Body size is manifestly one of the most important attributes of an organism from an ecological and evolutionary point of view. Size has a predominant influence on an animal's energetic requirements, its potential for resource exploitation, and its susceptibility to natural enemies. A large literature now exists on how physiological, life history, and population parameters scale with body dimensions (24, 131). The ecological literature on species interactions and the structure of animal communities also stresses the importance of body size. Differences in body size are a major means by which species avoid direct overlap in resource use (153), and size-selective predation can be a primary organizing force in some communities (20, 70). Size thus imposes important constraints on the manner in which an organism interacts with its environment and influences the strength, type, and symmetry of interactions with other species (152, 207). Paradoxically, ecologists have virtually ignored the implications of these observations for interactions among species that exhibit size-distributed populations. For instance, it has been often suggested that competing species

https://www.jstor.org/stable/pdf/2096954.pdf

The ontogenetic niche and species interactions in size-structured populations

Niche breadths in animal and plant populations are often measured without regard to the relative frequencies of the various resources available to the organisms. Recent papers have provided more accurate indices for use when available resources can be quantified. These indices may lack wide applicability or simple biological interpretations. We suggest that niche breadth be defined as the degree of similarity between the frequency distribution of resources used by members of a population and the frequency distribution of resources available to them. Similarity can be quantified with the familiar Proportional Similarity (PS) Index. This index, which measures objectively the similarity between two frequency distributions, reflects in satisfactory fashion the breadth of a population's niche. See full-text article at JSTOR

A Simple Measure of Niche Breadth

One of the ways that animals are thought to coexist is by differences in their body sizes. Very simply, different sized animals eat different sized foods or otherwise utilize different resources, until at some point there is enough nonoverlap to allow coexistence. This paper inspects the role of body size in competition, using MacArthur's (1972) theoretical framework. As well as describing patterns of convergence and divergence, the model developed here unifies two seemingly contradictory concepts that have become established over the past decade: (1) the concept that differences in body size promote a \"niche difference\" (Hutchinson 1959; Brown and Wilson 1956; Schoener 1965, 1967, 1970, 1974a and b; Grant 1968, 1972), and (2) the concept that differences in body size set up a competitive gradient whereby the larger can exclude the smaller (Brooks and Dodson 1965; Galbraith 1967; Hall et al. 1971; but see Dodson 1974 and Zaret 1975 for alternative view).

The Adequacy of Body Size as a Niche Difference

Body size is known to play a critical role in determining patterns of prey selection. In this study, we examined the diets of pumpkinseed sunfish (Lepomis gibbosus) from three Michigan lakes. Pumpkinseeds have highly developed pharyngeal jaws specialized for crushing gastropods, and in our study lakes gastropods consistently contributed >80% of the prey mass in pumpkinseed diets. However, the average dietary composition and prey selection among snail taxa and size classes varied considerably among dates and sites. We hypothesized that this variation was influenced by changes in the size structure of the snail community. We used laboratory studies to quantify the effect of snail (and fish) size on three important components of the predator—prey interaction: encounter rates, attack probabilities, and capture successes. We then used these laboratory data to predict prey selection observed in the field. For most of our field situations, a simple model based on size—specific encouter rates only) explained a large percent (71%) of the observed variation in prey selection. However, in those cases where some of the snails were resistant to predation, due to crushing resistance or gape limitation, this simple model was a very poor predictor of prey selection. A more complex model (based on encounter rates and size refuges) successfully explained 46% of the variance in these cases where snails were relatively invulnerable. Finally, we compared estimates of attack probabilities with predictions from optimal foraging theory and found qualitative agreement in that fish ignored prey of low profitabilities and became more selective as the quality of the environment improved. however, the incorporation of variable attack probabilities into the foraging model resolved only a small part of the observed residual variation in selectivities because snails with low profitabilities were already underrepresented in the diet due to their low encounter rates or capture successes. This study demonstrates that predator—prey interactions in size—structured populations can create apparently complex variation in prey selection, but that this variation can be largely understood within a framework that simultaneously considers the dynamics of prey size distributions and how components of the foraging interaction scale with body size of the predator and prey.

