Esperanza Huerta Lwanga, a soil scientist affiliated with both Wageningen University & Research in the Netherlands and El Colegio de la Frontera Sur in Mexico, has investigated the effects of microplastics on earthworms, creatures widely considered a boon to farming because of their ability to aid decomposition, add organic nutrients to the soil through their waste castings, and increase the aeration of soil. "When I was doing research on soil invertebrates' distribution at different home gardens in Tabasco, Mexico, I found microplastics. And in those soils with microplastics, there were not earthworms," Huerta Lwanga told EHN. This observation motivated her to study earthworms directly. In her subsequent experiments, she found that worms attempted to avoid microplastics, but when the soil concentration reached 7 percent, they began to ingest them along with the soil, concentrating the plastics in their castings, and transporting them through different layers of soil. In their 2018 paper, Huerta Lwanga's team cautioned that rainwater flows through earthworm burrows into groundwater, creating a clear conduit for microplastics to enter groundwater systems. Huerta Lwanga also said that microplastics caused an 8 percent to 25 percent mortality rate in earthworms depending on the dose. In their paper, she and her colleagues hypothesized that mortality may be partly caused by microplastics abrading the digestive tracts of earthworms, making it more difficult for them to absorb nutrients. Damage to the digestive tracts of earthworms that ingested microplastics has been documented by other researchers. Once microplastics enter an ecosystem, they can proliferate through trophic levels, such as when a bird eats an earthworm. Or when a person eats an apple.