Download Productivity vs Species richness

Productivity
What explains total diversity in a
community?
amount of sunlight turned into primary producers
(plants, algae, etc), often estimated using biomass
of primary producers
Keystone species can influence diversity
Equilibrium theory of island biogeography
Disturbance
Many empirical studies have found a hump-shaped
relationship between the productivity of a system
and the number of species in that system
Productivity
Productivity vs Species richness
Species richness
Number of species
Productivity
Biomass
Biomass
Habitat heterogeneity: the
relationship between species
richness and biomass varies
among microhabitats
Productivity and
Biodiversity
Habitat heterogeneity: the
relationship between species
richness and biomass varies
among microhabitats
Competition
Number of species
Productivity and
Biodiversity
Increase in
productivity
allows for
coexistence
of more
species
Competition
begins to
remove less
competitive
species
Biomass
What explains total diversity in a
community?
Habitat heterogeneity and
Biodiversity
Ecosystems with more heterogeneous habitats have
more potential niches, allowing the coexistence of more
species.
Keystone species can influence diversity
Number of species
Equilibrium theory of island biogeography
Disturbance
Productivity
Habitat heterogeneity
Habitat heterogeneity and Biodiversity
What explains total diversity in a
community?
Keystone species can influence diversity
-MacArtur and MacArthur (1961)
found that the bird diversity of a
habitat increased with the
complexity of the habitat’s
vegetation
Bird species
diversity
Habitat heterogeneity
Equilibrium theory of island biogeography
Disturbance
Productivity
-Similar relationships have been
demonstrated in other taxa
Habitat heterogeneity
Foliage height
diversity
Do ecosystems with high species diversity “function”
better?
Redundancy
Species richness
Species richness
Ecosystem function
Species richness
Ecosystem function
Are ecosystems with high species diversity more
stable?
More diversity is more stable
Ecosystem function
No relationship
Ecosystem function
Is biodiversity important for
ecosystem structure and function?
Idiosyncratic
Species richness
Do ecosystems with more
species function better?
Empirical evidence shows that in many
ecosystems there is a positive relationship
between productivity and species
richness. But some studies show that
there is either no correlation or a negative
correlation.
Are ecosystems with more
species more stable
Productivity
Time
Are ecosystems with more
species more stable
Productivity
Are ecosystems with more
species more stable
Productivity
Time
Time
Are ecosystems with more
species more stable?
Are ecosystems with more
species more stable?
Hypothetical relationship between species richness and invasion resistance
Ecosystems with more
species should be more
resistant to disturbances
and will recover faster than
species poor communities
Species richness
Species richness
Resistance to invasions
Variance of
Productivity
Species rich communities are less susceptible to invasion because they
use more of the available resources.
Resource availability
Hypothetical relationship between productivity and species richness
Species richness
Do ecosystems with high species diversity
“function” better ?
Do ecosystems with high species diversity
“function” better ?
What do the empirical data tell us?
1. Experiments in The Ecotron:
What do the empirical data tell us?
1. Experiments in The Ecotron
Naeem et al. (1994) created communities with 3
levels of biodiversity (low, medium, and high)
and examined the relationship between
biodiversity and ecosystem function in these
artificial communities.
The Ecotron is facility designed to
establish simplified experimental
communities
!
Are ecosystems with more
species more stable?
Do ecosystems with high species diversity
NATURE|Vol
441|1 June
2006 ?
“function”
better
What do the empirical data tell us?
1. Experiments in The Ecotron
% Change in
vegetation cover
high
medium
low
-High biodiversity communities
had denser canopies and higher
photosynthetic rates
-low diversity communities also
consumed less CO2
Time
Figure 3 | Effects of plant diversity on the relationship between mean
biomass and its temporal standard deviation. Each point shows the mean
biomass of a plot and its temporal standard deviation based on the five
annual samples collected from 2001 to 2005. Data for each species-number
treatment were fitted by a separate regression line. Data were not detrended.
The results show that high-diversity treatments have lower temporal
standard deviations (lower risks) for a given mean biomass (return).
Are ecosystems with more
species more stable?
