Effects of earthworm invasion on soil properties and plant diversity after two years of field experiment

Although belowground invasive species are probably equally widespread and as important as their aboveground counterparts, they remain understudied, and their impacts are likely to be stronger when these invaders act as ecosystem engineers and differ functionally from native species. This is the case in regions historically devoid of native earthworms, such as parts of northern North America, which are now experiencing an invasion by European earthworms. Although invasive earthworms have been reported to have multiple consequences for native communities and ecosystem functioning, this knowledge is mostly based on observational studies, and the mechanisms underlying their cascading impacts need to be investigated. Here, we thus investigated the sequence of events, i.e., ecological cascades following earthworm invasion, that have rarely been studied before, in a two-year field experiment. We expected that the changes in soil abiotic properties observed following invasion would coincide with changes in plant community diversity and community trait composition, as well as in alterations in above- and belowground ecosystem functions. To test these hypotheses, we set up a field experiment that ran for two years in a forest in Alberta (Canada) to investigate soil properties and understory plant community composition in response to invasive earthworms. Our study shows that invasive European earthworms alter several soil abiotic properties (i.e., soil nutrient content, and pH) after two years of experiment. Invasive earthworm effects varied with soil depth for some soil properties (i.e., soil pH, water-stable aggregates, nitrogen, and microbial basal respiration), but we did not find any significant earthworm effect on soil water content, bulk density, or the total soil microbial biomass independently of the soil layer. Moreover, invasive earthworms did not affect plant community composition and only slightly affected community diversity in this short-term experiment. The minor changes observed in plant functional group composition are thus potentially the first signs of invasive-earthworm effects on plant communities. Our research provides experimental evidence that previously reported observational effects of invasive earthworms on soil properties are indeed causal and already significant after two years of invasion. These changes in soil properties are likely to have cascading effects on plant community composition, functional diversity, and ecosystem functioning, but such effects may take longer than two years to materialize.