In the previous chapter, we dealt with the pro- 

 ductivity of communities ; in this chapter, we will be 

 concerned with the productivity of single species or 

 reprodiictivity. The rate at which a species repro- 

 duces and the frequency of its population turnover 

 can afifect the speed with which it occupies new areas, 

 becomes adapted to new niches, or evolves into new 

 races. In order to analyze the population dynamics 

 of a species, it is necessary to know its life history. 

 This involves the stages in its life cycle, mortality 

 rates of each stage, longevity, sex and age ratios, age 

 at which individuals become sexually mature, fecun- 

 dity, factors causing mortality, and so forth (Cole 

 1954). The proportion of different ages and sexes 

 gives the population a definite structure. All these 

 essential data may be conveniently summarized in the 

 form of life-tables. 



FECUNDITY 



15 



Ecological Processes 



and Community 



Dynamics: 



Species vary greatly in the characteristic 

 number of generations, broods, or litters produced 

 per year, and in the sizes of them. Protozoans often 

 divide so rapidly that they produce a new generation 

 every few hours. Plankton organisms, less fecund, 

 may produce a new generation every few days. Many 

 vertebrates breed but once a year ; some large animals 

 only once every two or three years. Several species of 

 small birds and mammals have two or more broods 

 per year. The female woodland white-footed mouse 

 in Michigan may produce three litters between early 

 April and early June, and two more between middle 

 August and early October (Burt 1940). Under fa- 

 vorable environmental conditions, rodents may con- 

 tinue to breed throughout the winter so that their 

 reproductive potential is enormous (Kalabukhov 

 1935). 



Innate capacity 



Reproductivity and 

 Population Structure 



The maximum size of a litter is determined by 

 the physiological and morphological characteristics 

 of the species. With mammals which produce vivip- 

 arous young, the size of the uterus and body cavity 

 as well as the number of mammary glands for suck- 

 ling the young after birth are limiting factors. With 

 birds there is a limit on the number of eggs that one 

 individual can cover and successfully incubate. In 

 species that do not take care of their eggs after lay- 

 ing, the number produced may be limited only by 

 the energy resources of the parent. This is indicated 

 in part by the inverse relation between number of 

 eggs produced and their average size (Lack 1954). 



210 



