Vertical heterogeneity of a forest floor invertebrate food web as indicated by stable-isotope analysis
Diverse populations of invertebrates constitute the food web in detritus layers of a forest floor. Heterogeneity in trophic interactions within such a species-rich community food web may affect the dynamic properties of biological communities such as stability. To examine the vertical heterogeneity in trophic interactions among invertebrates in litter and humus layers, we studied differences in species composition and variations in carbon and nitrogen stable-isotope ratios (δ13C and δ15N) using community-wide metrics of the forest floors of temperate broadleaf forests in Japan. The species composition differed between the two layers, and the invertebrates in the litter layer were generally larger than those in the humus layer, suggesting that these layers harbored separate food webs based on different basal resources. However, the δ13C of invertebrates, an indicator of differences in the basal resources of community food webs, did not provide evidence for separate food webs between layers even though plant-derived organic matter showed differences in stable-isotope ratios according to decomposition state. The minimum δ15N of invertebrates also did not differ between layers, suggesting sharing of food by detritivores from the two layers at lower trophic levels. The maximum and range of δ15N were greater in the humus layer, suggesting more trophic transfers (probably involving microorganisms) than in the litter layer and providing circumstantial evidence for weak trophic interactions between layers at higher trophic levels. Thus, the invertebrate community food web was not clearly compartmentalized between the detrital layers but still showed a conspicuous spatial (vertical) heterogeneity in trophic interactions.
KeywordsCompartment Food web structure Microhabitat
We thank Y. Takami, Y. Takeuchi and H. Nakagawa for statistics, M. Yoshida, M. Ito, H. Takeda, S. Saito, T. Tanigaki, H. Nishi, S. Yamamoto, S. Gotou for taxonomy, C. Hori, T. Takeyama and N. Nagata for technical assistance, and D. Gustafson for English text assistance. We also thank Prof. M. Hori and the members of the Laboratory of Animal Ecology, Kyoto University, for advice and discussion. This research was supported in part by grants-in-aid for Biodiversity Research of 21st Century COE (A14) and Global COE Program “Formation of a Strategic Base for Biodiversity and Evolutionary Research: from Genome to Ecosystem” from the Ministry of Education, Culture, Sports and Technology, Japan, and a grant-in-aid from the Japan Society for the Promotion of Science (No. 2037011 to TS and No. 19681002 to IT).
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