Abundance may change continuously and pro- 

 gressively in one direction over a long period of time, 

 or variations in abundance may take the form of ir- 

 ruptions, catastrophes, or cycles. An understanding 

 of how and why such changes in abundance occur 

 is of considerable academic interest, and is of the ut- 

 most importance for the economic management of 

 fish and game, preservation of wildlife, and in animal 

 husbandry, agriculture, and forestry. 



Minor fluctuations of less than 2:1 or 3:1 are 

 often the result of sampling errors in estimating the 

 true size of the population. When the ratio of popu- 

 lation sizes from one period to another is greater 

 than can be explained by errors of sampling, the 

 fluctuations have meaning for which we should know 

 the causes. Population ratios from one year to another 

 are commonly of the order of 10:1, 100:1, or in in- 

 sects, up to 10.000:1 or more (Solomon 1949). 



PROGRESSIVE CHANGE 



17 



Ecological Processes 



and Community 



Dynamics: 



Irruptions, 



Catastrophes, 



and Cycles 



Populations that continue to increase or de- 

 cline over a period of years are said to change pro- 

 gressively. The phenomenon is demonstrated as a 

 species invades a new habitat or region or is becoming 

 extinct. Progressive change in numbers also occurs 

 with seasonal growth of populations. Long-time cli- 

 matic change may produce gradual changes in abun- 

 dance and distribution. Thus the amelioration of 

 winter temperatures in northern Europe since the 

 mid-nineteenth century correlates with the north- 

 ward dispersal and increase in abundance of several 

 species of birds and mammals (Kalela 1949). 



IRRUPTIONS, OUTBREAKS, PLAGUES 



The phenomenon of a population suddenly 

 exploding to supersaturate an area is called an irrup- 

 tion, outbreak, or plague. These terms are considered 

 here to be synonymous and to represent the time 

 when an animal is abundant or injurious enough over 

 an appreciable area to be noticed and recorded by 

 untrained observers (Carpenter 1940b). The num- 

 ber of rodents may be in the hundreds or thousands 

 per hectare, of insects in the millions. Outbreaks are 

 known to have occurred since the beginning of re- 

 corded history in Europe, Africa, and North Amer- 

 ica, especially in insects and rodents. Plagues of 

 European meadow voles were recorded 18 times in 

 France between 1792 and 1931 (Elton 1942). 



The cause and control of plagues have concerned 

 man since civilization began. Biological control of 

 these outbreaks by introducing parasitoids, parasites, 

 bacteria, and viruses to infect the species concerned 

 has been attempted. Once a foreign insect has be- 



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