Invasional meltdown has been suggested between invasive plants and exotic earthworms. However, most reports have been made using indirect observations which do not necessarily reflect a positive feedback between both groups and do not distinguish between earthworm species. Only few reports suggest that these earthworms are able to sustain a positive interaction with invasive grasses. Yellow Bluestem (YB) is an invasive grass of growing interest in North America, with the potential to shift ecosystem function and structure. Invasive earthworms have been reported to drastically change ecosystem function and structure, as they change soil characteristics. This research aimed to determinate if there is indeed an invasional meltdown between European earthworm and YB in two different life stages. A mesocosme experiment was designed to observe the earthworm effects on the germination of YB. A second mesocosme experiment aimed to observe the effect of earthworms on the development of the seedlings of YB and the native grass Little Bluestem. Octolasion tyrtaeum was used as the invasive earthworm. The native earthworm Diplocardia spp. and no addition of earthworms were the controls. A field survey was performed to observe the distribution of invasive earthworms with relation to YB and native grasses.
Results/Conclusions
Germination rates of YB did not differ significantly between treatments (native vs invasive vs no earthworm), probably due to a low germination rate (0-12%). Nonetheless, O. tyrtaeum gained a higher proportion of mass than Diplocardia spp., suggesting a positive effect on the invasive earthworm that could be the result of seed predation. Little Bluestem (LB) seedlings were longer during the beginning of the experiment, but not at the middle. LB mortality was higher when O. tyrtaeum was present than YB alone. In addition, LB seedlings with O. tyrtaeum died substantially faster than YB seedlings in all treatments. This might result in an indirect positive effect on YB establishment as it survived for a longer time than LB. In the field survey, only the European Aporrectodea sp. was collected exclusively on soils dominated by YB. This suggested another indirect positive effect on the invasive earthworms. Although these are results do not fully confirm nor provide insight on the mechanisms behind the invasional meltdown, it did reveal that this system is more complex that initially thought. These results contrast with previous reports with different earthworm species, highlighting the importance of establishing earthworm identity.