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Factors that cause differential establishment among naturalized, invasive, and native species are inadequately documented, much less often quantified among different communities. We evaluated the effects of seed addition and disturbance (i.e., understory canopy removal) on the establishment and seedling biomass among two naturalized, two invasive, and two native species (1 forb, 1 grass in each group) within steppe and low elevation forest communities in eastern Washington, USA. Establishment within each plant immigrant class was enhanced by seed addition: naturalized species showed the greatest difference in establishment between seed addition and no seed addition plots, native and invasive species establishment also increased following seed addition but not to the same magnitude as naturalized species. Within seed addition plots, understory canopy disturbance resulted in significant increases in plant establishment (regardless of plant immigration class) relative to undisturbed plots and the magnitude of this effect was comparable between steppe and adjacent forest. However, regardless of disturbance treatment fewer invasive plants established in the forest than in the steppe, whereas native and naturalized plant establishment did not differ between the habitats. Individual biomass of naturalized species were consistently greater in disturbed (canopy removed) versus undisturbed control plots and naturalized species were also larger in the steppe than in the forest at the time of harvest. Similar trends in plant size were observed for the native and invasive species, but the differences in biomass for these two immigration classes between disturbance treatments and between habitats were not significant. We found that strong limitations of non-native species is correlated with intact canopy cover within the forest understory, likely driven by the direct or indirect consequences of low light transmittance through the arboreal and understory canopy. Considered collectively, our results demonstrate how seed limitation and intact plant ground cover can limit the abundance and performance of naturalized species in Pacific Northwest steppe and low elevation forest, suggesting that local disturbance in both habitats creates microsites for these species to establish and survive. Future studies evaluating interactions between multiple barriers to establishment using more representatives from each immigration class will further reveal how biotic interactions ultimately influence the demography and distribution of non-native plants within these communities.
This publication gives new records for all bumblebee species living in Iceland: B. hortorum, B. hypnorum, B. jonellus, B. lucorum, B. pascuorum, B. pratorum and B. terrestris. B. terrestris was detected outside the greenhouses for the first time. At 23 locations 217 specimens were collected. B. pratorum was only seen. The known strong expansion of B. lucorum (Kratochwil 2016) could be confirmed. The relation between B. jonellus and B. lucorum has constantly changed at the expense of of B. jonellus since B. lucorum occurred in Iceland. It is obvious that B. lucorum has already replaced the original species B. jonellus in some places. As a reason a strong competition of both species is given primarily (Kratochwil & Schwabe 2016). Factors effecting this competition are: the distribution of the neophyte Lupinus nootkatensis, the change in management of land as well as ethological but also morphological differences (Prys-Jones et al. 2016). A too low genetic diversity and a resulting suffering from diseases carried by other introduced bumblebee species (Prys-Jones et al. 2016) only can be supposed. Probably the influence of climatic change effects on B. jonellus in a negative way too (Kratochwil 2016, Kratochwil & Schwabe 2016). B. lucorum is just going to take the north top of Iceland. Only two locations could be found there without any bumblebees. In the future an ongoing expansion of B. lucorum is expected. B. jonellus will further be repressed and may become extinct in some places. A continuing expansion of B. terrestris can be expected.