322 Relations within the Species 



these fish had not only failed to gain but had actually lost weight 

 as a result of excessive competition. Another instance of the curtail- 

 ment of growth of sunfish brought about by overpopulation is shown 

 in Fig. 9.6. 



Beneficial Effects 



In the previous section we have seen that, if the population of a 

 species within any area continues to increase, a point will eventually 

 be reached at which harmful effects are produced. Before this point 

 is reached, the members of the population may receive definite bene- 

 fit from the presence of others of the same species. An isolated in- 

 dividual or a single pair of organisms is often not as able to deal 

 successfully with the environment as a group. This subject has been 

 extensively discussed by Allee (1931 and 1951). Moderate increase 

 in abundance of a plant or an animal may afford protection from 

 enemies or from physical features of the environment. It may ac- 

 celerate reproduction and improve survival. The degree and nature 

 of activity is also influenced by the density of the population ( Schuett, 

 1934). Learning by fishes and other animals has been shown to be 

 more rapid when other individuals are present than when they are 

 isolated (Welty, 1934). Groups of the same species also make pos- 

 sible a certain division of labor and the beginnings of social organi- 

 zation. Some of these effects of increased numbers that are bene- 

 ficial within the species will be considered in the following sections. 



Protection. Animals and plants in groups often protect each other 

 against harmful features of the environment without any special 

 group organization. In a thick stand of ti'ees a higher humidity 

 will be maintained and a better resistance to wind and water erosion 

 will be experienced than if the same individuals were growing in a 

 widely spaced manner. Furthermore, since the blowing away of 

 fallen leaves is largely prevented in a dense grove, the humus and 

 moisture content of the soil is increased. Groups of animals have 

 been shown to be less susceptible to poisons, oxygen lack, extreme 

 temperatures, and other environmental dangers than single indi- 

 viduals. 



The large number of honeybees living close together in a hive 

 enables these insects to modify the temperature inside their micro- 

 habitat to an extent that would be utterly impossible were the bees 

 living as solitary individuals. As winter conditions arrive, the bees 

 are able to maintain the temperature within their cluster in the hive 

 at 25 to 30°G by increased muscular activity. During hot days in 



