Exponential Increase and Decline

If the instantaneous rates of mortality and birth are consistently different for an extended period a population can show an exponential change in number. Figure 4 shows that the large prawn, Palaemon serratus, underwent an

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Figure 3 The change in annual abundance of the gurnard, Eutrigla gurnardus, in Bridgwater Bay, England. While there is some evidence for a small upward trend in numbers, the most striking feature of this time series is the greatly increased abundance in 1992.

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Figure 3 The change in annual abundance of the gurnard, Eutrigla gurnardus, in Bridgwater Bay, England. While there is some evidence for a small upward trend in numbers, the most striking feature of this time series is the greatly increased abundance in 1992.

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Figure 4 The change in annual abundance of the prawn, Palaemon serratus, in Bridwater Bay, England. This species underwent an exponential increase in abundance between 1987 and 2002. The population then suddenly fell back in abundance but seems to be rising again.

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Figure 4 The change in annual abundance of the prawn, Palaemon serratus, in Bridwater Bay, England. This species underwent an exponential increase in abundance between 1987 and 2002. The population then suddenly fell back in abundance but seems to be rising again.

approximately exponential increase in abundance between 1987 and 2002. This was demonstrated by plotting log number against time, which gave a good fit to a straight line. A major downward correction then occurred, which was at some stage inevitable as any population growing exponentially must eventually experience resource limitations. The increased abundance of this prawn is probably linked to recent climate warming and raised seawater temperature allowing increased growth and a longer reproductive season. While the long-term implications are still unclear, it seems likely that the exponential growth phase was a short-term response as the population increased to take possession of a larger

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Figure 5 The exponential decline in the annual abundance of eel, Anguilla anguilla, in Bridgwater Bay, England. This rate of decline has been observed at other localities across Europe.

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Figure 5 The exponential decline in the annual abundance of eel, Anguilla anguilla, in Bridgwater Bay, England. This rate of decline has been observed at other localities across Europe.

niche and will be followed by some degree of stabilization at a higher population level.

A clear example of exponential decline is shown by the common eel, Anguilla anguilla in Figure 5. This dramatic decline toward possible local extinction of a fish that was once highly abundant is probably linked to a multifactorial increase in mortality rate. Man has been responsible for over fishing, producing impediments to migration such as weirs and dams and finally introducing a deadly parasite. While such declines are becoming common because of human destruction, they can also occur naturally because of the outbreak of disease or climate change. This population decline must free resources allowing other species to increase as they use the resources previously taken by the eels.

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