0.5 1

Classical metapopulation model

Fraction of patches occupied

RateColonization

Extinction

Equilibrium metapopulation

size

Area effects

P

Ss

Sl

How does island area affect the number species on the island?

Cs

Es

El

Island species number

RateC

l

How does size affect colonization rate?

Ss

Sl

Ssmall

< Slarge

Distance effectsHow does distance from the mainland affect

the number species on the island?How does size affect colonization rate?

P

Sn

Sf

Cf

E

Cn

isn

Rate

Sf

Sn

Sfar

< Snear

S

What explains total diversity in a community?

Keystone species can influence diversity

Equilibrium theory of island biogeography

Disturbance

Disturbance

An event that moves the community

away from its equilibrium state

Disturbance events can be caused by biotic (e.g. disease) and abiotic factors (e.g. fire)

An event that moves the community

away from its equilibrium state

Disturbance

Equilibrium state?

In Southeast = long-leaf pine forest In Midwest = tall grass prairie

Disturbance vs. Equilibrium

When disturbances are very frequent, then

equilibrium may never happen.

Some species specialize on disturbance

Furbish!s lousewort

Succession

the process by which a community moves towards

its equilibrium state following a disturbance

What is the relationship between disturbance and species diversity?

How does diversity vary with the frequency of " "

" disturbance?

How does species diversity vary with the time since disturbance?

How does species diversity vary with the magnitude of disturbance?

What explains total diversity in a community?

Keystone species can influence diversity

Equilibrium theory of island biogeography

Disturbance

Productivity

Productivity

amount of sunlight turned into primary producers

(plants, algae, etc), often estimated using biomass

of primary producers

Many empirical studies have found a hump-shaped

relationship between the productivity of a system

and the number of species in that system

Productivity vs Species richness

Biomass

Spec

ies

rich

nes

s

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

Productivity and Biodiversity

Num

ber

of sp

ecie

s

Biomass

Increase in

productivity

allows for

coexistence

of more

species

Competition

begins to

remove less

competitive

species

What explains total diversity in a community?

Keystone species can influence diversity

Equilibrium theory of island biogeography

Disturbance

Productivity

Habitat heterogeneity

Habitat heterogeneity and Biodiversity

Ecosystems with more heterogeneous habitats have

more potential niches, allowing the coexistence of more

species.

Habitat heterogeneityNum

ber

of sp

ecie

s

Habitat heterogeneity and Biodiversity

Foliage height

diversity

Bird

sp

ecie

s

div

ers

ity

-MacArtur and MacArthur (1961)

found that the bird diversity of a

habitat increased with the

complexity of the habitat’s

vegetation

-Similar relationships have been

demonstrated in other taxa

What explains total diversity in a community?

Keystone species can influence diversity

Equilibrium theory of island biogeography

Disturbance

Productivity

Habitat heterogeneity

Is biodiversity important for ecosystem structure and function?

Do ecosystems with high species diversity “function”

better?

Are ecosystems with high species diversity more

stable?

Species richness

Eco

syst

em funct

ion

Species richness

Eco

syst

em funct

ion

Species richness

Eco

syst

em funct

ion

Species richness

Eco

syst

em funct

ion

No relationship More diversity is more stable

Redundancy Idiosyncratic

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.

Do ecosystems with high species diversity

“function” better ?

What do the empirical data tell us?

1. Experiments in The Ecotron:

The Ecotron is facility designed to

establish simplified experimental

communities

Do ecosystems with high species diversity

“function” better ?

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.

Do ecosystems with high species diversity

“function” better ?

What do the empirical data tell us?

1. Experiments in The Ecotron

low

medium

high

Time

% C

hange in

vegeta

tion c

over

-High biodiversity communities

had denser canopies and higher

photosynthetic rates

-low diversity communities also

consumed less CO2

Are ecosystems with more species more stable?

Tilman and Downing (1994) Biodiversity and stability in grassland.

Nature 367: 363-365.

©!2006!Nature Publishing Group!

