Occurrence and Population Density of the Endemic Species Cordulegaster buchholzi (Anisoptera: Cordulegastridae) on the Cyclades Islands in Greece

SIMPLE SUMMARY: Cordulegaster buchholzi is an endemic species of southern Greece with a centre of occurrence on the Cyclades Islands. Detailed research on the population of the species was carried out on the islands of Andros, Tínos and Náxos between the years 2010–2023. Based on larval findings, occurrence was found in only 19 streams on these islands. The species inhabits sloping, incised streams in narrow rocky valleys, the beds of which are stony, often even bouldery or with rocky banks. Streams are surrounded by forest belts or a segment of forests dominated by Platanus orientalis and Alnus glutinosa. The biotopes are strongly influenced by human activity; there is a significant impact of water use for irrigation, pollution from grazing cattle excrement, potential deforestation and forest fires. The range of the species (i.e., extend of occurrence) is only 100 km(2); the species is therefore among the rarest species of European dragonflies. ABSTRACT: Our research was focused on determining the geomorphological characteristics of streams, characteristics of sediment in streams, habitat, emergence sites and flight period. Larvae were recorded in 19 streams (altitude of 35–680 m a.s.l.), with an average minimum width of 44.2 cm, an average maximum width of 352.9 cm, an average minimum depth of 9 cm and an average maximum depth (in pools) of 55 cm, with an average stream gradient of 12 grades (range 0.6–45 grades). In terms of grain size, the sediment in these biotopes can be characterized as sandy gravel, medium-grained gravel with an admixture of fine sand and an admixture of coarse-grained gravel prevails (with dominancy of fraction 2–5 mm with a representation of 47%). The larval density reached 0.1–62.2 larvae per 1 m(2) of suitable sediment. Exuviae (100 exuviae found in total) occurred at an average of 66 cm horizontal distance from the shore and an average vertical height of 124 cm above the ground. The average total distance of larval movement was 190 cm. The emergence site was categorized as larvae-dominated tree trunks (57% of cases), rocks (51%) and overhanging rocks (11%). The flight period was recorded from 17th May to 15th July (literary record—to 15th August) with peak flight activity noted in the third quarter of June. Considering the size of the area—extent of occurrence, the population of C. buchholzi is strongly threatened; according to the IUCN categories it should be classified as endangered (EN).