2: Community composition & species identity

Question 1

Describe the criticisms of species richness values

Answer 1

Species richness → no identity of species, just a number
Community therefore has no ‘composition’
But, ecosystem function is driven by characteristics of the species (traits)
Even in controlled BIODEPTH experiments, a single level of species richness produced a large range of productivity

Question 2

Why does species identity matter? + examples

Answer 2

Because organism traits drive ecosystem function

Plant traits, e.g:
Growth rate
Leaf thickness
Life form (tree, shrub)
leaf nitrogen content
flowering date …

Question 3

Describe Grimes’ Mass ratio hypothesis (1998)

Answer 3

• Controls on ecosystem function are in proportion to biomass/productivity of that species
  = what’s most abundant will have the biggest influence on an ecosystem
• Richness of subordinate/transient species is of little influence

Question 4

Describe Scherer-Lorenzen et al. (2003) biodepth study

Answer 4

→ looked at the amount of nitrate that was in the water leaching from grasslands
- Negative relationship between plant growth and soil nitrate richness
- Shows species trait important = legumes (nitrogen fixing)

Question 5

Does increased species richness decrease nitrate leaching?

Answer 5

→ more complex
Legumes having an overwhelming effect
= Shows importance of community composition

Question 6

Describe Phoenix et al. (2008) study into limestone grassland mesocosms

Answer 6

• Long-term, mature communities / real soil / locally sourced wild plants
• 1 / 3 functional groups, 1 / 4 species per functional group
• 1,3,4 or 12 species
• NO legumes
  = significant ‘community effects’ on nitrate leaching
  = Grasses appear to play a key role in stopping leaching

No effect of:
Species richness
Functional group diversity
Species per functional group

Question 7

Describe Tilman et al. (2006) study on Ecosystem stability

Answer 7

→ Minnesota large scale experimental grassland
Showed higher levels of ecosystem stability with higher biodiversity
Stability = biomass/temporal standard deviation in biomass

Question 8

Describe the covariance effect

Answer 8

More species = higher chance that changes in the biomass of one species will be counteracted by another species

Question 9

Describe the portfolio effect

Answer 9

If you’ve got more species, they can’t vary in biomass so much
At community level it’s more difficult to change their biomass year on year
In a community with low biodiversity, some species may have very large biomass

Question 10

Why does ecosystem stability have important implications

Answer 10

Important in the context of climate change, we need to conserve biodiversity so communities can be more resistant and stable in the face of climate change