Evolutionary Ecology

The twenty-one papers in this volume, centred on the theme of Evolutionary Ecology, are presented by biologists from a range of research fields. The factor common to all contributors is that their work has, in some important way, been influenced by David Lack. David Lambert Lack (1910–73), whose life and works are summarised in a group of obituary notes in Ibis Volume 115 part 3 (1973), was primarily an ornithologist. He was also an evolutionist of distinction whose main field of study was the ecology and evolution of birds, and whose writings appealed strongly to a very wide range of biologists in many fields of research. Among the contributors to this volume are several former students and colleagues, who knew Lack personally and benefited directly both from his teaching and from his warm personality. Other contributors represent the majority — biologists all over the world who knew him only from his writings, but gained a better understanding of their own research from his concepts and approach.

We were taught in our earliest childhood that we ought to respect the rights and feelings of others. By the same token, the oldest code of human morals that has come down to us, the Ten Commandments of Moses, is chiefly concerned with obligations that are to be observed between people. Only the first three commandments relate to something different, enjoining reverence for the deity. The last seven all have to do with proper relationships between individuals in society; keeping holy the sabbath day the same as everybody else, honouring one’s father and mother, doing no murder, not committing adultery, not stealing nor bearing false witness nor coveting one’s neighbour’s possessions. To make the rules simple, Moses picked out a few particular obligations for mention, and many others, scarcely less important, are covered by the broader social virtues of being generous, temperate, just, honest, dutiful and brave.

Recent research on Red grouse in Scotland has concentrated on mechanisms which limit populations (Watson and Moss, 1972). This chapter concerns the adaptive value of territorial behaviour in population limitation, an aspect of evolutionary significance largely developed by David Lack, and of lasting interest to him.

What determines population density? Animal ecologists debated this question in the postwar years with as much fervour as bishops at an Ecumenical Council, and with as little thought of settling their differences experimentally. As one of the heretics, I was in schism with the orthodox doctrine of density dependence to which David Lack adhered (Nicholson, 1933; Smith, 1935; Lack 1954a). One of Lack’s contributions was putting this doctrine into an evolutionary context; it is therefore ironical that certain ideas he disapproved of (Chitty, 1952; Lack 1954b) should, after a long metamorphosis, have led to my outdoing him in attempting to tie together natural selection and the regulation of numbers (Chitty, 1967a; 1970). However gross the errors of this still-untested view, they are at least in a direction Lack approved of.

Because many species of animals live in diverse situations, it is unwise to generalise about what determines their population density. Some ecologists argue and others deny that the restricted fluctuations in numbers observed in nature can be explained only by some form of density-dependent regulation (Nicholson, 1954; cf. Andrewartha and Birch, 1954; see also Ehrlich and Birch, 1967). Some argue that animal numbers are usually limited by intraspecific competition for food (Lack, 1954a); others that once animals possess a piece of ground ‘they can do the actual food-getting in perfect peace and freedom, entirely without inter-ference from rivals’ (Wynne-Edwards, 1962, p. 12). Hairston, Smith and Slobodkin (1960) argued that because most green matter falls to the ground uneaten, the numbers of herbivores generally are not limited by food shortage; though in fact food may be limiting even when only a small fraction of the stock is eaten (Huf-faker, 1966). Many believe that ‘predation is centred upon over-produced young … upon what is identifiable as the more biologically expendable parts of the population’ (Errington, 1963, p. 184); yet Pearson (1971) suggests that carnivore predation alone is ‘responsible for the amplitude and timing of the microtine cycle’. Ehrlich and Birch (1967) expect regulatory factors to vary among populations of the same species and through time; while Chitty (1958), Watson and Moss (1970), and Krebs and Myers (1974) seek one ‘universal explanation’ for what prevents unlimited increase in small mammals. If there is any justification for these beliefs, they cannot be quite so incompatible as at first sight appears.

If the numbers of birds are regulated by availability of food, as the evidence suggests, species which live together in the same area must have evolved means of reducing interspecific competition for food. They might feed in different places, at different times, in different ways, or on different prey (Huxley, 1942); any two species may differ in more than one of these ways. How these separations are brought about may be difficult to determine because they may take effect only when food is short, the species overlapping in almost all respects when food is plentiful.

