Taxa–area relationships for microbes: the unsampled and the unseen

Abstract

The recent observation of a power-law relationship, S proportional A(z), between number of taxa, S, and area, A, for microbial eukaryotes and bacteria suggests that this is one of the few generic relationships in ecology, applicable to plants, animals and microbes. However, the rate of increase in the number of species with area varies from approximately the fourth (z = 0.26) to as little as the 50th root (z = 0.0019) in microbes. This is an enormous range for which no quantitative explanation has been proffered. We show by sampling from synthetic populations that the disparity between sample and community sizes in microbial community surveys means z can be considerably underestimated and accrual of rare taxa with increasing area will not be detectable. Significant microbial taxa-area relationships will only be observed when changes in community structure within samples correlate with area. Thus, the very low z values observed recently cannot be used as the sole evidence in support of any particular community theory of community assembly. More generally, this suggests that our search for patterns and laws in the microbial world will be profoundly influenced and, potentially distorted by the sample sizes that are typical of microbial community surveys.

Turnover of soil bacterial diversity driven by wide-scale environmental heterogeneity

“…The estimated z bacteria was 0.006 (±7 Â 10 À 4 , Po0.001), that is, within the lower range of turnover rates for microbes (0.002ozo0.26), as compiled by Woodcock et al 31 This is in agreement with the general trend observed for microbes, the values usually being well below than those of the macroorganisms (z macroorganisms Z0.1) 14 , even if a higher z bacteria (0.26) has been reported for small, discrete and highly heterogeneous ecosystems 30 known as hosting insular communities with a high community turnover 32 . The significant but low turnover observed on a wide scale in our study might partly be ascribed to the high average abundance per taxonomic unit, which induces low turnover rates 21 , and to technical limitations, particularly the low taxonomic resolution of DNA fingerprinting.…”

“…It is interesting to note that z bacteria values may attain those reported for larger organisms (ca. 0.02), even in large areas of contiguous habitats, suggesting that communities of both bacteria and macroorganisms are structured by analogous processes 31 . Recent microbial biogeography studies have revealed that the main environmental filters shaping spatial microbial diversity distribution are soil physico-chemical characteristics, land use and plant cover, whereas climatic and geomorphologic filters are less important 9,11,15,37 .…”

“…This results in an aggregation of species composition variations within the bands and may contribute to the low turnover rate observed 4 . In addition, DNA fingerprinting precludes consideration of the accumulation of new minor species ARTICLE with increasing distance, which may represent a large proportion of the total soil microbial species richness 33 and determine the slope of TAR 14,31 . The low z bacteria might also be related to the grain size of our sampling design (16 km  16 km: 256 km 2 ), which did not consider scales less than landscape, and smoothed significant local variations in soil microbial community composition that might potentially affect TAR 34 .…”

Links between soil microbial communities and plant traits in a species‐rich grassland under long‐term climate change

“…Prior to statistical analyses, the intensity of each TRF was converted to relative abundance based on the total intensity of all detected TRFs; for plot‐level analyses, we used the mean abundance of each TRF from the four microsites per plot. This approach provides a semiquantitative measure of abundance to assess differences in soil microbial community structure among sites but precludes measures of diversity (Bent, Pierson, & Forney, 2007) and excludes rare species (Woodcock, Curtis, Head, Lunn, & Sloan, 2006). …”

Improving soil bacterial taxa–area relationships assessment using DNA meta-barcoding

“…Challenging the widespread idea that microorganisms exhibit a cosmopolitan distribution, TAR is now commonly used in a majority of microbial biogeographical studies to assess microbial community turnover rate and its relative potential dependence on 'dispersal' and 'selection' (Angel et al, 2010;Martiny et al, 2011;Ranjard et al, 2013;Wang et al, 2013;Zinger et al, 2014). The estimated turnover rates for microbial communities in most studies range from 0.002 to 0.26 (Horner-Devine et al, 2004;Green and Bohannan, 2006;Woodcock et al, 2006), and are generally much lower than those estimated for macroorganisms (classical range: 0.1-0.25; Horner-Devine et al, 2004). In addition, Ranjard et al (2013) have shown that selection and limited dispersal are not mutually exclusive and that a non-negligible proportion of bacterial community variation on a broad scale might be explained by the latter.…”