BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



251 



unmolested ovules that ensures an ample 

 seed supply for the plant. The yucca flower 

 is adjusted to prevent self-fertilization, and 

 the moth is essential to the perpetuation of 

 the species. 



The biotic environments suppUed by the 

 fig and the yucca are thus extended to the 

 vvliole hfe history of the fertilizing agent, as 

 in the social bees. 



Seed dispersal appears to have had long 

 range effects on the interrelations of plants 

 and animals, tending toward mutualism, 

 and demonstrating anew that animals living 

 in the plant matrix also provide an animal 



Before turning to the further discussion 

 of the mutualism of animal with animal, 

 conspicuous examples of mutualism appar- 

 ently derived directly from commensalism 

 may be examined. The cowbird Molothrus, 

 familiar in North America, and the oxpecker 

 (Buphaga), attendant on the buffalo and 

 rhinoceros of Africa, render a considerable 

 service to their hosts by ridding them of ex- 

 ternal parasites and by reducing the plague 

 of biting and sucking insect predators, and 

 add to these functions the service of watch- 

 men, well known to hunters. The benefits 

 of the ccmstant food supply to the birds is 



Fig. 67. The cattle heron, Bubulcus, shown with the water buffalo, associates itself with the 

 larger wild and domestic mammals throughout its range, from North Africa to the East Indies. 

 The mutualist relation resembles that of the American cowbird. ( Drawing by W. J. Beecher. ) 



environment to which plants adapt them- 

 selves through the processes of evolution. 

 The great number of plants with chnging 

 seeds that become dispersed by mammals 

 and birds exhibit a kind of plant-animal 

 commensalism. The development of edible 

 fruits, in which a readily available food ma- 

 terial envelops a hard and resistant seed, 

 suggests that the dispersal of such seeds by 

 the animals feeding on fruits is a mutualis 

 tic relation; wild berries, in particular, are 

 most effectively dispersed by birds; but 

 there does not appear to be any develop- 

 ment of a strict taxonomic relation like that 

 in so many examples of mutualisticalh 

 paired species in which pollination is in 

 volved. Effective means of dispersal of 

 plants is important in the course of succes- 

 sion and in the maintenance of biotic com- 

 munities. 



evident, and the constancy of the association 

 is attested by the vernacular names of the 

 birds. A great many other birds enter into 

 this loose type of partnership with mam- 

 mals; one of the most conspicuous and un- 

 expected is that of the little white heron of 

 Africa and the Orient (Bubulcus) and the 

 larger hoofed animals, whether wild or do- 

 mestic. Eight of these birds have been ob- 

 served perched on the back of an African 

 buffalo and as many as twenty on the back 

 of an elephant. 



Marine animals exhibit the most astonish- 

 ing of partnerships in which mutualism ap- 

 pears to be directly derivable from commen- 

 salism. Decapod crustaceans, in particular, 

 tend to have the dorsal shell of certain spe- 

 cies covered by a specific type of sponge, 

 hydroid, or sea anemone, deriving benefit 

 from the resulting camouflage or nematocyst 



