Figure 1-17. Rapid growth with only slight 

 competition of seedlings produced this stand 

 of annual meadow barley (Hordeum brachy- 

 antherum) in a moist gully bottom, but run- 

 off from a high-intensity storm could wash 

 all the plants away. Utah. (U. S. Forest 

 Service.) 



"y 



-Mp 



as 700 miles.' ^2 Only one example will be cited: the rough, reticu- 

 late coats of the minute seeds of tobacco {Nicotiana spp.) appear to 

 be an outstanding adaptation to the invasion of disturbed sites, 

 which are the most common habitats of many species in this 



202 



genus 



Barriers such as mountain ranges or adverse climatic conditions 

 are important in forming limits to the distribution of species, and 

 therefore some plants are not found in habitats that would be suit- 

 able, but succeed when introduced artificially, e.g., European 

 weeds introduced into the United States, and vice versa. The 

 dispersal capacity of a species may be overrated, for there is no 

 real evidence that plants possessing specialized mechanisms are 

 more widely distributed than those without them.^^ In a limited 

 area without important barriers, however, dispersal tends to 

 maintain the several stands of a community-type uniform in 

 composition. Because of the effectiveness of dispersal of all sorts 

 of propagules, unoccupied habitats suitable for a given species 

 will soon be reached by individuals of that species, with the re- 

 sult that similar habitats in a restricted area will be occupied by 

 similar groupings of plants and animals. ^° 



Ecrological CHaracteristics of Species & Popmmlations • 39 



