03.5 



S — 

 e'2.5 



1.0 ^_ 

 2 



1915 I 1925 1935 | 1945 

 1920 1930 1940 1950 

 YEARS 



FIG. 1 01 Dispersal of the European starling in North Ame 

 between 1918 and 1949 (compiled trom Kessel 1953). 



acclimatize to new climatic conditions, and acquire 

 new behavior patterns. 



Manner and means of dispersal 



Animals find suitable habitats and niches in 

 various ways. In a uniform environment, dispersal 

 movements radiate in all directions from the home 

 area. The greater the density of individuals in the 

 home area, the more quickly distant areas are invaded, 

 and the farther away do individuals move. 



SOUTH 



FIG. 10-2 Relation of breeding localities to birthplaces among 

 robins. Numerals on concentric circles are distances from the 

 origin in miles (I mile = 1.6 kilometers). Data points beyond 

 the 200-mIle radius are not placed to scale (Farner 1945). 



A common method of achieving dispersal is the 

 broadcasting of enormous numbers of eggs, spores, 

 encysted stages, or young so that they scatter into a 

 wide variety of places in a more or less random man- 

 ner. Those that come by chance into suitable environ- 

 ments persist and become established ; those that enter 

 into unfavorable locations are destroyed or never de- 

 velop. Broadcasting is a wasteful procedure ; it is 

 especially characteristic of aquatic species. The fresh- 

 water clam annually produces hundreds, perhaps 

 thousands, of eggs. Only two fertilized eggs need ma- 

 ture that the two parents be numerically replaced 

 when they die, and thus the population of the species 

 be maintained at a constant level. Contrastingly, in 

 those forms, such as birds and mammals, that have 

 developed a high degree of parental care, the number 

 of eggs or young annually produced is commonly a 

 half-dozen or less, and the offspring exercise con- 

 siderable discrimination in their choice of suitable 

 habitat. 



Although the dispersal of broadcast offspring is 

 not under the control of the parents or the young, it is 

 not often truly random. Water and wind currents, 

 and other agents of dispersal may channel eggs 

 and spores in restricted directions. Such dispersal 

 is described as passive conveyance. In streams, all 

 agents of passive conveyance, except some other ani- 

 mals, direct dispersal downstream. Upstream dis- 

 persal must be the result of active locomotion. 



Eggs of insects, snails, fish, or other aquatic or- 

 ganisms will sink unless they are buoyed up by cur- 

 rents, possess flotation mechanisms, or are attached 

 to some floating object. Logs, masses of vegetation, 

 and other debris are sometimes torn loose from the 

 banks of rivers and float out to sea carrying the 

 smaller animals attached to or trapped on them. It is 

 estimated that over 300 debris rafts of significant size 

 are formed each century and float out to sea from the 

 mouths of the larger tropical rivers (Matthew 1915). 

 The passengers on such rafts may come to be colonists 

 of remote islands or even other continents. 



The dispersal of fresh-water animals from one 

 river to another is sometimes effected by erosion, 

 when the process permits one river system to rob a 

 branch of another river system (Crosby 1937). Fol- 

 lowing the recession of the continental glacier in 

 North America, the proglacial Great Lakes had outlet 

 down the Illinois and Mississippi Rivers. Later, a 

 new outlet was established over Niagara Falls, and 

 the Lakes became a part of the St. Lawrence River 

 system. 



Wind acts in a manner somewhat similar to water 

 currents. Some terrestrial spiders have evolved a spe- 

 cial mechanism allowing them to use mild air currents 

 for dispersal. The young climb up on a clod of earth 

 or other object and spin out long threads or flocculent 

 masses from glands in their abdomen. This continues 



46 Ecological processes and dynamics 



