Plant diversity and ecosystem function Chapter 13

Why do we care about biodiversity? Chapin III et al. 2000

How does biodiversity affect ecosystem function (or vice versa)? Regional species pool Local community structure Biotic factors Evolutionary processes Physiological constraints Historical events Dispersal abilities Habitat selection From Morin (2011) Abiotic factors Interspecific competition Herbivory Facilitation, mutualism Ecosystem function Productivity Nutrient retention Soil CO 2 flux Ecosystem function Productivity Nutrient retention Soil CO 2 flux

Diversity Alpha: # species at the local scale Beta (β): Difference between communities, species turnover Gamma (γ): # species at regional scale γ = β x α VS. HIGH LOW VS. α:α: β:β: Whittaker 1960

How many species can be lost before ecosystem function is impacted? Loreau et al. 2001

How many species can be lost before ecosystem function is impacted? Tilman and Downing (1996) Grassland in Minnesota Looked at drought resistance of plant communities with different levels of richness

How many species can be lost before ecosystem function is impacted? Naeem et al. (1994) 9, 15, and 31 species treatments – distributed among trophic levels Ecotron, UK

Naeem et al High diversity communities consumed more CO 2 and were more productive (greater light interception)

Ways in which biodiversity can affect ecosystem function Productivity - how much carbon is assimilated Stability - how much productivity fluctuates under different environmental conditions Invasibility - indirectly affects ecosystem processes

Biodiversity and Productivity Scale dependant Potential Mechanisms – Productivity and size correlate (Oksanen 1996) – Resource / energy constraints (Preston 1962, Wright et al. 1993) – Competition and dominant species (Grime 1973) – “Paradox of enrichment” (Rosenzweig 1971) – Reordering causes short term diversity losses

Biodiversity and Productivity - Neutral Oksanen 1996 Journal of Ecology

Biodiversity and Productivity – “Paradox of enrichment” With fertilization, productivity is generally increased, but species richness declines Speculated to be due to changes in which species is dominating the system Stevens and Carson 1999

Biodiversity and Productivity Diversity (H’) Knapp et al. 2002

Biodiversity and Productivity Found positive relationship between species richness and productivity in US and across Europe Loreau et al. 2001

Biodiversity and Productivity – “It depends” Adler et al. 2011

Biodiversity and Productivity - Scale Mittelbach et al On a regional scale, productivity and diversity were correlated in many more systems than at smaller scales

Biodiversity and Productivity The number of functional groups were shown to be more important than species richness (in this study at least) Hector et al. 1999

Biodiversity and Ecosystem Stability Dynamic stability – System returns to its “original” state after small perturbations – Robert May (1973) thought that increased diversity would actually lead to decreased dynamic stability – New theoretical models suggest that increased diversity will not increase dynamic stability but might not decrease it either.

Biodiversity and Ecosystem Stability Stability - How much does a system differ from one year to the next (usually in terms of productivity)? – Coefficient of variation – Stability – signal to noise ratio C v = σ/µ Stability = µ /σ

Biodiversity and Ecosystem Stability Signal to noise ratio Tilman et al Coefficient of variation Tilman et al. 1996

Biodiversity and Invasion Kennedy et al. 2002

Biodiversity and Invasion Native richness can be positively correlated with exotic species richness Observational pattern – No causation Resource rich vs. resource poor Beta diversity Stohlgren et al. 1999

Biodiversity and Invasion Environmental heterogeneity can influence invasion (Davies et al. 2005) Suggests beta diversity may be just as, if not more important in controlling exotic species Photo credit: Koerner, S.

Biodiversity and Invasion Invasive species can then have drastic effects on ecosystems – Reductions in biodiversity – Drastic changes to soil nutrient cycling (Ehrenfeld 2003) – Can change disturbance regimes (D’antonio and Vitousek 1992, Mack and D’antonio 1998)

Exercise 1 Dependent variable: Aboveground biomass (productivity) independant variable: Species richness Come up with a hypothesis Questions: 1.Describe the relationship between the number of plant species and plant biomass 2.How does the relationship between biomass and species richness change over time? 3.How does it stay the same? 4.How do the error bars change the way you interpret these results? Fargione and Tilman 2004 Teaching Issues and Experiments in Ecology (TIEE)

Sampling effect hypothesis With random assembly: – Higher diversity results in a higher chance that there will exist a dominant species in the community, thus increasing productivity. Loreau et al. 2001

Exercise 2 Dependant variable: % plots exceeding biomass of the monoculture plots Independent variable: Species richness Articulate hypothesis (Sampling effect) 1.Do the data support or reject the hypothesis? Does the answer depend on the year? 2.Why might a diverse plot contain more biomass than even the highest monoculture plot? Why might two species be better than one when it comes to biomass production? Fargione and Tilman 2004 Teaching Issues and Experiments in Ecology (TIEE)

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