Leaf litter and woody debris are both important terrestrial subsidy input to streams. Despite wood breaks down slower and is less nutritious than leaf litter, wood is an important structurally diverse and temporally variable physical habitat to affect stream community composition. Immersed wood provides important functions in the stream ecosystems in terms of refuge and substrate habitat for benthic invertebrates and food for xylophagous fauna. However, in stream ecosystems of tropical Asian regions, studies of benthic invertebrates’ colonization on wood are limited.
We performed a field experiment in a forest stream in Hong Kong to investigate the taxa associated with wood and artificial substrate. Out research questions were: (1) what invertebrates colonize submerged woody debris in the stream, (2) how invertebrates’ colonization differs between wood and artificial substrate, and (3) how the invertebrate community structure changes over time.
The experimental treatments were (1) wood in pool, (2) wood in riffle, and (3) PVC pipe in riffle. Each treatment had 6 replicates. Both six pools and six riffles were arranged in an alternative pattern. Three pieces of wood were put in each pool, while 3 pieces of wood and 3 PVC pipes were put in each riffle. Stream invertebrates colonized on the substrates were sampled on days 35, 56 and 79 of treatment setup.
Overall, 28 taxa of invertebrates were found colonizing submerged wood in the stream, Diptera being the most dominant order of insects, Trichoptera being the most diverse order. A few of xylophagous (wood-gouging) invertebrates included Chironominae, Orthocladiinae, Elmidae and Trichopteran Ganonema extensum. Benthic invertebrates generally preferred to wood than plastic as indicated by statistically significant differences in total invertebrate abundance, taxon richness and densities of several common taxa between substrate types and between habitat types.
Benthic invertebrates generally were in favor of (1) colonizing natural wood than artificial plastic in riffles, and in favor of (2) colonizing wood in riffles than that in pools. The low number of taxa on wood with respect to the species-rich benthic community in the stream implied that wood might not be the most desirable habitat for invertebrates in the local context and the habitat heterogeneity was the primary determinant of benthos distribution in the stream. The structurally-complex natural substrates should harbor more species than the experimentally-immersed fresh wood substrates that were less complex in structure.