Did you ever dig for earthworms when you were a kid? Chances are you’ve had some sort of contact with these critters, whether it was through using them as bait to hook a fish or watching them wriggle on the sidewalk after it rained. Earthworms are found all over the world in many ecosystems, though new research shows they are typically more prosperous in temperate regions. However, now scientists are wondering if that will change, due to temperature and precipitation shifts from climate change. A study contemplating this question was published recently in the journal Science by a team of 140 researchers from around the world, led by those at the German Centre for Integrative Biodiversity Research (iDiv) and Leipzig University (UL).
Earthworms are considered ecosystem engineers because they perform a variety of functions, from digging holes, mixing soil components and eating organic debris. Through these functions, they provide ecosystem services like nutrient cycling, decomposition, freshwater supply, carbon storage, climate mitigation and seed dispersal. Yet despite all of these important functions that earthworms provide, as well as their global abundance, little has been studied on the world distribution of the many species of earthworms.
First author of the study Dr. Helen Phillips is a researcher at iDiv and UL. Heading up the collaboration of scientists, she and her large team developed global maps of the distribution of earthworm diversity, abundance, and biomass, based on the data gathered from almost 7,000 sites around the world. The study resulted in the largest earthworm dataset worldwide to date, with information from 6928 sites in 57 countries.
Speaking to the impetus behind the project, Dr. Phillips commented, "Researchers have known for decades that for any given area in the tropics we would usually expect more species than in the same sized area in temperate regions. But until now, we had been unable to quantitatively investigate the same global patterns for earthworms, as there was no global earthworm dataset."
Phillips is referring to the well-studied ecological phenomenon that plant, insect and bird diversity (defined as the number of species within any given area) usually increases from high to low latitudes. In other words, the tropics is more biodiverse for above-ground species. The assumption was that earthworm diversity would follow this pattern. However, the researchers found that earthworm diversity, abundance, and biomass was actually highest in the temperate regions of Europe, northeastern USA and New Zealand.
As the authors write in the paper, “We found that local species richness and abundance typically peaked at higher latitudes, displaying patterns opposite to those observed in aboveground organisms. However, high species dissimilarity across tropical locations may cause diversity across the entirety of the tropics to be higher than elsewhere. Climate variables were found to be more important in shaping earthworm communities than soil properties or habitat cover. These findings suggest that climate change may have serious implications for earthworm communities and for the functions they provide.”
The development of this database paves the way for future studies to monitor global populations of earthworms, particularly as climate change threatens their abundance and biomass. As precipitation and temperature were shown to most greatly impact earthworm species, senior author Nico Eisenhauer explains, “Climate change could cause shifts in earthworm communities and change the functions and services ecosystems provide. Given their role as ecosystem engineers, we are concerned about potential cascading effects on other organisms like microbes, soil insects and plants." Because of this, they urge the scientific community to consider earthworms as a conservation priority.