516 



THE COMMUNITY 



land plants and animals which can invade 

 such overcut, artificially maintained areas. 

 Farming radically alters the grassland food 

 web by artificially maintaining a dense 

 population of certain cereal grains. With 

 respect to this last point, Howard (1925) 

 has made a classic comment: 



"As we bring more and more wild land into 

 cultivation we reduce the numbers and kinds 

 of plants growing on it. We cover each field 

 with the thousands of plants belonging to one 

 species, such as corn, wheat, potatoes or apples. 

 Each hill of corn or potatoes is planted where 

 dozens of vwld plants grew before. Insects 

 wliich formerly had dozens of plants to browse 

 upon now congregate on the few tender ones 

 which have been substituted and often damage 

 or destroy them. The kinds of insects which 

 find the new plants distasteful disappear 

 eventually, while the kinds which can thrive 

 on them continue to multiply as fast as their 

 food supply multipfies until some check is put 

 upon them." 



Intensive cultivation of the soil by farm- 

 ing also increases the possibihty of dust 

 storms (p. 468) and creates unbalance in 

 both the natural grassland food web and 

 the food web of which the farmer is a part. 

 Such agricultural practices also increase the 

 danger of erosion, which, with fire and 

 destruction of the watershed, must be re- 

 garded as an unusually grave danger to 

 community trophic relationships. The gen- 

 eral eflfects of erosion on community rela- 

 tionships have been emphasized recently by 

 Thornthwaite (1940a) in a symposium on 

 the ecology of man. 



Another aspect of food webs is that a 

 given species, in the course of its fife cycle, 

 occupies a regular sequence of feeding 

 niches or links, and these hnks may be in 

 the same, or different, food chains in the 

 food web. An example is afforded by the 

 Ufe history of the herring (Hardy, 1924). 

 Very young herring (7 to 12 mm. long) 

 feed upon larval moUusks, ciliate protozoans 

 (Tintinnopsis) , flagellate protozoans (Peri- 

 dinium), copepods (Pseudocalanus and 

 Harpacticidae), and other minute organ- 

 isms. When the young herring are 12 to 42 

 mm. long, they feed upon Pseudocalanus 

 almost exclusively. As these fish grow larger 

 (42 to 130 mm.) they feed upon Pseudo- 

 calanus, larvae of barnacles (Balanus), 

 larvae of decapods, mysid crustaceans, 

 sagittid worms, and other animals. Finally, 

 the adult herring feeds upon pteropod 



snails (Limacina), a genus of euphausid 

 crustaceans (Nyctiphanes), hyperiid amphi- 

 pods, and numerous copepods {Temora, 

 Calanus, and the fike). 



Consequently we find the herring an im- 

 portant predator on the North Sea plankton, 

 but its ecological position changes with its 

 physiological requirements. On the other 

 hand, its role is that of a plankton feeder 

 throughout. This demonstrates that Elton's 

 concept of the feeding niche, or the eco- 

 logical feeding category is not necessarily 

 the same thing as a link in a food chain. 



For purposes of clarity, the following 

 definitions will be used in the further dis- 

 cussion of the community: 



A food web (food-cycle of Elton, 1927, 

 p. 56) is the total complex pattern of feed- 

 ing relations of an independent, self-main- 

 taining major community in the sense of the 

 concept used in this book. This term em- 

 bodies the Darwinian web of fife or "web 

 of complex relations" (Darwin, 1859, p. 

 68) and has been called a "food-chain" by 

 some authors. 



A food chain is a linear series of feeders 

 and foods, as discussed previously (p. 508). 

 Such a condition seldom exists, as this 

 would reduce the food web of a community 

 to only a single thread of feeding relation- 

 ships. The food chain, as used here, refers 

 to a single strand of the whole web. 



A food link is a taxonomic entity in a 

 food chain; for example, a species or sub- 

 species at a particular stage of its fife 

 history. It becomes a food mesh when it 

 is considered in its total relation to the 

 community. 



A food mesh is a taxonomic entity in a 

 food web; for example a species or sub- 

 species at a particular stage of its life 

 history. 



A food niche is the feeding role of an 

 organism in a community, and has no fixed 

 taxonomic status. For example, a plankton 

 feeder can be one of a number of species, 

 and this category can be contrasted eco- 

 logically through a series of different 

 aquatic communities, or the several plank- 

 ton feeders of a single community can be 

 directly compared. This is Elton's concept 

 of the "niche" (1927, p. 64); e.g., "the 

 niche of an animal means its place in the 

 biotic environment, its relations to food and 

 enemies." We have broadened this useful 

 term to include plants as well as animals, 



