. Animal Ecology. Animal ecology. FIG. 16-8 Relation between population size of house ml< expressed by average amount of food consumed per day emigration of mice away from the colony. The colony was si with five pairs of mice in January (Strecker 1954). Emigration The pressure of overpopulations can be re- lieved by mass emigrations of individuals from par- ticular localities as well as by their death. It has been shown experimentally that such emigrations will ac- tually occur under conditions of crowding in a mouse population. It is of interest that those individuals which remained contin
. Animal Ecology. Animal ecology. FIG. 16-8 Relation between population size of house ml< expressed by average amount of food consumed per day emigration of mice away from the colony. The colony was si with five pairs of mice in January (Strecker 1954). Emigration The pressure of overpopulations can be re- lieved by mass emigrations of individuals from par- ticular localities as well as by their death. It has been shown experimentally that such emigrations will ac- tually occur under conditions of crowding in a mouse population. It is of interest that those individuals which remained continued their normal rates of re- production. This contrasts with the drastic reduction, even cessation, of reproduction in other colonies from which emigrations were prevented. Two species of aphids placed in their optimum niches, one at the top. the other at the bottom of a single barley plant, multiplied to saturation and dis- persed downward and upward on the plant until both species came to exist side by side. Continued repro- duction and overcrowding forced surplus individuals to emigrate to surrounding plants over cm away, leaving the two populations in equilibrium on the original plant. In another experiment where plants were within cm of each other, the aphids spread to the preferred sites on the second plant rather than to less favorable spots on the first plant (Ito 1954). Emigrations under natural conditions occur when there is overcrowding in the migratory locust, lem- ming, grouse, snowy owl (Gross 1947), snowshoe rabbit (Cox 1936), Arctic fox (Braestrup 1941), gray squirrel, and occasionally in other species (Heape 1931, Dymond 1947). The emigrations of the European lemming in the Scandinavian countries are spectacular (Elton 1942). Emigrations on a re- duced scale are known to occur also with lemmings in North America (Thompson 1955). Lemming emigrations do not invariably lead to death of whole armies as popularly believed, but to settlement of new areas, le
Size: 2171px × 1151px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollectionbiodive, booksubjectanimalecology