Although the phenomenon had barely been hinted at before Skutch’s review of 1961, co-operation by three or more adult birds in some aspect of reproductive effort is remarkably widespread as a usual feature of breeding biology, and is now known to involve at least 1.5 per cent of the world’s avifauna. I hold co-operative breeders to include the many birds with ‘helpers at the nest’, and the several species which breed in pairs but are group-territorial and provision the young communally, as well as the few in which several females lay in a common nest. For reasons given below we should exclude the half-dozen species that cooperate in building a supernest in which each pair rears its young unaided — those are best designated communal nesters. I include, however, birds in which later broods in the same season are regularly fed by their siblings of earlier broods, since this seems similar to their retaining family bonds and, when adult, helping parents at the nest in subsequent years. Details of social organisation and the nature of the co-operative effort vary considerably with species; they typically involve pre-breeding adult individuals helping a breeding pair with nest building, incubation, territorial defence, feeding nestlings and attending fledglings.

There has been considerable discussion about the adaptive nature of clutch size and whether or not the average clutch can be related to the maximum number of young that the parents can successfully raise (Lack, 1954; Wynne-Edwards, 1962). In nidicolous species, those which feed their young in the nest, it is possible to see at least some of the advantages to a bird of being conservative in the number of eggs that it lays. Although parent birds tend to bring more food to larger than to smaller broods, the increase in the amount of food brought is not proportional to the increase in the number of mouths to be fed. As a result, the individual young in large broods receive less food than those in smaller broods and leave the nest lighter in weight. In at least a few species the probability of survival has been correlated with the weight at fledging (Perrins, 1965). Thus the lowered survival rates of the young in large broods may be sufficient to outweigh the initial advantage that the large brood had in terms of greater numbers of young. In such circumstances, parents with broods of average size may be at least as productive as, and sometimes more productive than those with larger broods (Perrins and Moss, 1975).

David Lack (1947; 1954) suggested that clutch size in birds is adapted to correspond to the maximum number of young that parents can nourish. The studies that Lack’s pioneering work stimulated have revealed both supporting and contrary evidence (see Lack, 1954, 1966; Wynne-Edwards, 1962; Skutch, 1949; 1967; Klomp, 1970; Hussell, 1972). Lack’s hypothesis also has been criticised on theoretical grounds. Skutch (1949) suggested that predation could determine optimum clutch size if the loss of nests to predators were to increase as brood size increases, hence as feeding visits, begging and general activity about the nest increase. As Lack (1949) pointed out in response, whether predation or starvation is the principal cause of death, the optimum brood size is none the less that which produces the most young on average.

During his period of interest in swifts, David Lack drew attention to the importance of nesting behaviour and nest type as guides to the systematics of the Apodidae (Lack, 1956). This approach has been particularly fruitful among the Indo-Pacific assemblage known colloquially as ‘swiftlets’, recognised as a tribe Collocaliini by Brooke (1970) and as a subfamily Collocaliinae by Condon (1975). Among these swifts, in several instances, the type of nest built has proved to be critical in specific determination. Another characteristic exhibited only by the living bird that has claimed attention is the capacity of some (but not all) swiftlets to orientate in darkness by echolocation. Because the component frequencies of the click-like orientation sound fall largely within the range audible to man, it is detectable without instruments in field conditions. As a consequence, the distribution of this capacity among the swiftlets is now reasonably well known. Together with those included in this chapter, sound spectrograms have been published of the orientation clicks of six taxa, representing four species. Tests of the ability to detect and avoid obstacles in darkness have been made on three of these taxa. Data are thus available for evaluating the importance of echolocation in the ecology of these birds, and for considering its significance as a taxonomic character.

The evolution of a communication system depends on the existence of individuals which gain from it, that is the senders of the signals and their receivers. These two share a common interest about which they communicate. Common interests of this kind form the basis of the communication between such individuals as a sexual pair, parents and their offspring, and members of a flock or group which feed, roost or breed together. It is less often realised that individuals which are usually regarded as conflicting in their interests — for example, prey and predator, sexual rivals, a parasite and its host — may also share a common interest which may form the basis for the evolution of a communication system between them. Warning coloration, warning calls and other signals which are given by a prey species towards a predator (Smythe, 1970; Alcock, 1975) and also threat display among rivals, are some examples of signals exchanged between individuals which mainly conflict in their interest.

Of considerable interest to students of life history phenomena is the identification of circumstances that favour the evolution of life cycles in which a single episode of reproduction is followed by rapid degeneration and death of the reproductive individual. Among non-annual plants and animals, this kind of life history is relatively uncommon, but curiously has arisen independently in several groups (for example, palms, periodic cicadas, bamboos, salmon). In this chapter, we summarise evidence implicating pollinator foraging behaviour as the factor which has led to the evolution of “Big Bang” reproduction in yuccas and agaves. In addition we also consider the effects of variation in plant reproductive expenditure on the pollinators, and finally the way in which plants and pollinators may have coevolved.