Biodiversity and Ecosystem Functioning: Importance of Species Evenness in an Old Field

Abstract

Changes in land use, habitat fragmentation, nutrient enrichment, and environmental stress often lead to reduced plant diversity in ecosystems. However, it remains controversial whether these reductions in diversity will affect energy flow and nutrient cycling. Diversity has two components: species richness, or the number of plant species in a given area, and species evenness, or how well distributed abundance or biomass is among species within a community. We experimentally varied species evenness and the identity of the dominant plant species in an old field of Quebec to test whether plant productivity would increase with increasing levels of evenness, and whether relationships would be invariant with respect to species identity. Total and belowground biomass increased linearly with increasing levels of evenness after one growing season. These relationships did not depend on the identity of the dominant species. Relationships between aboveground biomass and evenness varied and depended on the identity of the dominant. Our results are largely consistent with the idea that human-influenced reductions in small-scale plant diversity, in this case evenness, will lead to indirect reductions in total primary productivity. Furthermore, because the evenness treatments were not confounded with species identity, our results suggest that diversity has an effect on plant productivity above and beyond the sampling effect (having a higher probability of species with higher growth rates in diverse communities) seen in studies that vary species richness.

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887
REPORTS
Ecology,
81(4), 2000, pp. 887–892
q
2000 by the Ecological Society of America
BIODIVERSITY AND ECOSYSTEM FUNCTIONING: IMPORTANCE OF
SPECIES EVENNESS IN AN OLD FIELD
B
RIAN
J. W
ILSEY
1
AND
C
ATHERINE
P
OTVIN
Department of Biology, McGill University, Montre´al H3A 1B1, Que´bec, Canada
Abstract.
Changes in land use, habitat fragmentation, nutrient enrichment, and envi-
ronmental stress often lead to reduced plant diversity in ecosystems. However, it remains
controversial whether these reductions in diversity will affect energy flow and nutrient
cycling. Diversity has two components: species richness, or the number of plant species
in a given area, and species evenness, or how well distributed abundance or biomass is
among species within a community. We experimentally varied species evenness and the
identity of the dominant plant species in an old field of Quebec to test whether plant
productivity would increase with increasing levels of evenness, and whether relationships
would be invariant with respect to species identity.
Total and belowground biomass increased linearly with increasing levels of evenness
after one growing season. These relationships did not depend on the identity of the dominant
species. Relationships between aboveground biomass and evenness varied and depended
on the identity of the dominant. Our results are largely consistent with the idea that human-
influenced reductions in small-scale plant diversity, in this case evenness, will lead to
indirect reductions in total primary productivity. Furthermore, because the evenness treat-
ments were not confounded with species identity, our results suggest that diversity has an
effect on plant productivity above and beyond the sampling effect (having a higher prob-
ability of species with higher growth rates in diverse communities) seen in studies that
vary species richness.
Key words: biodiversity and ecosystem functioning; biomass and plant species evenness; old
fields; plant community composition; productivity vs. plant diversity; Quebec; species evenness.
I
NTRODUCTION
Changes in land use, habitat fragmentation, nutrient
enrichment, and environmental stress often lead to re-
duced plant diversity in natural ecosystems (Mooney
et al. 1996, Chapin et al. 1997, Vitousek et al. 1997).
However, the scientific community has only recently
begun addressing the question of whether these reduc-
tions in biodiversity will affect energyflow and nutrient
cycling (Ehrlich and Ehrlich 1981, Ewel et al. 1991,
McNaughton 1993, Vitousek and Hooper 1993, Lawton
1994, Naeem et al. 1994, Sala et al. 1996, Tilman et
al. 1996, 1997
a, b,
Rusch and Oesterheld 1997, Sym-
stad et al. 1998, Stocker et al. 1999). Diversity has two
components: species richness, or the number of species
in a given area, and evenness, or how well distributed
abundance or biomass is among species within a com-
munity.
Manuscript received 8 September 1998; revised and accepted
9 April 1999.
1
Present address: Department of Biological and Earth Sci-
ences, Texas A & M University, Commerce, TX 75429, USA.
E-mail: Brian
Wilsey@tamu-commerce.edu
Several recent studies have found a decline in some
ecosystem processes with a reduction in species rich-
ness of experimentally established communities.
