Geology Reference
In-Depth Information
ies they contain to produce more biomass by capturing sunlight and to survive stress.
Tilman and his numerous assistants have set up hundreds of plots, each with a differ-
ent number of species chosen from the native flora of the immediate locality. Half-way
through one of these experiments, Minnesota experienced a severe drought, and to Til-
man's amazement the plots that survived best were those with the highest biodiversity.
This was evidence in favour of Clements and the organismic view, for the most diverse
plots seemed to have developed a powerful emergent protective network as their various
members melded their individual survival skills into a greater whole. But there were crit-
ics. They pointed out that because Tilman had fertilised his plots with different amounts
of nitrogen, the differences in drought resistance were due to this, and not to the effects
of species diversity.
To eliminate this possibility, Tilman established a more extensive experiment in
1994, using 489 plots of two sizes with different amounts of plant biodiversity, seeded
in identical soil and chosen from a maximum of four 'functional groups', each with dif-
ferent survival strategies: broadleaved perennial herbs, nitrogen-fixing legumes, warm
season grasses and cool season grasses. This time, the more diverse plots produced more
biomass, fixed more nitrogen, were better at resisting weed invasions and were less
prone to fungal infections. The best plots were those that hosted a variety of species
from each of the four functional groups. Once again, here is evidence that diverse biotic
communities resemble organisms with powerful emergent properties. But the news was
not all good, because Tilman found that the benefits of having extra species in the com-
munity peaked at around five to ten species. Beyond that, extra species didn't seem to
improve ecological performance—what mattered most was having at least one member
of each functional group. Some ecologists say that these results show that most species
in wild ecosystems are dispensable, and that the extinction crisis gives us nothing to
worry about. But how are we to know which species are expendable and which aren't?
Since we can't tell which are the keystone species, it makes more sense to protect as
many species as we can. Furthermore, there is almost certainly an 'insurance effect' at
work, in that more biodiverse communities are more likely to contain species that can
take over the jobs left vacant by any keystone species that have disappeared.
Tilman's approach has recently been extended by the BIODEPTH project, in which
plots with different amounts of native grassland biodiversity were set up in eight
European countries, from the cold north to the warm south. Despite the wide range
of climatic conditions, high biodiversity in each country was strongly correlated with
improvements in many key ecological functions such as nutrient cycling, resistance to
predators and biomass production—once again, good evidence in favour of the organis-
mic view (
Figure 42
).
