770 ANIMALS AND THEIR ENVIRONMENT 



relationships are often more important in determining the occurrence 

 and survival of organisms in nature than are the direct eftects of physi- 

 cal and chemical factors in the environment. 



One important attribute of a population is its density— the number 

 of individuals per unit area or volume, e.g., the number of animals per 

 square mile, of trees per acre in a forest, or millions of diatoms per 

 cubic meter of sea water. This is a measure of the population's success 

 in a given region. Frequently in ecologic studies it is important to know 

 not only the population density but whether it is changing and, if so, 

 what the rate of change is. Population density is often difficult to 

 measure in terms of individuals, but estimates such as the number of 

 insects caught per hour in a standard trap, the number of sea urchins 

 caught in a standard "sea mop," or the number of birds seen or heard 

 per hour, are usable substitutes. A method that will give good results 

 when used with the proper precautions is that of capturing, let us say, 

 lOU animals, tagging them in some way, and then releasing them. On 

 some subsequent day, another 100 animals are trapped and the pro- 

 portion of tagged animals is determined. This assumes that animals 

 caught once are neither more nor less likely to be caught again, and 

 that both sets of trapped animals are random samples of the popula- 

 tion. If the 100 animals caught on the second day include 20 tagged 

 ones, the total population of tagged and untagged animals in the area 

 of the traps is 500; x/100 = 100/20, hence x = 500. 



For many kinds of ecologic investigations, an estimate of the num- 

 ber of individuals per total area or volume, known as the "crude den- 

 sity," is not exact enough. Only a fraction of that total area may be a 

 habitat suitable for the population, and the size of the individual mem- 

 bers of a population may vary tremendously. Ecologists therefore cal- 

 culate an ecologic density, defined as the number, or more exactly as 

 the mass, of individuals per area or volume of habitable space. Trap- 

 ping and tagging experiments might give an estimate of 500 rabbits per 

 square mile, but if only half of that square mile actually consists of 

 areas suitable for rabbits to inhabit, then the ecologic density would 

 be 1000 rabbits per square mile of rabbit habitat. With species whose 

 individuals vary greatly in size, such as fish, live weight or some other 

 estimate of the total mass of living fish is a much more satisfactory 

 estimate of density than simply the total number of individuals present. 

 A graph in which the number of organisms, or its logarithm, is 

 plotted against time is a population growth curve (Fig. 37.6). Since such 

 curves are characteristic of populations, rather than of a single species, 

 they are amazingly similar for populations of almost all organisms from 

 bacteria to man. From a study of the human population growth curve 

 to date, and by comparing this curve to a general one, Raymond Pearl 

 estimated that the human population, about 2.2 billion in 1936, would 

 reach 2.65 billion in the year 2100 and would remain stable thereafter 

 unless there was some change in the ability of the earth to support 

 human life. Subsequent scientific discoveries may change somewhat the 

 estimated upper limit of the human population, but the principle that 



