182 



AGGREGATION, COMPETITION AND CYCLES 



eggs is also great under adverse weather conditions. The eggs of 

 many fislies are commonly dcstroj'ed by storms, floods, etc. 



Losses during young stages are of even greater importance than 

 loss of eggs. During periods of great abundance, the chinchbug may 

 be practically wiped out, owing to drowning of young by early spring 

 rains. Blegvad (1925) (Figs. 39 and 40) finds that the abundance of 

 bottom invertebrates in Danish waters is in part controlled by physi- 

 cal conditions during the transition period from the pelagic to the 

 bottom stage (cf. Johansen, 1927, 1929). 



A decline in the reproductive rate may be brought about by failure 

 to mate. Unsuccessful lamb raising may result from failure of sheep 



1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 



Fig. 40. — Showing the rough agi'eement between the three species of fish-food 

 invertebrates, in relation to the total invertebrate population of the same com- 

 munity. Such relations appear to be the usual rule. (Modified from Blegvad, 



1925.) 



to mate, on account of lack of sharp temperature changes from day 

 to night, which tend to stimulate the reproductive processes (Johnson, 

 1924). Foetal atrophy is common in the pig and rabbit, according to 

 Hammond (1921). A decline in the production of eggs is associated 

 with reduced numbers of grasshoppers (Parker, 1930). There are 

 other cases of reduced reproductive rates for which no apparent cause 

 has been assigned. 



Death of Well-developed or Mature Stages from Adverse Physical 

 Conditions. The later instars of insect larvae, weaned mammals, birds 

 out of the nest, etc., and adults of all types of animals often succumb 

 to unfavorable factors. Porpoises, fishes of various kinds, including 

 well-known species such as cod and plaice, crustaceans such as crabs, 

 lobsters, shrimp, and amphipods, are destroyed by extremely cold 



