476 



THE COMMUNITY 



ami) is found on open blackland prairie. 

 Taylor and Lay (1944) observed that cat- 

 tle grazing eliminates the vegetative cover, 

 and with the decrease in the herbaceous 

 stratum there is a corresponding increase 

 in breadth of the horizon, which benefits 

 the rabbits. This is an example of uncon- 

 scious cooperation between two species of 

 floor animals, although if the grazing be- 

 came too severe, the food supply for both 

 would be impaired. Hence the degree of 

 cooperation depends upon the activity level 

 examined in relation to the welfare of the 

 whole community. 



Overgrazed grassland may owe its re- 

 covery to intelligent husbandry, but its re- 

 covery may also result from much less satis- 

 factory events— for example, the rise in 

 parasite populations. This is demonstrated 

 in the Uganda Protectorate of Africa. In 

 the northern part of this district badly over- 

 grazed grasslands are making steady re- 

 covery since the invasion of the tsetse fly 

 (Thomas, 1943). In these areas, badly 

 overgrazed and secondarily gullied by ero- 

 sion following loss of herbaceous stratum, 

 destruction of cattle by fly-borne trypano- 

 somes resulted in grass regrowth in three 

 years to a stratum 3 feet high. In the 

 valley of the Kidepo River, near the 

 Uganda-Sudan boundary, the tsetse flies 

 invaded many years ago, and here the her- 

 baceous stratum is now 6 feet high. 



CONCEPT OF THE ECOTONE 



Structure of grassland at its boundaries 

 is affected by impingement of other com- 

 munities. This tension zone is not exclusive 

 to grassland, but is always formed where 

 two or more communities are in contact. 

 Such a transition is known as an ecotone. 

 Ecotones occur so generally that we may 

 discuss their formation and peculiarities in 

 the broad terms of an ecotone principle. 



First, it would appear that the principle 

 of the ecotone is biologically new at the 

 level of the community. Within the limits 

 of our technological equipment and meth- 

 ods, we have sharp boundaries with no 

 overlap between nucleus and cell, between 

 cell and cell, between organ and organ, 

 and, colonial organisms excepted, between 

 organism and organism. In bacteria, if the 

 assumption is made that nuclear material 

 is present, the absence of well-defined par- 



ticulate nuclei would appear to constitute 

 an exception. However, modern work in 

 which certain bacteria are examined 

 through ultraviolet light at high magnifica- 

 tions appears to demonstrate submicroscopic 

 nucleoids with sharp boundaries. Size of 

 nuclear particles is not involved; it is the 

 boundary which is important for this discus- 

 sion. For example, the multinucleate 

 Opalina has its nuclear bodies as sharply 

 set off from the surrounding cytoplasm 

 as obtains in a uninucleated Amoeba. It 

 would appear, therefore, that the ecotone 

 is a pecuharity of communities, or that at 

 the least the ecotone is so well developed 

 between communities that its presence is 

 a definitive characteristic. We are not speak- 

 ing here of any particular blade of grass, 

 rodent, or sapling, since these are organ- 

 isms and have their own sharp boundaries. 

 Rather, we are speaking of the total mar- 

 ginal zone as a highly integrated reality. 



As used here, the term "ecotone" refers 

 to this marginal area between communities. 

 It does not refer to extensive areas of 

 biome intergradation. For example, in 

 Africa, as one proceeds from the Congo 

 rain forest northward to the Sahara, there 

 are all gradations between rain forest, trop- 

 ical savanna, steppe, semidesert, and desert 

 (Hesse, Allee, and Schmidt, 1937). This 

 vast stretch of territory may be considered 

 a zone of intergradation involving thou- 

 sands of communities, but this stretch of 

 territory could not be considered an eco- 

 tone in the sense used here. Rather, each 

 of the thousands of communities involved 

 has its particular ecotone with those other 

 communities with which it is in contact. 

 Again Carpenter (1940a, p. 672) states 

 that the mixed-grass prairie is "in a 

 sense, a relatively broad ecotone or transi- 

 tion between the two other associations of 

 the biome, but nevertheless possesses cer- 

 tain characteristics peculiar to itself." By 

 the two other associations in the quotation 

 are meant the tall grass prairie and the 

 short grass prairie. In this case the mixed- 

 grass prairie is an intergradation in the 

 Hesse, Allee, and Schmidt sense, but is not 

 an ecotone in the sense used in the present 

 discussion. Consequently we could con- 

 ceive, throughout the range of the mixed- 

 grass prairie, of many separate mixed-grass 

 communities with stream-grassland eco- 

 tones, grassland-forest ecotones, tall grass- 