Naeem et al. (1994) found that species-poor commu-
nities had lower levels of primary productivity than did
species-rich communities, but that other ecosystem var-
iables varied in an idiosyncratic fashion in a laboratory
study. Tilman et al. (1996, 1997
b
) also found that plant
cover and biomass increased as species richness in-
creased in experimentally established plant commu-
nities in Minnesota (USA). However, the effects of di-
versity were relatively small compared to species com-
position effects (Tilman et al. 1997
b
). Other studies
have failed to find a relationship between functional-
group richness and primary productivity (Hooper and
Vitousek 1997, Hooper 1998, Hooper and Vitousek
1998), or an effect of species diversity on litter de-
composition (Wardle et al. 1997).
The studies of Naeem et al. (1994) and Tilman et al.
(1996, 1997
b
) have been criticized because the rela-
tionships between diversity and productivity can be
explained by species identity rather than diversity per
se (Aarssen 1997, Huston 1997). Huston and Aarssen

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Ecology, Vol. 81, No. 4
T
ABLE
1. Characteristics of the diversity treatments in an
experiment that varied plant community evenness through
increased abundance in the dominant species in experi-
mental plots in a Quebec old field.
Characteristic
Diversity treatment†
1.5:1:1 5:1:1 12:1:1
Species richness 3 3 3
Plant density 14 14 14
Dominant species (%)‡ 43 71 86
Evenness indices§
Simpson 0.96 0.61 0.45
Shannon 0.98 0.73 0.46
† Ratio of dominant plant species to the two subdominant
species.
‡ Relative abundance.
§ Simpson evenness index
5
1/( ), and Shannon even-
2
S
P
/
S
i
ness index
52S
P
i
ln(
P
i
)/ln
S
, where
S
5
number of species
and
P
i
5
proportion of total abundance of the
i
th species.
argued that in the species-rich plots, in which a larger
number of species were randomly selected from a pool
of species, it was more likely that species with high
growth rates were selected and that species rich plots
had higher productivity because of species identity
rather than diversity. However, Tilman et al. (1997
a
)
argued convincingly that this sampling effect was a
potential explanation for why productivity would in-
crease with species richness in nature, rather than an
experimental artifact.
In order to help resolve the debate on whether in-
creases in productivity are a function of diversity or
species identity, we tested if plant productivity would
increase with increasing levels of evenness, and if re-
lationships between productivity and evenness would
be invariant with respect to species identity. We con-
centrated on evenness as a measure of diversity rather
than richness because by varying evenness without
changing species richness we had the advantage that
diversity was not as confounded by species identity.
Therefore, it was a better test of whether effects are a
function of diversity or an indirect function of species
identity. In addition, in many cases, changes in even-
ness occur with little or no changes in species richness,
and this points to the importance of evenness as a com-
ponent of diversity (Magurran 1988, Elliot and Swank
1994, Vestergaard 1994, Piper 1995, Ducey et al. 1996,
Legendre and Legendre 1998). We chose plant pro-
ductivity as a response variable because it is perhaps
the most important ecosystem variable: net primary
productivity is positively associated with the produc-
tivities of higher trophic levels, herbivore consump-
tion, and herbivore biomass at a variety of scales (e.g.,
McNaughton et al. 1989, Frank and McNaughton 1992,
Pandey and Singh 1992, Siemann 1998).
M
ETHODS
The study was conducted in an old field on land
owned by the Morgan Arboretum of McGill University
near St. Anne de Bellevue, Quebec, Canada (45
8
30
9
,
74
8
W). The clearing was maintained by mowing and
had not been plowed in many years, if ever. The field
was dominated by a mixture of grasses and perennial
dicots, with timothy (
Phleum pratense
) and Kentucky
bluegrass (
Poa pratensis
) as the most common grasses,
and wild strawberry (
Fragaria virginiana
) and dan-
dlelion (
Taraxacum officionale
) as the most common
dicots.
