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Diversity and Productivity in a Long-Term Grassland Experiment



Plant diversity and niche complementarity had progressively stronger effects on ecosystem functioning during a 7-year experiment, with 16-species plots attaining 2.7 times greater biomass than monocultures. Diversity effects were neither transients nor explained solely by a few productive or unviable species. Rather, many higher-diversity plots outperformed the best monoculture. These results help resolve debate over biodiversity and ecosystem functioning, show effects at higher than expected diversity levels, and demonstrate, for these ecosystems, that even the best-chosen monocultures cannot achieve greater productivity or carbon stores than higher-diversity sites.
Diversity and Productivity in a
Long-Term Grassland
David Tilman,
* Peter B. Reich,
Johannes Knops,
David Wedin,
Troy Mielke,
Clarence Lehman
Plant diversity and niche complementarity had progressively stronger effects
on ecosystem functioning during a 7-year experiment, with 16-species plots
attaining 2.7 times greater biomass than monocultures. Diversity effects were
neither transients nor explained solely by a few productive or unviable species.
Rather, many higher-diversity plots outperformed the best monoculture. These
results help resolve debate over biodiversity and ecosystem functioning, show
effects at higher than expected diversity levels, and demonstrate, for these
ecosystems, that even the best-chosen monocultures cannot achieve greater
productivity or carbon stores than higher-diversity sites.
Recent demonstrations that greater plant di-
versity can lead to greater productivity (15)
have generated considerable debate (614).
This has been fueled by uncertainty about
which of many alternative hypotheses (6,9,
1520) is operating in nature and about the
number of species required to maintain eco-
system functioning (6,7,12,14). We report
results of a long-term experiment that allows
tests of these alternative hypotheses.
It has been hypothesized that productivity
may be greater at higher diversity because of
“niche complementarity” among particular
combinations of species and the greater
chance of occurrence of such combinations at
higher diversity (1520). Niche complemen-
tarity, which results from interspecific differ-
ences in resource requirements and in spatial
and temporal resource and habitat use, or
from positive interactions (21), is predicted to
allow stable multispecies coexistence and
sustainably greater productivity at higher di-
versity (17,18). Alternatively, it has been
hypothesized that reported diversity effects
might be short-lived transients caused solely
by the presence of some species with high
growth rates (6); be experimental artifacts
resulting solely from species pools containing
some low-viability species (6); or result from
the most productive species being the best
competitor (6,9,17). These “sampling ef-
fects” all result from the greater chance of
any given species being present at higher
diversity and from dynamics that cause a
single species to dominate and determine
ecosystem functioning (6,9,17,19).
Sampling and complementarity have dif-
ferent signatures (6,9,12,1720).Sampling
effects limit the maximal productivity of
higher-diversity plots to that of the best mono-
culture, giving an upper bound of variation in
community performance that is independent
of diversity. With niche complementarity, the
upper bound increases with diversity because
no monoculture is as productive as some
combinations of two species and no combi-
nation of Nspecies is as productive as some
combinations of N1 species.
In a 7-year experiment [(4), supplement A
(22)], we controlled one component of diversi-
ty, the number of plant species, in 168 plots,
each9mby9m.Weseeded the plots, in May
1994, to have 1, 2, 4, 8, or 16 species, with 39,
35, 29, 30, and 35 replicates, respectively. The
species composition of each plot was chosen by
random draw from a pool of 18 grassland pe-
rennials that included four C4 (warm-season)
grasses, four C3 (cool-season) grasses, four le-
gumes, four nonlegume forbs, and two woody
species. All species occurred in monoculture,
and all but three were in at least two monocul-
ture plots, allowing comparison of responses of
each species in monoculture to higher-diversity
combinations of these same species. We do not
use 76 additional plots that had functional
group compositions drawn from an augmented
species pool or 46 plots planted to 32 species
(4) because the additional species were not
grown in monoculture, and combining results
from different species pools could introduce
bias. We focus analyses on species number,
because it was directly controlled, and function-
al group composition (23), because of its hy-
pothesized importance. Other measures of di-
versity, including number of species per func-
tional group and the presence or absence of
species or functional groups, are highly corre-
lated with species number and show similar
In our grasslands, plant aboveground liv-
ing biomass, because it is all produced within
a growing season, is an index of primary
productivity. In contrast, total biomass
(aboveground plus belowground plant bio-
mass) measures carbon accumulated in living
tissues. Both aboveground and total biomass
increased highly significantly with species
number each year (Fig. 1, A and B, and Table
1), and functional group composition ex-
plained a highly significant amount of the
residual variation (Table 1). Moreover, when
the effect of each variable was determined
after controlling for effects of the other (type
III regressions), effects of functional group
composition predominated in the early years
[as for (24,25)], but species number had
highly significant positive effects on both
aboveground and total biomass by 1999 and
2000, showing the simultaneous importance
of species number and functional group com-
position in the long-term (Table 1).