!

four years before imposition of drought, more diverse plots hadgreater temporal stability25. In combination, these two studiesindicate that there might be short-term variability in effects ofdiversity on ecosystem stability but that, in the long term, higherplant diversity causes greater ecosystem stability.Our results support the predictions of competition theory that

greater diversity leads to greater ecosystem stability and lower speciesstability8,19,22.This theory predicts that greater ecosystem stability athigher diversity can result from increasingly negative covariance inthe abundances of competing species at higher diversity19,22 (covari-ance effect), from the manner in which temporal variance in speciesabundances scales with abundance19,22,26,27 (statistical averaging orportfolio effect), and/or from the manner in which species abun-dances scale with diversity19,22 (overyielding effect; greater ecosystemtotal biomass at higher diversity).The covariance effect requires that total covariance (temporal

covariance in abundances for each pair of species, summed acrossall possible pairs of species) decline as diversity increases. However,regression showed no dependence of total covariance for 2001–05 onspecies number (F1, 161 ! 0.83, P ! 0.36). Our results thus do notsupport the covariance effect, a conclusion also reached in an earlieranalysis of non-experimental data22. The portfolio effect22,26 requirestemporal variance, s2, in the abundance of a species to scale with itsbiomass, m, as s 2 ! c 1m

z, with 1 , z , 2, which it did, withz ! 1.60. This supports the role of the portfolio effect in stabilizinghigher-diversity plots. In multiple regression, summed varianceswere a declining function of species number (F 1, 160 ! 15.7,P , 0.0001) and an increasing function of total biomass(F1, 160 ! 127, P , 0.0001), which also supports the portfolio effect.The overyielding effect requires22 that total plot biomass, B, increasewith diversity, which it did each year of the experiment. For 2001–05,plots containing 16 species had, on average, 180%more biomass thanmonocultures. In total, these analyses strongly indicate that greaterecosystem stability at higher diversity resulted from portfolio andoveryielding effects.Rapidly increasing human population and consumption, and

concomitant demand for food and energy, are making societyincreasingly dependent on services provided by remaining naturaland managed ecosystems28,29. Biodiversity experiments have shownthat greater numbers of plant species lead to a greater productionof biomass1–6. Our results show that the long-term stability of an

ecosystem service—the annual production of biomass and thus ofpotential biofuels and livestock fodder1–6—also depends on bio-diversity. Biodiversity can therefore be an important element forthe reliable and sustainable provisioning of ecosystem services.

METHODSExperimental design. In a 7-ha field at Cedar Creek Natural History Area,Minnesota, USA, we controlled the number of plant species in 168 plots, each9m £ 9m. Plots were randomly assigned to be seeded with 1, 2, 4, 8 or 16perennial grassland species, with 39, 35, 29, 30 and 35 replicates, respectively, ofthe diversity levels. The composition of each plot was randomly chosen from aset of 18 perennials (four C4 grasses, four C3 grasses, four legumes, four non-legume forbs and two woody species). All plots received 10 gm22 of seed in May1994 and 5 gm22 in May 1995, with seed mass divided equally between species.Treatments were maintained by weeding three or four times each year. Weedswere removed while still small, with care being taken to minimize any disturb-ance. Plots were burned annually in spring before growth began. Five woodymonocultures are not included in analyses because burning effectively elimi-nated woody species from multispecies plots. Plots were annually sampled inmid-August for aboveground living plant biomass by clipping, drying andweighing four parallel and evenly spaced 0.1m £ 3.0m vegetation strips perplot from 1996 to 1999 and four 0.1m £ 6.0m strips per plot from 2000 to 2005.Different locations were clipped each year. Biomass from one strip per plot wassorted to species from 2001 to 2005. The Shannon diversity index, H 0 , usedabundances of each species, planted or weedy, in each plot by means of estimatesof percentage cover for 1996–2000 (four 0.5m2 subplots per plot) and sortedbiomass for 2001–2005. See ref. 4 for further details.Sampling effort. To eliminate potential bias from different sampling efforts forthe first four in comparison with the last six years, for each of the last six yearstwo clipped strips per plot were randomly chosen for an analysis of ecosystemstability. The full data gave similarly significant and positive effects of diversityon all three measures of ecosystem stability.Detrending and other analyses. Detrending was done, for each plot, by meansof linear regression of annually measured plot biomass on the logarithm of yearand used all ten years of plot data. The logarithm of year provided a generallybetter fit than year; both gave similar results. The standard deviation, jd, ofresiduals for each regression measures detrended variation. The detrendedtemporal stability, Sd, of a plot was Sd ! m/jd. Each plot had a single detrendedstability value for the ten-year period. In contrast, when data were divided intoshorter intervals that did not require detrending, there were multiple values of S,calculated as S ! m/j, per plot. We divided the data either into two subsets, eachfive years in duration (1996–2000 and 2001–05) or into five subsets, each twoyears in duration (1996–97, 1998–99, 2000–01, 2002–03 and 2004–05). Thesetemporal sequences of S values for each plot were analysed with the use ofrepeated-measures MANOVA.

