Species assemblages and community structure of adult caddisflies along a headwater stream in southeastern Ghana (Insecta: Trichoptera)

Abstract

Adult caddisflies were collected at 12 sites along a 5 km stretch of a forested headwater stream in southeastern Ghana in March and November 1993 and the catches are related to environmental variables. Some 34,000 specimens, belonging to 178 morphospecies in 43 genera and 11 families, were caught with Malaise traps and light traps. Many species demonstrate differences between sampling method, seasonal occurrence, and sex-ratios, with the light trap in the second wet sunny season (November) being most efficient with respect to the diversity of the catches. A Moran index analysis for global structures indicates a major transition in the fauna from the ravine with waterfalls to the forest, and secondly, a gradient through the forest. A Geary index analysis for local structures indicates further faunal turnovers in the lower reaches of the stream. Species-environment relationships were analyzed through correspondence analysis and co-inertia analysis, allowing ordination of both species and the environment into three zones. The first co-inertia axis reveals a transition between the waterfalls in the open ravine (sites 1–3), and the stream through the riverine forest (sites 4–12), while the second co-inertia axis reveals a gradient from the lotic stream in the moist semi-deciduous forest (sites 4–8) to more lentic stretches of the stream in a gradually more depleted forest (sites 9–12). The most important environmental factors related to the species transition are: riparian moss growth (r=0.94), leaves in the stream substratum (r=0.91), forest type (r=0.90), hygropetric surfaces (r=0.88), bedrock (r=0.87), lotic [riffle] (r=0.85) and lentic [pool] (r=0.85) stretches. The most important environmental factors related to the species gradient are: lotic [riffle] stretches (r=0.92), gravel (r=0.88) and leaves (r=0.85) in the stream substratum, forest type (r=0.81), and agricultural use (r=0.76). Three clusters representing three major environmental zones (zone I, II and III) along the stream are identified by projecting the average species positions on the co-inertia plane. By analyzing the Malaise trap samples significant indicator values are found for 29 species in zone I, 16 species in zone II, and 18 species in zone III. By analyzing the light trap samples significant indicator values are found for 17 species in zone I, 57 species in zone II, and 38 species in zone III. The high diversity of caddisflies in the sanctuary might be reflected both in the geological and climatological history of West Africa, as well as in the recognition of three major environmental zones, and the clean water with high diversity of stream microhabitats favorable for Trichoptera larvae. The closed forest seems to play an important role for the Trichoptera community, and the study demonstrates the importance of protecting forested headwater streams in order to maintain a sustainable aquatic biodiversity in tropical Africa.