Species but not genotype diversity strongly impacts the establishment of rare colonisers

Christian Schöb, Sara Hortal, Alison J. Karley, Luna Morcillo, Adrian C. Newton, Robin J. Pakeman, Jeff R. Powell, Ian C. Anderson and Rob W. Brooker

Environmental factors such as climate or land use are known to affect biodiversity. However, do we know how the existing diversity of an ecosystem affects the diversity of newly arriving organisms? Depending on the theory at hand we can argue that this effect should be positive or negative. On the one hand, we could expect that a more diverse existing community, with a range of species that each use resources differently, would deplete the resources more effectively and leave less available space for newly arriving species. Based on this theory, the effect of the species diversity of the existing community on the diversity of newly arriving species would be negative. On the other hand, we could expect that a more diverse existing community, with a range of species each having different effects on their surroundings, would create a more diverse environment that may in turn support more arriving species. Based on this theory, the diversity effect of the existing community on the diversity of newly arriving species would therefore be positive.

In this study, we tackled this issue by creating plant communities of different levels of genetic and species diversity. We created communities ranging from a single cultivar of barley (our most uniform community) up to communities consisting of five different cultivars of barley and five different weed species (our most diverse community) and assessed their impact on newly arriving species, i.e. species we added into these communities. We found that while species diversity of the existing community had a negative impact on the diversity of new arrivals, barley genetic diversity had no significant effect. Interestingly, the species diversity effect of the existing community was most negative for the rarest arriving species, while the most common arriving species were only weakly affected.

The negative “diversity-on-diversity” effect that we found suggests that diversification of crop fields could be a natural measure to suppress weeds and reduce herbicide usage. In contrast, the differential responses of the newly arriving species to the diversity of the existing communities could actually explain their commonness and rarity in nature and provide hints for the conservation of rare species.

Image caption: Barley weed mixture experiment. 
Read the paper here.

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