Aggregation does prevent competitive exclusion: A response to Green

Green (1986) raised a number of points concerning our simulation model of competition on a divided and ephemeral resource (Atkinson and Shorrocks 1981; Shorrocks et al. 1984), its analytical counterpart (Ives and May 1985), and the review of possible mechanisms giving rise to aggregation (Atkinson and Shorrocks 1984). This model allows a competitively inferior species to survive in probability refuges, that is, sites with no or few superior competitors that arise as a result of an aggregated distribution of individuals over breeding sites. Such refuges may occur even at equilibrium density, since aggregation increases crowding (Lloyd 1967), and global population density is limited by strong intraspecific competition in sites with high local density while low-density sites still exist. The model developed from field studies of drosophilid flies (Atkinson and Shorrocks 1977; Shorrocks 1982; Shorrocks and Rosewell 1987). In particular, Green's criticisms make use of the suggestion by Atkinson and Shorrocks (1984) that the observed negative-binomial distributions of drosophilid eggs over breeding sites could arise from a Poisson distribution of egg-laying visits by females to breeding sites, where eggs are laid in clutches, the size of which has a logarithmic distribution

Guild size in drosophilids: A simulation model

(1) This paper describes a simulation model which predicts the guild size of drosophilid flies living on a divided and ephemeral resource, without any traditional resource partitioning. (2) A distribution of empirical guild sizes was obtained from fifty-three field studies collected from all over the world. The resource bases used by the flies were fruit, fungi, sap fluxes, decaying leaves and flowers. The modal guild size was seven. (3) An acceptable range of parameter values for the model was obtained from a combination of field and laboratory experiments. Within this range the model predicted distributions of guild sizes slightly less than those observed in the field, with a modal size which varied between five and six

Competition on a divided and ephemeral resource: Testing the Assumptions. II. Association

(1) We test the assumption of the aggregation model of coexistence on a divided and ephemeral resource that there is no association between species. (2) We test associations by presence and absence from patches using 2 x 2 contingency tables and the chi-squared statistic. As a convenient index of association we use the four-point correlation coefficient phi = sqrt(chi-squared/N) where N is the number of observations (patches) in the corresponding contingency table and chi-squared is the uncorrected statistic. (3) The data, from laboratory experiments, field experiments and field collections shows no evidence of association between species pairs. Out of 157 comparisons, only eight show significant associations at the 5% level. (4) We incorporate phi into the original aggregation model for two-species competition and into the guild-size extension and examine its influence. Probability refuges and resource refuges are therefore brought together within one conceptual framework

Lichen World 2 : Lichen Biology and Surveying

This package is not primarily about lichens, but about measuring species diversity in ecological communities. It explores various aspects of measuring species diversity and abundance, using data from lichen communities growing on gravestones in graveyards in the Yorkshire Dales, England. 'Lichen World' includes a virtual sampling program, based on throwing quadrats and counting the number of species in each quadrat, which provides data that can be used to plot a variety of different species diversity indices and abundance models. This sampling program can be used to investigate basic ecological questions on sampling regimes and comparative methods of measuring species diversity, as well as more specific questions on lichen community ecology. Background information is supplied on the lichens and lichen communities sampled. Full instructions for the sampling program are provided in the courseware unit and should be consulted before use.

Lichen World 3 : Raw Data

This package contains the raw data sets that are used in the Biodiversity Simulator (found in the full version of 'Lichen World'), which samples the gravestones by 'throwing' quadrats of different sizes, and which provides data that are used to plot a variety of different species diversity indices and abundance models. Background information is supplied on the area of Yorkshire and the churchyards where the original study was carried out.

Lichen World 1 : Lichen Biology

This package provides background information on lichens and lichen communities, using examples from the larger 'Lichen World' full version to illustrate lichen ecology studies.

Interspecific competition is not a major organizing force in many insect communities

Part of the current dogma in ecology is that competition between species for limited resources is not only common but also a major organizing force in many communities largely because studies on vertebrates, particularly birds, have played a major role in creating the traditional framework of niche theory and resource partitioning. Other workers, particularly those studying insect communities, have suggested that significant interspecific competition is too rare and sporadic to be of major significance and have placed more emphasis on autecological processes. Efforts to resolve the controversy have concentrated on the question of whether or not competition is common in nature. Here we show that even where competition can be demonstrated, it need not have a major role in community organization