Impact of Fallopia on Soil Fauna

Many invasive plants have been shown to modify the structure of soil fauna and microflora communities (Wardle et al., 1995; Belnap & Philips, 2001; Kourtev et al., 2002; Standish et al., 2004; Herrera & Dudley, 2003; Ernst and Cappuccino, 2005; Li et al., 2006...). In our study, the total number of individuals under Fallopia japonica canopy was 50% lower (p<0.05) than in the uninvaded adjacent vegetation. These results are in agreement with those of Gerber et al. (2008) who studied the impact of invasive knotweeds in European riparian environments. A multivariate analysis allowed to clearly separate invaded and uninvaded plots in terms of fauna taxonomic groups (figures 6 and 7).

Moreover, faunistic assemblage was more homogenous in invaded than in uninvaded plots accordingly to the reduction of plant and litter diversity that contributed to decrease microhabitats diversity and resources heterogeneity (Dennis et al, 1998 ; Hassan, 2000 ; Oliver et al, 2000, Haddad et al, 2001). Despite a lower total density of soil fauna in invaded plots, we found an increase for some groups such as woodlice, millipedes and earthworms that play an important role in litter decomposition. The impact of Fallopia japonica on epigeic fauna has also been studied by Kappes et al. (2007) who found less herbivores and more carnivores (opilones) in invaded stands but an equivalent detritivorous fauna. In our study, some groups with affinity for shadow and humid environments (e.g., Isopods or diplopods) were more abundant under Fallopia while thermophilous organisms such as the ant Lasius flavus and the associated aphids were totally absent from invaded plots. Another interesting result concerned the differences in earthworms: species frequently observed in moist environment (i.e.: Lumbricus terrestris, Dandrobaena subribicunda) were only present in the invaded plots while grassland typical species (Lumbricus castaneus) (Bouché, 1972) were restricted to soil under the resident vegetation. Thus, soil fauna tend to shift from a typical herbaceous vegetation fauna to a typical forest fauna. Topp et al. (2008) who worked on the impact of Fallopia on soil dwelling beetles had the same conclusion. These changes in soil fauna communities should have contributed to the higher litter decomposition rate in invaded plots.

diOIPP

Diptara

cÎrÎes Isopods

„ . t Chilopodsl 1 Coleoptera \ 1

Molluscs \ 1 1

Arachnids ^Heteroptera \ 1

ActinidsTs. \\ \

Symphiles

Gamasids ~~~SI

HomopteJ»-"'''*'

Hymeinopei^^,,/ /

MhropSymphypJe°ne^

Figure 6. Soil fauna in a site invaded by Fallopia japonica. Factor coordinates of the variables (taxonomic groups) in the PCA based on correlations.

Figure 6. Soil fauna in a site invaded by Fallopia japonica. Factor coordinates of the variables (taxonomic groups) in the PCA based on correlations.

-6 -5-4-3-2-101234 Fact. 1 : 22.8%

Figure 7. Soil fauna in a site invaded by Fallopia japonica. Factor coordinates of cases (soil plots) in the PCA. Invaded plots (black) and uninvaded plots (white).

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