The Consequences of Competition Among Producers and Consumption by Herbivores
In Chapter 4 we considered how competition could in theory and in nature affect abundance of organisms. In this chapter we will briefly introduce models and concepts that predict the effects that consumers might have on their food species, especially producer species. Then we discuss some specific case histories that permit an assessment of the impact of grazing on marine producers.
KeywordsSalt Marsh Grazing Pressure Rocky Shore Kelp Forest Food Species
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- Herbivores may have many additional effects on vegetation, including influencing plant morphology. For example, some algae of the upper intertidal zone in rocky shores exist as upright morphs during the part of the year where grazing pressure is low, while crustose or boring morphs are dominant when grazers are more active (Lubchenco and Cubit, 1980).Google Scholar
- In Nova Scotia, the main kelp is Laminaria longicrurus and the sea urchin is Strongylocentrotus droebachiensis. Google Scholar
- S. alterniflora is a species that shows C4 metabolism. The name C4 derives from the fourcarbon 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. 85, 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.Google Scholar
- In terrestrial plants extensive support and vascular systems are needed to keep photosynthetic tissues exposed to adequate light and transport nutrients; in the water column water movement supplies these functions. The extensive, nongrowing tissues of terrestrial plants slow turnover rates (Table 2–3). This contrast is also important in chemical defense, since many of the deterrents are usually chemically associated with structural tissues.Google Scholar
- The larger size categories (2 cm to 20 m) often consist of active swimmers (nekton) in addition to the more passively drifting plankton. Such distinctions are not always clear, and different classifications have been forwarded (Sieburth et al., 1978).Google Scholar
- This is an instance of a property that emerges only when consideration is given to the community level, including algae, grazers, and predators. There is a growing awareness that there are significant interactions, not only between producers and herbivores but also among producers, herbivores, and carnivores, and that these three-way interactions can be crucial. This is the message, for example, of the review by Price et al. (1980) of interactions among terrestrial plants, insects, and their predators.Google Scholar