The initial saturating dependence of
aboveground and total biomass on species num-
ber (Fig. 1, A and B) became, by 2000, a linear
increase for species number 2. In 2000,
16-species plots had 22% greater aboveground
biomass total and 27% greater total biomass
than 8-species plots (both differences signifi-
cant; ttests: P0.018, P0.002, respective-
ly). The dependence of biomass on species
number and functional group composition be-
came progressively stronger, explaining about
one-third of variance in 1997 and two-thirds in
2000 (Table 1). This strengthening of the effect
of diversity and the increasingly steep and lin-
ear trends (Fig. 1, A and B) fail to support the
hypothesis (6) that diversity effects were short-
lived transients. Comparable and significant
(P0.01) dependences of total and
aboveground biomass on diversity and compo-
sition were observed when analyses used the
actual number of planted species observed in
each plot (Fig. 1C) or the Shannon diversity
index [supplement B (22)].
We tested the low-viability sampling hy-
pothesis by identifying the five species that
attained least total biomass in monoculture in
2000 and excluding from analysis plots con-
taining any combinations of just these spe-
cies. Total biomass was still significantly de-
pendent on species number and functional
group composition in the remaining 131 plots
[general linear model (GLM) type III regres-
sion: F
8.39, P0.001; F
11.6, F
4.36, P0.001 for each].
Similar results occurred when we excluded
from analysis of aboveground biomass plots
containing any combinations of the five spe-
cies with least aboveground biomass in mono-
culture (GLM type III regression: F
7.08, P0.001; F
10.6, P0.0014;
3.84, P0.001). In another anal-
ysis, we excluded the 30 plots with the lowest
total biomass in 2000 (total biomass 400 g
Department of Ecology, Evolution and Behavior, Uni-
versity of Minnesota, St. Paul, MN 55108, USA.
partment of Forest Resources, University of Minneso-
ta, St. Paul, MN 55108, USA.
School of Biological
Sciences, University of Nebraska, Lincoln, NE 68588,
School of Natural Resource Sciences, University
of Nebraska, Lincoln, NE 68583, USA.
*To whom correspondence should be addressed. E-
). Species number and composition still
had highly significant effects on total bio-
mass in the remaining plots (GLM type III
regression: F
5.05, P0.001;
12.3, P0.001; F
2.82, P
0.001). Similarly, when we excluded the 31
plots with lowest aboveground biomass
) in 2000, effects of composi-
tion and species number remained highly sig-
nificant (GLM type III regression: F
3.77, P0.001; F
10.5, P0.0016;
2.28, P0.0015). Similar results
occurred with lower and higher cutoffs (in-
cluding 50% higher) for aboveground and
total biomass. In total, the dependence of
biomass on species number and composition
was not explained solely by sampling effects
for a species pool containing some poorly
performing species.
We tested the sampling hypothesis that the
most productive species determined the effects
of diversity (6,9,17) by retaining in analyses of
year 2000 results only plots containing at least
one of the nine species with the highest mo-
noculture total biomass in 2000. Total biomass
remained significantly dependent on species
number and functional group composition in
these 145 plots, and in the subset of 95 plots
that contained at least two of these nine species
[type III regressions; supplement C (22)]. Sim-
ilar results occurred for aboveground biomass
in 2000 [supplement C (22)]. These analyses
fail to support the sampling hypothesis. Anoth-
er test comes from examining performance of
higher-diversity plots relative to the best mo-
noculture (Fig. 2). In 1999 and 2000, many
higher-diversity plots had greater aboveground
and total biomass than the single best-perform-
ing monoculture (Fig. 2). The percentage of
such plots was an increasing function of diver-
sity, on average for 1999 and 2000, with about
half of the 16-species plots having greater
aboveground or total biomass than the best
monocultures (Fig. 1D). The strength and re-
peatability of this increasing upper bound in
both aboveground and total biomass support the
importance of niche effects and refute the hy-
pothesis that sampling effects were the sole
explanation for the long-term effects of diver-
sity. The coexistence of most species, with
about 12 planted species per 2 m
persisting in
each 16-species plot (Fig. 1C), further supports
niche complementarity. However, in earlier
years, such as 1997, few high-diversity plots
had greater biomass than top monocultures, and
the percentage was independent of species
number (Fig. 1D), which is consistent with the
sampling hypothesis (6,7,12).