Received 20 December 2005; accepted 23 March 2006.

1. Naeem, S., Hakenson, K., Lawton, J. H., Crawley, M. J. & Thompson, L. J.Biodiversity and plant productivity in a model assemblage of plant species.Oikos 76, 259–-264 (1996).

2. Tilman, D., Wedin, D. & Knops, J. Productivity and sustainability influenced bybiodiversity in grassland ecosystems. Nature 379, 718–-720 (1996).

3. Hector, A. et al. Plant diversity and productivity experiments in Europeangrasslands. Science 286, 1123–-1127 (1999).

4. Tilman, D. et al. Diversity and productivity in a long-term grasslandexperiment. Science 294, 843–-845 (2001).

5. Hector, A., Bazeley-White, E., Loreau, M., Otway, S. & Schmid, B. Overyieldingin grassland communities: Testing the sampling effect hypothesis withreplicated biodiversity experiments. Ecol. Lett. 5, 502–-511 (2002).

6. Hooper, D. U. et al. Effects of biodiversity on ecosystem functioning: aconsensus of current knowledge. Ecol. Monogr. 75, 3–-35 (2005).

7. Elton, C. S. The Ecology of Invasions by Animals and Plants (Methuen, London,1958).

8. May, R. M. Stability and Complexity in Model Ecosystems (Princeton Univ. Press,Princeton, 1973).

9. McNaughton, S. J. Stability and diversity of ecological communities. Nature274, 251–-253 (1978).

10. Tilman, D. & Downing, J. A. Biodiversity and stability in grasslands. Nature 367,363–-365 (1994).

11. Tilman, D. Biodiversity: Population versus ecosystem stability. Ecology 77,350–-363 (1996).

12. Huston, M. A. Hidden treatments in ecological experiments: Re-evaluating theecosystem function of biodiversity. Oecologia 110, 449–-460 (1997).

13. Pfisterer, A. B. & Schmid, B. Diversity-dependent production can decrease thestability of ecosystem functioning. Nature 416, 84–-86 (2002).

Figure 3 | Effects of plant diversity on the relationship between meanbiomass and its temporal standard deviation. Each point shows the meanbiomass of a plot and its temporal standard deviation based on the fiveannual samples collected from 2001 to 2005. Data for each species-numbertreatment were fitted by a separate regression line. Data were not detrended.The results show that high-diversity treatments have lower temporalstandard deviations (lower risks) for a given mean biomass (return).

NATURE|Vol 441|1 June 2006 LETTERS

631

Are ecosystems with more species more stable?

Biomass varies less from

year to year in plots with

high species richness

Minnesota grassland plot experiment

Coe!cient of variation =standard deviation

mean

Are ecosystems with more species more stable?

Minnesota grassland plot experiment

Relationship between drought

resistance of vegetation in a

Minnesota grassland and plant

species richness prior to the

drought. Drought resistance was

measured as the log of the ratio

of plant biomass at the height of

the drought to plant biomass

before the drought. Data are

shown as means + SE (redrawn

from Tilman and Downing 1994).

Are ecosystems with more species more stable?

Minnesota grassland plot experiment: resource usage

Species richness

Res

ourc

e av

aila

bili

ty Tilman et al (1996, 1997) examined the effect of species diversity on productivity and soil nutrients.

Are ecosystems with more species more stable?

Minnesota grasslandplot experiment: resource usage

Plots with more species less nitrogen in their soil

lower resource availability

Are ecosystems with more species more stable?

Species richness

Res

ourc

e av

aila

bili

ty

Species richness

Res

ista

nce

to inv

asio

ns

Hypothetical relationship between species richness and invasion resistance

Species rich communities are less susceptible to invasion because theyuse more of the available resources.

Are ecosystems with more species more resistant to invaders?

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.

Is biodiversity important for ecosystem function?

Some studies show that species rich communities are more productive, but some other studies show alternative interpretation.

Some studies show that species rich communities are more stable and recover from disturbances faster and are less vulnerable to invasive species.

More studies are needed to allow generalizations beyond some model systems.