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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.