competing with grasses at the ecotones between grassland and 

 aspen groveland in northwestern Montana. ^^^ In moist years in 

 Kansas, as in 1940 to 1942, the prickly-pear cactus is so heavily 

 infested with insects that it may disappear from some communi- 

 ties, while in dry years it tends to increase. ^^^ And in Hawaii, the 

 introduced Procedidochares utilis, a tetritid gall fly, has eliminated 

 the weed Eupatorium adenophorum from some large areas, but has 

 had little effect in other places. ^^ Xhe selectivity of parasites, as 

 exemplified by this gall fly, is of major importance in the bio- 

 logical control of weeds. 



DifFerences of Species in Mutualistic and Commensal Relations 



In mutualistic relations both of the associated organisms pre- 

 sumably benefit. Nitrogen-fixation bacteria are most commonly 

 associated with legumes, but they are also found with alders and 

 the black locust, which may owe their special capacity to invade 

 bare areas to these bacteria in their root nodules, thus hastening 

 plant succession by supplying needed nitrogen. In commensal 

 relations, such as lichens and mosses growing on the bark of trees, 

 one of the plants benefits while the other may be injured very 

 slightly, or not at all (Figure 1-25). 



The association of a fungus with a root is known as mycorrhiza. 

 When the fungus is present inside the cells the association is endo- 

 trophic, occurring characteristically in the Ericaceae and Orchid- 

 aceae, and also in grasses and other plants. When the fungus is 

 growing on the surface of the root, as is common in woody plants 

 such as pines, spruces, beeches, oaks, aspens, and hazels, the 

 association is ectotrophic. Mycorrhizae are usually considered a 

 kind of mutualism in which the fungus appears to absorb carbo- 

 hydrates and possibly other organic substances from the root, 

 while the latter presumably obtains nitrogen and minerals from 

 the fungus. It appears that the fungus, in addition to absorbing 

 nitrogen compounds from the soil, can in some instances fix 

 atmospheric nitrogen. Some kinds of mutualism such as ecto- 

 trophic mycorrhiza may be the most successful type of parasit- 

 ism because there is no destruction of tissue, although hyper- 

 trophy may be induced. Other kinds of parasitism are "primi- 



Ecological CHaracteristics o£ Species &. Popislatioims • 53 



