In a paper published in Science in 1999, Andrew Hector and a team of collaborators reported the results of an experiment, replicated in eight European field sites, that showed that loss of plant species diversity leads to reduced above-ground plant biomass. Seventeen years after the paper was published, I spoke to Andrew Hector about the making of this project and what we have learnt since about the diversity-productivity relationship.
(Questions sent by email on 30 July 2016; responses received on 18 September 2016)
Citation: Hector, A., B. Schmid, C. Beierkuhnlein, M. C. Caldeira, M. Diemer, P. G. Dimitrakopoulos, J. A. Finn et al. 1999. Plant diversity and productivity experiments in European grasslands. Science 286: 1123-1127
Hari Sridhar: Please tell us a little about the motivation for setting up this multi-country experiment. Whose idea was it? Was it setup specifically to investigate niche complementarity and sampling effects?
Andrew Hector: Research on the link between biodiversity and ecosystem functioning only coalesced as a field following a conference in the early 90’s put together by Detlef Schulze and Hal Mooney. That meeting spawned several studies. Interestingly, the link between diversity and function can be traced all the way back to Darwin, but there was only sporadic study of it until the 90s. The multi-country approach of BIODEPTH was facilitated by the European Framework 4 – we had about a dozen groups in 8 countries led by John Lawton at the NERC Centre for Population Biology at Imperial College Silwood Park.
HS: You joined the BIODEPTH project soon after you completed your PhD. Can you share with us how you got into this project?
AH: Luckily, I spent much of the previous year working for Mick Crawley at Imperial College, monitoring on-going experiments and helping set up new ones, to earn money while I wrote my PhD up. That put me in a good position when the postdoc. came up for BIODEPTH.
HS: Please give us a sense of how this collaboration worked. How did people join this group? Did all of you meet anytime during the making of this study? Did you have regular online meetings?
AH: BIODEPTH was a success because of the great collaborative spirit - it was a real team effort and everyone in it made an important contribution to its success. The Framework 4 structure had meetings every 6 months and we worked our way around most of the sites.
HS: How did the writing of this paper happen? Did you do most of the writing? How were drafts shared and commented upon?
AH: The paper was born at one of the regular meetings. It was hard to explain to people how to send the data in for inclusion in the database so we went through the whole process in a mock example and then did a basic statistical analysis there and then. Seeing how quickly it could be done really motivated people to get the data sent in speedily.
HS: Did this paper have a smooth ride through peer-review? Was Science the first place you submitted this to? In what ways did the published version differ from the first submitted draft?
AH: Yes and no. Science were happy to have it but it had mixed reviews – some scientists were (and still are) quite against the whole idea that diversity can be important for functioning. To some degree, the final paper was a compromise between opposing reviewer opinions.
HS: In the Acknowledgements you thank "P. Heads and E.Bazeley-White" - can you tell us how these people helped? You also thank J. Nelder for advice on statistical analyses" - can you tell us more about this?
AH: Phil Heads managed the NERC Centre for Population Biology for John (he was one of his ex-PhD students) and Ellen managed the database (she is now at British Antarctic Survey). It was great to have both of them to help support the work. John Nelder was a very influential statistician. We hired him to hold a short workshop where we could bounce ideas for the analysis off him.
HS: You say you used "standardized protocols to establish experimental assemblages ". Can you tell us a little more about these protocols?
AH: The details are too technical to go into here. The key point was that each team followed the same approach at the different field-sites to make the data as comparable as possible.
HS: Did this paper attract a lot of attention - in academia and in the media - when it was published?
AH: Yes. Interestingly, the media did not find the idea controversial - it seems to make sense to people that biodiversity affects how ecosystems work – but some scientists did.
HS: What impact did this paper have on your career and the future course of your research?
AH: Obviously getting my second publication into Science was a huge break – I was very lucky to have had the opportunity.
HS: It is now 17 years since this paper was published - would you say that the main conclusions from this study still hold true?
AH: Yes, in general. We have realized many scientific results are not reproducible (‘the reproducibility crisis’) but the BIODEPTH results turned out to be very reproducible despite being controversial with some people. The experiment has been repeated in the US (Cedar Creek), Germany (Jena), the Netherlands and elsewhere and all results fall in the range seen in our study.
HS: If you were to redo these experiments today, would you do anything differently, given the advances in technology, theory and analytical techniques?
AH: We tried to manipulate both species richness and functional groups and although their effects cannot be totally separated (more groups means more species) it would have been nice to have teased them apart a bit more (although the experiment in the Netherlands did this for legumes, omitting them from their study). We used random mixtures of species due to some practical constraints. It might have been nice to use mixtures that reflected how species might be lost in reality, but this is hard to predict and depends on what is driving species loss. And, on the other hand, randomization is a key feature of good experimental design and a good place to begin.
HS: In the paper you say that this was "the most extensive experiment to date in terrestrial ecosystems." Since then, have there been bigger experiments on this topic?
AH: At single sites yes (Cedar Creek and Jena), and BIODEPTH seems to have been partly responsible for the current popularity of networks (coordinated distributed networks) like Nutrient Network and Drought-Net.
HS: Did the work presented in this paper serve as a motivation for Loreau & Hector 2001?
AH: Yes. Basically we had a pattern, but could not pin down the mechanism. The 2001 paper helped us to do this.
HS: In your paper you say that "There may also be transient effects at this early stage of the experiment that largely disappear by the following year". Can you tell us whether this has happened in subsequent sampling?
AH: Sadly, the EU framework 4 only allowed us to keep the 8 sites going for 3-4 years (some ran for longer) but longer term work at Cedar Creek and Jena has shown the effects generally get stronger over time as the experiments go on.
HS: At the time of this study, Trifolium pratense was the only species that had particularly marked effects on productivity. Since then have other important species been discovered?
AH: Actually, that result has to be taken in the context that red clover was one of the few species grown at all sites. I don’t doubt it has strong effects (it is a nitrogen fixer) but the design could not get at the effects of all species.
HS: What is the status of the plots used in this study? Do they continue to be used for these experiments? Have the sites in which these plots are located undergone any changes since the time of this paper?
AH: As I said above, sadly we could not keep the study going in the long term but the projects at Cedar Creek and Jena are still going.
HS: At the time you did this study, did you anticipate that it would be cited so much? Do you know what this paper has been mostly cited for?
AH: I didn’t really think about it but it was a new field and a controversial topic so it is not surprising. It is cited as evidence that biodiversity affects how ecosystems function.
HS: Have you ever read the paper after it was published? When you read it now, what strikes you the most about it?
AH: Not recently. I remember there is a mistake in it – at one point we say species richness affected diversity (when we meant productivity).
HS: What would you say to a student about to read this paper today? What should he or she take-away from it?
AH: That it is important to repeat the same study to see how repeatable or variable the result it.
HS: Is this your favourite paper among all the papers you have published? If yes, why? If no, and if you do have another favourite, which is it and why?
AH: It seems a long time ago now (it was!) but obviously it will always be one I remember. My other current favourites are: