Effects of microbivore species composition and basal resource enrichment on trophic-level biomasses in an experimental microbial-based soil food web |
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Authors: | Juha Mikola |
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Institution: | Department of Biological and Environmental Science, University of Jyv?skyl?, P.O. Box 35, FIN-40351 Jyv?skyl?, Finland, FI
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Abstract: | Previous theoretical and empirical evidence suggests that species composition within trophic levels may profoundly affect
the response of trophic-level biomasses to enhanced basal resources. To test whether species composition of microbivorous
nematodes has such an effect in microbial-based soil food webs, I created three microcosm food webs, consisting of bacteria,
fungi, bacterial-feeding nematodes (Acrobeloides tricornus, Caenorhabditis elegans), fungal-feeding nematodes (Aphelenchus avenae, Aphelenchoides sp.) and a predatory nematode (Prionchulus punctatus). The food webs differed in species composition at the second trophic level: food web A included A. tricornus and Aph. avenae, food web B included C. elegans and Aphelenchoides sp., and food web AB included all four species. I increased basal resources by adding glucose to half of the replicates of
each food web, and sampled microcosms destructively four times during a 22-week experiment to estimate the biomass of organisms
at each trophic level. Microbivore species composition significantly affected bacterivore and fungivore biomass but not bacterial,
fungal or predator biomass. Greatest bacterivore and fungivore biomass was found in food web A, intermediate biomass in food
web AB, and smallest biomass in food web B. Basal resource addition increased the biomass of microbes and microbivores but
did not affect predator biomass. Importantly, microbivore species composition did not significantly modify the effect of additional
resources on trophic-level biomasses. The presence of a competitor reduced the biomass of A. tricornus and Aph. avenae, in that the biomass of these species was less in food web AB than in food web A, whereas the biomass of C. elegans and Aphelenchoides sp. was not affected by their potential competitors. The biomass of Aph. avenae increased with additional resources in the absence of the competitor only, while the biomass of A. tricornus and Aphelenchoides sp. increased also in the presence of their competitors. The results imply that microbivore species composition may determine
the second-level biomass in simple microbe-nematode food webs, but may not significantly affect biomass at other levels or
modify the response of trophic-level biomasses to enhanced basal resources. The study also shows that even if the role of
predation in a food web is diminished, the positive response of organisms to increased resource availability may still be
hindered by competition.
Received: 22 June 1998 / Accepted: 28 August 1998 |
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Keywords: | Food web structure Resource availability Predation Nematode Soil microcosm |
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