TAXONOMU; COMPOSITION OF 

 COMMUNITIES 



I'rftlominance 



It is characteristic of the taxonomic structure 

 of comuninities tliat a few species furnisli the greatest 

 bulk of the population entirely out of proportion to 

 the rest of the species. Thus in stream riffles, two 

 species make up 85 per cent of the total riffles popula- 

 tions and another two species constitute a similar per- 

 centage of the mud bottom i)ool populations (Table 

 5-1 ). In the littoral zone of Lake F.rie. one species 

 furnishes 85 per cent of the population on cobble and 

 gravel bottoms, and another species makes up 68 

 per cent of the population on mud bottoms (Table 

 6-2). In populations of 79 species of birds nesting on 

 a large tract including advanced stages of succession, 

 5 per cent of the species (4 species) included 57 per 

 cent of the individuals, 10 per cent (8 species) for 

 56, and 50 per cent (40 species) for 96 per cent 

 (Evans 1950). The abundant species are ordinarily 

 of small size, herbivorous in their food habits, and at 

 the bottom of the food chains and pyramids. 



Variety of species 



W'iien a barren area is first colonized, a variety 

 of species may invade and become temporarily estab- 

 lished because competition is negligible. As the popu- 

 lations increase in size and individuals are crowded 

 together, competition for the limited resources sets 

 in, and the less well adapted species are eliminated 

 (Yoimt 1956). 



In general, all over the world, established com- 

 munities in extreme or impoverished habitats con- 

 sist of fewer species than do communities in fertile 

 and more favorable habitats. The tropical rain forest 

 is noteworthy for the great variety of species that it 

 possesses, while arctic areas have, comparatively, 

 greatly reduced faunas. Pioneer communities newly 

 established on bare areas regularly consist of fewer 

 species than later serai and climax ones (Tables 

 7-4, 8-3 ) . The reason for this is that infertile pioneer 

 communities and those in extreme habitats can sup- 

 port little variety in the way of niches, while com- 

 munities in rich habitats are able to develop a great 

 diversification of them. 



When an area occupied by a community is broken 

 up into a large number of small, different micro- 

 habitats and niches, few of them can be very exten- 

 sive, hence their average population is generally less 

 than that of species in extreme habitats where the 

 variety of niches is smaller. Tropical and climax 

 communities characteristically have a rich variety of 



species, but relatively low ]joi)ulations of most spe- 

 cies. In pioneer and extreme habitats, to which rela- 

 tively few species are adapted to survive, the species 

 present may become enormously abundant. In spite 

 of occasional exceptions, there tends to be an inverse 

 relation in the composition of communities between 

 the number of species and the average number of 

 individuals per species. 



Segregation of related species 



Closely related species usually have somewhat 

 similar adaptations and niche requirements : this may 

 affect their distribution in communities in relation to 

 each other. 



In a study of 55 animal and 27 plant communities 

 from a wide variety of habitats (Rlton 1946), it was 

 found that 86 per cent of the genera of insects and 

 other animals and 84 per cent of the genera of plants 

 were represented by a single species. The average 

 number of species per genus of animal and plant in 

 each community was 1.38 and 1.22 respectively, 

 compared with an average of 4.23 species per genus 

 for 11 large insect groups that occur in the British 

 Isles as a whole. This means that while closely re- 

 lated species may occur in the same regional fauna, 

 related species are more apt to be segregated into 

 adjacent communities or habitats than into the same 

 one. This conclusion has been disputed on a mathe- 

 matical basis because of the small size of the com- 

 munities and the small number of species involved 

 (Williams 1947) but is supported bv other studies 

 (Lack 1944, Bagenal 1951). 



It is true that in larger habitats or more diversi- 

 fied communities the larger number of niches ordi- 

 narily present should permit related species to occur. 

 Yet, where related species could occur in large trop- 

 ical communities of Africa, bird species belonging to 

 the same genus occurred together in the same com- 

 munity in only 26 per cent of the communities, and 

 if the weaver finches Ploceidae are excluded, only 16 

 per cent of possible overlap in habitat distribution is 

 realized. This segregation of related species into dif- 

 ferent communities applied to an equal e.xtent with 

 species belonging to the same family (Moreau 1948). 

 There are some genera of birds and other animals, 

 however, in which a large variety of related species 

 may be found together ; notably, certain wood war- 

 blers Dendroica, buntings Eniberisa, white-eyes Zos- 

 t crops, whistlers Pachycephala. weavers Ploceus, 

 hawks Accipiter. and the insects Drosophila, Anoph- 

 jlf-u^AecTes' ( Mayr 1947). Certain habitats, such as 

 : Lake^ Bailtal in Africa) are unusual in possessing a 

 great variety of closely related aquatic species 

 (Brooks 1951). 

 The tendency for related species to be segregated 



Niche segregation 255 



