The impact of rare species on natural assemblages

Summary 1 Compared with most terrestrial data sets used in development of theoretical models, what characterizes data sets from marine soft sediments is that they have a very high number of species. Marine data also show large numbers of rare species, even if the extent of the study is greatly increased from a few square metres to hundreds of square kilometres. 2 Fitting log-normal distributions to data on species abundance distributions (SADs) for marine benthic assemblages from the continental shelf of Norway, and for the well-studied tropical tree data of Barro Colorado Island, Panama (BCI), show patterns suggesting that more than one log-normal distribution occurs. 3 We have developed a simple model of SADs comprising two groups of log-normally distributed species, a group of rare species and a group of common species. The model appears to fit the marine data and also applies to the BCI tropical tree data and yet is simpler than Hubbell's Zero-Sum Multinomial (ZSM) distribution model. 4 The plots of the model results show that the marine data are dominated by the rare group of species at all scales studied. In the tropical tree assemblage a similar pattern to that in the marine data is apparent only at small spatial scales (5 and 10 ha), but not at the larger extent (25–50 ha). It is important to note, however, that marine data at all the scales studied are samples from a given area, whereas the terrestrial data are exact counts of all species of tree within the total 50 ha plot studied. 5 Most ecological data are likely to be based on samples rather than exact counts so the patterns found with the marine data are likely to be common, and our findings general ones. 6 A biological explanation for the two-group log-normal model is that recruitment of rare species from outside the studied area dominates benthic assemblages of soft sediments at all sampled scales. Within the tropical forest the dominance of recruitment processes are only apparent at small scales. 7 Thus, the interpretation of SAD patterns depends greatly on the type of data collected, i.e. whether they are exact counts or samples. We believe that structuring processes are similar in marine and many terrestrial systems and these processes probably conform to the mass-effects paradigm of the newly termed meta-community concept.

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