Dietary variation and food selection by mayfly grazers in a subtropical mountain stream

Abstract

Background: The classification of functional feeding groups of aquatic insects is often misleading in tropical/subtropical streams because their feeding habits are assumed to be the same as their temperate counterparts according to the mouthpart structure and foraging behavior. This study aimed to examine the diets and preferences of mayfly grazers (Baetis spp. and Rhithrogena ampla) in a subtropical mountain stream in the dry and wet seasons. Results: In the stream, epilithic algal communities on insect-excluded bricks (as a grazer-excluded control) were dominated by small adnate diatoms, most likely due to the high current velocity. Both grazers preferred understory and small adnate diatoms, Achnanthes spp. and Achnanthidium pyrenaicum, in both seasons. However, the stalked diatoms Gomphonema spp. were preferred only by Baetis, but not by Rhithrogena, in the dry season when the current velocity became relatively slower. The results of pairwise tests further showed that the algal compositions on the insect-excluded bricks coincided with the diets of both grazers in the wet season but were distinct from those on the bricks in the dry season. Seasonal variations in the algal compositions of the diets of Rhithrogena and on the insect-excluded bricks were apparent, but not in the diets of Baetis. Conclusions: The algal physiognomy was most likely attributed to the impact of the high current velocity in the stream. Our results suggest that the diet preference by mayfly grazers in the stream is potentially influenced by algal availability.