that an angler may need patience to make a 

 single catch. 



A study of the fauna in a summer forest 

 reveals a similar arrangement. Vast numbers 

 of aphids and other small herbivorous ani- 

 mals provide a broad food base for the 

 carnivorous species of the region; and a fairly 

 large population of spiders, beetles, and other 

 medium-sized carnivorous animals are able 

 to prosper on this base. But the number of 

 insectivorous birds, such as warblers, will not 

 be very great; and only two or three hawks 

 (Fig. 30-2) or other predatory birds will be 

 able to gain a living in a particular forest — 

 even though the range of flight of such hunt- 

 ing birds is extensive. 



Fig. 30-2. Red-shouldered hawk (Bu/eo lineatus) with 

 a field mouse in its talons. (Courtesy of the American 

 Museum of Natural History, New York.) 



In short, the smallest animals at the base 

 of a food chain, owing to the modesty of 

 their food needs and the efficiency of their 

 reproductive processes, tend to populate a 

 given habitat in great abundance, whereas 



Ecology and Evolution - 575 



the largest animals, at the apex of the food 

 chain, are much more restricted in numerical 

 abundance — with the medium-sized species 

 falling in between the two extremes. More- 

 over, the height of the pyramid is limited, 

 since the population of a dominant species 

 tends to be so sparse that no other animal 

 can gain a living by preying upon it. 



The Adaptation of Species to Species. During 

 evolution, many species had a profound in- 

 fluence upon the development of other spe- 

 cies. In this regard, the insects provide many 

 good examples. Insects have been abundant 

 since early in the Cenozoic era, at which 

 time the flowering plants began their evolu- 

 tion. Consequently, natural selection had 

 ample time to create a rich variety of modern 

 flowers. Thus, many present-day flowers pos- 

 sess both color and fragrance with which 

 they attract insects seeking food and, inci- 

 dentally, pollinating flowers. Likewise, the 

 prevalence of insects during such an extensive 

 evolutionary period accounts for many other 

 adaptations in modern plants and animals. 

 In this category we find the insect-trapping 

 leaves of certain plants (Fig. 14-2); the wings, 

 eyes, and mouth parts of insect-catching 

 birds; and the mouth parts and sensory or- 

 gans of many fish, reptiles, and mammals 

 that prey upon insects or insect larvae. And 

 although insects, owing to their unusual 

 prevalence, were particularly potent in shap- 

 ing the adaptations of other species, it seems 

 probable that every kind of animal and 

 plant, whether it persisted or became extinct, 

 has had some influence on the evolutionary- 

 destiny of neighboring species. 



Predatism: Mechanisms of Offense and De- 

 fense. Impacts between species have been of 

 many kinds, but only such adaptations as 

 have arisen in relation to predatism and 

 parasitism will be considered in the present 

 account. 



Even a cursory survey of familiar species 

 reveals a wide variety of adaptations that 

 enable animals to catch, kill, and feed upon 

 their living prey. Many one-celled animals 

 such as Amoeba have pseudopodia that en- 



