Biodiversity and Ecosystem Function

1. Biodiversity and Ecosystem Function

2. Species diversity is determined by: • Richness (number of species) • Evenness (relative balance among species)

3. What factors affect species diversity? • Time (ecological: for dispersal/colonization; evolutionary: for speciation/niche differentiation) • Spatial heterogeneity: multiple (micro)habitats • Resource competition (promotes specialization to avoid overlap in resource requirements) • Predation (prevention of dominance by any particular prey species) • Productivity: too much, too little, or just right? • Disturbance: non-equilibrium community dynamics; a mosiac of successional stages

4. Species diversity as a function of stress and disturbance intensity

5. How does species diversity affect ecosystem function? An experimental approach involving long-term plots

6. Cedar Creek LTER Plots(www.cedarcreek.umn.edu/about/history/)

7. Cedar Creek LTER Plots(http://www.cedarcreek.umn.edu/research/exper/e120/)

8. Productivity as a function of species diversity(Tilman et al., 2001, Science 294: 843-845)

9. Productivity as a function of functional group identity and diversity Tilman et al., 1997, Science 277: 1300-1302.

10. Plant cover as a function of species richness Tilman, Wedin and Knops, 1996, Nature 379: 718-720.

11. Species richness effects on soil nitrate concentration Tilman, Wedin and Knops, 1996, Nature 379: 718-720.

12. Species richness effects on plant cover and soil nitrate concentration in native prairie Tilman, Wedin and Knops, 1996, Nature 379: 718-720.

13. Species richness, functional composition, and functional diversity (Tilman et al., 1997, Science 277: 1300-1302) • Comparisons of the effects of diversity in experimentally manipulated plots at an N-limited, sandy site. • 5 functional groups were present: legumes, non-legume forbs, woody plants, C3 grasses, C4 grasses. • Functional composition (which groups were added) and functional diversity (no. of groups added) were the principal factors explaining N uptake, light interception, and plant productivity. Legumes + C4 grasses are winners.

14. Doesbiodiversity increase productivity? • “Virtually all” diversity-productivity experiments show that productivity increases with increasing species diversity. • Yet, there are many high productivity ecosystems with very low plant diversity (e.g., saltwater and freshwater marshes, bamboo forests, Pacific coniferous forests, eucalypt forests). • Conversely, there are many species-rich systems that have low levels of productivity (e.g., Mediterranean shrublands, chalk grasslands).

15. Why doesn’t the real world conform to results of the biodiversity–productivity experiments? • The biodiversity–productivity relationship may vary depending on the potential productivity of an environment. • Grime’s “hump-backed” productivity–species diversity curve. • Huston et al.: Climate and soil conditions play a much stronger role in determining productivity than does species diversity.

16. Artifacts of multi-species experiments(Huston et al., 2002) • Species-rich assemblages have a better chance of containing the one (or few) species that are most productive in a given environment. • By chance, experiments often don’t include all combinations of low- and intermediate-diversity assemblages, some of which might be highly productive. • Species-rich assemblages respond more noticeably to variation in soil conditions than do species-poor assemblages.

17. Doesbiodiversity increase stability? (Tilman & Downing, 1994, Nature 367: 363–365) • Stability has two components: resistance to perturbation, and speed of recovery following disturbance. • Net primary productivity in more diverse experimental plant communities was more resistant to, and recovered more quickly from, a major drought (1987-1988). • During the drought, the most species-rich plots produced ~50% of their pre-drought levels, whereas the most species-poor plots produced only ~12%.

18. Species richness effects on ecosystem and species stability Tilman, Reich & Knops, 2006, Nature 441: 629-632.

19. Are species-rich communities less ‘invasible’?

20. Observational (i.e. non-manipulated) studies present equivocal evidence for a biodiversity–resistance to invasion relationship. • Is the lack of a clear signal due to “co-varying extrinsic factors”? (soil disturbance; climate; fire; grazing; light, water, and nutrient levels) • What happens when extrinsic variation is controlled?

21. Species richness effects on ‘invader’ productivity “High diversity increases the competitive environment of communities and makes them more difficult to invade.” Naeem et al., 2000, Oikos 91: 97-108.

22. Species richness effects on ‘invader’ productivity and diversity Fargione & Tilman, 2005, Ecol. Letters 8: 604-611.

23. Root biomass effects on ‘invader’ productivity Fargione & Tilman, 2005, Ecol. Letters 8: 604-611.

24. The number of “external invading” plant species and the biomass of invading species were lower in high-diversity plots than in low-diversity plots. More effective exclusion of invading plants was correlated with greater light interception and N uptake by the community of sown species.

25. Does diversity increase both productivity and ‘sustainability’? • Comparisons of the effects of species richness in experimentally manipulated plots and adjacent grassland. • As species diversity increased, plant cover increased, N uptake increased, N leaching decreased, invasion by unwanted species decreased, and total productivity increased. • Does increased resource use efficiency equal sustainability?