Reports from the previous five years of staggering insect declines have opened debates concerning the fate of insects in the world. The decline in insect populations is often associated with habitat destruction, rampant use of pesticides, and climate change. But a recent study adds earthworms as unexpected suspects.
Science Daily reported that the introduction of invasive European earthworms into northern North America has adversely affected the insect fauna that lives in the soil. Soil ecologists found evidence for this claim, which indicates that there are factors affecting the changes in insect communities that have not been taken into account previously.
The Beginning of Earthworm Invasion
Professor Emeritus Ed Johnson from the University of Calgary explains that the last ice age that happened mainly in North America covered the continent with massive ice sheets, CBC reported. But as the world warms, the glaciers retreat and organisms start to recolonize, which means the ecosystem has reorganized itself without the earthworms.
However, they have been reintroduced intentionally by settlers and unintentionally in shipments, livestock, fishing bait or imported soil and plants.
Researchers first noticed the invasive earthworms in the aspen forest in the 1980s, which they identified as not native species in North America. They often find earthworms near cabins, fishing ports, and where horses stay, indicating that humans brought earthworms.
Johnson added that over the decades and generations of researchers who studied, watched, and tracked the invasion of earthworms, they noticed changes in the leaf litter, which means there is an abundance of earthworms in the area.
Earthworms Can Be Invasive Too
In the new research, titled "Aboveground Impacts of a Belowground Invader: How Invasive Earthworms Alter Aboveground Arthropod Communities in a Northern North American Forest," published in Biology Letters, the team looked at 60 plots in aspen and popular forest in Alberta, Canada.
They found that as earthworms wriggle and leaf litter is abundant, the diversity and abundance of invertebrates aboveground also decreases, National Geographic reported. The findings are indeed surprising because earthworms are widely considered a gardener's best friend for aerating and mixing soil and releasing locked-up casting nutrients that help plants grow well.
But the new suggests that earthworms in the forests of North America may not be the slimy angels that most people think. Instead, they could be the underground invaders with their relentless tunneling, feeding, and pooping, changing the physical, chemical, and biological properties of the soil that negatively affect native species.
Among the 13,000 invertebrates the team collected, they noticed a striking decline in the aboveground invertebrates. They are 18% fewer with a 27% reduction in overall biomass and there is a 61% decline in population at sites where there are high densities of invasive earthworms.
The team recommends further research to pinpoint how the worms are driving this decline. However, they believe that the main factor is likely to be the worms eliminating the leaf litter layer from the forest floor that serves as food for insects feeding on them.
Another pair of studies also showed that earthworms have negative impacts on ground-nesting birds and some salamanders that spend time in the leaf litter because the invasive worms devour the plant litter.
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