Rift valley-driven species richness, composition and phenology of Odonata in central Zambia

One hundred and seven species of damselflies and dragonflies were recorded at 66 localities in central Zambia during three expeditions between 2013 and 2017, with 104 species at 62 localities in the main study area, in broad environs of Chingombe. The relatively great richness and composition of the odonate fauna in this small remote area was a consequence of the close proximity of three quite different geomorphological units – the flat bottom of the rift valley, the steep mountainous slopes bordering it, and undulating uplands, with a combination of their climatic and hydrological conditions. An odonatological ‘river-continuum’ sequence was studied, from the upper and lower reaches of the mountain stream, through submontane streams and rivers down to the large river draining the rift valley. This spatial sequence was characterised by large-scale interpenetration of the odonate assemblages, with more specific fauna of the mountain stream and large river, and rich, but less specific that of submontane watercourses. Fluent replacement of some species with their related counterparts was recorded, i.e. more upland species of closed habitats by species related to semi-open lower-elevated watercourses. The range of species well adapted to temporary waters was rich but differed between the small waters of the tropical savannah bottom of the rift valley and larger and longer existing water bodies in seasonally inundated dambos in more subtropical uplands. The phenological pattern was generally well-structured due to two distinct peaks, the first in the first phase of the wet season, both in the large river and temporary waters, and the second at the end of the wet season, in the latter. Phenology of odonate assemblages in the temporary waters was based on two consecutive aspects dominated respectively by adults reproductively active in the early–mid season and tenerals emerging at the end of it. At the beginning of the wet season, the reaction of adult odonates to first heavier rains was extremely rapid and even preceded the formation of the seasonal water body. In the large river, the phenological shift towards the early wet season with a low-water phase and favourable habitat conditions was clearly discernible. In turn, the odonate fauna in another low-water phase, at the end of the wet season and before a cool phase of the dry season, was dramatically poor. In the smaller permanent watercourses, phenology of the odonate assemblages during the wet season was generally less patterned, without a dramatic change of generations and with the emergence more dispersed in time, though centred on the first half of the season. As an effect of replacing individuals and the prolonged occurrence of many species, the odonate assemblages, while richer in the first half of the rainy season, appeared more stable and continuous than those of the temporary waters and the large river.