Altitudinal variation of the life cycle of Poecilus fortipes (Coleoptera,Carabidae) in Eastern Siberia

Altitudinal variation of the demographic structure of populations of Poecilus fortipes (Chaud.) was studied in four mountain localities in the south of Eastern Siberia, and the life cycle of the species was reconstructed for each locality. The size of adults of both sexes decreased along the altitudinal gradient: the beetles from the Selenga middle mountains (600–650 m above sea level) were much larger, while those at the upper boundary of the forest-steppe belt in the Eastern Sayan (1500–1700 m asl) were smaller than others. Their linear dimensions decreased more distinctly in the transitional zone between middle and high mountains. The duration of the reproductive period did not depend on the altitude. However, the onset of oviposition shifted along the altitudinal gradient to the beginning of the vegetation season, due to its shortening and a decrease in the sum of effective temperatures. As the altitude increased, the mean number of eggs per female declined significantly from 15.9 ± 6.1 to 8.6 ± 5.8, whereas the maximum number of eggs increased from 26–28 to 40–43. On the contrary, the dates of emergence shifted gradually from the beginning and middle of the vegetation period to its middle and end. Populations from the Selenga middle mountains, the Vitim Plateau, and the lower margin of the forest-steppe belt in the Eastern Sayan mountains were characterized by a relatively low catch index and the absence of rebreeding old individuals, both these features increasing the risk of spontaneous local extinction. The annual life cycle with summer breeding was replaced along the altitudinal gradient with an obligate biennial life cycle with summer and early-summer breeding. This transition was observed as the altitude increased from 600–650 to 900–950 m asl while the sum of effective temperatures dropped below 1400°C, i.e., the value needed for implementation of the annual life cycle.