Abstract
There are many studies that demonstrate that the abundance and species composition of species in an environment are determined by a combination of competitive interactions among the species and the effect of predators on those species. As in the case of herbivores, the impact of predation may not be straightforward, as we can see in the following case histories.
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The fact that large urchins are present in high densities in Shemya seems unusual. Other studies referred to earlier show that stunted populations were more often the result of crowded, overgrazed conditions where the key predator was not present. Perhaps in Shemya there is enough horizontal transport of food from the rich rocky intertidal zone into the area where urchins are found (Simenstad et al., 1978) or cannibalism of young urchins so that stunting is prevented.
During the enslavement of Aleut hunters by Russian fur traders, the Aleut nearly eliminated the sea otter population (Kenyon, 1969). Thus, when they had to, these hunters could bring about a marked decrease in abundance of sea otters.
The genus of these snails has recently been changed to Nucella. We retain the earlier usage for convenience.
Recently it has become fashionable to question the validity of caging experiments in soft sediments, since caging is claimed to lead to artifacts, including changes in the sediments and fauna within the cages (McCall, 1977; Virnstein, 1977; Eckman, 1979). There is no
question that there are cage effects, but many of these changes are no doubt the result of changes in the fauna brought about by the lack of predators since organisms can markedly change the density and physiochemical properties of sediments (Aller, 1980a). The caging approach is too powerful a tool to dismiss too readily, especially when not many alternatives are available to directly study the effect of consumers. Cages should be used with careful attention to minimizing artifacts, and cage effects should be assessed using partial cages.
The supposition is that small predators about the same size as the macroinvertebrate prey are not effective as predators, as discussed in Chapter 5. This assumption needs some more careful examination, especially in soft sediments and in plankton communities.
Additional evidence is that the proportion of carnivorous zooplankton is much larger in depauperate tropical waters than in relatively rich subpolar waters (Vinogradov, 1970).
Such improvements in the “standard of living” after density has been lowered seem to be a rather general phenomenon. In the Baltic Sea, growth of flatfish increased after those populations were thinned out markedly in the 1920s (Persson, 1981). The phenomenon is not limited to marine organisms: food supply, wages, and quality of life in general improved for European people as a whole during 1350–1550, a period following severe outbreaks of the Black Death (Braudel, 1981, p. 193).
Actually blue whales are not rare in certain parts of the ocean (W. Watkins, personal communication), and since they have notable ability to move for very long distances, there is a good chance of recolonization of locally depleted areas if whaling pressure stops.
† 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 Peru, 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).
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© 1984 Springer-Verlag Berlin Heidelberg
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Valiela, I. (1984). The Consequences of Competition Among Prey and Consumption by Predators. In: Marine Ecological Processes. Springer Advanced Texts in Life Sciences. Springer, New York, NY. https://doi.org/10.1007/978-1-4757-1833-1_9
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DOI: https://doi.org/10.1007/978-1-4757-1833-1_9
Publisher Name: Springer, New York, NY
Print ISBN: 978-1-4757-1835-5
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