Experimental plots were established and plantedwith
three of the common plant species at different levels
of evenness. Each of the three plant species was in turn
used as the dominant species to test for the effects of
species identity (Table 1). In early May 1997, shortly
after spring snow melt, 45 plots (40
3
40 cm) were
established by hand removing the sod and roots. Re-
moving sod created a disturbed environment in our
plots, and although disturbances are not uncommon in
old fields, this probably led to increased nutrient avail-
abilities. However, live aboveground biomass in four
similar-sized plots in the natural vegetation averaged
158.1
6
27.5 g/plot (mean
6
1
SE
) and was in the same
range as the experimental plots (means ranged from
65.0 to 239.4 g/plot among the nine treatments). In
each of the plots, 14 plants were planted in such a way
that three levels of evenness were established: a ratio
of the dominant to others of 1.5:1:1 (high evenness),
5:1:1 (medium evenness), and 12:1:1 (low evenness);
treatments were randomly allocated to 15 plots per lev-
el. Within each of the evenness treatments, we then
randomly applied the identity treatments by having 15
(five within each of the three evenness levels) plots
dominated by
Poa pratensis,
15 plots dominated by
Fragaria virginiana,
and the final 15 plots dominated
by
Taraxacum officionale.
In all three evenness treat-
ments, the other two species were the subdominant
plants, so that all plots had three plant species (i.e.,
species richness was kept constant). Thus, nine groups
were created in a factorial design, consisting of three
levels of evenness and three levels of species identity,
with five replicates. The three plant species were ran-
domly selected from among the most common species
of the field. Species richness in the undisturbed veg-
etation outside our plots for a similar-sized plot (40
3
40 cm) averaged 5.5
6
0.6 species (
n
5
4 plots, pe-
rennials only), and an adjacent field averaged 4.0
6
0.2 species (
n
5
8 plots), so a species richness of 3 in
experimental plots was somewhat below normal.
After plots were laid out, the overwintering organs
of newly emerging plants from the rest of the field were
transplanted so that the dominant plant species was
located in a similar position in each plot across identity
treatments. Plants were watered three times after plant-
ing during the first two weeks of the study to facilitate
their establishment. Plants encountered natural water
availabilities thereafter. Edge effects (e.g., shading by
adjacent vegetation and encroachment by stolons and
roots outside the plots), which were similar for all plots,

April 2000 889
PLANT EVENNESS AND ECOSYSTEM FUNCTION
Reports
F
IG
. 1. Biomass, averaged across species composition
groups, in experimental plant communities in a Quebec old
field as a function of evenness (Simpson’s index). (a) Total
biomass (shoots
1
roots of all plants in the plot). (b) Root
biomass. Data are means
6
1
SE
;
n
5
15 plots. Results of
polynomial contrasts are included.
were minimized by periodically clipping and pulling
back adjacent vegetation. Plots were weeded weekly.
At the end of the growing season, in late September,
the plots were harvested. Plant height was measured
on two plants of each species and then all plants were
excavated so that measurements of above and below-
ground biomass could be made for each species. Each
plant was individually excavated with a combination
of gentle shaking and washing of the soil. By exca-
vating each individual plant separately, we included
only the root biomass of the experimental plants. Con-
currently, a measure of rooting depth was made on two
plants of each species from each plot. Roots were then
clipped from shoots, dried, and weighed.
From the measurements of rooting depth and plant
height, two other variables were derived: a measure of
variance in rooting depth, and a measure of variance
in plant height. These two variables were derived to
test for relationships with productivity. We tested if the
variance in height and rooting depth among plants
would be higher in the more diverse communities,
which would lead to more complementary resource use
and would provide an explanation for any possible in-
creases in productivity (Naeem et al. 1994, Tilman et
al. 1997
a,
Hooper 1998, Hooper and Vitousek 1998).
In both cases, a coefficient of variation was modeled
for each plot by assigning the mean height and rooting
depth (per species) for all 14 plants in each plot.
Because root and shoot biomass was very small at
the start of the experiment compared to the end-of-
season biomass (typical of herbaceous perennials, e.g.,
McNaughton et al. 1998), and because there is rela-
tively little carryover from year to year in this system,
the measurements of end-of-growing-season biomass
of the three species combined provides us with a useful
index of primary productivity. Productivity variables
(aboveground, belowground and total [
5
above
1
be-
lowground] biomass) were analyzed with two-way AN-
OVA (evenness and species identity). Variables were
log-transformed to improve normality before ANOVAs
were conducted. Relationships between productivity
and level of evenness were then tested with a priori
polynomial contrasts (first and second order). Second-
order (quadratic) contrasts helped to determine if re-
lationships either leveled off at high or low levels of
evenness or if they were highest or lowest at inter-
mediate levels of evenness. Comparison of means
among identity treatments were made with Duncans
multiple range tests. Relationships between productiv-
ity and the coefficient of variation (
CV
) of height and
rooting depth were analyzed separately with polyno-
mial regressions.
