Gordon H. Orians
Bio: Gordon H. Orians is an academic researcher from University of California, Berkeley. The author has contributed to research in topics: Agelaius & Population. The author has an hindex of 19, co-authored 32 publications receiving 2315 citations.
TL;DR: All features of social systems are considered to be the products of natural selection just as are any physiological or morphological adaptations.
Abstract: The conspicuousness of adaptive radiation in morphology tends to conceal the fact that often the slight differences between closely related species give no clues to their widely differing ecologies, because many of the important differences between species are the result of behavioral and not morphological adaptations. This study analyses the role of social organization of the Red-winged Blackbird (Agelaius phoeniceus) and the Tricolored Blackbird (A. tricolor) in the different ways in which these two species exploit their environment. Knowledge of avian social systems began with natural history studies, but certain phases, such as territoriality, early attracted special consideration. In the 1930s, social systems began to be studied from the viewpoint of the comparative ethologist, who is primarily interested in the motivational and evolutionary aspects of behavior patterns, but whose publications contain a wealth of information about many ecological features of avian social systems. The mathematical approach to population parameters has provided a basis for considering the consequences of changes in social system characteristics upon basic population parameters, but biologists have in general been suspicious of this approach, which seemed to rest upon assumptions of doubtful biological validity. The result is rather widespread failure to realize the significance of certain features of social systems in quantitative terms, and failure to record and publish relevant information. Finally, the study of social * Present address: Dept. of Zoology, Univ. of Washington, Seattle 5, Washington. systems from the modern ecological viewpoint has lagged behind other approaches because few observers have made use of the background of a century of Darwinian thinking in evaluating their observations. In this study I have considered all features of social systems to be the products of natural selection just as are any physiological or morphological adaptations. To the question whether or not differences between social systems are adaptive, three types of answers are possible. Firstly, it may be assumed that the particular features of a social system are surely adaptive. Secondly, it may be assumed that the traits are purely fortuitous, without selective significance. Thirdly, it may be assumed that the particular traits are not adaptive but that they are associated with other, as yet unrecognized, differences which are adaptive (Maynard Smnith 1958). In this paper I shall attempt to interpret as far as possible the characteristics of social systems in the light of the first of these three assumptions. The second is rejected because it is sterile as a basis for research and because the widespread and consistent differences to be discussed cannot be without selective significance. The third can never be easily accepted, for unless this statement of faith is followed by attempts to discover the traits of adaptive significance and their connection with the supposedly unadaptive trait, nothing is really explained. Furthermore, no such case involving polygenic traits has been shown to be true, and separation of desirable from undesirable traits will almost certainly occur with time. Because the closely related and morphologically similar Red-winged and Tricolored blackbirds differ
TL;DR: It is suggested that Darwinian selection at the level of the individual permits an understanding of the known structure of avian communities and that there is no need at present to invoke new selective mechanisms at thelevel of the community or ecosystem.
Abstract: Territories of birds, usually defended against conspecific individuals, are sometimes defended against individuals of other species. Since such behavior is demanding both of time and energy, natural selection should favor ecological should favor ecological divergence, the establishment of overlapping territories, and the reduction of aggression. Lack of divergence in modes of exploitation could mean that insufficient time has elapsed for the changes to be completed or that the environment imposes some limitation preventing the evolution of the required degree of divergence. Such environmental limitation can be predicted in (a) structurally simple environments, (b) when feeding sites are strongly stratified in structurally complex vegetation, or (c) when the presence of other species in the environment prevents divergence in certain directions. The known cases of interspecific territoriality in birds are analyzed and shown to be largely in accordance with these predictions, although several cases of overlapping territories in situations where interspecific territoriality has been predicted provide relationships worthy of further study. We suggest that Darwinian selection at the level of the individual permits an understanding of the known structure of avian communities and that there is no need at present to invoke new selective mechanisms at the level of the community or ecosystem.
TL;DR: Results of tests indicated that early successional annuals were significantly more palatable than early succession perennials, and no correlation was found between palatability and evolutionary association of the herbivores with the plant species.
Abstract: Theoretical considerations and evidence from the literature suggest that early successional plant species should make a lesser commitment of resources to defense against herbivores, and should then provide better food sources for generalized herbivores than later successional and climax plants. Commitment to defense by plants is estimated by determining short-term palatabilities to two slug species; one native to western Washington, Ariolimax columbianus, and one introduced from Europe, Arion ater. Results of tests with 100 plant species of three growth forms and from different seral stages indicated that early successional annuals were significantly more palatable than early successional perennials which were sig- nificantly more palatable than later successional species. No correlation was found between palatability and evolutionary association of the herbivores with the plant species.
TL;DR: The number of species of birds recorded was not measurably affected by the tree species diversity or the length of the dry season, but a larger proportion of the total forest species known to occur in the region as a whole was found in the drier areas, whereas only about 50% were recorded in the wetter areas.
