Antibiosis 383 



species B and at the same time an antibiotic secreted by B is harmful 

 to A. 



In the foregoing paragraph the term competition was used in a re- 

 stricted sense referring to the inchrect rivahy of two species striving 

 for some necessity from a Hmited supply. The aggression between 

 species that prey on one another or exhibit some other form of direct 

 antagonism may also be considered competition in the broad sense. 

 The ecological effects of competition as both direct and indirect 

 antagonism will be considered both from the point of view of the in- 

 dividual and of the population and the species. 



All gradations exist between relations that are harmful to one 

 species and neutral to the other, those that are harmful to one and 

 beneficial to the other, and those that are harmful to both parties. 

 Associations between species that are at first neutral or even bene- 

 ficial may change during the life of the individuals to become harmful 

 to one species, as already mentioned, and this relationship may change 

 further to bring disadvantage to both parties. For example, the 

 strangling fig starts life as a harmless epiphyte on a palm or other 

 tree, but after a time its dangling roots reach the ground. Deriving 

 nutrients from the soil and energy from sunlight, the fig rapidly en- 

 velops the trunk of its host with a network of anastomosing roots 

 (Fig. 10.10). The luxuriant growth of the upper part of the fig grad- 

 ually shades out the palm, and, after the death of the host tree, the fig 

 eventually falls to the ground from lack of support. In similar fashion 

 a parasite or a predator may bring about its own destruction under 

 certain circumstances by killing its host or consuming all of its prey. 



Antibiosis 



Many substances produced by organisms, or conditions resulting 

 from their metabolism, are generally harmful to others. Thus the 

 generation of carbon dioxide or of organic acids may harm more sen- 

 sitive species to such an extent that they are unable to continue to 

 live in the area, and excessive shading by one kind of vegetation will 

 kill off species intolerant of low illumination. The term antibiosis 

 applies more particularly to the production of materials that are 

 specifically antagonistic to other species. Such antibiotic substances 

 are known to be generated by many kinds of fungi and bacteria. The 

 action of penicillin, streptomycin, aureomycin, and other antibiotics 

 produced by fungi in destroying various pathogenic bacteria is fa- 

 miliar to the reader through the practical use of these substances— 

 either natural or synthetic— in medicine. 



