Abstract
The previous chapter highlighted the influence of consumers on their food resources, and defined some consequences of such top-down controls. Here, we discuss how resources might influence producers and consumers, effects that can cascade upwards through food webs. We first review selected cases of bottom-up control, then look at evidence of interaction of top-down and bottom-up controls. We end the chapter by examining how both top-down and bottom-up controls are increasingly subject to external forcing by climate-driven change and human activities.
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Notes
- 1.
The physical oceanography underlying these processes is explained in texts by Mann and Lazier (2006) and Knaus (2005).
- 2.
The effects of the rotation of the earth on wind-driven currents are reversed in the two hemispheres, so in the western margin of continents northerly winds result in upwelling in the northern hemisphere, while southerly winds lead to upwelling in the southern hemisphere. The Coriolis effect does not lead to high productivity in eastern margins of continents because the deflection moves nutrient-poor oceanic water toward the coast. This may also result from northerly winds in the southern hemisphere and southerly winds in the northern hemisphere. An example of the latter is the “downwelling” in the Washington, OR coast during winter (Duxbury et al. 1966).
- 3.
El Niño (The Child) episodes start toward the end of the calendar year (hence the connection to the Nativity), when warmer water intrudes into coastal areas of Perú, preventing upwelling of cold, rich water. The result is markedly lowered primary production and massively reduced abundance of anchoveta. The hydrodynamics of El Niño are connected to events going on at surprisingly distant areas of the tropical Pacific (Wyrtki 1975).
- 4.
The five major upwelling regions of the world that support very large fisheries include the Canary Current (off NW Africa), the Benguela Current (off southern Africa), the California Current (off California and Oregon in the USA), the Humboldt Current (off Peru and Chile), and the Somali Current (off Somalia and Oman). There are many other upwelling areas of lesser extent scattered around the coastlines of the world.
- 5.
Some anthropogenic activities, especially release of wastewater, also release land-derived organic matter into marine environments. This input increases oxygen demand, provides a source of externally furnished energy, alters availability of pollutants, and generally enhances the biogeochemical changes brought about by increased delivery of nutrients to marine environments. At a global scale, these inputs are small compared to anthropogenic CO2 inputs, but organic inputs may be important at regional scales (Spitzy and Ittekot 1991).
- 6.
S. alterniflora is a species that shows C4 metabolism. The name C4 derives from the four-carbon compound that is the first product of carbon fixation rather than the three-carbon compound typical of the more usual Calvin cycle metabolism. Such C4 species have a number of remarkable biochemical, physiological, and ecological properties (Black 1971, 1973). One property of C4 plants relevant here is that they are relatively free of herbivores (Caswell et al. 1973), and one reason for this is their relatively low nitrogen content (Fig. 8-6, top). The experimental fertilization increases the percentage nitrogen of S. alterniflora to that of the C3 plants, and the ensuing response of the herbivores shows one reason why C3 plants are more attractive to herbivores.
- 7.
A similar contrast in controls associated with larger and smaller scales was reported for anchoveta and zooplankton in the Peruvian upwelling (Ayón et al. 2008). Hunt (2006) makes the same point, and suggests that the relative effect of top-down and bottom-up controls may be spatial-scale dependent.
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Valiela, I. (2015). Food Web Structure and Its Controls II. In: Marine Ecological Processes. Springer, New York, NY. https://doi.org/10.1007/978-0-387-79070-1_11
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DOI: https://doi.org/10.1007/978-0-387-79070-1_11
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