Principles of terrestrial ecosystem ecology.pdf

specialization might increase the efficiency of
resource use by the community if some species
use resources that would otherwise not be
tapped by other species.
In experimental grassland communities, for
example, plots that were planted with a larger
number of species had greater plant cover and
lower concentrations of inorganic soil nitrogen
than did low-diversity plots (Fig. 12.8) (Tilman
et al. 1996). The more diverse plots might use
more resources because species have complementary
patterns of resource use; in other
words, species might differ in the types of
resources, the location of their roots, or their
timing of uptake. Alternatively, diverse plots
might use resources more effectively because
they are more likely to have a species that is
highly effective in capturing resources
(sampling effect) or are more likely to include
species with complementary patterns of
resource use (Hooper et al., in press). In other
cases, low-diversity ecosystems are quite efficient
in using soil resources. Crop or forest
monocultures, for example, are often just as
productive as mixed cropping systems
(Vandermeer 1995) and mixed-species forest
stands (Rodin and Bazilevich 1967). Although
there are many examples of a positive relationship
between species number and productivity
or efficiency of resource use, this does not
always occur. The effect of species richness
Nitrate in rooting zone (mg kg -1 )
0.4
0.3
0.2
0.1
0
5 10 15 20 25
Species richness treatment
Figure 12.8. Effect of the number of plant species
sown on a plot on the nitrate concentration in the
rooting zone. Measurements were made 3 years after
the plots were sown. Data are means ± SE. (Redrawn
with permission from Nature; Tilman et al. 1996.)
Diversity Effects on Ecosystem Processes 275
frequently saturates at a much lower number of
species (5 to 10) than characterize most natural
communities. Determining the circumstances
and mechanisms in which species number influences
ecosystem processes is an active area of
ecosystem research (Hooper et al., in press).
Diversity of functionally similar species
stabilizes ecosystem processes in the face of
temporal variation in environment. In ecosystems
in which functionally similar species
differ in environmental response, this can
buffer ecosystem processes from environmental
fluctuations (McNaughton 1977, Chapin and
Shaver 1985).Tropical tree species, for example,
differ subtly in their growth response to nutrients
(Fig. 12.9). Conditions that favor some
species will likely reduce the competitive
advantage of other functionally similar species,
thus stabilizing the total biomass or activity by
the entire community. In other words, in compensation
for the reduced growth by some
species, other species grow more. For example,
in one study, annual variation in weather caused
at least a twofold variation production by each
of the major vascular plant species in arctic
tussock tundra. Years that were favorable for
some species, however, reduced the productivity
of others, so there was no significant difference
in productivity at the ecosystem scale
among the 5 years examined (Chapin and Shaver
1985). Directional changes in environment can
also cause less change in total biomass than in
the biomass of individual species for similar
reasons; some species respond positively to the
change in environment, whereas other species
respond negatively. This stabilization of biomass
and production by diversity has been observed
in many (but not all) studies (Cottingham et al.
2001), including grasslands, in response to the
addition of water and nutrients (Lauenroth
et al. 1978) and to grazing (McNaughton 1977);
in tundra, in response to changes in temperature,
light, and nutrients (Chapin and Shaver
1985); and in lakes, in response to acidification
(Frost et al. 1995). This stability of processes
provided by diversity has societal relevance.
Many traditional farmers plant diverse crops,
not to maximize productivity in a given year but
to decrease the chances of crop failure in a bad
year (Altieri 1990). Even the loss of rare species