The study of interspecific competition has long been one of ecology's most fashionable pursuits. Stimulated in part by a simple theory (Lotka 1932; Volterra 1926; Gause 1934; Hutchinson 1959; MacArthur and Levins 1967), ecologists gathered numerous data on the apparent ways species competitively coexist or exclude one another (reviews in Schoener 1974b, 1983). As is typical in science, most of the early data were observational, and the few experimental studies were mostly performed in the laboratory rather than in the field. Though never lacking its doubters, the belief in the natural importance of interspecific competition is now being severely questioned (review in Schoener 1982). Many of the putatively supportive observations have been challenged as being statistically indistinguishable from random contrivance. Most such attacks have been rebutted, but not without some modification of original conclusions (e.g., papers in Strong et al. 1983). New observations have been gathered for certain systems, suggesting a lack of competitively caused patterns and catalyzing the variable environment view of Wiens (1977) in which competition is seen as a temporally sporadic, often impotent, interaction. Other critics have charged that the lack of experimental field evidence for competition would preclude its acceptance regardless of the quality of observational data. Indeed, results of some of the earlier field experiments are in part responsible for competition's presently beleaguered state. Connell (1975), after reviewing the field experiments known to him through 1973, concluded that predation, rather than competition, appears to be the predominant ecological interaction and should be given "conceptual priority." Shortly afterward, Schroder and Rosenzweig (1975) showed experimentally that two species of desert rodents overlapping substantially in habitat did not appear to affect one another's abundances. This result was interpreted as contradicting a crucial assumption of competition theory, almost its linchpin: the greater the resource overlap between species, the greater the competition coefficient, a measure of the intensity of interspecific competition (relative to intraspecific competition; MacArthur and Levins 1967; review in Roughgarden 1979).

Field experiments on interspecific competition

In a strictly defined sample of competition studies using controlled field experiments, covering 215 species and 527 experiments, competition was found in most of the studies, in somewhat more than half of the species, and in about two-fifths of the experiments. In most of these experiments interspecific competition was not distinguished from intraspecific competition. In the few studies in which the two were separated, interspecific competition was the stronger form in about onesixth of all experiments done. When competition was demonstrated, intraspecific competition was as strong or stronger than interspecific in three-quarters of the experiments. Some evidence from this literature survey suggests that negative results may be underrepresented, so that the absolute values of these figures may be too high. Since this bias should apply also to studies of all taxa, habitats, or other interactions it should not greatly affect estimates of the relative prevalence of competition. Since these estimates come fr...

On the prevalence and relative importance of interspecific competition: evidence from field experiments

We present a quantitative literature review to assess the extent to which field experi- ments with plants have addressed questions about patterns of competition over time and space, consequences of competition for community structure, and comparisons of competitive ability among species We outline the necessary treatment comparisons and statistical analyses to answer each question and then describe the number of experiments that meet these criteria and their results Although we found a total of 101 experiments in 89 studies, 63% of these experi- ments only addressed whether competition significantly affected some component of individual fitness of a single species at a single time and site Despite the limited data base to address more complex questions about competitive interactions, we did find consistent results for a few of the questions we reviewed Where tested, competition always had significant effects on distribution patterns (five experiments), on relative abundances (two experiments), and on diversity (four experiments), consistent with the notion that competition has strong effects on community structure On the other hand, intraspecific competition was not usually stronger than interspe- cific competition for either competitive effect (four experiments) or response (three experi- ments), which suggests that resource partitioning may not be an important mechanism of coexis- tence in plants

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Patterns and Consequences of Interspecific Competition in Natural Communities: A Review of Field Experiments with Plants

Summary 1 Theoretical models have predicted that the relative importance of facilitation and competition will vary inversely across gradients of abiotic stress, with facilitation being the dominant interaction under high abiotic stress conditions. A critical reappraisal of current theoretical models is needed because experimental studies both support and refute their predictions. 2 A quantitative meta-analysis of field and common garden studies evaluating the effect of abiotic stress (low vs. high) on the net outcome of plant‐plant interactions in arid and semi-arid environments was performed to evaluate the degree of empirical support for these models. We created four separate data sets corresponding to the categories of response variables commonly used to measure plant performance (survival, density, growth and fecundity). 3 The analyses showed that both the selection of the estimator of plant performance and the experimental approach followed have a strong influence on both the net outcome of plant‐plant interactions and the effect of abiotic stress on such outcome. The effect of neighbours on the survival and growth of target plants was not significant at either stress level, but that on the density and fecundity of target plants was positive (facilitation) and negative (competition) at the low and high abiotic stress level, respectively. Density data showed that the net effect of neighbours was positive and negative at low and high abiotic stress levels, respectively, whereas other estimators suggested that the net effect of neighbours did not differ with stress level. None of our meta-analyses indicated that the magnitude of the net effect provided by neighbours, whether positive or negative, was higher under high abiotic stress conditions, and facilitation does not therefore appear to increase in importance with abiotic stress. 4 As the predictions of current theoretical models regarding the relationship between the net outcome of a plant‐plant interaction and abiotic stress do not hold in arid and semi-arid environments, different models are needed. These should consider different sources of abiotic stress separately, and should be valid for performance measurements, such as survival, that integrate plant responses over time. The incorporation of these features into theoretical models will undoubtedly improve their predictive capabilities.

Is the change of plant–plant interactions with abiotic stress predictable? A meta‐analysis of field results in arid environments

The importance, and even the existence, of competition among plants in arid ecosystems has often been questioned. An influential statement of Shreve (113) asserted that interspecific competition does not occur in deserts, and Went (145) denied that competition between desert plants occurs at all. Neither provided evidence for his assertions, although Shreve pointed out the diversity of habits and phenologies found among desert species. He may have been responding to the strong emphasis placed on competition by Clements and his followers (e.g. 27). The importance of competition in natural communities has recently been debated (28, 109, 127). These reviews suggested that terrestrial plant communities are among the communities in which competition is relatively important. However, the majority of studies upon which this conclusion is based were made in humid regions. Grime (53) suggested that competition is less important in "high stress" habitats (in which he included dry habitats), but he presented little evidence from true arid or semiarid environments. This paper reviews the available evidence for competition in plant communities in arid and semiarid regions; as is demonstrated, competition certainly occurs in these communities and involves many different species. In several instances it appears to be important in the determination of community structure. Competition may be less frequent in these communities, though not less important on that account. This review also addresses several other questions concerning the role of competition in these communities, including: the role of competition in determining the absence, or presence and abundance, of species in a community, and their spatial arrangement; which

The Role of Competition in Plant Communities in Arid and Semiarid Regions

(3) The results of a controlled removal experiment showed that water-related interference does occur at this site when water availability is low, and that at present, interference between species is usually more intense than that within a species. These results, together with nearest-neighbour analyses, revealed that regular distributions may not necessarily indicate present interference among individuals of the same species, but rather past interference. (4) The importance of episodic seedling establishment and longevity in horizontal pattern development are also discussed.

P. J. Fonteyn

Papers

An experimental analysis of structure in a desert plant community