eciually parasitic on the lar\ ae oi Imtli species is iiitro- 

 chiced into the mixed culture, tlie populations of botli 

 species decline markedly below their normal levels in 

 single pure cultures, and they both continue to exist 

 together indetinitely (L'tida 1953). 



Closely related insect species may differ in their 

 adaptations to climatic factors. As the weather dif- 

 fers between different localities during different 

 months or from one year to another, two or more 

 species may reach high abundance at different times. 

 correlated with prevailing weather conditions, in what 

 otherwise appears to be the same ecological niche 

 (Ross 1957). 



Xicbes of different species may overlajj during 

 most of the year but be clearly defined during critical 

 stages in the life cycle. Plethodontid salamanders 

 wander around freely most of the time, but during 

 the reproductive period they are sharply segregated. 

 In the Xorth Carolina mountains, Dcstnotinathtis 

 qitadraiiiaciilatiis is predominant in clear rocky 

 streams, D. phoca lays its eggs in muddy streams on 

 the under surface of rocks or logs, while D. fusciis 

 carolincnsis. which also occurs in muddy streams, 

 deposits its eggs in the moss and mud on the upper 

 surfaces of logs (Xoble 1927). 



A niicrohabitat may be occupied simultaneously 

 by two or more species if their combined ]5opulations 

 do not exceed the carrying capacity of the micro- 

 habitat. This may occur when the populations of the 

 species involved are limited by conditions in some 

 other part of their niches. Thus the niches of house 

 wren, bluebird, black-capped chickadee, tufted tit- 

 mouse, white-breasted nuthatch, and tree swallow- 

 overlap because they all nest in small cavities in trees 

 or bo.xes. Usually the number of nesting sites is ample 

 to the demand, so that there is no competition for 

 them. The population of each species is restricted by 

 factors other than available nest sites, perhaps by 

 food supply, unfavorable climate, parasites, or preda- 

 tors. The wren feeds mostly on the ground under 

 bushes, the bluebird in open fields, the chickadee on 

 the smaller tree branches, the tufted titmouse on the 

 larger branches and on the ground, the white- 

 breasted nuthatch on the trunk of trees, and the tree 

 swallow in the air. Competition occurs between them 

 only when one or more species temporarily increases 

 in abundance so that there are not enough nest-sites 

 to go around. 



When competitive species occupy the same micro- 

 habitat, they sometimes set up mutually exclusive ter- 

 ritorial relations, based on responses to similarity of 

 body form and behavior (Simmons 1951 ) or call notes 

 (Dilger 1956, Lanyon 1957). This divides the space 

 and reduces competition. 



Because of parasites, unfavorable weather, preda- 

 tors, or other causes, species do not continuously satu- 

 rate their habitats. It is only when they do that com- 



petition becomes clearly evident. Cumpclition in the 

 house wren takes various forms including destruc- 

 tion of eggs and young and even killing of adults. In 

 a 19 year study of a 6-hectare (15-acre) plot, such 

 drastic competitive acts, both intra- and interspecific, 

 occurred only when the number of male birds was 

 more than 10. When the number of breeding house 

 wrens was reduced to 10 or less, in consequence of 

 heavy over-winter mortalities, no such competition 

 occurred (Kendeigh 1941b). 



The niche relationships and tiie amount (jf c()m])eti- 

 tion between species sometimes varies geographically. 

 The vertical range of the salamander Plcthodon c/lut- 

 inosiis in the southern Appalachians is sharply defined 

 from those of three different subspecies (jordani, 

 shcrmani, iiictcalfi) of Plethodon jordani, but com- 

 pletely overlaps the ranges of two other subspecies 

 (clcutsonae, mclaventris) (Hairston 1951). 



During a spruce budworm insect outbreak in the 

 coniferous forests of northern Ontario, three species 

 of warblers — bay-bl^easted, Tennessee, and Cape 

 May — greatly increased in numbers, partly because 

 of their aggressiveness and partly because they were 

 more accustomed to feeding on this type of food. 

 Other warblers were held in check and one species, 

 the magnolia warbler, actually decreased in numbers 

 because of competition with the three favored spe- 

 cies. Competition between birds for song posts and 

 territorial space was considerable with nearly a third 

 of all conflicts observed occurring between individuals 

 of different species (Kendeigh 1947). It is probable 

 that niche segregation of species becomes established 

 at times of stress or crises like this, and after the be- 

 havior pattern once becomes fixed in the species, only 

 sporadic attacks of one species on another are there- 

 after sufficient to check random variations away from 

 the standard pattern. At times other than those of 

 stress, direct conflict between species is not often 

 observed so that its importance in segregating species 

 to particular niches is occasionally not fully appre- 

 ciated (Lack 1944, Udvardy 1951, Andrewartha and 

 Birch 1954). 



Cause's rule 



The evidence presented in this chapter demon- 

 strates the important concept that has come to be 

 known as Cause's Rule or, more recently, the "com- 

 petitive exclusion principle" (Hardin 1960) : an eco- 

 logical niche cantwt be siinultanroiisly and completely 

 occupied by stabilized populations of more than one 

 species. In other words, two or more species with 

 closely similar niche requirements cannot exist indefi- 

 nitely in the same area. Two species w-ith expanding 

 populations attempting to occupy the same niche will 

 sooner or later come into competition for possession 



Niche segregation 253 



