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J.H. Lawton
2.2.2 The BIODEPTH Experiment
Recently, I and colleagues throughout Europe have created the BIODEPTH experiment (Biodiversity and gcosystem frocesses in Ierrestrial Herbaceous Communities). There are eight field sites (Umea in Sweden, Sheffield and Silwood in the
UK, Cork in Ireland, Bayreuth in Germany, Zurich in Switzerland, Lisbon in Portugal, and Lesbos in Greece). At all the field sites, we set up replicated model
ecosystems with five levels of plant species richness, from monocuItures to
polycultures with a maximum richness per m 2 characteristic of natural vegetation
for each site. We used the same basic protocol everywhere, and weeded the plots to
maintain treatments (i.e. this is essentially Tilman's Cedar Creek experiment replicated across eight different sites). We now have three years data on the effects of
species richness on ecosystem processes from BIODEPTH.
We asked whether the same within-site relationship between plant species richness and ecosystem processes would be found at every site, despite the very wide
range of environments, soil-types and herbaceous plant communities embraced by
the experiment. Our expectation was that we would find the same relationship
(because there are no theoretical grounds for believing otherwise), and that it would
again basically be like Fig. lb, differing only in the positions of the curves from
individual sites. And (as by now should be clear) we expected to find a totally
different relationship plotting data across-sites.
The results of the experiment are still being worked up. Preliminary conclusions (A. Hector, B. Schmidt, J.H. Lawton and the BIODEPTH consortium, unpublished) are that both our major predictions are supported. If we fit one statistical
model to the within-site responses from the second year's data, we find that for all
eight sites, plant biomass increases linearly with the log of plant species richness
(i.e. biomass rises curvilinearly with untransformed plant richness, as in Fig. Ib).
There is no significant site x treatment interaction, meaning that all sites fit the
same model, but sites differ from one another in overall productivity, because of
inherent differences in soil fertility, the weather, and the length of the growing
season. As well as within-site effects of plant species richness, there are also effects
of plant functional group, particularly the presence of legumes, but only at some
sites. Finally, taking the richest experimental communities from each site, and plotting a between-site graph of biomass vs plant species richness reveals no clear
pattern. Between- and within-site effects, are, as predicted, totally different.
We do not yet know the mechanism(s) underpinning the strong and consistent
within-site effects, but preliminary analyses suggest that the results cannot be explained solely by the sampling hypothesis, suggesting that complementarity or positive interactions playa role. Notice that all sites can be fitted to one statistical
model, showing a linear increase in biomass with the log of plant species richness.
In other words, ecosystem processes do not reach an asymptote; in this experiment,
increasing plant species richness always leads to an increase in ecosystem performance, albeit at a decelerating rate as richness increases.
I emphasise that these are preliminary conclusions. Nevertheless, the BIODEPTH
results point to a strong and consistent emerging picture of the way in which changes
J.H. Lawton
2.2.2 The BIODEPTH Experiment
Recently, I and colleagues throughout Europe have created the BIODEPTH experiment (Biodiversity and gcosystem frocesses in Ierrestrial Herbaceous Communities). There are eight field sites (Umea in Sweden, Sheffield and Silwood in the
UK, Cork in Ireland, Bayreuth in Germany, Zurich in Switzerland, Lisbon in Portugal, and Lesbos in Greece). At all the field sites, we set up replicated model
ecosystems with five levels of plant species richness, from monocuItures to
polycultures with a maximum richness per m 2 characteristic of natural vegetation
for each site. We used the same basic protocol everywhere, and weeded the plots to
maintain treatments (i.e. this is essentially Tilman's Cedar Creek experiment replicated across eight different sites). We now have three years data on the effects of
species richness on ecosystem processes from BIODEPTH.
We asked whether the same within-site relationship between plant species richness and ecosystem processes would be found at every site, despite the very wide
range of environments, soil-types and herbaceous plant communities embraced by
the experiment. Our expectation was that we would find the same relationship
(because there are no theoretical grounds for believing otherwise), and that it would
again basically be like Fig. lb, differing only in the positions of the curves from
individual sites. And (as by now should be clear) we expected to find a totally
different relationship plotting data across-sites.
The results of the experiment are still being worked up. Preliminary conclusions (A. Hector, B. Schmidt, J.H. Lawton and the BIODEPTH consortium, unpublished) are that both our major predictions are supported. If we fit one statistical
model to the within-site responses from the second year's data, we find that for all
eight sites, plant biomass increases linearly with the log of plant species richness
(i.e. biomass rises curvilinearly with untransformed plant richness, as in Fig. Ib).
There is no significant site x treatment interaction, meaning that all sites fit the
same model, but sites differ from one another in overall productivity, because of
inherent differences in soil fertility, the weather, and the length of the growing
season. As well as within-site effects of plant species richness, there are also effects
of plant functional group, particularly the presence of legumes, but only at some
sites. Finally, taking the richest experimental communities from each site, and plotting a between-site graph of biomass vs plant species richness reveals no clear
pattern. Between- and within-site effects, are, as predicted, totally different.
We do not yet know the mechanism(s) underpinning the strong and consistent
within-site effects, but preliminary analyses suggest that the results cannot be explained solely by the sampling hypothesis, suggesting that complementarity or positive interactions playa role. Notice that all sites can be fitted to one statistical
model, showing a linear increase in biomass with the log of plant species richness.
In other words, ecosystem processes do not reach an asymptote; in this experiment,
increasing plant species richness always leads to an increase in ecosystem performance, albeit at a decelerating rate as richness increases.
I emphasise that these are preliminary conclusions. Nevertheless, the BIODEPTH
results point to a strong and consistent emerging picture of the way in which changes