Nelson G. Hairston

Community Structure, Population Control, and Competition

In summary, then, our general conclusions are: (1) Populations of producers, carnivores, and decomposers are limited by their respective resources in the classical density-dependent fashion. (2) Interspecific competition must necessarily exist among the members of each of these three trophic levels. (3) Herbivores are seldom food-limited, appear most often to be predator-limited, and therefore are not likely to compete for common resources.

The Experimental Test of an Analysis of Field Distributions: Competition In Terrestrial Salamanders

Experimental removal of the terrestrial salamander Plethodon jordani from marked plots in the Great Smoky Mountains and the Balsam Mountains of North Carolina, USA resulted in statistically significant increases in the abundance of P. glutinosus in both areas. The increase ap- peared earlier in the Smokies, where the two species overlap by only 70-120 m. Removal of P. glutinosus, the less abundant species, did not result in a statistically significant increase in P. jordani, but did increase the proportion of young in that species. Again, the effect was more pronounced in the Smokies than in the Balsams, where the altitudinal overlap is 1220 m (4000 ft). Stronger compe- tition in the narrow zone of overlap had been predicted on the basis of field distributions, but a measurable level of competition in the wide zone of overlap was surprising. The experiments permitted the direct calculation of coefficients of competition and carrying capacities for both species in both areas. Competition coefficients were higher in the Smokies, but carrying capacities were very similar in the two areas. The experimental approach eliminated the assumptions required for calculating competition coefficients by resource partitioning, and since the experimental design was published before the experiments were begun, the circularity implicit in retrospective analyses was avoided.

Species Abundance and Community Organization

The prevalent conviction among ecologists that natural communities represent important and meaningful assemblages of organisms has prompted a diverse series of analyses. The present paper is an examination of some of the analyses as applied primarily to soil arthropods, with an explanation of some of the difficulties that have been encountered. The thesis contained herein is that the numerical abundance and spatial distribution of all species found must be taken into account before an understanding of community organization can be obtained. Analyses and definitions of the community have been used to approach the subject from several more or less distinct points of view-function, location, and biotic composition. Most studies have dealt with only one of these aspects, but some assumption has usually been made about the others. Macfadyen (1957) has recently reviewed the various community definitions that have been offered. In the present paper, particular attention is paid to the aspects of community structure that can be discovered from the occurrence and relative abundance of species, usually confined to a taxonomic group not larger than a phylum. The species composition of communities has been used as an approach to community analysis for at least 50 years. The data have been viewed from the standpoints of species frequency, species per unit of area, the spatial distribution of individuals, and the numerical abundance of species. In all of these, attempts have been made, graphically and mathematically to express some aspect of community structure.

An Experimental Test of a Guild: Salamander Competition

The guild of plethodontid salamanders of the deciduous forest of the southern Appa- lachians shares the common resource of food, which consists of insects and other invertebrates inhabiting the forest floor and low vegetation. Since in practice, use of the guild concept implies competition for the shared resource, the validity of accepting a guild as delimited for the terrestrial Plethodontidae was tested by field experiments covering the 5 yr 1974-1978. The test consisted of regularly removing the most abundant species, Plethodonjordani, from one set of experimental plots, and the second most abundant species, P. glutinosus, from another set of plots. Each of these two species was favorably affected by the removal of the other. P. glutinosus increased significantly in abundance where P. jordani was removed. Removal of P. glutinosus resulted in a significant increase in the proportion of young P. jordani. The remaining species, including the congeneric P. serratus, were not affected by the removal of either of the two most abundant species. The results call into question the common assumption that competition is the organizing force in all coexisting assemblages of species which share common resources. The data also imply that food is not the resource for which P. jordani and P. glutinosus compete. Nest sites may be the critical resource, but much information is needed before they can be implicated.

Predation and Competition in Ambystoma Larvae: Field and Laboratory Experiments

The rate of predation of Ambystoma opacum larvae on A. maculatum larvae was shown to vary with the size and density of the predator in the laboratory. An increased density of both the predator and the prey in field enclosures increased the daily mortality rate of the prey A. maculatum, but did not significantly decrease the number of survivors. Intraspecific competition in both species was shown to be density-dependent in the laboratory. The growth of A. opacum was inhibited by an increased density of conspecifics in field enclosures. Interspecific competition was demonstrated in the laboratory and in field enclosures. The data suggest that the two species are probably more affected in nature by the predator-prey relationship than by interspecific com- petition.

