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キーワード:
funcitonal feeding groups; shredders; stream; Stenochironomus; decomposition
要旨:
Leaf litter is a major component in the organic matter budgets of streams worldwide. Shredding invertebrates are widely considered to be of central importance for the breakdown of allochthonous organic material in temperate-zone streams. However, various authors report an absence of this group in tropical streams. Various phenomena, including hydraulic disturbance, chemical leaf quality, and biotic control through macroconsumers, may cause variable shredder performance in streams. Our paper discusses the hypothesis that biogeographic distribution and the life-cycle strategies of the shredders are additional
factors affecting the contribution of invertebrates to lotic decomposition processes. As a case study, we analyzed the type of organic matter inputs, the community of benthic invertebrate shredders, and the decomposition of temperate zone leaves (alder, Alnus glutinosa) in the Breitenbach (BRB), a temperate stream in Hesse, Germany, and in the Córrego Tenente Amaral (CTA), a neotropical Cerrado stream in Mato Grosso, Brazil. Quantities of natural leaf-litter inputs were comparable in the 2 systems (CTA: 820 g m-2y-1, BRB: 700 g m-2y-1), but the diversity of trees varied from 60 species/ha in CTA to 5 in BRB. Several shredding species were found in CTA, but in very low abundances. Larvae of the shredding calamoceratid caddisfly Phylloicus sp. were found only in lateral pools and not in the stream channel at CTA. Earlier decomposition experiments with naturally fallen native leaves did not indicate any importance of shredders at CTA, but green leaves of A. glutinosa were rapidly consumed by mining chironomids (Stenochironomus sp.). Decomposition rates were faster in CTA (–k=0.035 ± 0.006) than in BRB during wintertime (0.0095 ± 0.0021). We conclude that detritivorous neotropical insects may have 2 different species traits. Either they have mass occurrences during short periods when their food source is available and they must be adapted to a secondary, permanently available resource on which they can survive for the rest of the time (facultative specialists), or they are restricted to the few sites that provide a permanent food source (localized specialists). Apart from these special cases, however, the community in the neotropical stream was largely composed of omnivores.