The increasing importance of complemen-
tarity and the increasingly linear effects of
species number raise another question. Did
complementarity occur among most spe-
cies—i.e., did most species contribute to in-
creasing community biomass—or is there a
smaller set of species with complementary
interactions, with this set being increasingly
likely to co-occur at higher diversity (19)?
We used analysis of variance (ANOVA) to
determine the simultaneous effects of the pres-
ence or absence of each species (entered as
main effects) on aboveground or total biomass
(one test per year, with 4 years for total bio-
mass, 5 years for aboveground biomass). Three
or four species had significant (P0.05) pos-
itive effects on aboveground or total biomass in
most years. Among legumes, Lupinus perennis
had significant effects in all nine tests, Lespe-
Fig. 1. The dependence of (A) plant aboveground biomass and (B) total biomass (aboveground plus
belowground living plant mass) on the number of planted species. Data are shown as the mean
SE. (C) The relation between the number of species planted in a plot and the actual number
(mean SE) of planted species visually observed ina2m
area of each plot. (D) The percentage
of all plots of a given planted diversity level, on average for 1999 and 2000 combined, or on average
for 1997, that had greater biomass than the single monoculture plot with the greatest biomass.
Fig. 2. The dependence of aboveground (Aand B) and of total (Cand D) biomass of each plot on
planted species number for 1999 and 2000. The broken line shows the biomass of the top
monoculture for a given year. The solid line is a regression of biomass on the logarithm of species
number. Logarithm of species number was used in the figure because it gave slightly better fits, but
was not used in Table 1 because it often gave slightly lower R
values than species number.
26 OCTOBER 2001 VOL 294 SCIENCE www.sciencemag.org844
deza capitata in six tests, and Petalostemum
purpureum in two tests. Schizachyrium scopa-
rium and Sorghastrum nutans, both C4 grasses,
were significant in five tests each. These are
five of the six most abundant species in mix-
tures. A rarer forb also had a significant effect.
Similarly, when plots were characterized by the
presence or absence of functional groups in
ANOVAs, in 2000 there were significant posi-
tive effects of legumes (P0.001), forbs (P
0.05), and C4 grasses (P0.01) on
aboveground biomass, and significant positive
effects of legumes (P0.001) and C4 grasses
(P0.001) on total biomass. For aboveground
biomass, only the legume C4 grass interac-
tion was significant (P0.05), and it was
positive. For total biomass, the legume C4
grass interaction was marginally significant
(P0.068) and biased toward positive (26),
suggesting complementarity or facilitation
among legumes and C4 grasses (4). However,
even after controlling for the presence or ab-
sence of all functional groups, there were pos-
itive (P0.02) effects of species number on
both aboveground and total biomass in 2000,
indicating that biomass also depended on spe-
cies number rather than on just the presence of
functional groups.
Although these analyses suggest that the
presence of about five dominant species
might explain much of the effects of diversi-
ty, there may be small but additive effects of
rarer species. To test this possibility, we
ranked all species on the basis of their abun-
dance ( percentage cover) in 16-species plots
in 2000, and created 17 new diversity indices.
Each index states how many of the Nmost
abundant species (N2, 3,. . .18) had been
planted in each plot. We then determined
which diversity index (log transformed) ex-
plained the most variance in aboveground or
total biomass in 2000. Aboveground biomass
was most dependent on how many of the 9 to
13 most abundant species were planted in
each plot [supplement D (22)], showing that
many rarer species contributed detectably to
aboveground biomass. However, total bio-
mass in 2000 was most dependent on how
many of the four most abundant species were
planted [supplement D (22)], likely because
three of these four are C4 grasses, the species
that accumulate the greatest root mass.