R
ESULTS
Total biomass (above
1
belowground) increased lin-
early with increasing levels of evenness after one grow-
ing season (
F
5
3.2,
P
,
0.05, Fig. 1a). It also varied
significantly among the three species identity treat-
ments, with
Fragaria
- and
Poa
-dominated plots having
greater productivity than
Taraxacum
(
F
5
5.6,
P
,
0.01, Fig. 2). The relationship between evenness and
biomass was consistent across species identity treat-
ments (evenness
3
identity interaction,
F
5
1.8,
P
5
0.15) (Table 2).
Because aboveground and belowground biomass
have different ecological roles and food webs, the data
were then analyzed separately for above- and below-
ground biomass. Again, belowground biomass in-
creased linearly with increasing levels of evenness (
F
5
4.2,
P
,
0.02, Fig. 1b). Belowground biomass was
not affected by species identity (
F
5
1.2,
P
5
0.31).
The relationship between root biomass and evenness
was consistent across the identity treatments (evenness
3
identity interaction,
F
5
0.8,
P
5
0.51).
The relationship between evenness and aboveground
biomass, however, was not a simple function of even-
ness (main effect,
F
5
1.2,
P
5
0.31) but rather de-
pended on the species identity of the plots (Fig. 3).
The main effect of species identity was highly signif-
icant (
F
5
24.0,
P
,
0.01) as was the interaction be-
tween identity and evenness (
F
5
6.2,
P
,
0.01). When

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BRIAN J. WILSEY AND CATHERINE POTVIN
Ecology, Vol. 81, No. 4
F
IG
. 2. Total biomass (shoots
1
roots of all plants in the
plot), averaged across evenness treatments, in experimental
communities dominated by
Fragaria virginiana, Poa praten-
sis,
and
Taraxacum officionale.
Data are means
1
1
SE
,
n
5
15 plots. Bars with the same lowercase letter above are not
significantly different at
P
5
0.05.
F
IG
. 3. Aboveground biomass as a function of evenness
(Simpson’s index) in experimental old-field plant communi-
ties dominated by (a)
Fragaria virginiana
, (b)
Taraxacum
officionale
, and (c)
Poa pratensis
in Quebec. Data are means
6
1
SE
;
n
5
5 plots.
T
ABLE
2. End-of-season biomass in experimental plantcom-
munities in a Quebec old field as a function of evenness
(Simpson’s index).
Evenness
0.45 0.61 0.96
Fragaria
dominated
Root biomass 119.0 (15.4) 171.0 (19.4) 230.9 (24.7)
Total biomass 301.2 (27.3) 410.5 (30.3) 381.5 (35.3)
Taraxacum
dominated
Root biomass 163.3 (43.8) 140.5 (19.5) 220.2 (34.9)
Total biomass 246.5 (57.5) 205.5 (33.8) 358.3 (27.5)
Poa
dominated
Root biomass 195.0 (42.9) 214.7 (29.9) 209.5 (26.5)
Total biomass 319.9 (54.4) 336.9 (26.1) 348.7 (41.0)
Note
: Total biomass
5
belowground (roots) and above-
ground biomass. Data are means, with
SE
in parentheses;
n
5
5 plots.
one-way ANOVAs were done separately for each spe-
cies identity treatment, in plots dominated by
Fragaria,
intermediate evenness plots had the highest biomass,
and biomass was lower in high- and low-evenness plots
(
F
5
8.0,
P
,
0.01, Fig. 3a). In plots dominated by
Taraxacum,
the highest-evenness plots had the highest
biomass, but the other two evenness levels had roughly
the same levels of biomass (
F
5
5.9,
P
,
0.02, Fig.
3b). There was no relationship between aboveground
biomass and evenness when
Poa
was dominant (
F
5
0.2,
P
5
0.87, Fig. 3c).
