Thomas W. Schoener

Resource Partitioning in Ecological Communities

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?

Field Experiments on Interspecific Competition

Rare until recently, field-experimental studies of interspecific competition now number well over 150. Competition was found in 90% of the studies and 76% of their species, indicating its pervasive importance in ecological systems. Exploitative competition and interference competition were apparent mechanisms about equally often. Few experiments showed year-to-year variation in the existence of competition, though more did in its intensity; many were not long-term. The Hairston-Slobodkin-Smith hypothesis concerning variation in the importance of competition between trophic levels was strongly supported for terrestrial and freshwater systems. In particular, producers, and granivores, nectarivores, carnivores, and scavengers taken together, showed more competition than did phytophagous herbivores and filter feeders. In marine systems, virtually no trend was detectable one way or the other. Large heterotrophs competed more than small ones in most comparisons, and other properties possibly deterring predation, such as stinging behavior, seemed also characteristic of species competing frequently. Among terrestrial plants and certain terrestrial animals but not all, experiments carried out in enclosures were more likely to show competition than unenclosed experiments. A greater ecological overlap implied a greater tendency to compete, as determined experimentally, when niche dimensions were food type or microhabitat; the opposite was true for macrohabitat. A substantial number of studies showed asymmetry in their species response to competition; larger species were significantly more often superior than smaller ones, though a variety of other apparent reasons for asymmetry also existed. The integration of competition theory into field experimentation has only just begun.

Nonsynchronous Spatial Overlap of Lizards in Patchy Habitats

Sympatric native Anolis species with similar structural habitats but contrasting climatic habitats are closer in head and body size on species—rich than on depauperate islands. In two localities, sympatric Anolis species with differential occurrences in sun or shade sought lower, more shaded perches during midday, resulting in partly nonsynchronous utilization of the vegetation by the two species. The second observation may be related to the first in the following way: nonsynchronous spatial overlap could dictate relatively great resource overlap for species coinhabiting patchy or edge areas, requiring great differences between the species in prey size in addition to those in climatic habitat. The extent of such overlap on small depauperate islands could be greater if these contained a greater proportion of patchy or edge habitats (with respect to insolation), or if climatic preferences were broader and more overlapping than on large, species—rich islands. In each locality, the relatively more shade—inhab...

The Ecological Significance of Sexual Dimorphism in Size in the Lizard Anolis conspersus

Adult males of Anolis conspersus capture prey of significantly larger size and occupy perches of significantly greater diameter and height than do adult females; similarly, these three dimensions of the niche are significantly larger for adult females than for juveniles. Adult males on the average eat a smaller number of prey, and the range in size of prey is larger. The relationship between the average length of the prey and that of the predator is linear when the predator size is above 36 millimeters, but becomes asymptotic when it is below that value. Subadult males as long as adult females eat significantly larger food than do the latter, but only in the larger lizards is this correlated with a relatively larger head. Anolis conspersus selects prey from a wide range of taxa and shows no obvious intraspecific specialization not connected to differences in microhabitat and prey size. The efficiency of this system for solitary species is pointed out.

An empirically based estimate of home range

The minimum-convex-polygon method for estimating home-range area, in which the outermost points are connected in a particular way, is extremely sensitive to sample size. Existing methods for estimating home-range area that correct for sample size fail to encompass all the important kinds of biological variation in the home-range utilization. (The home-range utilization describes the relative degree to which different units of space are frequented by an animal.) Although previous methods have assumed specific unimodal distributions, such as the bivariate normal, home-range utilizations may resemble funnels or pies as well as hills. A regression method is introduced that uses data from well-sampled individuals whose true home ranges are assumed approximately known to predict home-range areas for less well-sampled individuals. Appendix 5 summarizes this method. Sizes of home ranges estimated by the regression method are half or less than sizes estimated by previous methods in which utilization distributions are assumed to be all of a particular statistical type.

Simple Models of Optimal Feeding-Territory Size: A Reconciliation

How optimal territory size changes with changes in food density and intruder pressure is in theory complex. Qualitative predictions (table 1) may change depending upon the goal of the territory holder (time minimization, energy maximization), the possible constraints on energy maximization (time availability, processing capability), and if and how food density and intruder pressure covary. Predictions for the amounts of time spent feeding and defending (tables 2, 3) are similarly variable. Often, the qualitative effect of environmental changes will be indeterminate without detailed specification of the functions describing territorial costs and benefits.

THE EVOLUTION OF BILL SIZE DIFFERENCES AMONG SYMPATRIC CONGENERIC SPECIES OF BIRDS

Recently, Hutchinson (1959), Klopfer and MacArthur (1961), and Klopfer (1962) have introduced the use of the quantitative comparison of bill size differences among groups of sympatric congeneric species of birds in the study of the evolution of niche overlap and size. In his analysis of character displacement, Hutchinson (1959) has shown that the ratio of the size of the larger to smaller trophic appendages of congeneric species generally falls between 1.2 and 1.4 where they are sympatric, but is less where they are allopatric. Klopfer and MacArthur (1961), using Ridgways data, found a much smaller ratio for certain associations of sympatric tropical birds. This paper presents the results of an analysis of bill length, in which representatives of 46 bird families inhabiting temperate, subtropical, and tropical zones are compared. Several models are proposed to explain interfamilial, regional, and intrafamilial differences. Finally, the implications of this study for the concepts of niche overlap and behavioral stereotypy are discussed.