In summary, diversity effects were neither
transients, nor explained in the long-term solely
by other sampling-effect hypotheses, nor solely
by the presence of legumes on a low-Nsoil.
Rather, niche complementarity contributed sig-
nificantly. Plant species number [as in (15,20,
21)] and functional group composition [as in (4,
5,24,25)] became simultaneously and approx-
imately equally important in our long-term ex-
periment. Compared with the average of the
single best species in monoculture, our 16-spe-
cies plots had 39% greater aboveground bio-
mass and 42% greater total biomass on average
for 1999 and 2000. Moreover, 16-species plots
in 1999 and 2000 had 2.7 to 2.9 times greater
aboveground and total biomass than the average
for all species in monoculture (Fig. 1A). The
nonsaturating increase in aboveground biomass
with diversity likely reflects niche effects among
about 9 to 13 species and their greater chance of
co-occurrence at higher diversity (19), whereas
such effects among about four species seem to
account for total biomass responses.
The demonstration that diversity effects
strengthened through time and were not the
result solely of sampling effects or functional
group composition should resolve aspects of
the biodiversity debate (614). Moreover,
our results suggest, for ecosystems in which
niche complementarity occurs, that even with
the wisest choices, monocultures may be less
productive and accumulate less living carbon
than many higher-diversity species combina-
tions. Our results show that ecosystem pro-
cesses are simultaneously influenced by di-
versity and composition, but long-term work
in additional ecosystems is needed to deter-
mine the generality of our results, to better
understand the effects of nonrandom commu-
nity assembly and disassembly, and to better
determine the implications of biodiversity for
ecosystem management.
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groups taken 1 to 5 at a time (2
1), of which 28
combinations occurred in the 168 plots. The combi-
nation that occurred in a plot is referred to as its
functional group composition.
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for plots with neither C4 grasses nor
legumes; 832 g m
for C4’s but not legumes; 564 g
for legumes but not C4’s; 1234 g m
for both.
27. We thank the National Science Foundation (NSF/
DEB 0080382 and NSF/DEB 9629566) and the
Andrew Mellon Foundation for support; J. Fargione,
S. Pacala, S. Levin, A. Dobson, and J. Reichman for
comments; and N. Larson, C. Bristow, and L. John-
son for assistance.
5 March 2001; accepted 20 August 2001
Table 1. Analyses, using general linear models, of the effects of number of
planted species (continuous variable; entered first using type I SS) and of
functional group composition (categorical variable; entered second) on
total biomass and on aboveground biomass, showing results for each year
when measured. N168. Overall model df 28 and error df 139.
Species number df 1, composition df 27. The last columns show type
III effects of species number (entered second, after functional group
Variable analyzed Year
Overall Species number
(entered first)
Functional group
comp. (entered
Species number
(entered second)
Fvalue PFvalue PFvalue PFvalue P
Total biomass 1997 0.32 2.26 0.001 9.80 0.002 2.06 0.004 3.88 0.051
Total biomass 1998 0.47 4.37 0.001 43.8 0.001 2.91 0.001 2.38 0.13
Total biomass 1999 0.60 7.31 0.001 94.2 0.001 4.09 0.001 7.11 0.009
Total biomass 2000 0.68 10.5 0.001 152. 0.001 5.27 0.001 12.3 0.001
Aboveground biomass 1996 0.41 3.28 0.001 15.5 0.001 2.83 0.001 3.80 0.053
Aboveground biomass 1997 0.39 3.02 0.001 12.3 0.001 2.60 0.001 0.52 0.47
Aboveground biomass 1998 0.49 4.80 0.001 31.8 0.001 3.81 0.001 2.56 0.11
Aboveground biomass 1999 0.56 6.27 0.001 90.7 0.001 3.15 0.001 14.9 0.001
Aboveground biomass 2000 0.61 7.80 0.001 111. 0.001 3.97 0.001 10.7 0.001
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Precipitation variability and nitrogen (N) deposition caused by anthropogenic activities could profoundly impact ecosystem productivity and carbon cycling. In desert ecosystems, vegetation is sensitive to changes in precipitation and N deposition. However, the impacts of large changes in precipitation, especially with a concurrent increase in N content, on plant community remain unclear. In this study, we carried out experiments to monitor the impacts of five precipitation levels and two N levels on the plant community function and composition from the Junggar desert in Central Asia during the period 2018–2019. Our results showed that: (1) Aboveground net primary production (ANPP) significantly increased with increasing precipitation, it followed a positive linear model under normal precipitation range, and nonlinear mode under extreme precipitation events; (2) N application led to an increase in ANPP, but did not significantly improve the sensitivity of ANPP to precipitation change; (3) Changes in N content and precipitation, and their impacts on ANPP were mainly driven by plant density. These results provide a theoretical basis for predict the future dynamics of terrestrial vegetation more accurately under climate change and increasing nitrogen deposition.
Phenotypic variation of individuals within populations can be influenced by not only genetic diversity and environmental variation experienced by these individuals but also environmental variation experienced by their parents. Although many studies have tested impacts of phenotypic diversity caused by genotypic or species diversity on productivity, no study has assessed the effects of phenotypic diversity induced by parental environmental variation on productivity. To address this novel question, we conducted two experiments with the widespread, fast-growing, clonal, floating plant Spirodela polyrhiza. We first grew mother (ancestor) ramets of S. polyrhiza under different environmental conditions to obtain descendent ramets with different phenotypes. Then, these ramets were used to construct descendent populations with different levels of phenotypic diversity caused by ancestor environmental variation and examined the effect of phenotypic diversity on population productivity. Environmental variation (changes in nutrient availability, plant density and light intensity) had significant effects on descendent populations of S. polyrhiza. However, descendent phenotypic diversity induced by ancestor environmental variation had no significant effect on total biomass or number of ramets of the descendent populations and such an effect did not depend on the nutrient availability that the descendent populations experienced. Although our results failed to support the idea that phenotypic diversity induced by ancestor environment variation can influence descendent population productivity, we propose that this novel idea should be tested with more species in different ecosystems.
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Note describing the design of the Haiti Timber Re-Introduction Program, a community-based agroforestry promotion program founded in Haiti in 2005 by the Friends of Hopital Albert Schweitzer Haiti. Integrated into the hospital's community development program that had a 50 year history of supporting local mountain communities, the program has grown steadily through demonstration plots, adult education, and empowering people to plant trees on their own land to improve their quality of life. It is ongoing.
Wind damage in a forest stand can result in varying soil effects depending on the pre-history of the site, but areas with storm-felled trees can generally be expected to show more nitrate leaching than undamaged stands. Previous fertilization in such areas, especially with nitrogen (N) fertilizer, may further increase nitrate leaching. This study examined the effect of partial felling of a 42-year-old Norway spruce stand in the Skogaby experimental forest in Sweden during Storm Gudrun in 2005. Nitrate leaching was measured one year before and six years after the storm, in three experimental treatments: fertilization-irrigation with complete nutrient admixture (IF), fertilization with N-free nutrient admixture (V), and an untreated control (0). The 0 and IF treatments had some undamaged replicate plots, but V plots had no trees left after the storm. Compared with undamaged plots and the pre-disturbance level, nitrate leaching was significantly higher in all storm-felled plots, and in the soil solution nitrate dominated strongly over ammonium. Leaching peaked during the second and third post-storm years (2006–2007) and decreased to near pre-storm levels during the fifth and sixth years (2009–2010). Total nitrate leaching 2005–2010 was estimated to be 414, 233, and 218 kg N ha⁻¹ in the damaged IF, 0, and V plots, respectively. Total nitrate leaching in undisturbed plots in the IF and 0 treatments was 37 and 0.3 kg N ha⁻¹, respectively. Ground vegetation coverage, biomass, and biomass N increased with time and were negatively correlated with nitrate discharge. However, plant uptake of N only partly explained the significant decline in nitrate leaching between 2006 and 2010. This decrease could also be explained by N immobilization in fungi decomposing woody roots with low N concentrations.
