Enrichment and diversity loss: a mechanism tested

To paraphrase Thomas Henry Huxley: How stupid of us not to have thought of that!

In what has to be one of the most elegant and simple experiments I've seen in a long time, Yann Hautier, Pascal Niklaus and Andy Hector tested a basic mechanism of why nutrient enrichment results in species loss. This is a critically important issue as it has been repeatedly shown that while adding nitrogen to plant communities causes increases in productivity, species go locally extinct. We may bare witness to local diversity declines because human activity has greatly increased nutrient deposition. This pattern has been observed for a couple of decades, but the exact mechanism has never been adequately tested, with some camps believing that enrichment increases below-ground competition for other resources that become limiting, or above ground for light.

As reveled in the most recent issue of Science, Hautier et al. performed an exceedingly simple experiment; they added light to the understory of plant communities with or without nitrogen additions. They made two compelling observations. First, when communities were enriched without elevated light, they lost about 3 of the 6 initial species compared to the control, while light addition in the enriched communities maintained the 6 member community (as did a light only treatment). The second result was that the light plus nitrogen treatment obtained much higher biomass than either the nitrogen or light only treatments, and in fact the light only treatment did not significantly increase productivity, meaning that the communities are not normally light-limited. Further, they failed to detect any elevated belowground competition for other resources.

These results reveal that nutrient enrichment causes diversity loss because increased plant size increases light competition and plants that grow taller with elevated nitrogen are better light competitors. An old problem solved with the right experiment.

Hautier, Y., Niklaus, P., & Hector, A. (2009). Competition for Light Causes Plant Biodiversity Loss After Eutrophication Science, 324 (5927), 636-638 DOI: 10.1126/science.1169640

Conserve now or wait for the data?

E. O. Wilson, referring to the ethical imperative we should apply to the conservation of life, said “The ethical imperative should be, first of all, prudence. We should judge every scrap of biodiversity as priceless while we learn to use it and to come to understand what it means to humanity” (pg. 351, The Diversity of Life). Although, I would argue we should aim to learn biodiversity’s value, both intrinsic and extrinsic, as opposed to what it solely means to humanity, his point is protect now, study later. The reason being that there is still so much to learn in order to adequately assess the Earth’s biological riches, by the time we inventory and map a fraction of biodiversity, we would have lost numerous unique regions and species. Of course the opposing point of view is that we need detailed information in order to best use limited resources to best protect biodiversity. This is a major philosophical divide. In a recent, important paper by Hedley Grantham and colleagues published in Ecology Letters, the question of how long should we wait to take conservation actions was empirically tested.

The authors used simulations based on 20 years of habitat loss data from the biologically-rich Fynbos region of South Africa and knowledge about spatial distribution of Protea diversity. Protea surveys (The Protea Atlas) have been carried out over 20 years, inventorying 40,000 plots and recording 381 species within the Proteaceae. They began their simulations with no information about Protea diversity patterns and included annually increasing knowledge, set against annual habitat destruction. They showed that waiting to make conservation decisions after only 2 years resulted in species loss, because habitat loss far outweighed any advantage to gaining more information. Further, more detailed information did not appear to increase the effectiveness of conservation decisions over cruder habitat-level maps.

The philosophical divide between protect now-learn later versus the need for detailed information to maximize resources appears bridgeable. It seems that by just accumulating some rough data may go a long way towards making those important conservation decisions. Of course, the irony is that this study needed 20 years of data to adequately assess this.

Grantham, H., Wilson, K., Moilanen, A., Rebelo, T., & Possingham, H. (2009). Delaying conservation actions for improved knowledge: how long should we wait? Ecology Letters, 12 (4), 293-301 DOI: 10.1111/j.1461-0248.2009.01287.x