The increasing ease with which we can apply molecular techniques has lead to a major expansion in studies exploring patterns of genetic diversity. At the same time, increasing recognition of the extent and possible role of such genetic diversity has, perhaps, given conservation science a significant headache. Although it might be a hostage to fortune, much of the argument in favour of conserving species diversity has focused on the role that it plays in maintaining ecosystem properties such as productivity, stability and resistance to invasion. Recent syntheses suggest that the ecosystem function–diversity link shows some common characteristics across studies, including reduced process rates with declining diversity and changes in some aspects of stability (Hillebrand & Matthiesen 2009). However, there are clear indications that such relationships are context-specific and temporally variable (Hooper et al. 2005, Fargione et al. 2007). At least, though, our understanding of this link and its context-specificity is relatively well advanced when compared to understanding the role of genetic diversity. Just what is genetic diversity doing, and how much of it do we need to conserve, and under what circumstances, in order to protect the long-term functioning of ecosystems?
The new paper by Kotowska et al. (2010), published in this issue of Journal of Ecology, not only addresses the link between genetic diversity and an important aspect of ecosystem function, namely primary productivity, but in a novel twist it also follows this effect through from the primary producers to the level of consumers. Specifically the authors examine how genetic diversity of a model species – Arabidopsis thaliana – influences productivity of synthetic communities as well as offtake by, and biomass and survival of, the cabbage looper – Trichoplusia ni. Importantly the study also attempts to address the context-specificity of such effects by manipulating the fertility levels and planting densities under which the synthetic plant communities are grown.
Nine genotypes of A. thaliana – differing in their functional traits – were grown both in monoculture and a full nine-species mixture, under varying fertilization and density levels, both with and without Trichoplusia. Primary productivity was increased by increasing plant genetic diversity, but was reduced in the presence of insects. The positive plant biomass response may be a consequence of non-additive effects (i.e. niche partitioning or facilitation) and - because of biomass reduction when insects were present - not a result of indirect insect-mediated effects (as has been suggested in other studies). Notably, herbivore biomass was also higher in the more genetically diverse mixtures (as was herbivore survival). However, greater plant productivity only partially explained the increased biomass of the herbivore, suggesting perhaps a dietary mixing effect. Some of these responses were context-specific – for example the amount of herbivory may be influenced by changes in the palatability of different genotypes under different levels of fertility. It is clear, however, that the positive effects of genetic diversity on biomass, as well as reductions in plant and insect mortality, are consistent across varying levels of fertilization and planting density.
Notably, despite the (acknowledged) point that trait differences between species in communities are generally much greater than between genotypes within species, these effects were still quite sizable when being driven simply by within-species genetic diversity. Admittedly these genotypes do not normally co-occur, so it is worth asking how often this level of genetically-driven within-species trait diversity might occur within a natural community. Furthermore, although these patterns broadly tally with some field-based studies, they have been demonstrated under controlled conditions and hence need to be explored in a more ‘natural’ setting, as identified by the authors. Irrespective, this study is clearly an important step forward in helping us to go beyond measuring how much genetic diversity might be out there, toward an understanding of what it might be doing.
Associate Editor, Journal of Ecology
- Fargione, J., Tilman, D., Dybzinski, R., Lambers, J.H.R., Clark, C., Harpole, W.S., Knops, J.M.H., Reich, P.B. & Loreau, M. (2007) From selection to complementarity: shifts in the causes of a biodiversity-productivity relationships in a long-term biodiversity experiment. Proceedings of the Royal Society of London Series B- Biological Sciences, 274, 871-876.
- Hillebrand, H. & Matthiesen, B. (2009) Biodiversity in a complex world: consolidation and progress in functional biodiversity research. Ecology Letters, 12, 1405-1419.
- Hooper, D.U., Chapin, F.S.I., Ewel, J.J., Hector, A., Inchausti, P., Lavorel, S., Lawton, J.H., Lodge, D.M., Loreau, M., Naeem, S., Schmid, B., Setälä, H., Symstad, A.J., Vandermeer, J. & Wardle, D.A. (2005) Effects of biodiversity on ecosystem functioning: a consensus of current knowledge. Ecological Monographs, 75, 3-35.
- Kotowska, A.M., Cahill, J.F. & Keddie, B.A. (2010) Plant genetic diversity yields increased plant productivity and herbivore performance . Journal of Ecology, 98, 237–245.
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