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Quality is an important aspect of durum wheat in the processing sector. Thus, recognizing the variability of quality and agronomic traits and their association is fundamental in designing plant breeding programs. This study aimed to assess the variability, heritability, genetic advance, and correlation of some agronomic and quality traits among 420 Ethiopian durum wheat genotypes and to identify the promising genotypes with distinct processing quality attributes to produce superior quality pasta. The field experiment was conducted at two locations (Sinana and Chefe Donsa) using an alpha lattice design with two replications. Analysis of variance, chi-square test, and Shannon–Weaver diversity index revealed the existence of highly significant (p < 0.001) variation among genotypes for all studied traits. The broad-sense heritability values were ranging from 46.2% (days to maturity) to 81% (thousand kernel weight) with the genetic advance as a percent of the mean ranging from 1.1% (days to maturity) to 21.2% (grain yield). The phenotypic correlation coefficients for all possible pairs of quantitative traits showed a significant (p < 0.05) association among most paired traits. The gluten content (GC) and grain protein content (GPC) were negatively correlated with grain yield and yield-related traits and positively associated with phenological traits, while yield and phenological traits correlated negatively. The frequency distributions of amber-colored and vitreous kernels, which are preferable characters of durum wheat in processing, were highly dominant in Ethiopian durum wheat genotypes. The identified top 5% genotypes, which have amber color and vitreous kernel with high GC and GPC content as well as sufficient grain yield, could be directly used by the processing sector and/or as donors of alleles in durum wheat breeding programs.
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We evaluated the effects of plant functional group richness on seasonal patterns of soil nitrogen and phosphorus cycling, using serpentine grassland in south San Jose, California. We established experimental plots with four functional types of plants: early-season annual forbs (E), late-season annual forbs (L), nitrogen-fixers (N), and perennial bunchgrasses (P). These groups differ in several traits relevant to nutrient cycling, including phenology, rooting depth, root:shoot ratio, size, and leaf C:N content. Two or three species of each group were planted in single functional group (SFG) treatments, and in two-, three-, and four-way combinations of functional groups. We analyzed available nutrient pool sizes, microbial biomass nitrogen and phosphorus, microbial nitrogen immobilization, nitrification rates, and leaching losses. We used an index of “relative resource use” that incorporates the effects of plants on pool sizes of several depletable soil resources: inorganic nitrogen in all seasons, available phosphorus in all seasons, and water in the summer dry season. We found a significant positive relationship between increasing relative resource use (including both plant and microbial uptake) and increasing plant diversity. The increase in relative resource use results because different functional groups have their maximum effect on different resources in different seasons: E’s dominate reduction of inorganic nitrogen pools in winter; L’s have a stronger depletion of nitrogen in spring and a dominant reduction of water in summer; P’s have a stronger nitrogen depletion in summer; N-fixers provide additional nitrogen in all seasons and have a significant phosphorus depletion in all seasons except fall. Single functional group treatments varied greatly in relative resource use; for example, the resource use index for the L treatment is as high as in the more diverse treatments. We expected a reduction of leaching losses as functional group richness increased because of differences in rooting depth and seasonal activity among these groups. However, measurements of nitrate in soil water leached below the rooting zone indicated that, apart from a strong reduction in losses in all vegetated treatments compared to the bare treatment, there were no effects of increasing plant diversity. While some single functional group treatments differed (P ≤ L, N), more diverse treatments did not. Early- and late-season annuals, but not perennial bunchgrasses, had significant positive effects on microbial immobilization of nitrogen in short-term (24 h) ¹⁵N experiments. We conclude that: (1) total resource use, across many resource axes and including both plant and microbial effects, does increase with increasing plant diversity on a yearly timescale due to seasonal complementarity; (2) while the presence of vegetation has a large effect on ecosystem nitrogen retention, nitrogen leaching losses do not necessarily decrease with increasing functional group richness; (3) indirect effects of plants on microbial processes such as immobilization can equal or exceed direct effects of plant uptake on nutrient retention; and (4) plant composition (i.e., the identity of the groups present in treatments) in general explains much more about the measured nutrient cycling processes than does functional group richness alone (i.e., the number of groups present).
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THE functioning and sustainability of ecosystems may depend on their biological diversity1-8. Elton's9 hypothesis that more diverse ecosystems are more stable has received much attention1,3,6,7,10-14, but Darwin's proposal6,15 that more diverse plant communities are more productive, and the related conjectures4,5,16,17 that they have lower nutrient losses and more sustainable soils, are less well studied4-6,8,17,18. Here we use a well-replicated field experiment, in which species diversity was directly controlled, to show that ecosystem productivity in 147 grassland plots increased significantly with plant biodiversity. Moreover, the main limiting nutrient, soil mineral nitrogen, was utilized more completely when there was a greater diversity of species, leading to lower leaching loss of nitrogen from these ecosystems. Similarly, in nearby native grassland, plant productivity and soil nitrogen utilization increased with increasing plant species richness. This supports the diversity-productivity and diversity-sustainability hypotheses. Our results demonstrate that the loss of species threatens ecosystem functioning and sustainability.
