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The Influence of Functional Diversity and Composition on Ecosystem Processes

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Science  29 Aug 1997:
Vol. 277, Issue 5330, pp. 1300-1302
DOI: 10.1126/science.277.5330.1300

Abstract

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.

Although the organisms living in an ecosystem control its functioning (1-4), it has not been clear how much of this control is determined by the identities of the species present (4, 5), by the number of species present (2, 4, 6,7), by the number of different functional roles that these species represent (1, 2, 8), or by which functional roles are represented (4, 9). The effects of species or functional diversity are expected to increase with the magnitude of the differences among species or functional groups (10). These differences are also expected to influence the magnitude of the effects caused by compositional differences. However, the relative effects attributable to diversity versus composition are unclear.

We performed a field experiment in which plant species diversity (defined as number of plant species added to plots), functional diversity (defined as number of functional groups added to plots), and functional composition (defined as which functional groups were added to plots) were directly controlled (11). Our 289 plots, each 169 m2, were planted and weeded to have either 0, 1, 2, 4, 8, 16, or 32 perennial savanna-grassland species representing 0, 1, 2, 3, 4, or 5 plant functional groups. Grassland-savanna plants were classified into functional groups on the basis of intrinsic physiological and morphological differences, which influence differences in resource requirements, seasonality of growth, and life history. Legumes fix nitrogen, the major limiting nutrient at our site (7). Grasses with the three-carbon photosynthetic pathway (C3) grow best during the cool seasons and have higher tissue N than do grasses with the C4 pathway, which grow best during the warm season. Woody plants have high allocation to perennial stem and low growth rates, and forbs do not fix N and often have high allocation to seed.

When analyzed in separate univariate regressions, species diversity had significant effects on plant productivity (Fig.1A) and on three of five other response variables measured in the third year of study (12, 13, 14). Functional diversity significantly influenced plant productivity (Fig.1B) and all other variables (13, 14). Species diversity had a highly significant effect (P < 0.001) in a one-way multivariate analysis of variance (MANOVA) that included all six response variables, as did functional diversity in a similar MANOVA.

Figure 1

(A) Dependence of 1996 aboveground plant biomass (that is, productivity) (mean and SE) on the number of plant species seeded into the 289 plots. (B) Dependence of 1996 aboveground plant biomass on the number of functional groups seeded into each plot. Curves shown are simple asymptotic functions fitted to treatment means. More complex curves did not provide significantly better fits.

In multiple regressions of each of the six response variables on both species and functional diversity, functional diversity was significant in all six cases, but species diversity was not (Table1) (14). Plant productivity and plant total N significantly increased, and soil NO3, soil NH4, plant percent N (% N), and light penetration significantly decreased as functional diversity increased. A two-way MANOVA that included all six response variables showed highly significant effects of functional diversity (Wilk's lambdaF = 7.58; df = 6, 277; P < 0.0001) but no significant effects of species diversity (Wilk's lambdaF = 0.12; df = 6, 277; P = 0.99). Similar results were obtained in alternative analyses (14), including a two-way MANOVA that used observed species and functional diversities from 1996 (15). Thus, the functional group component of diversity is a greater determinant of ecosystem processes than the species component of diversity.

Table 1

Dependence of ecosystem variables on diversity treatments as determined by multiple regression. Values shown are regression parameters. A separate regression was performed for each ecosystem variable. Regressions have df = 2, 283 to 2, 286. NS,P > 0.05; *, 0.05 ≥ P > 0.01; **, 0.01 ≥ P > 0.001; and ***, P < 0.001 for tests of significant difference of parameter value from 0.

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The independent effects of functional composition can be tested by ANOVAs in which each of the 32 possible functional compositions (16) is nested within the appropriate level of functional diversity. There were highly significant effects of both functional diversity (Fig. 1B) and functional composition (Fig.2) on plant productivity, plant % N, plant total N, and light penetration (Table2). Soil NH4 and soil NO3 depended on functional diversity but not on functional composition. Thus, for four of the six variables, both functional composition and functional diversity had significant impacts. A two-way MANOVA that included all six variables found highly significant effects of both functional diversity and functional composition (14,17).