Competition and the form of habitat shift

This study derives simple models of competition and applies them to the description and prediction of habitat shift. Habitat shift is a change from one species population to another in the habitats occupied by most of their individuals. n nInterference models in which individuals are limited in their time for feeding by other individuals can give rise to competition equations structurally identical to Lotka-Volterra models. These models have linear zero-isoclines. Interference models in which individuals as a group are energy-limited produce nonlinear, concave downward, zero-isoclines. These are asymptotic to the axes. Unlike the linear-isocline case, such models always produce a unique stable equilibrium. Asymptotes may be made nonzero by inclusion of a refugium into the isocline equation. n nExploitation models in which individuals of different species feed with finegrained selection on the same resources but gain different energies from these give parallel linear isoclines. They cannot produce a stable point equilibrium, whether consumption per individual is fixed or not. A simple model, in which individuals have flexible requirements and feed with fine-grained selection, has a unique stable equilibrium if each species has some resources not usable by the other. This model has concave downward isoclines. If one species utilizes only resource kinds included within those of another, both can coexist if the first is sufficiently better at obtaining the shared resources and if there are enough such resources. n nSystems of equations specifying growth of both consumers and resources can sometimes give isoclines that are linear (MacArthur, R. H. 1968, The theory of the niche in “Population Biology and Evolution,” Syracuse Univ. Press) or that are of the same curvilinear form as isoclines for the pure consumer systems. n nOne way to relate competition to habitat shift is to assume the isocline equations hold separately for each of a set of habitats. Then if estimates of carrying capacities are available, models for isoclines can be constructed and competition parameters computed by regression techniques, using data for each habitat as a separate point. Relative carrying capacity in Locality S (with competitors) can be estimated as follows. Measure the relative abundance of the species in its “fundamental” niche in Locality A (without competitors); then weigh it by the ratio of available resources in Locality S to available resources in Locality A. This method provides a possible structure for formalizing how competition transforms the “fundamental” into the “realized” niche. n nThe method is applied to a shift in structural habitat (perch height and diameter) in some Caribbean lizards. Habitat availability is measured as relative surface area, estimated from vegetation using a new sampling technique. Results are: (1) nonlinear isoclines fit better than linear ones; (2) concave downward isoclines fit much better than convex downward isoclines; (3) an exploitative model fits better than all others, including interference and descriptive models; and (4) competitors of different sizes are more likely to fit interference than exploitative models; those of the same size show the opposite; and species best fitting interference models are more often the smaller member of pairs of differently sized competitors.

Alternatives to Lotka-Volterra competition: models of intermediate complexity.

Abstract A family of one-level differential-equation competition models in which two populations are limited by the energy flowing into the system generates the following results. For competitors on the same and only resource: 1) Purely exploitative competition leads to exclusion; which species wins depends on relative abilities to appropriate and extract energy from the resource, and the relative death and maintenance rates. 2) If conspecific interference (e.g., deaths or energy loss from fighting, cannibalism, or display) is sufficiently high relative to abilities to exploit the common resource, competition for the same resource can lead to coexistence. 3) If heterospecific interference is sufficiently high relative to abilities to exploit the common resource, competition for the same resource can lead to a priority effect, in which the outcome depends on initial population sizes. 4) Depending on whether situation (2) or (3) prevails, an increase in the amount of the common resource can convert an outcome in which one species always wins into one giving coexistence (2) or a priority effect (3). 5) If species are similar to one another in their abilities to appropriate and extract energy from the common resource and show reciprocity in intererence costs, competition can have multiple outcomes; either one species wins or the species coexist, depending on initial values. For competition on the same resource, but with each species monopolizing an exclusive resource as well: 1) Purely exploitative competition always leads to a unique point coexistence. 2) If interference is added to the system described in (1), two points of coexistence, separated by a saddle (an “unstable equilibrium”) are possible. This is favored by a) a small yield from the exclusive resources relative to the common one; and b) strong interspecific relative to intraspecific interference.

Population growth regulated by intraspecific competition for energy or time: Some simple representations☆

Abstract An examination is made of some of the ways populations can grow in response to changes in their own density. Under two different assumptions on birth and death rates, models for single-species population growth that incorporate intraspecific competition by interference but not exploitation are of logistic form. Where an individuals net energy input from feeding is inversely proportional to population size, population growth follows a convex curve, whether interference is included or not. Data of Smith (1963) on Daphnia populations are fit well by this kind of curve. Combination of the two kinds of growth can produce S -shaped curves whose inflection is displaced from that value—half the carrying capacity—given by the logistic; an upward displacement is favored by a high ratio of metabolic and replacement costs to feeding input. Inflection points from real curves are much more often higher than expected from the logistic. Nonmonotonic growth curves can arise when there is instantaneous feedback between consumers and resource availability; certain of these equations are of logistic or convex form at equilibrium. The possible effect of r - and K -selection on the biological parameters, such as feeding efficiency, used to construct the monotonie equations is discussed, and the equations are extended to 2-species competition. Table III characterizes some simple single-species growth curves.