Minnesota grassland plot experiment
four years before imposition of drought, more diverse plots had
greater temporal stability25. In combination, these two studies
indicate that there might be short-term variability in effects of
diversity on ecosystem stability
butvaries
that, less
in the
long term, higher
Biomass
from
plant diversity causes greater
yearecosystem
to year instability.
plots with
Our results support the
predictions
of competition theory that
high
species richness
greater diversity leads to greater ecosystem stability and lower species
stability8,19,22.This theory predicts that greater ecosystem stability at
higher diversity can result from increasingly negative covariance in
the abundances of competing species at higher diversity19,22 (covariance effect), from the manner in which temporal variance in species
abundances scales with abundance19,22,26,27 (statistical averaging or
portfolio effect), and/or from the manner in which species abundances
with diversity19,22 (overyielding effect; greater ecosystem
standard scale
deviation
Coefficient of variation =
total biomass
at higher diversity).
mean
The covariance effect requires that total covariance (temporal
covariance in abundances for each pair of species, summed across
all possible pairs of species) decline as diversity increases. However,
regression showed no dependence of total covariance for 2001–05 on
species number (F 1, 161 ¼ 0.83, P ¼ 0.36). Our results thus do not
LETTERS
Tilman and Downing (1994)
Biodiversity
and stability
in grassland.
annual
ecosystem
service—the
Nature 367: 363-365.
production of biomass and thus of
potential biofuels and livestock fodder1–6 —also depends on biodiversity. Biodiversity can therefore be an important element for
the reliable and sustainable provisioning of ecosystem services.
METHODS
Experimental design. In a 7-ha field at Cedar Creek Natural History Area,
Minnesota, USA, we controlled the number of plant species in 168 plots, each
9 m £ 9 m. Plots were randomly assigned to be seeded with 1, 2, 4, 8 or 16
perennial grassland species, with 39, 35, 29, 30 and 35 replicates, respectively, of
the diversity levels. The composition of each plot was randomly chosen from a
set of 18 perennials (four C4 grasses, four C3 grasses, four legumes, four nonlegume forbs and two woody species). All plots received 10 g m22 of seed in May
1994 and 5 g m22 in May 1995, with seed mass divided equally between species.
Treatments were maintained by weeding three or four times each year. Weeds
were removed while still small, with care being taken to minimize any disturbance. Plots were burned annually in spring before growth began. Five woody
monocultures are not included in analyses because burning effectively eliminated woody species from multispecies plots. Plots were annually sampled in
mid-August for aboveground living plant biomass by clipping, drying and
weighing four parallel and evenly spaced 0.1 m £ 3.0 m vegetation strips per
plot from 1996 to 1999 and four 0.1 m £ 6.0 m strips per plot from 2000 to 2005.
Different locations were clipped each year. Biomass from one strip per plot was
sorted to species from 2001 to 2005. The Shannon diversity index, H 0 , used
abundances of each species, planted or weedy, in each plot by means of estimates
of percentage cover for 1996–2000 (four 0.5 m2 subplots per plot) and sorted
biomass for 2001–2005. See ref. 4 for further details.
Minnesota grassland plot experiment
Sampling effort. To eliminate potential bias from different sampling efforts for
the first four in comparison with the last six years, for each of the last six years
two clipped strips per plot were randomly chosen for an analysis of ecosystem
stability. The full data gave similarly significant and positive effects of diversity
on all three measures of ecosystem stability. Relationship between drought
vegetation
in a
Detrending and other analyses. Detrending resistance
was done, forofeach
plot, by means
and
plant
of linear regression of annually measured plotMinnesota
biomass on grassland
the logarithm
of year
and used all ten years of plot data. The logarithm
of year
provided
a generally
species
richness
prior
to the
better fit than year; both gave similar results.
The standard
deviation,
j d , ofwas
drought.
Drought
resistance
residuals for each regression measures detrended
variation.
measured
as theThe
logdetrended
of the ratio
. Each
plot
had a single
detrended
temporal stability, S d, of a plot was S d ¼ m/j dof
plant
biomass
at the
height of
stability value for the ten-year period. In contrast, when data were divided into
the drought to plant biomass
shorter intervals that did not require detrending, there were multiple values of S,
before the drought. Data are
calculated as S ¼ m/j, per plot. We divided the data either into two subsets, each
shown
as means
+ SE
(redrawn
five years in duration (1996–2000 and 2001–05)
or into
five subsets,
each
two
from
Tilmanand
and
Downing
1994).
years in duration (1996–97, 1998–99, 2000–01,
2002–03
2004–05).
