Species assemblages and community structure of adult caddisflies along a headwater stream in south-eastern Ghana (Insecta: Trichoptera).

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 (r¼0.94), leaves in the stream substratum
(r¼0.91), forest type (r¼0.90), hygropetric surfaces (r¼0.88), bedrock (r¼0.87), lotic [riffle] (r¼0.85)
and lentic [pool] (r¼0.85) stretches. The most important environmental factors related to the species
gradient are: lotic [riffle] stretches (r¼0.92), gravel (r¼0.88) and leaves (r¼0.85) in the stream substratum,
forest type (r¼0.81), and agricultural use (r¼0.76). 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.