340 



POPULATION ECOLOGY: 



POPULATION EXTINCTION 



The exact causes of species extinction are poorly 

 understood. However, if one ignores times when re- 

 moval of life was relatively widespread, it seems that 

 the two primary causes are change in heredity and 

 change in environment. Hereditary shifts lead to ex- 

 tinction if there is a greater and greater increase in 

 lethal and detrimental genes. There is a belief that 

 gene mutations can lead to racial senility and explain 

 the death of species that were inhabiting seemingly 

 suitable environments. The second cause, variable 

 environment, may provide ecological conditions un- 

 der which a species no longer can perpetuate itself. 

 These environmental fluctuations can be either 

 catastrophic and sudden, as in volcanic eruptions 

 and the hardly slower activities of man, or they can be 

 gradual, as in slowly changing climates. 



The most dramatic extinction is that beyond the 

 population level, that afifecting entire communities 

 and even much of the life of the globe. Such devasta- 

 tion is recorded from the end of the Paleozoic and 

 Mesozoic Eras. The possible details of these events 

 are presented in the discussion of biogeographical dy- 

 namics in Chapter 1 9. 



POPULATION FLUCTUATIONS 



Populations are rarely static. Most have some 

 seasonal, periodic, and/or noncyclic variations in 

 numbers. The main factors causing these fluctua- 

 tions are birth and death. The reproductive rate is 

 related to the life history and habits of each species 

 in its environment, and mortality rate is adjusted to 

 each reproductive rate. 



REPRODUCTIVE RATE 



Reproductive rates are affected by the biotic po- 

 tential as represented by types of reproduction, 

 fecundity, fertility, and reproductive periodicity. 

 They are affected by environmental resistance as 

 represented by the complete environment, especially 

 population density. Each of the four general types of 

 reproduction has a different potential insofar as re- 

 productive rates are concerned. Sexual reproduction 

 provides organisms with great hereditary variation, a 

 gene pool upon which selection can act to produce 



generations of individuals ever more well-suited to 

 their environment. Parthenogenesis, virgin birth, al- 

 lows rapid production of individuals that often are 

 suited to their environment. Self-fertilizing hermaph- 

 roditism regularly results in a low reproductive rate 

 when environmental changes occur, because the he- 

 redity of the offspring is like that of the parent and 

 there is little opportunity for individual hereditary 

 types to adjust to their changing habitat. Asexual re- 

 production influences reproductive rates much as does 

 parthenogenesis, because genetically there is little dif- 

 ference between the two phenomena. 



Fecundity and fertility are kindred functions. 

 Fecundity is the rate of formation of the units of 

 reproduction, usually eggs and sperm, and fertility is 

 the rate of production of living offspring. There are 

 both individual and species rates of fecundity and 

 fertility. These dififerent rates stem from variations in 

 the ages at which young reach maturity and in the 

 sizes of individuals at maturity. Within a species 

 larger organisms regularly have more oflTspring than 

 do smaller individuals. Other somewhat related 

 variables are environmental, annual, and geographic 

 effects upon the bearing of young. Although the lat- 

 ter three effects can be independent of one another, 

 environmental factors normally have annual and geo- 

 graphic variations that act upon fertility. For ex- 

 ample, in the "average" environment, with its sea- 

 sonal diversity and geographically "unique" features, 

 reproduction often is during a particular time of the 

 year; in other geographic areas it may be at different 

 times. Also, a single locale's ecological variations 

 can implement reproduction. In one instance an 

 early spring might advance the reproductive season; 

 in another, unfavorable weather might either reduce 

 the number of or even eliminate offspring. 



Reproductive periodicity, the bearing of young at 

 approximately the same time each year, was said to 

 be modified by environmental factors. However, most 

 organisms have a definite season within a year or 

 definite number of years between reproductive 

 periods. Whether offspring are produced one or more 

 times a year or less frequently, reproduction in 

 temperate and arctic plants and animals tends to be 

 during the spring, the best time for the appearance of 

 offspring. During the spring, the surrounding fac- 

 tors, especially food, water, and temperature, are 

 most favorable for the young, which are more sus- 

 ceptible to environmental extremes than are the 

 adults. 



