Structural complexity of species assemblages and spatial scale of community organization: A case study of marine benthos

In this paper, definitions and measures of complexity with regard to biological communities are briefly considered. A new topological approach that considers the community's complexity in terms of groups of species coherently varied in space or time is proposed. For a given set of samples, the number of such groups is related to the minimal number M of axes necessary to represent the original configuration of the data set. I interpret this “minimal dimensionality of structure” as the number of independent factors of structural variability and its normalized value M/MMAX as a measure of the organizational complexity of a community. The M value can be estimated as the number of significant axes obtained by ordination procedures. The percentage of total variance explained by these axes, T, is used as measure of structural rigidity. This approach is applied to data on the multi-scaled spatial distribution of marine benthic ciliates and macrofauna. Both the M and the T values obtained by principal component analysis show significant scale-dependence with an evident threshold at some critical area, with values of zero below this threshold, then increasing sharply as the area extends beyond the threshold. The critical scale of community organization ranges from hundreds of meters to kilometers for macroorganisms, whereas several meters are sufficient when considering ciliates.