Scotland (Brian 1952). On decaying stumps, the 

 number of nests of Formica jusca, Myrmica scabri- 

 nodis, M. rubra, and Leptothorax acervorum were 

 found to be in the ratio of 23:20:12:7, but on the 

 ground away from stumps, the ratio was 1 :13:18:1. 

 The apparent restriction of F. jusca and L. acer- 

 vorum to stumps is probably attributable to the fact 

 that they are not mound-builders ; rather, they com- 

 monly make galleries in wood. L. acervorum makes 

 its galleries in wood that is too hard for the other 

 species to work ; galleries so small that the larger 

 species are excluded. A difference between species 

 in point of position occupied on the same stump was 

 also observed. F. jusca tended to occupy the warmer 

 southerly sites but often extended over the entire 

 stump. M. scabrinodis was widely distributed, but 

 there was a tendency to concentration on the east side 

 of the stumps. M. rubra occurred principally in the 

 cooler, moister north and west sectors. Away from 

 the stumps, M. scabrinodis was especially character- 

 istic of small knolls resulting from the decay of 

 stumps no longer favored by M. jusca, while M. rubra 

 extended into cooler moister soils. F. jusca was the 

 most aggressive and socially dominant species, un- 

 challenged in its occupancy of the best sites. M. rubra 

 tended to occupy the second-best sites, leaving them 

 only when they became temporarily unsuitable. M. 

 scabrinodis often came into the vacated sites and 

 when once established could not be dislodged. 



In earlier chapters, we have described how the 

 characteristics of the soil, which the females first test 

 with their ovipositors, determine where grasshoppers 

 and tiger beetles will lay their eggs. Of 18 types of 

 rodents studied in Utah, 4 were found only in rocky 

 situations, 2 only in gravelly soils, and 2 only in 

 sandy soils ; the other 10 were less limited by the 

 type of soil (Hardy 1945). In aquatic habitats, spe- 

 cies segregate according to whether the bottom is 

 rock, sand, or mud. The swift current limits the in- 

 habitants of streams to species possessing clinging 

 structures and proper orienting behavior. 



Congeneric species of ectoparasitic mites and fleas 

 on small mammals commonly are distributed between 

 several different host species. When they occur on 

 the same host, they are segregated by species on dif- 

 ferent parts of the body, a given species is present 

 only when the host occurs in a particular type of 

 vegetation, or they occur at different seasons (Jame- 

 son and Brennan 1957). 



Tiie importance of microclimate in niche segrega- 

 tion of species is shown by a study made in Danish 

 bogs (N0rgaard 1951). The low humidities and 

 high temperatures obtaining at mid-day on the sur- 

 face of the peat mat restrict one species of spider to 

 the stalk region of the sphagnum. Another species of 

 spider tolerates these conditions, so the two species 

 divide the habitat between them. 



Microclimate is often a major factor in determin- 

 ing whether a species can maintain itself against 

 competition in a particular microhabitat. This has 

 been shown experimentally. When equal numbers 

 of two related species of beetles are introduced into 

 the same flour container and placed at a particular 

 combination of temperature and relative humidity, 

 one species becomes established, the other is elimi- 

 nated. The particular species favored in the various 

 microclimates are as follows (Park 1954) : 



34°C-70 % R.H. 

 34°C-30 % R.H. 

 29°C-70 % R.H. 

 29°C-30 % R.H. 

 24°C-70 % R.H. 

 24°C-30 % R.H. 



Tribolium castancum 

 Tribolium conjusum 

 Tribolium castaneum 

 Tribolium conjusum 

 Tribolium conjusum 

 Tribolium conjusum 



Similar reversal of dominance has been found to 

 take place at high and low temperatures with dif- 

 ferent species of grain beetles (Birch 1953), Dro- 

 sophila flies both in Europe (Timofeeff-Ressovsky 

 1933) and in North America (Moore 1952), two in- 

 sect parasitoids using the same host (DeBach and 

 Sisojevic 1960), and turbellarian flatworms (Beau- 

 champ and Ullyott 1932). Usually the species fa- 

 vored by a given micro-climate has a higher rate of 

 population growth at that particular temperature or 

 liumidity. 



There is a positive correlation between high oxy- 

 gen tensions required by trout for saturating their 

 blood hemoglobin and the oxygen-rich waters that 

 they select. The restriction of these fish to cold waters 

 is correlated with the fact that a rise in temperature 

 decreases the oxygen-loading capacity of the hemo- 

 globin. Catfish and carp, common to warm waters of 

 low oxygen high carbon dioxide content, have hemo- 

 globin that loads and unloads at low oxygen tensions 

 and is less sensitive to changes in carbon dioxide 

 tension and temperature (Prosser ct al. 1950). 



Arctic mammal species differ in thickness and 

 density of fur. which insulate against loss of body 

 heat, and this determines whether they can sleep 

 above ground and be active during the winter or 

 whether they must confine themselves to nests and 

 runways below the snow level (Scholander et al. 

 1950). 



In central Illinois, the short-tailed shrew is largely 

 subterranean in habit and occurs in moist habitats; 

 the woodland white-footed mouse is nocturnal and 

 inhabits the forest floor ; the prairie vole is restricted 

 to grassland and the most arid of the habitats of the 

 three species. There is a connection between the 

 amount of water available in the habitats and the level 

 of water exchanges in the animal. At 19°C, for in- 

 stance, the rate of water absorption and loss in the 

 shrew is twice that of the mouse. The rate of total 



246 Ecological processes and dynamics 



