Transcript Biodiversity per se does not increase productivity – other factors are

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)