Tuesday, 6 July 2010

July review for F1000

Facilitation cascade drives positive relationship between native biodiversity and invasion success. Altieri AH, van Wesenbeeck BK, Bertness MD, Silliman BR Ecology 2010 May 91(5):1269-75

Our Review 
Mark Lonsdale: Faculty of 1000 Biology, 5 July 2010 
Copyright F1000  

This article is interesting because it provides plot-scale support for the role of native species in facilitating invasions that helps explain the positive relationship between native species diversity and invasibility typically seen at the landscape scale.

The 'invasion paradox' [1] in ecology stems from a debate started back in the 1950s when Elton examined the relationship between native species diversity and invasive species establishment [2]. The paradox is that, on the one hand, many landscape-scale studies have shown a positive relationship between native diversity and invasibility, whilst, by contrast, many studies at the smaller experimental scale have shown a negative relationship, usually attributed to increased competition from more native species.

The present study focuses on the intertidal landscape of New England cobble beaches, particularly the ecosystem comprising invasive Asian shore crab (Hemigrapsus sanguineus) and the native plant community in which "cordgrass ameliorates solar stress and provides an attachment surface for ribbed mussels (Geukensia demissa) which, in turn, provide stable hard substrate and crevice space for other organisms" (p. 1270). The authors demonstrate the relationship between higher invasive crab densities and greater diversity of native species of algae and invertebrates (facilitated by the presence of cordgrass and ribbed mussels, i.e. a 'facilitation cascade'). The authors show that the abiotic stress (desiccation, thermal and instability) in the system is moderated by the presence of the cordgrass and ribbed mussels. This, in turn, increases the native species richness, which, in turn, reduces the mortality of the Asian shore crabs at larger scales. The authors claim that such 'facilitation' has "played an important but unrecognized role" in many studies that have focused on the role of competition when examining relationships between native biodiversity and invasiveness (p. 1273). This study clearly shows that there is a need to encompass multiple trophic levels, multiple spatial scales and multiple theories about the interactions driving invasibility (i.e. not just 'competition') when trying to understand drivers of invasive species establishment.

This study has shown positive effects of increasing native diversity on the success of a single invasive species. The next step will be for future studies to examine whether this can be translated to a positive effect of increasing native diversity on success for a range of invasive species, and thus on invasive species diversity (something that can only be inferred from these results). This would then connect more directly with empirical studies at the landscape scale, which have shown that higher native species diversity can lead to higher invasive diversity.

References: [1] Fridley et al. Ecology 2007, 88:3-17. [2] Elton CS, The ecology of invasions by animals and plants. London: Methuen, 1958.

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