Aboveground impacts of a belowground invader: how invasive earthworms alter aboveground arthropod communities in a northern North American forest

Abstract

Declining arthropod communities have recently gained a lot of attention, with climate and land-use change among the most frequently discussed drivers. Here, we focus on a seemingly underrepresented driver of arthropod community decline: biological invasions. For approximately 12 000 years, earthworms have been absent from wide parts of northern North America, but they have been re-introduced with dramatic consequences. Most studies investigating earthworm-invasion impacts focus on the belowground world, resulting in limited knowledge on aboveground-community changes. We present observational data on earthworm, plant and aboveground arthropod communities in 60 plots, distributed across areas with increasing invasion status (low, medium and high) in a Canadian forest. We analysed how earthworm-invasion status and biomass impact aboveground arthropod community abundance, biomass and species richness, and how earthworm impacts cascade across trophic levels. We sampled approximately 13 000 arthropods, dominated by Hemiptera, Diptera, Araneae, Thysanoptera and Hymenoptera. Total arthropod abundance, biomass and species richness declined significantly from areas of low to those with high invasion status, with reductions of 61, 27 and 18%, respectively. Structural equation models suggest that earthworms directly and indirectly impact arthropods across trophic levels. We show that earthworm invasion can alter aboveground multi-trophic arthropod communities and suggest that belowground invasions might be underappreciated drivers of aboveground arthropod decline.

Less overall, but more of the same: drivers of insect population trends lead to community homogenization

“…invasive earthworms directly and indirectly affect higher trophic levels mediated by plants, herbivores and detritivores [35]. Total arthropod abundance, biomass and species richness decreased significantly even at low levels of invasion.…”

“…Many invasive species negatively interact with or even displace native species [ 34 ], but the impacts on ecosystems can be complex and often indirect. In a Canadian forest, invasive earthworms directly and indirectly affect higher trophic levels mediated by plants, herbivores and detritivores [ 35 ]. Total arthropod abundance, biomass and species richness decreased significantly even at low levels of invasion.…”

Invasive earthworms can change understory plant community traits and reduce plant functional diversity

“…Lathyrus ochroleucus , Vicia americana ). This forest has been intensively studied for decades in the context of earthworm invasion, and the general description of the climate and soil type of the valley are detailed in previous publications 36 , 39 , 50 , 53 , 100 : the climate is considered as continental with short dry summers and cold winters (Mean annual temperature of 3.4°C and precipitation of 638 mm) and the soil as orthic grey luvisol. Notably, earthworm invasion is still ongoing in some areas of this forest, and thus parts of it are still free of earthworms.…”

“…We also determined earthworm densities in all ten plots per area by extracting earthworms from a subplot (0.50 x 0.50 m 2 ) using a combination of the hand sorting and mustard extraction methods. 36 , 39 Briefly, five liters of mustard solution, consisting of 100 g of mustard powder dissolved in 10 liters of tap water and mixed with 20 ml of vinegar, were poured into the subplot after having excavated the upper ∼10 cm of soil. Topsoil was hand sorted, and earthworms were immediately collected and washed with tap water for 15 minutes.…”

“… 32 Recent studies showed that invasive earthworms affect soil microorganisms, soil invertebrates, 33 , 34 , 35 , 36 as well as native plant communities, 37 , 38 and soil surface/vegetation-dwelling invertebrates. 31 , 36 , 39 The ecological mechanisms behind their impacts are numerous, and the direction and magnitude of the effects vary with earthworm diversity and community composition. 35 , 38 , 40 …”