Showing papers in "Annual Review of Ecology, Evolution, and Systematics in 1973"

Resilience and Stability of Ecological Systems

Abstract: Individuals die, populations disappear, and species become extinct. That is one view of the world. But another view of the world concentrates not so much on presence or absence as upon the numbers of organisms and the degree of constancy of their numbers. These are two very different ways of viewing the behavior of systems and the usefulness of the view depends very much on the properties of the system concerned. If we are examining a particular device designed by the engineer to perform specific tasks under a rather narrow range of predictable external conditions, we are likely to be more concerned with consistent nonvariable performance in which slight departures from the performance goal are immediately counteracted. A quantitative view of the behavior of the system is, therefore, essential. With attention focused upon achieving constancy, the critical events seem to be the amplitude and frequency of oscillations. But if we are dealing with a system profoundly affected by changes external to it, and continually confronted by the unexpected, the constancy of its behavior becomes less important than the persistence of the relationships. Attention shifts, therefore, to the qualitative and to questions of existence or not. Our traditions of analysis in theoretical and empirical ecology have been largely inherited from developments in classical physics and its applied variants. Inevitably, there has been a tendency to emphasize the quantitative rather than the qualitative, for it is important in this tradition to know not just that a quantity is larger than another quantity, but precisely how much larger. It is similarly important, if a quantity fluctuates, to know its amplitude and period of fluctuation. But this orientation may simply reflect an analytic approach developed in one area because it was useful and then transferred to another where it may not be. Our traditional view of natural systems, therefore, might well be less a meaningful reality than a perceptual convenience. There can in some years be more owls and fewer mice and in others, the reverse. Fish populations wax and wane as a natural condition, and insect populations can range over extremes that only logarithmic

The structure of lizard communities

Abstract: Strictly speaking, a community is composed of all the organisms that live together in a particular habitat. Community structure concerns all the various ways in which the members of such a community relate to and interact with one another, as well as community-level properties that emerge from these interactions, such as trophic structure, energy flow, species diversity, relative abundance, and community stabil­ ity. In practice, ecologists are usually unable to study entire communities, but instead interest is often focused on some convenient and tractable subset (usually taxonomic) of a particular community or series of communities. Thus one reads about plant communities, fish communities, bird communities, and so on. My topic here is the structure of lizard communities in this somewhat loose sense of the word (perhaps assemblage would be a more accurate description); my emphasis is on the niche relationships among such sympatric sets of lizard species, especially as they affect the numbers of species that coexist within lizard communities (species den­ sity). So defined, the simplest (and perhaps least interesting) lizard communities would be those that contain but a single species, as, for instance, northern populations of Eumeces msciatus. At the other extreme, probably the most complex lizard commu­ nities are those of the Australian sandridge deserts where as many as 40 different species occur in sympatry (20). Usually species densities of sympatric lizards vary from about 4 or 5 species to perhaps as many as 20. Lizard communities in arid regions are generally richer in species than those in wetter areas; therefore, because almost all ecological studies of entire saurofaunas have been in deserts (l8, 20, 25), this paper emphasizes the structure of desert lizard communities. As such, I review mostly my own work. Other studies on lizard communities in nondesert habitats are, however, cited where appropriate. Historical factors such as degree of isolation and available biotic stocks (particu­ larly the species pools of potential competitors and predators) have profoundly shaped lizard communities. Thus one reason the Australian deserts support such very rich lizard communities may be that competition with, and perhaps predation pressures from, snakes, birds, and mammals are reduced on that continent (20).

Ecological Inferences From Morphological Data

Abstract: To some extent ecologists have differentiated into two groups: one whose primary concern is theory, another whose primary concern is collection of data. Fretwell's engaging analysis of research strategies in ecology (19:x-xix) suggests that special­ ization as theorist or data collector increases prestige, which is almost certainly favored by selective biology faculties in the current era of restricted budgets and PhD oversupply. One corollary of such specialization has been the tendency of theoreticians to test models, not by direct field observation of ecological systems, but by measurement of museum specimens for a morphological characteristic indicating the organism's ecology. Certain types of ecologic analysis over large biogeographic areas can probably be done in no other way short of several man-lifetimes of study (46, 57, 74). How valid is this methodology? The purpose of this review is to scrutinize critically some of the studies in which ecological generalizations have been made or tested on the basis of morphological measurements and to examine the validity of the relationship assumed between morphology and ecology. As is demonstrated in the next section, this area of concern involves a wide range of phenomena, some recently and well reviewed elsewhere. I therefore concentrate attention on two related aspects of this topic: the relationship of prey size to predator size and the phenomenon of character displace­ ment understood in terms of niche width, separation, and overlap.

Structure of Foliage Canopies and Photosynthesis

Abstract: All living things on the earth, including plants, rely principally upon the photosynthate produced by plants for their daily food, and accordingly are strongly affected by the variation of plant photosynthesis over the globe. The distribution of solar energy with latitude determines to a great extent the geographic variation of photosynthetic activity of plants. Its latitudinal change in turn sets broad geographic limits to the different forms of terrestrial life, affecting the energy flow and the cycle of materials in ecosystems. On the other hand, the morphological and physiological characteristics of plants are thought to result from their evolutional adaptation to environmental conditions during the geological past. The morphological features as characterized by the geometrical structure (or architecture) of plant canopies have a great influence upon the processes of action and reaction between plants and their environment through the modification and interception of fluxes of radiation, heat, carbon dioxide, etc. Consequently it is obvious that the canopy structure is determinant of the photosynthetic productivity of plant canopies. The canopy structure as well as the physiological properties of leaves with respect to photosynthesis and respiration, therefore, can play an important role in the competition between plants. Since the relation between the photosynthetic activity and the structure of plant canopies was elucidated by Monsi & Saeki (100) in 1953, a great number of both theoretical and experimental studies have been done on this problem. Studies in this field have been greatly stimulated by the activities of the International Biological

Enzyme Polymorphism and Biosystematics: The Hypothesis of Selective Neutrality

Abstract: In the last few years there has been an explosion of information concerning electrophoretic variation at enzyme loci. These data are being increasingly employed in attempts to elucidate biosystematic and phylogednetic relationships. As the evolutionary role of these allozyme polymorphisms is not well understood, the assumptions inherent in such approaches warrant careful consideration. The following review addresses itself to an examination of the possible role of selection in maintaining enzyme polymorphisms in natural populations. Selected for discussion here are those papers which seem to me to bear importantly upon central issues; the literature citations are not intended to be comprehensive or complete. EXPERIMENTAL ASPECTS OF ELECTROPHORETIC ANALYSIS Before discussing either the patterns of allozyme variation which have been observed or their possible evolutionary significance, it is necessary to consider what has been examined: the classes of protein variants detectable by current methods, the organisms which have been examined for such variants, and the enzyme reactions which have been used as screens. It is necessary to state carefully the experimental question which allozyme surveys pose in order to evaluate possible limitations and bias in the results obtained. Experimental approaches involving electrophoretic analysis have dealt with three related sorts of questions: (a) those concerning relative amounts of variation; (b) those concerning the genetic nature of polymorphic variation; (c) those employing comparisons of variant types. Work in each of these areas may entail important assumptions about the nature of the variation. When assessing the levels of electrophoretic variation in a natural population, the assumption is generally made, implicitly or explicitly, that electrophoretically de