These
temporal sequences of S values for each plot were analysed with the use of
repeated-measures MANOVA.
Are ecosystems with more
species more stable?
Received 20 December 2005; accepted 23 March 2006.
1.
Naeem, S., Håkenson, K., Lawton, J. H., Crawley, M. J. & Thompson, L. J.
Biodiversity and plant productivity in a model assemblage of plant species.
Are ecosystems with more
species more stable?
Are ecosystems with more
species more stable?
!
Minnesota grassland plot experiment: resource usage
Minnesota grassland
plot experiment:
resource usage
LETTERS
NATURE|Vol 441|1 June 2006
Resource availability
Tilman et al (1996, 1997)
examined the effect of
species diversity on
productivity and soil
nutrients.
Plots with more species
less nitrogen in their soil
lower resource availability
!
Species richness
Figure 2 | Dependence of ecosystem temporal stability from 1996 to 2005
on realized species number. All species, whether planted or weedy, were
ranked by proportional abundance in the sorted 0.6 m2 clipped strip of 2005.
Proportional abundances were summed, in order from the most abundant
species, to determine the realized species number, which is the number of
more abundant species comprising 90% of the total aboveground biomass of
a plot. Ecosystem stability was also significantly dependent on realized
species number determined with cutoffs of 75% (P , 0.0001) and 99%
(P ¼ 0.002). As in Fig. 1a, one data point (realized species number of 7.2,
ecosystem stability of 15.76) is not shown but was included in all analyses.
species stability (log transformed) was a declining function of
planted species number (F 1, 159 ¼ 63.5, P , 0.0001; Fig. 1b) and of
(F 1, 159
¼ 57.1, P , 0.0001). Species stabilities were not
e H June
NATURE|Vol 441|1
2006
detrended, but results were similar if detrended.
Figure 1 | Dependence of temporal stability of each plot on experimentally
We used stepwise regression to evaluate the influence of root mass,
imposed species-number treatment. a, Ecosystem temporal stability for
functional group composition (presence or absence of C3 grasses, C4
the decade from 1996 to 2005 was an increasing function of the number of
grasses, legumes or non-legume forbs), weedy species biomass, initial
planted species. Ecosystem stability is the ratio of mean plot total biomass to
soil fertility (initial total soil nitrogen) and species number on
its temporal standard deviation, determined after detrending. The
ecosystem stability. In both forward addition and backward elimiregression line and its 95% confidence interval are shown (untransformed
nation analyses, the same three variables were retained, with ten-year
data: F 1, 159 ¼ 43.7, P , 0.0001). To reduce the difference in y axis scale
detrended ecosystem stability remaining positively dependent on
between the two parts of this figure, a single data point (species number of
16, ecosystem stability of 15.76) is not shown but was included in all
species number (F 1, 159 ¼ 16.2, P , 0.0001) and also being positively
analyses. b, Plot-average species temporal stability, determined with species
dependent on root mass (F 1, 159 ¼ 23.0, P , 0.0001) but negatively
biomass data for 2001–2005, was a declining function of the number of
dependent on the presence of legumes (F 1, 159 ¼ 4.42, P ¼ 0.037).
planted species. The regression curve and 95% confidence intervals are
The positive effect of root mass probably occurred because roots are
based on a fit of log(species stability) on log(species number), with
the perenniating structure of these herbaceous perennial species, and
F 1, 159 ¼ 72.3, P , 0.0001.
higher root mass should provide a larger store of nutrients and
energy to buffer growth in response to environmental variation.
