Ratio-dependent response of a temperate Australian estuarine system to sustained nitrogen loading

Bishop, Melanie J., Kelaher, Brendan P., Smith, Marcus P. Lincoln, York, Paul H., and Booth, David J. (2006) Ratio-dependent response of a temperate Australian estuarine system to sustained nitrogen loading. Oecologia, 149 (4). pp. 701-708.

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Abstract

Classical resource- and the less studied ratio-dependent models of predator–prey relationships provide divergent predictions as to the sustained ecological effects of bottom-up forcing. While resource-dependent models, which consider only instantaneous prey density in modelling predator responses, predict community responses that are dependent on the number of trophic levels in a system, ratio-dependent models, which consider the number of prey per consumer, predict proportional increase in each level irrespective of chain length. The two models are only subtly different for systems with two or three trophic levels but in the case of four trophic levels, predict opposite effects of enrichment on primary producers. Despite the poor discriminatory power of tests of the models in systems with two or three trophic levels, field tests in estuarine and marine systems with four trophic levels have been notably absent. Sampling of phytoplankton, macroinvertebrates, invertebrate-feeding fishes, piscivorous fishes in Kooloonbung Creek, Hastings River estuary, eastern Australia, subject to over 20 years of sewage discharge, revealed increased abundances in all four trophic levels at the disturbed location relative to control sites. Increased abundance of phytoplankton at the disturbed site was counter to the predictions of resource-dependent models, which posit a reduction in the first trophic level in response to enrichment. By contrast, the increase in abundance of this first trophic level and the proportionality of increases in abundances of each of the four trophic groups to nitrogen loading provided strong support for ratio dependency. This first evidence of ratio dependence in an estuarine system with four trophic levels not only demonstrates the applicability of ecological theory which seeks to simplify the complexity of systems, but has implications for management. Although large nutrient inputs frequently induce mortality of invertebrates and fish, we have shown that smaller inputs may in fact enhance biomass of all trophic levels.

Item ID: 33234
Item Type: Article (Research - C1)
ISSN: 1432-1939
Keywords: bBottom-up control; nutrient enrichment; predator–prey models; ratio dependence; resource dependence
Funders: Australian Research Council (ARC), University of Technology, Sydney, NSW Department of Land and Water Conservation, Hastings Council
Date Deposited: 17 Jun 2015 03:53
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 70%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 30%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960503 Ecosystem Assessment and Management of Coastal and Estuarine Environments @ 100%
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