Changes in populations

I have long felt that our population is one that has survived massive upheavals over the last few hundred years, the black death, emigration, starvation, war and tuberculosis .There is little to support this , but there is an interesting paper written on fish which is heavily fished in Nature vol 452 pp 835-839..
Christian N. K. Anderson et al have written a paper in Nature ”Why fishing magnifies fluctuations in fish abundance “ showing that fished populations can fluctuate more than unharvested stocks. However, it is not clear why. They distinguish among three major competing mechanisms for this phenomenon, by using the 50-year California Cooperative Oceanic Fisheries Investigations larval fish record. First, variable fishing pressure directly increases variability in exploited populations. Second, commercial fishing can decrease the average body size and age of a stock, causing the truncated population to track environmental fluctuations directly. Third, age-truncated or juvenescent populations have increasingly unstable population dynamics because of changing demographic parameters such as intrinsic growth rates. They find no evidence for the first hypothesis, limited evidence for the second and strong evidence for the third. Therefore, in California Current fisheries, increased temporal variability in the population does notarise from variable exploitation, nor does it reflect direct environmental tracking. More fundamentally, it arises from increased instability in dynamics.
This is an empirical example of how selective harvesting can alter the basic dynamics of exploited populations, and lead to unstable booms and busts that can precede systematic declines in stock levels.
Maybe the culling of human populations has the same effect.
Anderson et al 2008, Why fishing magnifies fluctuations in fish abundance Nature vol 452 pages 835-839.

Martin Eastwood
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