There was a relationship between variation (
CV
)in
height among plants, and total biomass (Fig. 4). As the
variation in height among plants increased—as would
be found initially in higher-evenness plots—the total
biomass of the plots increased until an intermediate
level of variation was found; the total biomass leveled
off thereafter (quadratic,
P
,
0.01,
r
2
5
0.23,
n
5
42
plots). There was no relationship between variation in
rooting depths and total biomass (
P
.
0.10,
n
5
45
plots).
D
ISCUSSION
In contrast to previous experiments on the effects of
plant diversity on plant productivity, we varied even-
ness rather than species richness. Because our diversity
treatment (evenness) was not as confounded with spe-
cies identity as were earlier experimental studies
(Naeem et al. 1994, Tilman et al. 1996, 1997
b
), it sug-
gests that diversity might have a direct effect on plant
productivity above and beyond the sampling effect.
That is, diverse communities are not more productive
merely because they have a larger probability of con-

April 2000 891
PLANT EVENNESS AND ECOSYSTEM FUNCTION
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F
IG
. 4. Relationship between modeled variation in height
among plants and total biomass.
taining species with higher growth rates. Relationships
between total productivity and evenness tended to be
more linear than were the relationships between pro-
ductivity and species richness found by Naeem et al.
(1994) (see Chapin et al. 1998: Fig. 6) and Tilman et
al. (1996, 1997
b
). I. Nijs and J. Roy (
unpublished data
)
also found linear relationships between plant produc-
tivity and community evenness in a modeling study.
We also found that total biomass effects were largely
influenced by an index of root productivity (peak bio-
mass), which was not measured in earlier studies. This
emphasizes the importance of measuring belowground
processes in ecological studies.
Naeem et al. (1994) hypothesized that more-diverse
communities are more productive because a greater
proportion of light is captured by the plant community
as a whole. A similar explanation (termed the ‘‘gen-
eralized niche model’’) was mentioned as one of the
three possible explanations for the results of Tilman et
al. (1997
a
). Hooper (1998) reviewed a variety of mech-
anistic explanations for why diverse communities
might be more productive than less diverse commu-
nities; these primarily involved greater complementary
use of resources in space and time and thus a shift in
community interactions from strong competition to
weak competition or facilitation. Our results were
somewhat consistent with the hypotheses of Naeem et
al. (1994) and the generalized niche model of Tilman
et al. (1997): we found a relationship between variation
in height among plants and total biomass. We found
that total biomass increased as variation in plant heights
increased, but did not increase as variation in rooting
depths increased. Thus, differences in the interception
of light were probably more important than nutrient
uptake in explaining the plant community responses to
evenness. An outcome such as this would be most like-
ly in communities where intraspecific competition for
light is greater than interspecific competition, and es-
pecially in plant communities where there are differ-
ences in plant architecture among species (Bazzaz
1996). However, the proportion of light captured by
the experimental communities was not measured di-
rectly, and further work is required to test whether this
or other mechanisms are behind the relationship be-
tween evenness and productivity.
Although there was a relationship between the even-
ness treatments and total biomass after statistically con-
trolling for species identity, it should be emphasized
that there were also differences among the species iden-
tity groups (i.e., there was a main effect of species
identity, Hooper and Vitousek 1997, Tilman et al.
1997
b
). Thus, both diversity and the identity of the
plant species in the community, together with factors
that were not included in this study, are important in
predicting plant productivity in Quebec old fields, and
perhaps other plant communities.
Our results have implications for the management of
early-successional environments (e.g., hay fields). En-
vironmental changes from management of a preferred
species, fertilization, or other human-caused simplifi-
cations of plant communities often have direct effects
on ecosystem processes, including primary productiv-
ity (Vitousek et al. 1997). The results presented here
suggest that reductions in evenness, which also occur
in response to these environmental changes (e.g.,
Grime 1979, Silverton 1980, Tilman 1987, Carson and
Barret 1988), may have indirect effects on plant pro-
ductivity.
A
CKNOWLEDGMENTS
We thank Catherine Sartoros, Anna Loan-Wilsey, and Mar-
garet Kraenzal for help in the field. Gray Stirling made some
very useful comments on an earlier version of the manuscript.
Financial support was provided by the Ministe`re de
l’E
´
ducation du Que`bec and NSERC of Canada. This manu-
script benefitted from the comments of Ragan Callaway and
two anonymous reviewers.
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