These GEITP pages recently discussed the evolutionary history and divergence of plant genomes, it seems reasonable to note the evolutionary history and divergence of earthworm genomes. The importance of earthworms has long been recognized for their effects on terrestrial systems — as diverse as tropical rainforests and our backyards. Because of burrowing activities, earthworms are appreciated for stabilizing soil particles into aggregates, increasing soil porosity, and elevating rates at which water infiltrates the soil during rainfall; these animals also reduce erosion of surface soils from hillslopes and accelerate movement of gases into, and out of, the soil.
Earthworms speed up decomposition of organic matter — by ingesting >30 times their own weight in soil per day, they can rapidly mix large amounts of leaf litter into underlying soil layers, increasing the release of plant nutrients. The presence of earthworms obviously enhances plant growth, because of all the processing they do, to the soil. Our ecological understanding of global biodiversity patterns (e.g. latitudinal diversity gradients) is largely based on the distribution of above-ground taxa — yet many soil organisms have shown global diversity patterns that differ from above-ground organisms.
Provisioning of ecosystem functions by earthworms is likely dependent on the abundance, biomass, and ecological group of the earthworm species (see diagram on p 425 of editorial); consequently, understanding global patterns in community metrics for earthworms is critical for predicting how changes in their communities may alter ecosystem functioning. Small-scale field studies have shown that soil properties such as pH and soil carbon influence earthworm diversity. For example, lower pH values constrain the diversity of earthworms by decreasing calcium availability, and soil carbon provides resources that sustain earthworm diversity and population sizes. In addition to many interacting soil properties — a variety of other drivers can shape earthworm diversity, such as climate and habitat cover. However, to date, no framework has integrated a comprehensive set of environmental drivers of earthworm communities to identify the most important drivers on a global scale.
Authors [see attached article and editorial] have developed a global-scale atlas of earthworms. It has been appreciated that earthworms may have greater diversity across the tropics, compared with that in temperate regions. Authors postulated that earthworm diversity might not follow global patterns seen above-ground, but rather may increase with latitude (i.e. heading from the poles toward the equator). Because of the relationship among earthworm communities, habitat cover, and soil properties on local scales — authors found soil properties (e.g. pH and soil organic carbon) to be key environmental drivers of earthworm diversity. Authors compiled a worldwide dataset of sampled earthworm communities from 6,928 sites in 57 countries(!!) as a basis for predicting patterns in earthworm diversity, abundance, and biomass. They found that the richness and abundance of local species typically increased at higher latitudes — displaying patterns opposite to those observed in above-ground organisms. Authors determined that changing climate properties were more influential in shaping earthworm communities than soil properties, or than habitat cover.
Science 25 Oct 2019; 366: 480-485 & editorial pp 425-426