5CTNDJFMAMJJAS0 NOV 



FIG. 13-2 Monthly variation in kinds of food consumed by an 

 herbivore, the mule deer, in California and Oregon (Interstate 

 Deer Herd Committee 1951, courtesy California Division of 

 Fish and Game). 



ton is of very minor importance in the diet of the fish. 

 Forage fish were not present. 



The vulnerability to a predator of a prey species 

 is directly proportional to its relative abundance 

 among the other species available. Voles are pre- 

 ferred by the red fox over white-footed mice, but 

 both mice and voles are preferred over moles, shrews, 

 and snakes. Predator preferences among them, how- 

 ever, occur only when all are abundant. When prey 

 species are reduced in numbers and difficult to get, 

 little or no predator preference occurs (Scott and 

 Klimstra 1955). 



There is some evidence that predators — particu- 

 larly insectivorous birds — when searching for prey 

 concentrate on one or a few species at a time. By a 

 kind of learning, they acquire what may be called 

 specific searching images for these species and thereby 

 mostly disregard other species. When a new species 

 becomes numerous in an area, they feed on it at first 

 only as the result of chance encounters. To obtain 

 a preference for the new species, they must become 

 conditioned to it gradually and must learn the proper 

 cues of where and how to search it out (Tinbergen 

 1960). 



The importance of availability is illustrated by 

 seasonal variations in the kinds and amounts of food 

 consumed by animals. Predator species that remain 

 in one habitat throughout the year must adjust their 

 feeding to the kinds of food available in each season. 

 Species that are unable to do this are compelled to 

 migrate, hibernate, or make other adjustments to sur- 

 vive the unfavorable seasons. Thus birds that are 

 strictly insectivorous may occur in a given area only 

 during the warm part of the year, leaving it before 



insects disappear. Omnivorous species commonly 

 change from an insect diet in summer to a diet of 

 seeds and fruits in the autumn and are often non- 

 migratory. 



Seeds and fruits are most abundant and easily ob- 

 tained in the autumn. During the winter they decom- 

 pose, become buried in the snow or softened ground, 

 or are consumed. In Michigan, the weed seeds avail- 

 able on agricultural lands in March are only nine 

 per cent of those available in October (Baumgras 

 1943). The most critical time of the year, as far as 

 food supplies are concerned, is early spring, the time 

 before new vegetation and hibernating prey animals 

 appear. Abundance of seeds also varies with fertility 

 of soil, which thus influences survival, density, and 

 distribution of animal populations. 



Animals are subject to considerable variation in 

 the abundance and kinds of food available to them 

 from year to year. In a four-year study of the yield 

 of fruit and seeds from 27 species of trees and shrubs 

 in West Virginia, only 33 per cent of the species pro- 

 duced a crop every year, 29 to 33 per cent failed to 

 produce a crop in three of the four years, and 22 per 

 cent failed twice within the four years (Park 1942). 

 When herbivorous species vary in abundance, because 

 of variation in food supply or other factors, carnivo- 

 rous species that prey upon them often vary in direct 

 proportion. 



Protective devices 



Few kinds of plants are well equipped with de- 

 fense mechanisms against the grazing and browsing 

 of animals. Trees and shrubs may be deformed or 

 killed by excessive browsing because the tender ter- 

 minal twigs and buds are destroyed. Grasses are not, 

 for they grow from the base of the leaves ; they may 

 even be benefited by moderate cropping. Some spe- 

 cies of trees, shrubs, and cacti are protected from 

 browsing by prickles or thorns. This protection is 

 important to the plants for survival in deserts and in 

 grazing subseres of humid regions. Some plants are 

 noxious or toxic, and animals quickly learn to avoid 

 them. 



When coloration renders it inconspicuous in its 

 normal environment, an animal is said to have con- 

 cealing or cryptic coloration (Cott 1940). When the 

 coloration or markings reproduce the general tone or 

 characteristics of background, it is called protective 

 resemblance; disruptive, when the markings break up 

 the outlines of an animal and replace it by some ir- 

 regular configuration so that the animal is not recog- 

 nized as prey. The white collar of the killdeer, ob- 

 served casually from a distance, tends to the human 

 eye to separate the head as a distinct object from the 

 rest of the body. Obliterative coloration, or coiinter- 



192 Ecological processes and dynamics 



