POPULATION DYNAMICS IN NORTHERN MARINE BIRDS 



127 



The second defect in the concept of carrying 

 capacity is that it presupposes a stable envi- 

 ronment. During the early decades of the 20th 

 century most climatologists believed that a 

 departure from the norms of a regional cli- 

 mate set processes in motion which would re- 

 turn the climate to normal. During the last 

 decades, however, climatologists and 

 oceanographers have shown clearly that envi- 

 ronments are continuously in flux. 



An A ttack on Density-dependent 

 Mortality 



Some theorists rejected the concept of 

 carrying capacity as soon as it was formu- 

 lated. Andrewartha and Birch (1954) pre- 

 dicted fluctuations would be undamped by in- 

 herent population mechanisms but rather 

 would be controlled by external forces indif- 

 ferent to the density. Their supporting data 

 were drawn from field studies of insects in 

 arid climates. Some of their ideas are directly 

 relevant to seabirds; for example, their asser- 

 tion that in many cases limits to carrying 

 capacity of the habitat are not set in a way re- 

 sponsive to the density of the population. The 

 number of occupiable ledges on a seabird cliff 

 are fixed and when they are full no more birds 

 can breed there regardless of the amount of 

 food available. For another example, some 

 biological processes act in a way that rein- 

 forces fluctuations. Predation can act in this 

 way in the relatively closed system of a sea- 

 bird colony; i.e., the smaller the prey popula- 

 tion the larger the percentage taken by the 

 predators. The importance of predation as a 

 selecting factor is shown by the adaptations 

 marine birds and waterfowl make to avoid it. 

 The fact that large colonies of seafowl are 

 usually concentrated on isolated, predator- 

 free islands is one obvious case (Lack 1966). 



Although their ideas are useful in under- 

 standing changes in many species, primarily 

 insect populations, the generality of An- 

 drewartha and Birch's (1954) hypothesis is 

 weakened because it conflicts with detailed 

 studies of seabirds which show that in many 

 cases local food resources do limit breeding 

 success. Ashmole (1963) showed this for tropi- 

 cal terns, and Hunt (1972) for some colonies of 

 herring gulls on the New England coast. 



Nettleship (1972), studying the effects of 

 herring gulls on Atlantic puffins, showed that 

 the effect of harassment and stealing food 

 from the parents was to reduce the amount of 

 food brought to the young and thus reproduc- 

 tive success. In those parts of the colony 

 where gulls were numerous or where the 

 puffins were at a disadvantage in escaping 

 from gulls (i.e., flat rather than steep slopes) 

 the reproductive success of puffins was sig- 

 nificantly lower than in areas away from the 

 gulls. 



The literal application of Andrewartha and 

 Birch's general ideas also conflicts with obser- 

 vations on subtle adaptations some waterfowl 

 have made to counter predation. 



Barry (1967) described the density-avoiding 

 adaptations of arctic-nesting geese to evade 

 predation specifically by foxes. Black brant 

 (Branta nigricans) nest on low coastal or delta 

 islands seeking to escape by remoteness. 

 Snow geese (Chen caerulescens) are colonial 

 on large, flat areas, seeking protection in num- 

 bers. White-fronted geese (Anser albifrons) 

 are solitary nesters on inland swamps, seek- 

 ing to be "over-dispersed" among scrub 

 willow. 



Common eiders, black scoters (Melanitta 

 nigra), tufted ducks (Aythya fuligula), and 

 other ducks select gull colonies as nesting 

 habitat. Although there is little doubt that 

 the ducks choose gull colonies for nesting, 

 there is some doubt as to the reasons. Finnish 

 biologists (summarized by Bergman 1957; 

 Hilden 1965) have concluded generally that 

 gulls protect the duck nests from predation 

 by hooded crows (Corvus corone). 



The Assumption that Fluctuations 

 Are Generally Present 



Recently theorists have built models based 

 on assumptions that fluctuations are a 

 general characteristic of population dy- 

 namics, such as Gilpin's (1975) model describ- 

 ing multiphased oscillations. He took account 

 of the fact that fluctuations (and models) be- 

 come more complex as more species and non- 

 linear effects are included. May and Leonard 

 (1975) emphasized that the effect of non- 

 linearities is to make it impossible to speak 

 even in principle of the equilibrium point of a 



