beriiatioii and out of liihcniation. In tlic autumn, for- 

 est species migrate downward and may he found in 

 peak numbers first in the slirubs, then in the herbs, 

 then in their hibernacula in the ground. Many insects 

 of tlie forest-edge, meadows, and agricultural crops 

 also hibernate in the forest, usually a few meters in 

 from the South-exposed edge where they derive some 

 heat from the winter sun. These insects usually mi- 

 grate into the forest in the same stratum, herb or 

 shrub, in which they occur during the summer, then 

 downwards into the soil. These flights into the forest 

 occur with declining temperatures and are sometimes 

 spectacularly large numbers of individuals. As they 

 come out of hibernation in the spring the direction of 

 movement the insects take is just the reverse that taken 

 in the autumn ; i.e., upward into their proper stratum, 

 then horizontally outward into open country (Weese 

 1924). 



Daily migralions 



Ascent of plankton towards the surface at night 

 and descent to greater depths during the day occur 

 both in the sea and in lakes. The lake-dwelling culicid 

 larva Chaobonts lies on the bottom during the day but 

 becomes pelagic at night. .Snails, slugs, and millipedes 

 in the deciduous forest lie quiescent under logs or 

 litter during the day, but come out at night to crawl 

 around on the forest floor or even climb up on the 

 vegetation to a height of perhaps a few meters. Al- 

 though these excursions are restricted in range, they 

 are more or less regular and periodic, and may be 

 thought of as migrations. 



The two species of catadromous eels, of the western 

 hemisphere and of luirope, migrate to the open sea 

 in the region northeast of the West Indies in order 

 to reproduce. The immature stage of the eel, not the 

 adults, returns to the two continents. 



ECESIS 



Dispersal or migration of individuals into 

 new areas is without great ecological significance un- 

 less those individuals become established and can build 

 up significant populations. The process by which or- 

 ganisms become established in new areas is called 

 ccesis. Ecesis will occur there if a species disperse 

 into a habitat favorable to it, and if the species can 

 then secure its proper niche or become adjusted to a 

 new niche, new competitors, predators, parasites, and 

 disease organisms. Ecesis, to go to completion, re- 

 quires first the establishment of individuals in an 

 area, then the growth of the population that they form, 

 and finally or simultaneously, the maturing of com- 

 munity structure with the invasion of many other 

 species. 



Ecesis is often of a temporary nature. Temporary 

 ecesis is the rule with migrant species, as the periodic 

 change of location is normal in their life behavior. 

 Ecesis as range expansion is not surely permanent 

 until the species demonstrates that it can survive criti- 

 cal years in weather cycles. For instance, northward 

 dispersal of tropical forests is, after a point, thwarted 

 by frosts that occur only at rare intervals. Insects 

 may continuously expand their ranges for a period 

 of years, only to be forced back hundreds of miles 

 by a severe drought or cold spell. 



Metamnrphic migrations 



Aquatic larvae and naiads of several orders of 

 insects eventually change into adults that leave the 

 aquatic habitat and become aerial. The adult stage is 

 often short-lived. The eggs are deposited in water, or 

 the immature stages return again to water to begin 

 the cycle over again. The length of the cycle may be 

 part or the whole of a year, or a longer period. The 

 seventeen-year cicada is a well-known insect whose 

 nymphs spend 17 years underground feeding on juices 

 from the roots of trees. The adults appear above 

 ground in large numbers in late May, mate, lay their 

 eggs on twigs of various trees, and then disappear, all 

 in a few weeks. The eggs hatch in about six weeks, 

 and the nymphs drop to the ground and bury them- 

 selves for another long period of years. 



The anadromous Pacific salmon ascends fresh- 

 water streams but once to spawn. The breeders die ; 

 it is their offspring that return to the ocean to develop 

 for a period of years before they make the migration. 



Growth of populations 



The growth of a species population from a 

 single individual or pair of individuals is governed 

 by the same laws that govern the growth of the in- 

 dividual itself from zygote to adult organism. When 

 either the size of the individual or the cumulative 

 growth of the population is plotted against time, a 

 characteristic sigmoid curve or logistic curve results. 

 This phenomenon was first demonstrated by Verhulst 

 in 1839. This curve has been used to describe the 

 population growth of such diverse organisms as yeast, 

 Paramecium, Drosophila, Tribolinm, and man (Pearl 

 1927, Park 1939), and even the growth of commu- 

 nities in particular habitats when many species are 

 simultaneously present. Under natural conditions, 

 however, growth of animal populations is subject 

 to so many variable factors, including the change from 

 one morphological stage to another in the life cycle of 

 many species, and the change in the physical environ- 

 ment both daily and seasonally, that the curve is often 



Dispersal, migration, and ecesis 159 



