346 



POPULATION ECOLOGY: 



valleys in a manner that can be likened to man's 

 following a highway. However, this cannot explain 

 how certain other species home. For example, the 

 young that hatch during the breeding season might 

 precede the adults to the nonbreeding grounds. 

 Also, other species migrate high and at night, con- 

 ditions under which land features are not likely to be 

 used; and some species migrate over water, out of 

 sight of the land. 



Various attempts have been made to explain the 

 mechanism of such homing. Most modern hypoth- 

 eses of cause and effect are related to a peculiar 

 fan-shaped structure, the pecten, within the eye of a 

 bird. All that is definitely known about the structure 

 is that it is present in the eye, an organ that must be 

 involved in any mechanism of migration. However, 

 the pecten also is present in some reptiles that hardly 

 move at all. Although this might appear to consti- 

 tute a problem in the hypothesis, it does not. There 

 are various examples of like structures that have dif- 

 ferent functions in different organisms. 



However, even if one assumes that the pecten does 

 perceive the stimuli that enable navigation, the prob- 

 lems are just beginning. What is perceived? Here 

 again, hypotheses vary because data on different 

 birds indicate a number of possibilities. The better- 

 documented conclusions imply orientation by stars, 

 sun, and/or the earth's magnetic field. Another 

 possibility, apparently less likely, is the use of the 

 Coriolis force, the force of the earth's rotation 

 which deflects northern hemisphere air masses to the 

 right (clockwise) and southern hemisphere air masses 

 to the left (counterclockwise). 



Once again, even if one assumes that the pecten re- 

 ceived stinnuli from a particular set of navigational 

 aids, how are these stimuli used in finding a migra- 

 tion area.' In cases where young birds migrate with 

 the adults, the answer might be learning. However, 

 how can one explain the navigation of young birds 

 that first migrate by themselves? The only possible 

 answer at the present time is hereditary behavior pat- 

 tern, or "instinct," perhaps a tidy way of clouding 

 ignorance. 



EMIGRATION AND IMMIGRATION 



Emigration is associated with overpopulation, but 

 there are cases of permanent departure without prior 

 increase in numbers. Many of the latter emigrations 

 are related to such things as reduced food supply or 



modified climate, but there are other seemingly un- 

 explanable movements. No matter what the cause, 

 home desertion often ends in death; however, in some 

 species the movement assumes a semipermanent 

 wandering state, or nomadism, and in others causes a 

 permanent range extension. However, death is prob- 

 able because the species is not likely to find an 

 available suitable habitat. It is true that traveling 

 animals might reach a locality already occupied by 

 their species, but the new arrivals rarely can find 

 room in which to live (immigrate), or can compete 

 with their resident kinsmen. 



POPULATION COMPOSITION 



The components of groups up to and including a 

 single species are determined mostly by reproductive 

 and mortality rates, but also by movements of in- 

 dividuals into and out of an area. Moreover, seasonal 

 and other periodic fluctuations in rates and/or move- 

 ments cause the makeup of populations to be ever 

 changing. 



Of most importance in reproduction and mortal- 

 ity control of population structure is the length of 

 life span. At one life-span extreme is the single age 

 class found in ephemeral annual plants, species that 

 locally are progressively germinating, growing, 

 flowering, fruiting, shedding seeds, or dying, but 

 usually never more than one of these functions is 

 present at a given time. The next life-span step is 

 the population of biennial plants with two age 

 classes, the first-year and second-year forms. In 

 animal populations the short-lived species may be as 

 simple as biennials, or even annuals. For example, 

 many invertebrates resemble annuals in that a 

 single generation is hatched, reproduces, and dies 

 each year. 



More complex age classes are found in perennial 

 plants and long-lived animals. When some members 

 become reproductively mature each year, and es- 

 pecially over a period of each year, the makeup of 

 the population contains many age classes of im- 

 matures and reproductives. In some cases, the com- 

 plex organizations also have one or many age classes 

 of postreproductives, as is the case in man. 



Variations exist within, as well as among, species. 

 Within species, changes in population structure may 

 be found by comparing geographically isolated sub- 



