Invasion of the Earthworms | The economist

EARTHWORMS’ GOOD Image as an aerator, drainer and fertilizer for fields and gardens belies a darker secret. They are indeed fierce competitors with other invertebrates, voraciously consuming rotting plant matter and tiny organisms such as protists, nematodes, bacteria and fungi that could otherwise sustain a wide variety of bottom dwellers.

This much is known, not least from research in northern North America. Worms were exterminated here during the last Ice Age, which ended 12,000 years ago. They returned only a few centuries ago in the form of European invaders. What has been little studied is the consequences of this subterranean slaughter for surface creatures. But that has just been corrected by Malte Jochum of the University of Leipzig in Germany and his colleagues in a recently published study biology letters.

dr Jochum and his team worked in a forest overlooking Barrier Lake in Alberta, Canada, where earthworm invasion has been tracked for three decades. This made it possible to identify areas with low (on average four worms per square metre), medium (43 worms per square metre) and high (106 worms per square metre) population.

Within each abundance zone, the researchers marked 20 plots with individual plots of two square meters. They identified all of the plants in it and used vacuum-suction collectors to “suck up” as many of the above-ground arthropods (mostly insects and spiders) in each plot as possible. They then sorted, identified and measured these animals to calculate their abundance, biomass and species richness.

In total, the collectors collected 13,037 individual arthropods – and discrepancies between the plots were quickly apparent. Those with abundant earthworms were stripped of arthropods compared to those where worms were rare. Plots with the most worms had 61% fewer individual arthropods, 18% fewer arthropod species, and a 27% reduction in total arthropod biomass compared to those with the fewest.

The so-called detritivores, which compete directly with the worms for food, suffered the worst. There were 80% fewer of them in areas with high worm density compared to those with low worm density. But herbivores and omnivores also suffered. There was only good news for carnivorous arthropods like spiders. Their numbers were 41% higher in worm-rich plots than in worm-poor plots.

Why predators performed well is unclear. Some may have eaten worms directly. Others may have exploited the stripping of cover for their prey, caused by worms dragging leaf litter underground for later consumption.

The repercussions of this carnage further up the food chain, among invertebrate amphibians, reptiles, birds and mammals, remain a mystery. Species that like to eat worms are likely to do better, while those that prefer their meals to be crunchy rather than mushy will fare worse. Whether this matters broadly is debatable. But for those who prefer their pristine ecosystems, this work confirms that earthworms are certainly not the pure commodity some people make them out to be.

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This article appeared in the Science & Technology section of the print edition under the heading “News from the Underground”

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