Previous research has shown non-native earthworms can alter functions and structure of ecosystems across North America. These changes include alterations to soil structure and chemistry, changes in plant communities including facilitating non-native plant invasions, shifting relative abundance of microbial functional groups, and disruption of intra- and inter-radical arbuscular mycorrhizal (AM) fungi. In tallgrass prairies of the southern Great Plains, both native and non-native earthworms occur together. Compared to previously earthworm-free regions, few studies have investigated effects of non-native earthworms in regions where native earthworms are currently present, with even fewer investigating earthworm invasions in tallgrass prairie.
We conducted a field study to assess potential effects of non-native earthworms on plants and plant-fungal symbioses in tallgrass prairies in north-central Oklahoma. At 100 points with similar time since fire, grazing intensity, and soil type, we assessed: (a) earthworm abundance and species composition using the hand-sifting method from a 33 cm squared by 30 cm depth subplot, (b) plant species composition as percent cover within a 1 m squared subplot directly adjacent to each earthworm subplot, and (c) intra-radical AM hyphal abundance and total soil microbial biomass of four 2 cm soil cores (0-10 cm depth) collected within the plant subplot.
Results/Conclusions
Non-native earthworms were found in 45 of 100 plots with 73 individuals identified (all Aporrectodea trapezoides or Aporrectodea spp.). Native earthworms were present in 85 of 100 plots, with 451 individuals identified (various species in Diplocardia genus). Therefore, non-native earthworms are present in this native prairie but are not the dominant species. Abundances of neither native nor non-native earthworms were significantly associated with percent vegetation cover, vegetation biomass, vegetation richness, or intra- or inter-radical AM fungal abundance. However, higher numbers of total earthworms (native and non-native combined) were inversely related to native vegetation cover and positively related to non-native vegetation cover.
Our research suggests total abundance of earthworms, but not specific species groups, may influence vegetation composition in tallgrass prairies, potentially reducing cover of native plants and promoting abundance of non-native plants. Non-native earthworms (A. trapezoides) may have similar effects on tallgrass prairie ecosystems as native earthworms (Diplocardia spp.) because both occupy the same ecological niche (endogeic) and have similar feeding habits. This may result in non-native earthworms having less-severe effects on ecosystems than in previously earthworm-free regions. Continued examination of earthworm effects in tallgrass prairies is important, as further increases in non-native earthworms may lead to greater ecosystem alterations.