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

Coenagrion hastulatum faces threats due to degradation of its habitats caused by a combination of factors that are amplified by ongoing climate change. These factors involve acidification, 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, predation from invasive fish species, lack of or poor management plans and removal of tree cover essential for the species to escape hot summer temperatures. The European range has become fragmented with isolated occurrences mainly on higher elevations. It has especially declined in the lowlands, where many populations are lost, and it is threatened in Austria, Belgium, Bulgaria, the Czech Republic, France, Germany, Hungary, Slovakia, Slovenia, Switzerland and in the UK. In the north, it still thrives in the boreal zone but based on distribution trends the decline of the species is also clearly visible in countries situated in the core of its range, such as Finland, Lithuania, and Sweden. Based on occupancy modelling and distribution trends the species has been inferred to have declined by at least 40% during the past 10 years. The threats are not likely to cease during the next 10 years. The species is hence considered to be Vulnerable (A2c+3c) in Europe as well as in the EU27.

C. hastulatum has a large distribution area in the European region spanning the temperate and boreal zones from isolated occurrences in France and Scotland in the west to the Urals in European Russia. It is widespread and regionally very common in northern and north-eastern Europe where it mainly occurs in lowlands. It becomes progressively rarer with more patchy distribution west and south and increasingly associated with more upland terrain. In the Balkans, in the Pyrenees and in the Massif Central there are isolated occurrences as relicts after the ice age and these populations are completely restricted to higher altitudes (Boudot et al. 2015).

The global range extends east to the Kamchatka Peninsula.

C. hastulatum is rather common in suitable habitats in large parts of Northern Europe such as in Fennoscandia, some of the Baltic states and probably also in northern Belarus and European Russia. It has a scattered occurrence throughout most of the rest of its European distribution, with isolated disjunctions especially along the fringes of its southern range and at higher altitudes. It has shown a sharp decline for a long time in Western and Central Europe, especially in the lowlands, and it is severely threatened in Austria, Belgium, Bulgaria, the Czech Republic, France, Germany, Hungary, Slovakia, Slovenia (not found this century), Switzerland and in the UK. Based on distribution trends the decline of the species is also clearly visible in countries situated in the core of its range, such as Finland, Lithuania, and Sweden.

C. hastulatum is found mainly in water bodies which are nutrient-poor to moderately nutritious, not acidified. The habitats have a very varied and rich vegetation structure with both small and large floating leaf plants (especially Potamogeton polygonifolius and Nymphaea alba) as well as low and high emergent vegetation (such as Carex rostrata, C. lasiocarpa, Eriophorum angustifolium and Schoenoplectus lacustris). These vegetation structures must be present together and ensure a high degree of cover in the riparian zone (on average 75%). In fens where the species is absent, this percentage of vegetation cover is usually less than 40% (Termaat 2005). In smaller and shallower aquatic environments such as bog puddles, C. hastulatum can be very numerous. It is one of the most common dragonflies in forests in almost the entire Nordic region where it occurs to the tree line. Sink populations can occur along slow-flowing watercourses in Scandinavia. In larger lakes and in more nutrient-rich and open environments such as in distinct cultivated landscapes, the species is rare or absent. In areas with strong populations, it can however locally be abundant in more nutrient-rich environments such as man-made habitats in open agricultural landscapes, garden ponds and quarries (Billqvist et al. 2019). To the south, it becomes gradually more restricted to higher elevations, up to 2,500 metres, with most sites situated between 900 and 1,900 metres in Switzerland (Boudot et al. 2015).

Coenagrion hastulatum belongs to the group of species that are first on the wings in the spring. It occurs near the edges or in vegetation. It rarely spends time above open water and is therefore only rarely seen perching on straws or on floating vegetation in the water. It is also found adjacent to wetlands where it stays low in vegetation in clearings, among shrubbery, and along edge zones on bogs and in marshes. The eggs are laid in floating vegetation such as Potamogeton natans. The larvae are found in shallow areas with sparse vegetation, preferably in strands of Equisetum fluviatile and various Potamogeton species. The larval development lasts one to four years, but normally two to three. The exuviae are found from a few centimetres to a few decimetres up in aquatic vegetation or along the edges of waters (Billqvist et al. 2019).

The previous European assessment stated that C. hastulatum had shown a decline throughout western and southern Europe, but as it was widespread and still common in the north and east of its range, the decline was not perceived as serious on a European scale. It was however noted that present and future global warming and multiple drought periods could represent a major threat to breeding habitats throughout western and southern Europe. There, some localities could also be at risk of degradation by livestock, and the species in Germany was already locally declining due to agriculture-induced eutrophication and emissions (Boudot 2010). Since then, this decline has accelerated and 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. 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 extinction of C. hastulatum. Eutrophication, acidification, and desiccation have in recent decades been aggravated by climate change and these factors also reinforce each other. Many subpopulations in the lowlands and along the fringes of the distribution range are already lost. The ones that are remaining 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 distribution trends the decline of the species is also clearly visible in countries situated in the core of its range, such as Finland, Lithuania, and Sweden.

The main cause of the decline is the degradation of habitats. Nutrient-poor sites become less and less nutrient-poor, which has meant that larvae might suffer from an increased intra-guild predation from thermophilic species that previously were rare or did not occur in these environments. Loss of buffering (acidification), or eutrophication and desiccation changes the vegetation which gradually is replaced by other plants, such as Juncus effusus. The structure-rich vegetation, critical as it forms the habitat of the larvae, disappears. In addition, Sphagnum dries out during the summer so that the process of raising peat is halted. Water levels that are too high in the winter months can also have a negative effect because non-floating peat moss vegetation then drowns. The problem of eutrophication can have many different causes. In addition to the deposition of ammonia and sulphate from the atmosphere, leaf litter and eutrophic surface water can also cause a strong increase in available nutrients (Termaat 2005).

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 or semi-open habitats into forests. 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). In some areas, recreational pressure and clearing of the habitats has also resulted in the disappearance or a decline of the species (Termaat 2005).

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. When a bog is trenched and dewatered, the peat is oxygenated, the levels of pH changes and metals and nutrients that have been bound in the peat are released into the water. Because of the contact with oxygen that occurs during desiccation, extra organic material is broken down and the sulphide is converted into sulfuric acid. Fens that are fed by groundwater can become desiccated when the groundwater level drops too far in the summer months. This natural process is often enhanced by water extraction or irrigation of agricultural land and heightened by climate change (Rova and Paulsson 2015, Termaat et al. 2019). Higher water temperatures lead to a lower oxygen availability for the larvae which leads to higher mortality, change in larval development and flight period. 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 C. hastulatum this implies that higher mean temperatures alone can explain a rapid decline.

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.

C. hastulatum still thrives in the northern parts of its distribution and also locally 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 C. hastulatum 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.

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 of nutrient-poor habitats 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 scrub and trees adjacent to sites in open habitats in areas where hot summer days is 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. Sufficient buffering and a stable water level are crucial for the development of the specific vegetation the larvae are dependent upon. If the level is too low for longer periods, other vascular plants can establish themselves and out-compete the plant societies. A stable water table also makes it difficult for encroaching trees and shrubs to survive.

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