Life-history correlates of body-size differences between two populations of the Salamander, Desmognathus monticola

Etude de deux populations de salamandre Desmognathus monticola provenant de deux localites proches de la Caroline du Nord (USA).Mesure des parametres concernant la croissance, la maturite et la fecondite pour distinguer effets du milieu et divergence genetique.

Studies on the Organization of Animal Communities

Animal communities may be considered organized if any property of a natural assemblage of species can be predicted. Inasmuch as some prediction is always possible, the occurrence of organization will scarcely be debated under this definition, but the degree to which it exists and the relationships involved are more open to question. It will be agreed, however, that the degree of organization will determine the accuracy and generality of the predictions that are made. The relationships that force order upon natural communities may be placed into two broad categories: energy transfer and competition. In its general form, the transfer of energy through communities is reasonably well understood, although much remains to be learned. This organizing force will not be considered directly in the present paper. Competition is less obvious, and its importance and even its existence have been the subject of intense debate (Nicholson 1933; Andrewartha & Birch 1954; Birch 1957; various papers and discussions in Population Studies: Animal Ecology and Demography, Cold Spring Harbor Symposium on Quantitative Biology, Vol. 22, 1957). There are a few generally accepted observations which appear to lead to a strong conclusion that competition for resources is widespread in natural communities (Hairston, Smith & Slobodkin 1960). These authors use three principal observations: bound energy is fossilized at an ecologically insignificant rate; in terrestrial communities, most of the organic matter produced by green plants falls to the ground uneaten; and when herbivores are protected either by the removal of predators or by being introduced into regions which their predators and parasites have not reached, they frequently become numerous enough to deplete the vegetation. From these observations, it is deduced that as whole groups, the terrestrial decomposers, producers and predators are limited by their own depletion of their respective resources. Terrestrial herbivores appear to be limited in abundance by predation and parasitism. Now, it can be regarded as an axiom that when the populations of two or more species are limited by the supply of a common resource, those species are in competition. Inasmuch as the numerous species of decomposers are not completely segregated in their needs, it follows that competition must exist among these species. The same reasoning applies to terrestrial plants and to predators. Hairston, Smith & Slobodkin repeatedly point out that individual exceptions for particular species by no means invalidate their conclusions. Thus, many carnivore species are at least partly regulated by territoriality, but predators and parasites must in general control the populations of herbivores, and in general be limited by the supply of these. Otherwise, the herbivore populations would expand to the point where the vegetation is depleted. It is concluded that competition for resources exists among decomposers, among producers, and among carnivores in terrestrial communities. Therefore, it seems worth while to examine those approaches to the organization of

The evolution of competing species of terrestrial salamanders: niche partitioning or interference?

SummaryOne of the central assumptions of evolutionary ecology is that interspecific competition is a potent evolutionary force acting on coexisting species. There are few animal species that provide an opportunity for an experimental analysis of the evolutionary consequences of the phenomenon. We have taken advantage of the fact that two species of terrestrial salamander,Plethodon glutinosus andP. jordani, have different altitudinal distributions on two mountain ranges in North Carolina. Field removal experiments showed that interspecfic competition was much stronger in the Great Smoky Mountains than in the Balsam Mountains, and transplant experiments between the two mountain ranges showed that neither species from the Balsam Mountains had a measurable effect on its congener in the Smokies, although both species from the Smokies had strong negative effects on the Balsam congeners. Other experiments were conducted on the behavioral and ecological changes that have (or have not) evolved in the two areas. Our studies show that increased interspecific interference was the major evolutionary response of these largePlethodon species to interspecific competition, and that partitioning of food or microhabitat was not involved.

The Interpretation of Experiments on Interspecific Competition

Commentaires sur les articles de Choener 1983 et Connell 1983 (ibid., 122, 240-285, 661-696)