The impact of rare species on natural assemblages

Abstract

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 wellstudied 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.

Scale-dependent distribution of soft-bottom infauna and possible structuring forces in low diversity systems

“…Our values, which ranged from 9 to 21% singletons, are well in accordance with the 16% reported by Rumohr et al (2001), but clearly lower than the 27% reported by Ellingsen (2001), the 30% reported by Cao et al (1998) and the > 30% reported by Holme (1953). These numbers are not fully comparable, since rare species have a low probability of being recorded, and thus their characterisation and observed distribution is directly linked to sampling intensity (Brown 1984, Gray et al 2005. With the exception of the 70 samples in Rumohr et al (2001), the sampling intensity in the other studies mentioned was clearly lower than in our study (total sample numbers ranged between 4 and 24).…”

“…When unravelling scale-dependent matters, it is also important to pay attention to the role of a single (rare) species in addition to the dominant ones (Cao et al 1998, Schlacher et al 1998, Ellingsen et al 2007, Fontana et al 2008. Soft-sediment communities often consist of relatively high fractions of rare species compared with other environments (Gray et al 2005).…”

Species diversity, functional complexity and rarity in Arctic fjordic versus open shelf benthic systems

“…Gray et al (2005) documented the domination of rare species in species−abundance distributions in marine soft-bottom systems explored at various spatial scales. They noted that soft-bottom habitats are populated by small-sized species that do not occupy the whole habitat space available, and are thus open to constant immigrations (resulting in rare species occurrences).…”

Diversity of sand flies in domiciliary environment of Santarém, state of Pará, Brazil: species composition and abundance patterns in rural and urban areas

“…The variable frequency of occurrence (species presence along sampling period) was based on the ratio between the number of years that a given sand fly species occurred and total period sampling (six years) and expressed as percentage. Species captured in only one or two years along the study (0-25% frequency class) were defined as rare; moderately common species (hereafter, only common species) were sampled in three or four years of the study (26-75% frequency class) and very common species were collected in five years of study or all years (76-100% frequency class) (Gray et al 2005).…”