Weedy biomass had no significant (P . 0.05) effects on stability and
was neither added nor retained in the forward or backward stepwise
(F 1, 155 ¼ 16.5, P , 0.0001), stability had a weak tendency to
regressions, respectively. Similarly, in repeated measures analyses
increase through time (F 4, 152 ¼ 2.24, P ¼ 0.067) and there was no
using two-year or five-year ecosystem stabilities, weed biomass had
species-number £ time interaction (F 4, 152 ¼ 0.60, P ¼ 0.66). Simino significant effects (P . 0.1) but ecosystem stability remained an
lar repeated-measures MANOVAs, of both two-year and five-year
increasing function of numbers of species planted (P , 0.001). This
stabilities, that used realized species number as the independent
indicates that any disturbance that might have been associated with
variable yielded similar results. The greater ecosystem stability of
Figure 2 | Dependence of ecosystem temporal stability from 1996 to 2005
differences between treatments in weeding intensity did not influence
higher-diversity plots resulted from their having lower temporal
on realized species number. All species, whether planted or weedy, were
results. Diversity did affect invading weedy species. After cessation of
standard deviations, for a given mean plot biomass, than plots with
ranked by proportional abundance in the sorted 0.6 m2 clipped strip of 2005.
weeding in subplots, total numbers of plant species and total biomass
lower diversity (Fig. 3). In total, on average across the decade
Proportional abundances were summed, in order from the most abundant
increased more at lower diversity than at higher diversity24.
of measurement, ecosystem stability was significantly positively
species, to determine the realized species number, which is the number of
The strength and consistency of the long-term stabilizing effects of
dependent on plant diversity, and this result was robust with respect
more abundant species comprising 90% of the total aboveground biomass of
diversity on ecosystem productivity that we observed contrast with
to data detrending and the intervals over which stability was
on realized
effects observed when a short-term drought was imposed on a
determined.a plot. Ecosystem stability was also significantly dependent mixed
species
number
determined
with
cutoffs
of
75%
(P
,
0.0001)
and 99% experiment13. In that study, the proportion of abovebiodiversity
In contrast to ecosystem stability, stabilities of individual species
(P ¼ 0.002).
As in with
Fig. 1a,
data record
point (realized
species number
of 7.2,biomass lost after an 8-week drought was independent
ground plant
(log transformed),
determined
ourone
five-year
of abundances
ecosystem
15.76)
shownfunction
but wasof
included
indiversity
all analyses.
13
, indicating, by a metric analogous to ours, no effect of
of
of each species
plantedstability
in each of
plot,
wereisa not
declining
the
diversity on short-term proportional resistance stability. Because
number of planted species (F 1, 988 ¼ 134.3, P , 0.0001) and, simimore diverse plots had greater biomass, the absolute biomass loss was
larly, of effective species number, e H (F 1, 988 ¼ 83.6, P , 0.0001).
at greater
as showing lower
We also calculated
average, (log
for each
plot, of the species
speciesthe
stability
transformed)
was stabilities
a declininggreater
function
of diversity, which was interpreted
13
et al that
2006,
absolute
resistance
of all species
planted
in the Tilman
plot, and(F
found
the plot-average
¼
63.5,
Pnature
, 0.0001; Fig.
1b) and
of stability at higher diversity . However, during the
planted
species
number
0
LETTERS
Are ecosystems with more species
more resistant to invaders?
0
0
1, 159
e H (F 1, 159 ¼ 57.1, P , 0.0001). Species stabilities were not
©!2006!Nature Publishing Group!
detrended, but results were similar if detrended.
We used stepwise regression to evaluate the influence of root mass,
functional group composition (presence or absence of C3 grasses, C4
grasses, legumes or non-legume forbs), weedy species biomass, initial
soil fertility (initial total soil nitrogen) and species number on
ecosystem stability. In both forward addition and backward elimination analyses, the same three variables were retained, with ten-year
detrended ecosystem stability remaining positively dependent on
species number (F 1, 159 ¼ 16.2, P , 0.0001) and also being positively
dependent on root mass (F 1, 159 ¼ 23.0, P , 0.0001) but negatively
dependent on the presence of legumes (F 1, 159 ¼ 4.42, P ¼ 0.037).
The positive effect of root mass probably occurred because roots are
the perenniating structure of these herbaceous perennial species, and
higher root mass should provide a larger store of nutrients and
energy to buffer growth in response to environmental variation.