Abstract: Resident birds were censused in seven plots in Costa Rica ranging from 0 to 6 months dry season and from sea level to 2,380 m in elevation. Included were one highland and two lowland sites that were dominated by one or two species of trees. All stands were tall, undisturbed forests, but some were surrounded by agricultural land. The number of species of birds recorded was not measurably affected by the tree species diversity or the length of the dry season. However, a larger proportion (about 90%) of the total forest species known to occur in the region as a whole was found in the drier areas, whereas only about 50% were recorded in the wetter areas. This suggests that "between—habitat" diversity" may be greater in the areas with less severe dry seasons. The highland sites and markedly fewer species than the lowland sites, and a greater percentage of the species foraged by moving actively than was the case in the lowland sites. The large number of tropical lowland forest species that hunt by sitting and w...
01 Jan 1980
TL;DR: Orians as discussed by the authors used models derived from Darwin's theory of natural selection to predict the behavior and morphology of individuals as well as the statistical properties of their populations, including habitat selection, foraging behavior, territoriality and mate selection.
Abstract: The variety of social systems among the New World blackbirds (Family Icteridae) and the structural simplicity of their foraging environment provide excellent opportunities for testing theorics about the adaptive significance of their behavior. Here Gordon Orians presents the results of his many years of research on how blackbirds utilize their marsh environments during the breeding season. These results stem from information he gathered on three species during ten breeding seasons in the Pacific Northwest, on Red-winged blackbirds during two breeding seasons in Costa Rica, and on three species during one breeding season in Argentina. The author uses models derived from Darwin's theory of natural selection to predict the behavior and morphology of individuals as well as the statistical properties of their populations. First he tests models that predict habitat selection, foraging behavior, territoriality, and mate selection. Then he considers some population patterns, especially range of use of environmental resources and overlap among species, that may result from those individual attributes. Professor Orianns concludes with an overview of the structure of bird communities in marshes of the world and the relation of these patterns to overall source availability in these simple but productive habitats.
TL;DR: In the majority of natural communities succession is frequently interrupted by major disturbances, such as fires, storms, insect plagues, etc., starting the process all over again, but if not interrupted, it eventually reaches a stage in which further change is on a small scale as individuals die and are replaced.
Abstract: The sequence of species observed after a relatively large space is opened up is a consequence of the following mechanisms. "Opportunist" species with broad dispersal powers and rapid growth to maturity usually arrive first and occupy empty space. These species cannot invade and grow in the presence of adults of their own or other species. Several alternative mechanisms may then determine which species replace these early occupants. Three models of such mechanisms have been proposed. The first "facilitation" model suggests that the entry and growth of the later species is dependent upon the earlier species "preparing the ground"; only after this can later species colonize. Evidence in support of this model applies mainly to certain primary successions and in heterotrophic succession. A second "tolerance" model suggests that a predictable sequence is produced by the existence of species that have evolved different strategies for exploiting resources. Later species will be those able to tolerate lower levels...
TL;DR: A conceptual model of the evolution of plant defense is concluded, in which plant physioligical trade-offs interact with the abiotic environment, competition and herbivory.
Abstract: Physiological and ecological constraints play key roles in the evolution of plant growth patterns, especially in relation to defenses against herbivores. Phenotypic and life history theories are unified within the growth-differentiation balance (GDB) framework, forming an integrated system of theories explaining and predicting patterns of plant defense and competitive interactions in ecological and evolutionary time. Plant activity at the cellular level can be classified as growth (cell division and enlargement) of differentiation (chemical and morphological changes leading to cell maturation and specialization). The GDB hypothesis of plant defense is premised upon a physiological trade-off between growth and differentiation processes. The trade-off between growth and defense exists because secondary metabolism and structural reinforcement are physiologically constrained in dividing and enlarging cells, and because they divert resources from the production of new leaf area. Hence the dilemma of plants: Th...
TL;DR: To conclude with a list of questions appropriate for studies of resource partitioning, questions this article has related to the theory in a preliminary way.
Abstract: To understand resource partitioning, essentially a community phenomenon, we require a holistic theory that draws upon models at the individual and population level. Yet some investigators are still content mainly to document differences between species, a procedure of only limited interest. Therefore, it may be useful to conclude with a list of questions appropriate for studies of resource partitioning, questions this article has related to the theory in a preliminary way. 1) What is the mechanism of competition? What is the relative importance of predation? Are differences likely to be caused by pressures toward reproductive isolation? 2) Are niches (utilizations) regularly spaced along a single dimension? 3) How many dimensions are important, and is there a tendency for more dimensions to be added as species number increases? 4) Is dimensional separation complementary? 5) Which dimensions are utilized, how do they rank in importance, and why? How do particular dimensions change in rank as species nuimber increases? 6) What is the relation of dimensional separation to difference in phenotypic indicators? To what extent does the functional relation of phenotype to resource characteristics constrain partitioning? 7) What is the distance between mean position of niches, what is the niche standard deviation, and what is the ratio of the two? What is the niche shape?
TL;DR: Resource availability in the environment is proposed as the major determinant of both the amount and type of plant defense, and theories on the evolution of plant defenses are compared with other theories.
Abstract: The degree of herbivory and the effectiveness of defense varies widely among plant species. Resource availability in the environment is proposed as the major determinant of both the amount and type of plant defense. When resource are limited, plants with inherently slow growth are favored over those with fast growth rates; slow rates in turn favor large investments in antiherbivore defenses. Leaf lifetime, also determined by resource availability, affects the relative advantages of defenses with different turnover rates. Relative limitation of different resources also constrains the types of defenses. The proposals are compared with other theories on the evolution of plant defenses.