Responses of an African wading bird community to resource pulses are related to foraging guild and food-web position

Cumming, Graeme S., Ndlovu, Mduduzi, Mutumi, Gregory L., and Hockey, Philip A.R. (2013) Responses of an African wading bird community to resource pulses are related to foraging guild and food-web position. Freshwater Biology, 58 (1). pp. 79-87.

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Abstract

The immediate impacts of nutrient inputs on aquatic systems are well documented, but the ways in which resource pulses affect the wider food web of water-associated vertebrates remain obscure. 2.We monitored the wading bird community of Barberspan, a natural freshwater lake and Ramsar wetland in South Africa, before, during and after the addition of a pulse of nutrients in the form of a sewage overspill from an upstream processing facility. We counted waders at 13 points around the lake over 3years, every 2 months from March 2007 to March 2010, and sampled water quality during all counting periods from January 2008 to January 2010. 3.We used our data to test the hypothesis that wading birds that forage directly at lower trophic levels and/or on prey populations that have fast turnover rates, such as those of phytoplankton and invertebrates, will be more heavily influenced by nutrient addition than birds that forage on species with lower turnover rates and/or at higher trophic levels (such as frogs and fish). 4.During the sampling period Barberspan experienced a significant, nutrient-driven decline and subsequent recovery in dissolved oxygen and pH. This trend was mirrored by significant changes in the wading bird community. Partial Mantel tests and Canonical Correspondence Analysis (CCA) showed that the nutrient pulse had marked short-term, negative impacts on both the diversity and the abundance of medium-sized, shoreline foragers such as scolopacids (e.g. sandpipers). 5.Our analysis supports the proposal that both food-web position and the turnover rate of the prey population are strong influences on ecological responses to resource pulses. Analysis of time series of principal components that describe community composition suggested that recovery of the prey base was rapid and that the bird community was able to respond via immigration. These results must, of course, be considered provisional in the absence of replicated experimental data. 6.More generally, we interpret our results as suggesting that two different mechanisms act in different directions to determine the sensitivity of secondary and tertiary consumers to changes in their prey. First, for 'earlier' consumers (i.e. that forage lower in the food web), there are (on average) fewer generalist consumers and fewer stored nutrients in the intervening trophic levels. This increases the sensitivity of earlier consumer populations to changes in the composition of the primary consumer community. Second, the dynamics of prey populations lower in the food web tend to be faster, making recovery faster and serving to decrease the sensitivity of earlier consumers to perturbations. These dynamics may obscure the impacts of nutrient pulses in cases where additional analysis of system trajectories is not undertaken.

Item ID: 40957
Item Type: Article (Research - C1)
ISSN: 1365-2427
Keywords: Barberspan, eutrophication, nutrients, resilience, shorebird
Funders: USAID, Wildlife Conservation Society, DST/NRF Centre of Excellence at the Percy FitzPatrick Institute, University of Cape Town
Projects and Grants: Global Avian Influenza network for Surveillance (GRIPAVI) project
Date Deposited: 19 Nov 2015 01:31
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 50%
05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050199 Ecological Applications not elsewhere classified @ 50%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960501 Ecosystem Assessment and Management at Regional or Larger Scales @ 50%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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