EVOLUTION OF INTERSPECIES INTEGRATION AND ECOSYSTEM 



699 



DISOPERATION 



Instances of direct mutual harm between 

 species are not known to us, but a few 

 cases of mutual elimination thiough over- 

 exploitation exist (see p. 395; Gause, 

 1934a). Because of negative selection, dis- 

 operation would be most likely when two 

 organisms come in contact for the first time 

 and have no common evolutionary history 

 within the same community. Unless one of 

 the species is preadapted (p. 642) to the 

 other, disoperation may occur. 



Many introduced organisms that become 

 pests or cause epidemic disease are ex- 

 amples of such disoperation. The simplest 

 cases are those in which man does not in- 

 sert himself in a dominant role within the 

 community. 



The chestnut bark disease or chestnut 

 blight, caused by an ascomycete fungus 

 (Endothia parasitica), originally a parasite 

 of oriental chestnuts (Castanea mollissima, 

 C. Henryi, and C. sequinii), was intro- 

 duced into the United States from China 

 about 1904. This foreign fungus has since 

 nearly wiped out the native chestnut 

 {Castanea dentata), once the most abun- 

 dant tree in eastern deciduous forests. 

 With the death of the host, the fungus also 

 lost its major food supply. The parasite 

 survives on other species of American 

 chestnuts and also on species of oaks, but 

 is less common on these hosts and has not 

 eliminated these species (Craighead, 1916). 



Cases of drastic disoperation are not 

 easily found in nature, because of the ob- 

 vious long-continued selection against such 

 a relationship. Indications of previous elim- 

 ination of populations are fairly abun- 

 dant, although the factors involved may 

 only be surmised. With the exception of 

 bank and cliff swallows, which nest in or 

 on relatively inaccessible steep banks or 

 cliffs, all colonial ground-nesting birds, in- 

 cluding penguins, auks, murres, skimmers, 

 petrels, tropic birds, gannets, cormorants, 

 pelicans, and flamingos, now breed onlv on 

 islands or otherwise inaccessible situations. 

 It seems possible that such a distribution is 

 the result of survival in regions where dis- 

 operative relations do not occur, and elim- 

 ination in continental regions might have 

 taken place through disoperation between 

 such ground-nesting birds and their preda- 

 tors. However, we have no knowledge con- 



cerning the secondary elimination of the 

 predators or what other factors may have 

 been influential in such instances. 



EXPLOITATION 



Exploitation with unilateral benefit to 

 one of the species is characteristic of the 

 majority of known cases of coaction be- 

 tween two species, and the eflFect on the 

 struggle for existence is marked. 



Characteristic community organization 

 exemplified by the food web (pp. 508- 

 522) and pyramid of numbers (pp. 522- 

 525) is largely the result of exploitation. 

 The food relations within the food web are 

 usually the result of long evolutionary 

 action. For example, insects whose food is 

 limited to a species or higher taxonomic 

 category of plants are usually adjusted to 

 special biochemical traits of their hosts, and 

 therefore have evolved a coactive relation- 

 ship. The evolution of gall structures formed 

 by plant tissues under the stimulation of 

 the gall-producing animal is an example of 

 unilateral exploitation of the morphogenetic 

 capacities of the plant host without benefit 

 to the plant (Agar, 1943, p. 186). 



Mordvilko (1928) states that the evolu- 

 tion of plant lice (aphids) often runs paral- 

 lel to the evolution of their plant hosts (see 

 Patch, 1938). The primary host of each 

 species of Adelginae is always a spruce 

 (Picea), and the secondary host is always 

 another conifer (Abietineae). The phylo- 

 genetic origin of genera of aphids is some- 

 times associated with a change of host 

 genera (Mordvilko, 1934). For example, 

 the genus Euceraphis on the birch (Betula) 

 gave rise to the genera Drepanosiphum 

 and Drepanaphis on maples (Acer). 



Along with their hosts, specialized her- 

 bivorous or phytophagous insects exhibit a 

 rather clear successional sequence in the 

 Indiana dunes series, although the ecologi- 

 cal factors are different from those affect- 

 ing the succession of predatory and 

 scavenging ant species. For example, the 

 pine spittle insect (Aphrophora paraJlela) 

 is fovmd on several species of pine and on 

 the Norway spruce; the clustered midrib 

 gall {Cynips nigricens) is found on the 

 white and burr oaks, and the woolly leaf 

 gall (Callirhiftis Janata) is found on the 

 under side of the leaves of several species 

 of the red oak group. The gall wasp genus 

 Neuroterus has seventeen species in New 



