Comparing internal and external drivers in the southern Benguela and the southern and northern Humboldt upwelling ecosystems
AbstractTrophic models of three upwelling ecosystems, the southern Benguela (South African), southern Humboldt (Chilean) and northern Humboldt (Peruvian) systems, have been fitted to catch, abundance and fishing mortality time-series. Three drivers were considered during the model fitting: internal forcing by means of the trophic flow controls between the various interacting species groups, and two kinds of external forcing, namely fishing and the environment. The southern Benguela model was fitted to time-series data from 1978 to 2003, the southern Humboldt model to data from 1970 to 2004, and the northern Humboldt to data for a shorter period, 1995–2004. Fishing has been relatively carefully managed in the southern Benguela during the period modelled and previous studies found that most of the resource variability was attributed to internal trophodynamic forcing and to environmental forcing rather than to fishing. By comparison, fishing has been shown to have played a relatively major role in driving ecosystem changes observed in the southern and northern Humboldt models. Bearing in mind the different roles played by each of the drivers in these ecosystems, flow controls between interacting species groups, which improved the fits of the models, were compared across the three ecosystems to determine to what extent the three models supported the hypothesis that upwelling ecosystems function as wasp-waist systems. Secondly, environmental forcing was examined by searching for hypothetical forcing functions, affecting different levels of the foodweb, which improved the model fits. This was an attempt to start to uncover the processes that may be involved in linking the environment to observed ecosystem dynamics and changes in these upwelling ecosystems. Model results confirmed the important ecological role played by small pelagic fish in the studied upwelling ecosystems. For example, the fit of the southern Benguela model to time-series data of catch and abundance was similarly improved when anchovy/sardine–prey and anchovy/sardine–predator interactions were externally forced, supporting the waspwaist hypothesis. In addition, although physical drivers and conditions may differ in their nature or merely their frequency and intensity between systems, and different fishing strategies operate in each of the three ecosystems, model results suggest that these effects are transferred through the ecosystems and manifest themselves as ecosystem changes and observed resource dynamics largely via interactions with small pelagic fish.
Keywords: Benguela; drivers; ecosystem dynamics; environmental forcing; fishing; Humboldt; trophic model; upwelling
African Journal of Marine Science 2008, 30(1): 63–84