Alessandra R. Kortz1,2 and Anne E. Magurran2
1International Institute for Applied Systems Analysis (IIASA), Schlossplatz 1, Laxenburg, 2361, Austria
2Centre for Biological Diversity, School of Biology, University of St Andrews, Fife, KY16 9TH, United Kingdom
The world’s ecosystems are experiencing unparalleled rates of biodiversity change with invasive species implicated as one of the drivers that restructure local assemblages (Dornelas et al. 2014). Here we focus on the processes leading to biodiversity change in the plant assemblage of a biodiversity hotspot, the Brazilian Cerrado. The null expectation that invasion leads to an increase in local species richness (Buckley & Catford 2016) is supported by our investigation of the grass layer in two key habitats (campo sujo and campo úmido). Overall, we recorded 80 plant taxa, belonging to 17 botanical families, of which 76 species are native. The four species invasive to Brazil – the pine P. elliottii (slash pine) and three grass species: Urochloa decumbens (signal grass, previously named as Brachiaria decumbens), Melinis repens (natal grass) and Melinis minutiflora (molasses grass) are established in the Cerrado (Zenni & Ziller 2011). We show, for two Cerrado habitats, that – as predicted by Buckley & Catford (2016) – local richness is correlated with invasive richness. In other words, sites that contain more invasive species also tend to have higher overall richness. However, at the same time, sites with more invasive species are less different from one another in terms of their composition than they would be if only native species were present. The reason for this is quite straightforward. Although there are few invasive species relative to native species, these taxa are widespread and occur in more localities than many native species. Thus, invasive species, even if relatively few in number (4/76), can reduce β-diversity and consequently homogenize biodiversity at regional (γ- diversity) scales. By focusing on changes in β-diversity in a known local system we detected a key mechanism of β-diversity change which would have been missed at the macroecological scale. This mechanism can help explain the paradox that species richness is not declining in many local assemblages, yet compositional change is exceeding the predictions of ecological theory (Gotelli et al. 2017). As we have shown, any increase (or stasis) in species richness cannot be viewed as benign unless the accompanying changes in composition (and potential shifts in ecosystem function; Hobbs et al. 2006) are considered. As such, our results emphasize the importance of quantifying both α-diversity and β-diversity in assessments of biodiversity change in the contemporary world.
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Zenni R. D. & Ziller S. R. (2011) An overview of invasive plants in Brazil. Rev. Bras. Bot. 34: 431–446.