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Humans are modifying both the identities and numbers of species in ecosystems, but the impacts of such changes on ecosystem processes are controversial. Plant species diversity, functional diversity, and functional composition were experimentally varied in grassland plots. Each factor by itself had significant effects on many ecosystem processes, but functional composition and functional diversity were the principal factors explaining plant productivity, plant percent nitrogen, plant total nitrogen, and light penetration. Thus, habitat modifications and management practices that change functional diversity and functional composition are likely to have large impacts on ecosystem processes.
In this chapter we are concerned with the significance of biodiversity in the functioning of a particular type of ecosystem. In the broadest sense all the concepts and principles covered in the other chapters of this book are relevant to this discussion. Our main purpose is thus to establish a context for considering the role and significance of biodiversity in the functioning of agricultural systems.
Increasingly, those studies which are aiming to study the relationship between biodiversity and ecosystem function are utilising experimental designs in which species diversity is varied, with the species composition at each level of diversity being determined randomly from a predetermined species pool. Studies utilising such designs have been criticised on the basis that they are confounded by 'sampling effect' (SE) or 'selection probability effect', i.e. that the treatments which have the highest diversity have a greater probability of being dominated by the most productive species of the entire species pool; however it has also been claimed that SE is a legitimate mechanism by which diversity effects may express themselves in nature. Firstly I show, using an example of a recently published study claiming to show a diversity effect, how SE can result in the identification of apparent relationships between diversity and ecosystem properties which have little meaning in the real world. I then point out that if we accept SE is a diversity mechanism operating in nature, it is firstly necessary to assume that biological communities are randomly assembled with regard to the ecosystem property being measured; this assumption is inconsistent with conventional concepts about how biological communities are organised. Finally I discuss other experimental approaches which may remove the likelihood of results of biodiversity studies from being confounded by problems associated with SE. None of those studies in which SE may contribute to the observed outcome have successfully shown a result which cannot be ascribed to artifact, and alternative experimental approaches are required in order to better understand how biodiversity loss affects ecosystems in nature.
The study of the relationship between species richness of a plant community and its productivity has received much attention, recently renewed by the concern on the loss of biological diversity at a global scale. Here, we briefly review some indices widely used in agronomic and competition experiments to compare monocultures and mixtures, and compare them to other, more recently designed ones. These various indices are then calculated for two experiments. In the first experiment, two grass and two legume species were grown at six levels of nitrogen availability, either in monocultures or in mixtures of the four species in a substitutive design; in the second experiment, five grass species were grown at 16 levels of total nutrient availability, either in monocultures or in mixtures of the five species in an additive design. These data clearly show that the conclusions drawn from the experiments depend on the index used to compare the experimental communities. We argue that a clear test of whether the productivity of communities increases with species richness requires that: (1) all species present in the multispecies assemblages also be grown in monocultures under the same environmental conditions, and (2) the productivity of these assemblages be compared to the most productive monoculture. We conclude that there are as yet very few cases where superior productivity of multispecies assemblages as compared to monocultures has been clearly shown.
Three markedly different models of multispecies com- petition—one mechanistic, one phenomenological, and one statis- tical—all predict that greater diversity increases the temporal stability of the entire community, decreases the temporal stability of individ- ual populations, and increases community productivity. We define temporal stability as the ratio of mean abundance to its standard deviation. Interestingly, the temporal stability of entire communities is predicted to increase fairly linearly, without clear saturation, as diversity increases. Species composition is predicted to be as im- portant as diversity in affecting community stability and productivity. The greater temporal stability of more diverse communities is caused by higher productivity at higher diversity (the "overyielding" effect), competitive interactions (the "covariance" effect), and statistical av- eraging (the "portfolio" effect). The relative contribution of each cause of temporal stability changes as diversity increases, but the net effect is that greater diversity stabilizes the community even though it destabilizes individual populations. This theory agrees with recent experiments and provides a degree of resolution to the diversity- stability debate: both sides of the longstanding debate were correct, but one addressed population stability and the other addressed com- munity stability.