Figure 2

Effects of functional composition on 1996 aboveground plant biomass (productivity) in plots containing at least one legume species (Legume), at least one C4 grass species (C4 grass), at least one of each (C4 grass plus legume), or only species from other functional groups (Other). Mean and SE are shown, using all plots containing 1, 2, or 3 functional groups.

Table 2

Dependence of response variables on functional diversity treatments and functional composition based on ANOVAs. Functional composition was nested within each level of functional diversity. A separate analysis was performed for each ecosystem response variable.

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On average, across the six ANOVAs of Table 1, species and functional diversity together explained 8% of the variance in response variables, whereas functional composition and diversity together explained 37% (Table 2), suggesting that composition is the greater determinant of ecosystem processes.

To determine if particular functional groups were responsible for the effects of functional diversity, we repeated the multiple regressions of Table 1, but replaced functional diversity with five dummy variables, each describing a functional group as either absent from a plot or represented by at least one species. For each of the six ecosystem variables, there were significant (P < 0.05) effects of the presence of particular functional groups and no significant effects of species diversity. Only C4 grasses and legumes significantly affected productivity (Fig. 2) and light penetration (P < 0.001 for each, overall r2 = 0.19 for C4 grasses and 0.27 for legumes). Plant % N depended on all five functional groups (P < 0.05 for all, r2 = 0.57). The other ecosystem variables were significantly dependent only on either legumes (plant total N) or C4 grasses (soil NH4, soil NO3). On average, across plots containing two, four, or eight species, the presence of one or more C4 grass species led to a 40% increase in productivity, and the presence of one or more legume species led to a 59% increase. The greater biomass from legumes is consistent with their ability to fix N. The greater biomass from C4 grasses is consistent with their lower tissue N concentrations.

Another multiway MANOVA, in which the five independent variables were the species diversity within each functional group (number of plant species within a functional group planted in a plot) and the dependent variables were the six ecosystem responses, showed significant (P < 0.01) effects of species diversity within each functional group except woody plants. Thus, both the presence of some functional groups and the number of species within most functional groups had significant effects on ecosystem processes.

The increase in productivity with diversity was partially caused by overyielding of species, especially C4 grasses, in high-diversity plots. Specifically, a regression for each species of log(percent cover) on log(species richness) revealed significant (P < 0.05) overyielding at high species diversity (that is, slopes significantly less negative than –1) for 14 of the 34 species, but significant underyielding at high diversity for only four species. All eight C4 grasses significantly overyielded (Andropogon gerardi, Bouteloua curtipendula, B. gracilis, Buchloe dactyloides, Panicum virgatum, Schizachyrium scoparium, Sorghastrum nutans, and Sporobolus cryptandrus), as did the C3 grass Elymus canadensis, the legumesLespedeza capitata and Petalostemum villosum, the forb Aster azureus, and the woody plantsQuercus ellipsoidalis and Q. macrocarpa. Thus, many species inhibited themselves in monoculture and low-diversity plots more than they were inhibited by other species in high-diversity plots. This is consistent with several mechanisms of niche differentiation and coexistence (18), suggesting that such mechanisms may explain the increase in productivity with diversity (10).

Other studies have shown that the number of species (2, 6,7, 19), the number of functional groups (8), or ecosystem species composition (20, 21) influence various ecosystem processes. Our results show that composition and diversity are significant determinants of ecosystem processes in our grasslands. Given our classification of species into functional groups, functional diversity had greater impact on ecosystem processes than did species diversity. This suggests that the number of functionally different roles represented in an ecosystem may be a stronger determinant of ecosystem processes than the total number of species, per se. However, species diversity and functional diversity are correlated; each was significant by itself, as was species diversity within functional groups; and either species or functional diversity may provide a useful gauge of ecosystem functioning.

Our results show a large impact of composition on ecosystem processes. This means that factors that change ecosystem composition, such as invasion by novel organisms, nitrogen deposition, disturbance frequency, fragmentation, predator decimation, species extinctions, and alternative management practices (20, 21), are likely to strongly affect ecosystem processes. Our results demonstrate that all species are not equal. The loss or addition of species with certain functional traits may have a great impact, and others have little impact, on a particular ecosystem process, but different processes are likely to be affected by different species and functional groups.

  • * To whom correspondence should be addressed. E-mail: tilman{at}lter.umn.edu

REFERENCES AND NOTES

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