384 Relations between Species 



Bacteria, molds, and actinomycetes that produce antimicrobial sub- 

 stances have now been shown to be widespread in nature. Burk- 

 holder ( 1952 ) reports that about half of the species of actinomycetes 

 and half of the lichens as well as large numbers of higher plants pro- 

 duce substances that inhibit certain molds and bacteria. Under 

 some natural conditions antibiotics are believed to protect the species 

 producing them from bacterial marauders, but this ecological aspect 

 of the subject is largely unexplored. 



Antagonistic substances formed by algae have been shown to af- 

 fect other algae adversely in the laboratory and undoubtedly also do 

 so in nature under some circumstances. Some substance accumulat- 

 ing in cultures of the green alga CJiIorella vulgaris was found to in- 

 hibit the growth of the diatom Nitzschia frustrulum to a greater ex- 

 tent than its own growth; similarly, an antibiotic produced by dense 

 cultures of Nitzschia retarded the division rate of Chlorella grown in 

 the same culture or in conditioned water filtered off from the diatom 

 culture. Interspecies antagonism of this sort probably exerts control 

 on the abundance of different kinds of phytoplankton in ponds or 

 other aquatic areas, and in some instances it may influence the sea- 

 sonal sequence of species commonly observed in nature ( Rice, 1949 ) . 



Similar products of plant growth have been found to be harmful 

 to certain animals. Substances produced by senescent cultures of 

 Chlorella and of the diatoms Navicula and Scenedesmus inhibit the 

 filter feeding of Daphnia in laboratory tests. A reaction of this sort 

 may account in part for the curtailment of the growth of zooplankton 

 sometimes observed in natural waters containing senescent popula- 

 tions of phytoplankton ( Ryther, 1954t ) . Pond "blooms" of blue-green 

 algae, especially of the genus Microcystis, are known to produce toxic 

 substances, such as hydroxylamine, which cause the death of fish and 

 even of cattle that drink the water (Prescott, 1948). In the marine 

 environment toxins produced by huge populations of certain micro- 

 organisms, popularly known as "red tide," cause the catastrophic de- 

 struction of fish and other animals. An outbreak of the dinoflagellate 

 Gymnodinium brevis off the west coast of Florida in 1946-1947 re- 

 sulted in the wholesale death of fish throughout an area of several 

 thousand square miles (Gunter et al., 1948). 



Certain land plants are similarly harmful to animals, but they usu- 

 ally exert their poisonous effects only when eaten. For example, a 

 s(?midesert bush (Halogeton glomeratus) found in Nevada and neigh- 

 boring states kills the sheep that eat it. The spongy leaves of this 

 weed are filled with oxalic acid, which combines with the calcium 

 in the blood and causes death within a few hours. 



