Briones et al. 2007

Briones MJI, Ineson P, Heinemeyer A. 2007. Predicting potential impacts of climate change on the geographical distribution of enchytraeids: a meta-analysis approach. Global Change Biology 13: 2252-2269.

These authors conducted a meta-analysis of all studies describing population abundances of enchytraeids. This meta-analysis required certain standards of error reporting and sample sizes for the analysis, thus many papers were not included. The authors seem inordinately enthusiastic about their meta-analysis, going to great lengths to describe both meta-analyses in general, and their own approach.

These authors focused on enchytraeids because they are often the dominant-biomass organisms of organic soils. Organic soils are not well defined in this paper, but are apparently those with very high carbon contents, thus these soils are important in the context of global climate change because changes to these systems could result in large changes in these soils’ roles as either carbon sinks or sources. Biomass of enchytraeids in organic soils can exceed 50% of all animal biomass in the soil, often dominated by one or a few species, feeding primarily on bacteria and detritus.

As an additional layer of analysis, these authors focused on one species of enchytraeid, Cognettia sphagnetorum, commonly found in European organic soils such as marshlands. The majority of studies analysed were situated in Europe, principally the UK and other parts of north-western Europe. The authors repeatedly describe this geographic bias, but do not seem otherwise concerned.

In general, high population sizes of enchytraeids were associated with Hungary (one site), alpine meadows, tropical grasslands, tropical rainforests, moorlands, moder and brown-earth soils, slightly acidic soils (pH 4 – 6), temperate rainy climates with moisture all year, and regions with moderate or cold summers. Mean annual temperature (I think that’s what the undefined acronym “MAT” stands for) higher than 16°C was strongly associated with reduced population sizes, and the loss of the focal species C. sphagnetorum. MAT higher than 10°C appears to be an inflection point, with reduced population sizes above that limit. Additionally, small population sizes were associated with warm dry summer climates (e.g. Mediterranean) and cold snowy tundra climates.

The authors present a confusing and possibly meaningless discussion of the results of their geographic analysis. They describe the range of population densities in their studies (more than 500 000 m-2 down to less than 5 000 m-2), then state these differences in mean densities were not significant under the Wilcoxon test. If the means are not different, they’re not different, so why bother to report them, except to describe the error associated with comparing across ecosystems? They ran a regression analysis using these population densities, after stating the data were not normally distributed; I do not recall how sensitive to departures from normality regression analysis may be.

The authors state that there was no association between enchytraeid population density and depth in soil, then go on to rather confusingly describe the most enchytraeid-rich soil depth horizons. Apparently, enchytraeids are generally concentrated in the top 3 to 4 cm, with very few individuals found deeper than 12 cm. Enchytraeids generally seem to require permanent high moisture levels.

The focal species apparently reproduces asexually by fragmentation; many of the studies included in the meta-analysis describe numbers of individuals that are “whole” or “regenerating”. This curious and surprising life-history trait is never referenced in this paper, seemingly treated as common knowledge among enchytraeidologists. I know of few animals that habitually reproduce this way.

Overall, I found this a confusing and disappointing paper, though I admire their attempt to reconcile a highly heterogeneous dataset. The reference list contains probably the majority of available papers on Enchytraeidae, and may be very useful in that context.