Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Biodiversity per se does not increase productivity – other factors are
Document related concepts
no text concepts found
Transcript
Diversity Productivity Relationships Species Richness Seminar October 21, 2003 Observational Studies • Generally see a negative relationship between diversity and productivity – Fynbos, chalk grasslands: ↓Prod, ↑Diversity – Cattails, cornfields: ↑Prod, ↓Diversity • Mittelbach et al (2001) - Summary of trends in natural systems – Unimodal (hump-shaped) curve • Contradicts results of experimental studies Observational Studies • Hump-shaped curve – most common pattern found for vascular plants Gurevitch, Scheiner, and Fox 2002 We will… • Focus on experimental studies • Focus on studies that manipulate diversity Why is this interesting? • To reconcile contradictory patterns found in natural and experimental systems • To assess the impacts of species loss on ecosystem function • To assess the impacts of climate change and fragmentation on ecosystem function Two Schools • Camp 1: An increase in biodiversity causes an increases productivity – Naeem et al: Ecotron, UK – Tilman et al: Cedar Creek, MN – Hector et al: Europe Two Schools • Camp 1: An increase in biodiversity causes an increases productivity – Naeem et al: Ecotron, UK – Tilman et al: Cedar Creek, MN – Hector et al: Europe • Camp 2: Biodiversity per se does not increase productivity – other factors are involved – Huston et al – Wardle et al Camp 1: Ecotron • Naeem et al (1994): Ecotron experiments – 1m2 plots – Manipulated diversity (9, 15, 31spp) with low diversity plots having a subset of species in high diversity plots, 4 trophic levels – Measured several ecosystem processes • Respiration, Decomposition, Nutrient and water retention, Plant productivity Camp 1: Ecotron • Naeem et al (1994): Ecotron experiments – Results => loss of biodiversity resulted in impairment of ecosystem processes: • Higher diversity plots consumed more CO2 • Higher diversity plots had higher plant productivity Camp 1: Tilman • Tilman (1999): Camp 1: Tilman High diversity plots were more productive than the best monocultures Graphic from John Bruno Tilman’s study of the effect of plant diversity on productivity Manipulated plant diversity (0 to 16 species) by adding seeds and extensive weeding Measured productivity as biomass (above and below ground) Slide from John Bruno Camp 1: Hector et al • Eight field sites across Europe with replicates at each site • Established plots that varied in species richness • Overall result => loss of average aboveground biomass as diversity is lost • Assemblages with fewer functional groups had lower productivity Claims that “a single general relationship” may exist between species richness and productivity Hypotheses • Niche Complementarity • Facilitation • Sampling Effect Hypotheses • Niche Complementarity – Species are able to use different resources, or use the same resources in different ways – More species results in more efficient use of available resources, thus increasing overall productivity Hypotheses • Facilitation – Species impart a beneficial effect on each other such that productivity of a species in the mixture will be higher than that of the species grown in monoculture Hypotheses • Sampling Effect – The more species there are, the higher the chance of including highly productive species Common Criticisms from Camp 2 • Experimental design – Soil heterogeneity – Control for over yielding • Interpretation of hypotheses Common Criticisms from Camp 2 • Soil heterogeneity Hector et al (1999) Common Criticisms from Camp 2 • No control for over yielding – two of the sites contained more species than were grown in monoculture Fridley (2001) Common Criticisms from Camp 2 • Interpretation of hypotheses – Niche complementarity and facilitation are dependent upon species composition, not diversity Common Criticisms from Camp 2 • Interpretation of hypotheses – Niche complementarity and facilitation are dependent upon species composition, not diversity – The sampling effect is a hidden treatment, and therefore an artifact of experimental design, not a mechanism Common Criticisms from Camp 2 • Interpretation of hypotheses – Niche complementarity and facilitation are dependent upon species composition, not diversity – The sampling effect is a hidden treatment, and therefore an artifact of experimental design, not a mechanism – The sampling effect assumes that natural communities are randomly assembled Common Criticisms from Camp 2 • Interpretation of hypotheses – Niche complementarity and facilitation are dependent upon species composition, not diversity – The sampling effect is a hidden treatment, and therefore an artifact of experimental design, not a mechanism – The sampling effect assumes that natural communities are randomly assembled => Tilman (1999) beginning to accept alternative mechanisms to explain productivity, but still defines them as effects of diversity Questions • How transferable are experimental results to natural systems? e.g. Sampling Effect? Questions • How transferable are experimental results to natural systems? e.g. Sampling Effect? • Why focus on productivity as the response to diversity in natural systems? Questions • How transferable are experimental results to natural systems? e.g. Sampling Effect? • Why focus on productivity as the response to diversity in natural systems? • Consequences of the reverse: Does increasing productivity decrease diversity? (Rosenzweig’s Paradox of Enrichment)