Effects of Body Size on the Predator‐Prey Interaction Between Pumpkinseed Sunfish and Gastropods

On Isle Royale, Michigan, moose cows which have calves show a different distribution and diet from solitary adults and yearling moose. Solitary adults and yearlings follow a feeding pattern predicted from the location of high nutrient plant growth. In the presence of wolves, they feed on the ridges of the main island, where they take advantage of plants that leaf early; only later in the season do they move to the small outlying islands where plant phenology is delayed by the cold water of Lake Superior. Cows with calves deviate sharply from this pattern. They remain on the wolf-free small islands throughout the growing season. While on the islands, cows with calves eat a poorer quality diet than other moose. They switch to eating high quality spring and summer foods later than other moose and they eat significantly fewer high preference shrubs and significantly more herbs and low preference shrubs than other moose. This study suggests that in order to avoid predators, cows with calves sacrifice the high quality diet available on the main island. These data also suggest that the wolves not only affect prey numbers by direct kills but may also indirectly influence prey numbers by altering the diet of their prey. In this case, the reproductive cows, those individuals that contribute most directly to growth of the population, avoid predators but frequent poor feeding areas.

Diet shifts in moose due to predator avoidance.

The parameters of prey capture and competition are remarkably simple to measure in the larval ant lion (Neuroptera: Myrmeliontidae). There exists a thirteen- to fifty-fold difference in food availability (mg. prey available/mg. predator) between the largest and smallest individuals. The larva grows to the optimal size as a food getter, but no larger before metamorphosing. A special distribution of ant lions minimizing competition is hypothesized and observed. The ant lion does not conform to the concepts of limiting similarity or niche variation. THE ANT LION has long been a naturalist's delight in its exotic method of prey captLre. The larva digs a conical pit in loose sand and lies buried at the vertex. When an insect falls in, the ant lion attempts to grab it with its long ice-tong-shaped mandibles. If it fails and the prey starts to scramble up the side of the pit, the ant lion tosses sand with violent flips of its head, creating land slides that carry the prey back into its jaws. After extracting the fluids, it throws the carcass out of the pit, along the rim. Interest in the ant lion died together with interest in simple natural history, for the last nontaxonomic paper was Wheeler's (1930) monograph. What has not been appreciated is the ant lion's elegance for ecological study. Here I show that the searching capacity, prey size range, and success of capturing prey within this range can all be measured by taking the diameter of the pit; the nutritional history, prey size distribution and abundance by collecting the carcasses around the rim of the pit; and the competitive effect of neighbors, in percent prey intercepted, by the spatial configuration of the pits. Also, while not studied here, it is worth noting that when the animal disperses it does so half submerged in the sand, leaving an easily visable trough as a record of its movements.

Prey Capture and Competition in the Ant Lion

Optimal foraging theory predicts that the energy value of a marginally acceptable item equals the overall energy available to the animal in the environment (calories per unit time). If true, then the selective behavior of the consumer may be used as a measure of this important parameter. In many arthropod predators the time spent feeding on an individual item should vary inversely with energy availability. This hypothesis is successfully tested using two species of tiger beetles (Cicindelidae). In many vertebrate predators the minimal acceptable food size should vary directly with energy availability. This hypothesis is not tested here, although some support for it exists in the literature. If valid, the simplicity of the method described allows the assessment of global patterns of energy availability for some taxa. Preliminary information on tropical-temperate gradients for birds is given. THE AMOUNT OF ENERGY (food) available to animals in nature is one of ecology's fundamental parameters. Unfortunately, it is also one of the most difficult to measure. Are tropical birds more food limited than temperate zone birds? What are the seasonal patterns of energy availability? What makes one habitat more attractive than another? These questions are all easier to ask than to answer. However, animals themselves assess energy availability, and often modify their behavior on the basis of their 'estimates.' This paper explores the possibility of using the behavior of the consumer to deduce the energy available to it in the environment. OPTIMAL FORAGING THEORY Most theories of selective feeding assume that the organism acts on the following principle: It searches for food, and on encountering an item "asks" itself the question, will it gain more energy by processing the item, or would it gain more by neglecting it and continuing to search (see Schoener 1974 for a review; this concept, however, does not take into account long-term considerations, such as discussed by Katz ( 1974) ) ? In other words, every item is judged by its 'e(x)/t(x)' ratio, where: e(x) -the 'energy profit' of the item (x) (this is a close correlate of biomass, adjusted for the amount of energy it took to capture it, indigestible matter content, etc.: it is the net energy received from that item); and t(x) -the amount of time it takes to process that item (this is the time during which the feeder is unable to search for, or process, other items). If the food is not particulate, these factors can be expressed per unit biomass. Also, it is clear that uch variables as success of capture can be incorporated into these terms. For example, the time expended on unsuccessful captures per successful capture can be included in the t (x) term, and the energy expended on unsuccessful captures per successful capture can be included in the e(x) term. By this process the animal partitions its set of encountered prey into a subset which it will accept and a subset which it will ignore. The main factor determining which subset a particular items falls into is the abundance of more desirable items (desirability defined by its e ( x) /t (x) ratio); but the animal itself may be spatially confined to a territory or to a refuge in order to escape predation, and in limiting he rea it can search, this restriction will ultimately affec what it will accept. The animal may have seve al feedin modes (e.g. hawking or foliage gleaning for birds) or a choice of several feeding patches. It is assumed that the organism will operate in the mode and patch that offers the most energy and, within these, follow the same principle. The result is some conglomerate optimization of many strategies, with the animal acquiring some amount of energy, E, over the total time spent feeding, T (its daily feeding bout, for instance), or an average of E/T calories per unit time. In the most realistic sense, this figure expresses the amount of energy available to the animal. But it is so subjective and involves so many compromises that it would be very difficult for the ecologist to measure it. It is now necessary to investigate the set of items that are on the boundary line of being accepted or rejected. The question once again is whether the animal will gain more energy by processing the item, I Present address: Department of Zoology, University of Washington, Seattle, Washington 98195, U.S.A. 96 BIOTROPICA 8(2): 96-103 1976 This content downloaded from 157.55.39.111 on Sat, 17 Sep 2016 05:47:56 UTC All use subject to http://about.jstor.org/terms or whether it could more profitably spend that time searching. If the animal processes the item it will get e(x)/t(x) calories per unit time throughout the processing time. If it continues searching it will receive, on the average, E/T calories per unit time. The boundary condition for accepting an item is thus:

https://www.jstor.org/stable/info/2989628

Deducing the Energy Available in the Environment: an Application of Optimal Foraging Theory

A population of the ripple bug Rhagovelia scabra (Bacon) (Hemiptera: Veliidae) in the Rio Guacimal, Costa Rica, is structured by interference competition into three components: (1) optimal habitat users, who occupy fast-flowing areas such as the heads of pools; (2) suboptimal habitat users, who occupy stiller water; and (3) non-users, who form dense lethargic clusters by the margins of the stream. The distributions are enforced by the dominant animals and lead to a priority of prey capture. Dominance is determined mostly by age, sex, and disability. ECOLOGISTS RECOGNIZE TWO FORMS of competition (Miller 1967): Exploitation, in which an organism interacts only with its resources, and interference, in which an organism interacts directly with its competitors, excluding them from resources and thereby obtaining more for itself. The theoretical consequences of exploitation are fairly well worked out, and in fact form the foundation of the Lotka-Volterra equations on which most competition models are based. However, the theoretical consequences of interference are less well understood, and, moreover, seem to lead to a greater variety of outcomes (Schoener 1976). Many questions present themselves. Does interference simply space animals out over the available habitat (Huxley 1934) or does it generate a have/have-not situation in which one segment of the population lives amidst relative plenty while another starves, disperses, or lives in areas that will not support offspring (Brown 1969) ? If the latter, what determines the proportions of the population falling into these classes and how do they change with fluctuations in food supply? Does interference constitute a form of population regulation (see Rogers and Hassel 1974. Rogers and Hubbard 1974, Wynne-Edwards 1962, Wilson 1977 for different approaches), and, if so, is regulation the essence of, or coincidental to, its evolution? How are subordinate animals maintained in the population if they are so severely selected against by interference? This paper describes a very simple field study on interference competition and some of its consequences in a population of the tropical ripple bug Rhagovelia scabra Bacon (Hemiptera: Veliidae). The data are of interest for two reasons. First, they show how a surprisingly 'vertebrate' form of population structure can exist in an insect using simple behavior patterns. The ripple bug's population structure closely resembles that which Brown (1969) describes for birds. Second, working with insects allows manipulations that are difficult, if not impossible, for vertebrates, on which most studies of this type are conducted. The research site was on the Rio Guacimal, a high mountain stream near Monte Verde, Guanacaste Province, Costa Rica. The work was conducted during June-July 1976. The methods are very simple and are described along with the results.

David Sloan Wilson

Papers

Prey Capture and Competition in the Ant Lion

Deducing the Energy Available in the Environment: an Application of Optimal Foraging Theory

Interference competition in a tropical ripple bug (Hemiptera: Veliidae).