No evidence of facilitation between invasive Rhamnus cathartica (European buckthorn) and invasive earthworms in west central Minnesota

Biological invaders are seen as a real and growing threat to biodiversity, and there is much interest in the role that positive interactions among invasive species may play in promoting further invasions. Both the invasive tree Rhamnus cathartica (European buckthorn) and invasive Eurasian earthworms are reported to alter forest dynamics across large parts of the northern portion of the eastern deciduous forest of North America. Some evidence suggests that the two invaders may be aided by mutual facilitation, and it has recently been hypothesized that interactions between the two organisms may form the basis of a larger multi-species "invasional meltdown." We examined covariance among R. cathartica and earthworm distributions over a seven year period at six forested sites near the prairie-forest border in west central Minnesota and found no evidence to support facilitation. In fact, the correlation between R. cathartica and earthworm abundance and biomass was weak: 95% confidence intervals for Kendall's tau correlation coefficients in some cases indicated a negative association among the two organism types, often suggesting no association, and never allowing for a positive association greater than τ= 0.38. Individual earthworm species varied in their correlation with R. cathartica, but most correlations were negative and none were substantially positive. We thus suggest that the relationship between these invaders may be more complicated than previously thought and question whether earthworms and R. cathartica can be seen as the foundation of an invasional meltdown.