Download Biodiversity and Ecosystem Function

Biodiversity and ecosystem
functioning
Diane Srivastava, UBC Zoology
In the 20 years since the
Conventional on Biological
Diversity was signed at Rio
…what have we learnt about biodiversity loss?
…what have we done about biodiversity loss?
Ecological - Economic conflicts prior to Rio
Human activity:
• Habitat loss
• Pollution
• Overharvesting
ECOLOGICAL LOSS
ECONOMIC WIN
Conversion of
Ecological Capital to
Economic Capital
Biodiversity loss
Ethical and aesthetic costs
not measured in $
Economic growth
measured in $
how to compare?
Reasons to conserve species diversity:
• Ethical and aesthetic
• Evolutionary capital
• Functional importance in ecosystems
& ecosystem services to humans
Ecosystem functions are biological
processes that involve the flow of energy and
nutrients in, out and through food webs.
-
Carbon fixation
Water purification
Pollination
Decomposition
Pest suppression
Production of biomass
Nitrogen fixation
Energy flow through food webs
Ecosystem services are those ecosystem
functions that benefit humans.
The value of the world’s ecosystem services: $33 trillion
Costanza et al. 1997 Nature 387: 253-260.
Ecological - Economic conflicts post Rio
Human activity:
• Habitat loss
• Pollution
• Overharvesting
ECOLOGICAL LOSS
Biodiversity loss
Ecosystem service costs
measured in $
Global wetlands: $3.4 billion
ECONOMIC WIN
Conversion of
Ecological Capital to
Economic Capital
Economic growth
measured in $
can compare!
Example 1: Pollination
Ecosystem service value: $195 billion for global
agriculture
Pollinator diversity is declining globally (e.g local
diversity of native bees in Europe is half of pre-1980
values)
Distribution of bee-dependent plants is declining (e.g.
12- 22% in Europe since 1980s)
But does pollinator diversity really increase pollination?
Yes! Pollinator diversity increases
pollination and crop yield
Pumpkin, Indonesia
Coffee, Indonesia
Watermelon, California
Farms near forests in Costa Rica have 20% greater yield due to
higher native pollinator diversity, translating into $60,000 more
income for the farmer
Example 2: Water purification
Nitrate uptake
Ecosystem service value: New York City $7 billion
Algal species diversity
Example 3: Carbon fixation
Terrestrial plants sequester 2.6 x 109 g C per year
offsetting 30% of atmospheric carbon emissions
Ecosystem service value: set by carbon credits and
other instruments, globally in the trillions
So plants are valuable – but do we need so many species?
Yes! Most (86% of 272)
experiments
show
that plant
Random loss of species
can reduce
function
increases plant
(in this case,diversity
plant biomass)
production ( = C fixation)
Tilman, D. et al. 2001. Science 294: 843-5.
Example 4: Stability
Ecosystem service value: economic prosperity depends
on predictable rates of return on investment.
Marine systems:
Loss of biodiversity
concomitant with
increase in risks
Example 4: Stability
A system with no consumptive resistance:
Mountain pine beetle attack of monoculture forest
Overall – biodiversity increases
ecosystem functioning
Meta-analysis of 111 experiments (Cardinale, Srivastava
et al. Nature 2006)
Biomass production and resource consumption increases
with diversity of:
•
•
•
•
plants
detritivores
herbivores
predators
in both terrestrial and aquatic systems
Losing biodiversity is like losing rivets on an
airplane – eventually catastrophic failure
Plant production
Nutrient uptake
Decomposition
Mechanisms: (1) Niche complementarity
Competitive exclusion principle: Species need
to be different that each other in order to
coexist
Optimal stream flow
Species differ in their use of microhabitats and
resources, and so complement each other
functionally
Mayfly
Stonefly
Sensitivity to disturbance
Mechanisms: (2) Facilitation
• Species may help each other with
function. For example, clover fixes
nitrogen which grasses use for
biomass production.
• A field with both clover and grass may
therefore be more productive than one
with just one of these species.
• e.g.: Farmers often “intercrop” to get
higher yields.
Mechanisms: (3) Portfolio effect
• If species fluctuate independently, their net
biomass (or function) may not fluctuate much
as individual fluctuations may cancel each
other out.
• Thus more diverse communities may have
lower variability than depauperate
communities.
• Rationale taken from economics: to reduce
risk, investors “diversify” their stocks.
Problem # 1
All BDEF experiments are in small, homogeneous
plots or bottles. The real world might be different!
Problem # 1….is not that much of a problem
If anything, adding realistic amounts of spatial
heterogeneity strengthens the BDEF relationship
Increasing heterogeneity
Heterogeneous
streams
Homogeneous
streams
Problem # 2
Most BDEF experiments are based on random-loss.
But we know extinctions aren’t random!
Tend to lose first:
• Pollution sensitive species
• Rare species
• Large species
• Predators
Problem # 2….is not that much of a problem
Expected order of species loss usually exacerbates
the effects of species loss on ecosystem function*
Functionally important species often the most
vulnerable
Nutrient
release from
marine
sediments
Species richness
* 9 out of 11 studies that formally compare real and random-loss
scenarios, Duffy et al. 2009
Problem # 3
Sure, diverse ecosystems function better than the
average monoculture, but not better than the best
monoculture
We could optimize carbon sequestration by replacing
native forests with ecualyptus monocultures!
best
Problem # 3…true only for one function!
If we consider optimizing multiple ecosystem
functions, there is no “magic” monoculture
In the 20 years since the
Conventional on Biological
Diversity was signed at Rio
…what have we learnt about biodiversity loss?
High certainty about impacts for humanity
…what have we done about biodiversity loss?
Not enough, over to you today!