Diversity Productivity Relationships Species Richness Seminar October 21, 2003

Diversity Productivity Observational vs Experimental Two schools of thought Hypotheses of Camp 1 Criticisms of Camp 2

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 –1m 2 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 CO 2 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

Experimental Studies Hector et al. Hector et al (1999)

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

Two Schools Camp 2: Biodiversity per se does not increase productivity – other factors are involved –Huston et al –Wardle et al

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)