution per year (van Grunsven 2020). In Flanders, it has long been in decline, and this have continued with more than 28% during the period 2010–2020 (De Knijf et al. 2021).

There is no trade or use of this species.

L. dubia still thrives in the northern parts of its distribution and locally also in its central European range, but the impact of future climate change and nitrogen deposition is difficult to estimate but is likely to continue in the near future. The diminishing fringe populations and isolated occurrences throughout Europe require immediate attention. To ensure that L. dubia 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. 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 at least in Fennoscandia 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.

The alien invasive species Pumpkinseed Sunfish (Lepomis gibbosus) originates from eastern North America and has been observed in 24 countries in the EU and is established in all but one of these. It is estimated to be able to establish populations in all European countries and also in colder areas, as demonstrated by its wide range in Ukraine. It has also been found in the Nordic region, where many oligotrophic dragonfly species have their strongholds. It does not yet pose a threat to them there, but further climate change might change that. Humans have been shown to be the main vector in the spread of Pumpkinseed Sunfish and the probability of introduction is related to pond availability. For nature conservation planning, it is wise to choose basins that are least susceptible to introductions. Therefore, it is important to assess the availability of ponds and thus the vulnerability to introductions. Isolated waters with Pumpkinseed Sunfish are more often located near human settlements and infrastructure than would be expected based on randomly selected sites. This suggests that planning conservation practices are best done at distances greater than 250 m from human settlements and 100 m from roads and trails to minimise the chances of introductions (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 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 of mires is 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.