The second defect in the concept of carrying capacity is that it presupposes a stable environment. During the early decades of the 20th century most climatologists believed that a departure from the norms of a regional climate set processes in motion which would return the climate to normal. During the last decades, however, climatologists and oceanographers have shown clearly that environments are continuously in flux.
An Attack on Density-dependent Mortality
Some theorists rejected the concept of carrying capacity as soon as it was formulated. Andrewartha and Birch (1954) predicted fluctuations would be undamped by inherent population mechanisms but rather would be controlled by external forces indifferent 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 assertion that in many cases limits to carrying capacity of the habitat are not set in a way responsive 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 reinforces fluctuations. Predation can act in this way in the relatively closed system of a seabird colony; i.e., the smaller the prey population 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 understanding changes in many species, primarily insect populations, the generality of Andrewartha 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 tropical 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 reproductive 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 significantly lower than in areas away from the gulls.
The literal application of Andrewartha and Birch's general ideas also conflicts with observations 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 numbers. White-fronted geese (Anser albifrons) are solitary nesters on inland swamps, seeking 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; Hildén 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 dynamics, such as Gilpin's (1975) model describing multi-phased oscillations. He took account of the fact that fluctuations (and models) become more complex as more species and nonlinear effects are included. May and Leonard (1975) emphasized that the effect of nonlinearities is to make it impossible to speak even in principle of the equilibrium point of a community. They pointed out that even though the model is deterministic (i.e., assumes that the system will come to equilibrium) the oscillations are so complex that they may appear to be random, and it may be a very long time before the system returns to a position near its starting point. "On the other hand a truly random ecological system could always be fitted by a suitably ingenious limit cycle. This suggests that ecological analysis which does not consider component processes must be viewed with great suspicion" (Gilpin 1975). May and Leonard (1975) and Gilpin are both making a familiar point—that neither the logic nor the interactions described in a formula will describe biological reality unless the assumptions are correct. They are also making a different point—that an ingenious mathematician can create a formula to describe almost any operation (whether its workings are systematic or random), and the formula may seem to work.
Gilpin's moral is that one cannot learn very much that is helpful by studying fluctuations as such. One must study the factors controlling populations. This is a very old idea.