Trophic Levels and Relations 469 



"making a living" within each functional category. Among herbivores 

 that feed on trees, for example, some fill the niche of eating the leaves 

 whereas others use the twigs, sap, bark, or roots as a source of food. 

 In different geographical regions each type of niche is often filled by 

 quite different species. Activity in some niches requires extreme and 

 often bizarre specialization of anatomy, physiology, or behavior. 

 Minute differences in function are exhibited by the niches of birds that 

 obtain food organisms from crevices in bark. Small woodpeckers 

 characteristically fly to the base of a tree and climb up the trunk 

 looking for grubs. When they reach the top, they fly diagonally down- 

 ward to the base of the next tree and again work their way upward. 

 In contrast, nuthatches common in eastern United States ordinarily 

 fly to the top of a tree and work toward the base, clinging upside 

 down to the trunk. In this way woodpeckers readily remove grubs 

 and other food from crevices easily reached from below, whereas the 

 nuthatches obtain their prey from cracks in the bark more accessible 

 from above. In the Galapagos Islands a finch that has evolved toward 

 the woodpecker type fills this niche, but having no barbs on its tongue 

 it wedges grubs out of crevices by means of a thin twig held in its 

 beak. 



Trophic Levels and Relations. Each successive level of nourish- 

 ment as represented by the links of the food chain is known as a 

 trophic level. The plant producers within an ecosystem constitute 

 the first trophic level, the herbivores form the second trophic level, 

 and the primary carnivores represent the third level. Additional links 

 in the main food chain, and in side chains such as those formed by 

 parasites, constitute further trophic levels. Three types of pyramidal 

 relations may be found among the organisms at different trophic levels 

 in the ecosystem, resulting in the production pyramid, the pyramid of 

 biomasses, and the pyramid of numbers. 



At each step in the food chain a loss of energy and of material from 

 the system takes place because the processes of assimilation and 

 growth are not 100 per cent efficient. This means that the organic 

 matter produced per average unit of time, and the energy represented 

 by it, become less at each successive trophic level. The production 

 rates of the components of a self-sufficient ecosystem may thus be 

 thought of as forming a pyramid. The base of this production 

 pyramid is represented by the organic synthesis of the green plant 

 component, and higher levels are represented by the growth rates of 

 the herbivore and carnivore components. 



Frequently, the size, growth rate, and longevity of the species 

 making up a particular ecosystem are such that the living weight, or 



