into different communities or regions is an old con- 

 cept and may be stated: given any species in any 

 region, the nearest related species is not likely to be 

 found in the same region nor in a remote region, but 

 in a neighboring district separated from the first by 

 a barrier of some sort (Jordan 1905). The principles 

 and reasons underlying this law require an under- 

 standing of speciation, which will be taken up in the 

 next chapter. 



Ecological equivalents 



Communities in general are of fundamentally 

 similar organization, and the facts we have already 

 considered about food chains, trophic levels, and 

 pyramids of numbers and biomass attest this. Con- 

 sequently, a coniferous forest has many niches similar 

 to those available in a deciduous forest, a grassland 

 has some niches that are much the same as forest 

 niches, a pond has some of the same niches as do 

 lakes. The niches in a forest, a grassland, or a pond 

 on one continent are very much the same as those to 

 be found in similar communities on other continents. 

 Communities could well be analyzed, compared, and 

 evaluated in respect to the niches of which they are 

 composed rather than the species they contain. Spe- 

 cies are taxonomic units, niches are ecological units. 

 Similar niches in different communities or in dif- 

 ferent regions are commonly occupied by species pos- 

 sessing similar but not necessarily identical habits, 

 adaptations, and adjustments (Table 8-3). Such 

 species are called ecological equivalents (Friedmann 

 1946, Dirks-Edmunds 1947). Equivalent species are 

 not necessarily closely related taxonomically. 



The explanation of why a particular species has 

 come to occupy a particular niche in a particular part 

 of the world requires a knowledge of where it orig- 

 inated, how it dispersed, and how it evolved its 

 present adjustments and characteristics. When popu- 

 lations invade new regions and environments and 

 become geographically isolated, they commonly differ- 

 entiate into new species. Each species thus becomes 

 the product or visible expression of a particular com- 

 bination of environmental factors, interactions, and 

 locality, and as such the species is the practical taxo- 

 nomic unit with which the ecologist must deal. The 

 process of speciation is thus of great ecological inter- 

 est since it is an adaptive process that leads to the fill- 



ing in of empty niches and to the most efficient and 

 complete utilization of the environment (Mayr 1949). 



SUMMARY 



The ecological niche is a particular com- 

 bination of microhabitat and biotic relations required 

 for the existence of a species. The niche of a species 

 is defined by the features of the substratum and mi- 

 croclimate to which that species is peculiarly ad- 

 justed, the time of day and season of year when it is 

 mainly active, the type of shelter or cover it requires, 

 the manner in which it uses vegetation in its repro- 

 ductive performance, the type of food it consumes, 

 and the predators that prey on it. 



Animals, particularly higher types, have more or 

 less stereotyped behavior patterns associated with 

 their restriction to particular niches. These behavior 

 patterns may be genetically inherited and subject to 

 evolutionary development, or they may be trans- 

 mitted to succeeding generations through training or 

 conditioning (imprinting) of the young. 



Segregation of species into different niches is 

 doubtless the result of interspecific competition. Ac- 

 cording to Cause's rule, an ecological niche cannot 

 be simultaneously and completely occupied by stabi- 

 lized populations of more than one species. Occu- 

 pancy of different niches reduces interspecific com- 

 petition, furnishes the species with microhabitats to 

 which they are especially adjusted, reduces confusion 

 and disturbance, and permits a greater variety of spe- 

 cies to occur in the same region. 



Communities commonly have a predominance of a 

 few species composing the bulk of the populations. 

 Communities in extreme or impoverished habitats 

 have fewer species than those in fertile or favorable 

 ones. Closely related species tend to be segregated 

 into different but adjacent regions separated by bar- 

 riers. 



Since communities in various parts of the world 

 are fundamentally similar in organization and struc- 

 ture, species occupying similar niches in them make 

 similar adjustments. Although not necessarily re- 

 lated taxonomically, these species are ecological equiv- 

 alents. Understanding how a particular species has 

 come to occupy a particular niche in a particular part 

 of the world requires a knowledge of speciation, 

 which will be considered in the following chapter. 



256 Ecological processes and dynamics 



