What’s the difference between an alien and a native plant across a landscape?

Impact studies in plant invasion ecology have consistently reported decreases in species richness associated with high abundances of invasive aliens, and interpreted it as a sign of negative impact of the alien. But how frequently do such correlative patterns occur across a landscape? Undoubtedly,  one could potentially detect similar patterns for some native plants as well. Do alien species show consistently different patterns from native species, or is the geographical origin of a species irrelevant compared to other factors such as growth forms?

In a study published this month in Journal of Ecology, we put this question to the test across a highly invaded landscape of New Zealand. We carried out the equivalent of 261 separate impact studies, examining correlations between species richness and the abundance of a given focal species across the landscape. We equally targeted native (146 species) and alien (115 species) plants, and compared how frequently we detect patterns suggestive of impact. We found that alien species showed indeed different patterns from natives, with more negative correlations with species richness compared to null models.

The observed number of significant positive (upward bars) and negative (downward bars) relationships was compared to expected frequencies according to null model 2, which randomly reassigns alien or native status to the total species pool. Significantly higher frequencies than expected are indicated (***P < 0.001, **P< 0.01, *P < 0.05)
Number of significant positive (upward) and negative (downward) correlations of alien and native focal species abundance with components of species richness across Banks Peninsula.

Importantly, we examined not only correlations with total species richness, but also with the native and alien components of richness. We show that these two components vary differently along gradients of invasion. Separately analysing relationship with alien and native components of richness can help distinguish between situations where aliens may be acting as the primary drivers in plant community changes or simply passengers. This is an essential first step in designing further experimental studies to determine the underlying ecological processes and potential ecosystem impacts of alien species.

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