Arthropod 'rain' into tropical streams: the importance of intact riparian forest and influences on fish diets

Terrestrial arthropods might represent an important energy source for stream predators, but these trophic linkages have seldom been studied in the tropics. Terrestrial arthropod inputs (essentially, arthropod 'rain') into four streams with different riparian vegetation (two draining shrublands and two draining forests) were measured over three consecutive seasons (dry, wet, dry) from 2005 to 2007 in monsoonal Hong Kong. Predatory minnows, Parazacco spilurus (Cyprinidae), were collected and their consumption of terrestrial arthropods was estimated. Inputs of arthropods were dominated by Diptera, Collembola, Formicidae and aerial Hymenoptera, accounting for >= 73% of the arthropod abundance. Seasonal variation was marked: numbers in the dry seasons were approximately half (47-57%) those in the wet season, and biomass fell to one-third (33-37%) of the wet-season value. Shrubland streams received 19-43% fewer individuals and 6-34% less biomass than shaded forest streams. An analysis of fish diets in three of the four streams showed that terrestrial insects and spiders were more important prey in the two forest streams, accounting for 35-43% of prey abundance (39-43% by volume) v. 28% (27%) in the shrubland stream. Because riparian vegetation is the source of terrestrial arthropod inputs to streams, degradation of streamside forests that reduce these inputs will have consequences for the diets of stream fishes.