Weedy biomass had no significant (P . 0.05) effects on stability and
was neither added nor retained in the forward or backward stepwise
(F 1, 155 ¼ 16.5, P , 0.0001), stability had a weak tendency to
regressions, respectively. Similarly, in repeated measures analyses
increase through time (F 4, 152 ¼ 2.24, P ¼ 0.067) and there was no
using two-year or five-year ecosystem stabilities, weed biomass had
species-number £ time interaction (F 4, 152 ¼ 0.60, P ¼ 0.66). Simino significant effects (P . 0.1) but ecosystem stability remained an
lar repeated-measures MANOVAs, of both two-year and five-year
increasing function of numbers of species planted (P , 0.001). This
stabilities, that used realized species number as the independent
indicates that any disturbance that might have been associated with
variable yielded similar results. The greater ecosystem stability of
between treatments
in weeding intensity did not influence
higher-diversity plots
resulted
from their
having
temporalrichdifferences
Some
studies
show
thatlower
species
communities
are more
results. Diversity did affect invading weedy species. After cessation of
standard deviations, for a given mean plot biomass, than plots with
productive,
someacross
other
show inalternative
weeding
subplots, total numbers of plant species and total biomass
lower diversity (Fig.
3). In total,but
on average
the studies
decade
increased more at lower diversity than at higher diversity24.
of measurement,interpretation.
ecosystem stability was significantly positively
The strength and consistency of the long-term stabilizing effects of
dependent on plant diversity, and this result was robust with respect
diversity on ecosystem productivity that we observed contrast with
to data detrending and the intervals over which stability was
effects observed when
short-term drought was imposed on a
determined.
Some studies show that species richmixed
communities
are amore
biodiversity experiment13. In that study, the proportion of aboveIn contrast to ecosystem stability, stabilities of individual species
and
faster
are
ground
plant and
biomass
lostless
after an 8-week drought was independent
(log transformed),stable
determined
withrecover
our five-yearfrom
record ofdisturbances
abundances
of diversity13, indicating, by a metric analogous to ours, no effect of
of each species planted
in each plot,
a decliningspecies.
function of the
vulnerable
towere
invasive
diversity on short-term proportional resistance stability. Because
number of planted species (F 1, 988 ¼ 134.3, P , 0.0001) and, simimore diverse plots had greater biomass, the absolute biomass loss was
larly, of effective species number, e H (F 1, 988 ¼ 83.6, P , 0.0001).
studies
tostabilities
allow generalizations
beyond
greater at greater diversity,
which was interpreted as showing lower
We also calculatedMore
the average,
for eachare
plot, needed
of the species
absolute resistance stability at higher diversity13. However, during the
of all species planted in the plot, and found that the plot-average
630
Figure 1 | Dependence of temporal stability of each plot on experimentally
imposed species-number treatment. a, Ecosystem temporal stability for
the decade from 1996 to 2005 was an increasing function of the number of
planted species. Ecosystem stability is the ratio of mean plot total biomass to
its temporal standard deviation, determined after detrending. The
regression line and its 95% confidence interval are shown (untransformed
data: F 1, 159 ¼ 43.7, P , 0.0001). To reduce the difference in y axis scale
between the two parts of this figure, a single data point (species number of
16, ecosystem stability of 15.76) is not shown but was included in all
analyses. b, Plot-average species temporal stability, determined with species
biomass data for 2001–2005, was a declining function of the number of
planted species. The regression curve and 95% confidence intervals are
based on a fit of log(species stability) on log(species number), with
F 1, 159 ¼ 72.3, P , 0.0001.
Is biodiversity important for
ecosystem function?
0
some model systems.
630
!
!
© 2006 Nature Publishing Group!
Species Diversity and Invasion
Resistance in a Marine Ecosystem
John J. Stachowicz, Robert B. Whitlatch, Richard W.
Osman. 1999. Science 286:1577-1579
Theory predicts that systems that are more diverse should be
more resistant to exotic species, but experimental tests are
needed to verify this. In experimental communities of sessile
marine invertebrates, increased species richness significantly
decreased invasion success, apparently because species-rich
communities more completely and efficiently used available
space, the limiting resource in this system. Declining
biodiversity thus facilitates invasion in this system,
potentially accelerating the loss of biodiversity and the
homogenization of the world's biota.