Our dispersal rate data so far have described the 

 outward diffusion of a local population through an 

 area already occupied by the species. Once it has sur- 

 mounted a barrier the dispersal rate of a species into 

 an area previously unoccupied by it should be faster. 

 The European starling was introduced into North 

 America about 1890. From a central locus around 

 New York City it spread at an accelerating pace until, 

 in 1940, it had become established over 6,500,000 sq 

 km (2,500,000 sq miles) (Wing 1943), a mean rate 

 of about 130,000 sq km (50,000 sq miles) per year. 

 With amelioration of the climate in Finland during 

 recent years, the lapwing spread northward between 

 1899 and 1954 at a mean annual rate of 7 km (4.3 

 miles) (as computed from Fig. 9 in Kalela 1955) ; the 

 roe deer, between 1850 and 1945, at a mean annual 

 rate of about 9.5 km (5.8 miles) (computed from Fig. 

 1 in Kalela 1948). The Norway rat invaded south- 

 western Georgia and virtually displaced the previously 

 established black rat at a rate of about 430 sq km 

 (167 sq miles) per year (Ecke 1954). By way of 

 contrast, it has taken the fresh-water amphipod Gani- 

 inarus pulex the last 6000 years to disperse across 12 

 river systems from southern England into Scotland 

 (Hynes 1954). 



Causes of dispersal 



It is the case that the reproductive rate of any 

 species is so great that if all offspring survived the 

 world would be overrun with that species within rela- 

 tively few generations. Because species produce a sur- 

 plus of young in most years, there is continuous pres- 

 sure on individuals to move into all suitable niches, 

 and to seek out new areas in which to settle. The im- 

 pact of large numbers of individuals struggling for 

 survival is described as population pressure, and is 

 doubtless the most potent force inducing dispersal. It 

 should be recognized, however, that population pres- 

 sure is not uniformly constant year after year. When 

 because of poor breeding conditions or catastrophe 

 there is a reduction in the over-all population of a 

 species, that decimated species may withdraw into its 

 optimum habitat and be less put upon to exploit new 

 or less desirable areas. In years favorable to the pro- 

 duction of large surpluses of young, a species will 

 often be found in less favorable habitats, even regions 

 it would not otherwise occupy at all (Kluyver and 

 Tinbergen 1953). The broadcasting of eggs or off- 

 spring, or the passive conveyance of them to other re- 

 gions, varies directly with the size of the population 

 producing them, and is hence as much an expression 

 of population pressure as the active search for new 

 areas engaged in by individuals under their own loco- 

 motion. 



Animals cannot disperse successfully, if at all, into 



new areas to the characteristics of which they are not 

 structurally, functionally, and behaviorally adapted. 

 If an area the characteristics of which have excluded 

 a species changes so that the species is adaptive to it 

 with the equipment it has, that species can success- 

 fully invade. The American robin, song sparrow, 

 chestnut-sided warbler, house wren, and prairie 

 horned lark have invaded Georgia only in recent years 

 as the logging of forests, initiation of early serai, 

 grassy, and shrubby stages, and extensive general cul- 

 tivation of the land have produced habitats meeting 

 the requirements of these birds (Odum and Burleigh 

 1946). If an area the characteristics of which have 

 excluded a species remains unchanged but the species 

 acquires new structural, functional, or behavioral traits 

 by which it can adapt to that area, the species can in- 

 vade the area. If its newly acquired traits let the spe- 

 cies remain adaptive to its former range, its range is 

 expanded by inclusion of the newly-invaded area. 

 If the food supply fails, homesites or vegetation be 

 destroyed, or a pernicious change in climate occur 

 there, animals may be forced to leave an area to 

 which they were well-adapted and disperse, more or 

 less temporarily, into an area to which they are less 

 well-adapted. The snowy owl, for instance, depends 

 heavily on lemmings and mice for food in its usual 

 range, the Arctic tundra. In apparent correlation with 

 the cyclic decline of the lemming population there, 

 the owls invade our northern states. 



DISPERSAL PATHWAYS 



Theories of how animal and plant groups 

 have dispersed over the face of the earth are based 

 fundamentally on the hypothesis that the continental 

 land masses were at one time intimately connected 

 and only later drifted apart to their present locations, 

 or the hypothesis that the continents have been perma- 

 nently fixed in their present positions throughout geo- 

 logical time. 



Continental drift theory 



This theory postulates that throughout the Pa- 

 leozoic and most of the Mesozoic the presently dis- 

 tributed continents were grouped into two great land 

 masses : a northern Laurasia was separated from a 

 southern Gondwana by the vast sea of Tethys, al- 

 though narrow connections between the two might 

 have occurred for short periods. In the Cretaceous 

 the land masses fragmented, and the fragments sub- 

 sequently drifted apart. Laurasia is supposed to have 

 split into North America, Greenland, Europe, and 

 most of Asia ; Gondwana, into South America, Africa, 



150 Ecological processes and dynamics 



