Biodiversity and Ecosystem Function by yyc62487

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									Biodiversity and Ecosystem
         Function

      Dr. Mathew Williams
        What is biodiversity?
• OED: ―biodiversity Ecol., diversity of plant and
  animal life, as represented by the number of
  extant species‖
• Ricklefs & Miller: Biodiversity includes a
  number of different levels of variation in the
  natural world: genetic, species, ecosystem
• Begon et al. ―The term may be used to describe
  the number of species, the amount of genetic
  variation or the number of community types
  present in an area‖.
• But…most studies do focus on species diversity
Are Aspects of Ecosystem
Functioning Dependent on
      Biodiversity?
    Functional Consequences of
           Biodiversity:

Numbers and   Organismal   Ecosystem
Kinds of      traits       Processes
Species
      Traits and Ecosystem
             Function
• Traits may mediate energy and material
  flow directly
• Traits may alter abiotic conditions (limiting
  resources, disturbance, microclimate)
    Trait Expression Is
     Determined By:
•   Species richness
•   Species evenness
•   Species composition
•   Species interaction
•   Temporal and spatial variation
Photo: Arctic LTER
                                              Leaf area index
                         2.0           Pteridophyte
                                       Evergreen
 LAI, one-sided (m m )
-2




                                       Deciduous
                         1.5           Graminoid
2




                         1.0


                         0.5


                         0.0
                               5   1    2   H 94-4 3   4     6 HV 13 14 15 16 9
                         Barrens Wetland Heath MNT         Moist acidic tundra Shrubs
                                           2
                                           r =0.93
                                  3.2


Total foliar N (g m ground area
                              )
                                  2.8

                                  2.4

                                  2.0
-2




                                  1.6

                                  1.2

                                  0.8

                                  0.4

                                  0.0
                                     0.0       0.4   0.8   1.2    1.6   2.0   2.4
                                                            LAI
     The Ecosystem/Ecology
            Divide
• Key ecosystem types in Arctic tundra show clear
  differences in key species and functional types
• But at the ecosystem level there are clear patterns
  in the landscape irrespective of species
  composition
• Bulk measures like LAI and foliar N are good
  descriptors of process rates
• So, are species important?
        An exercise in pairs
• Identify specific examples of links between
  biodiversity and ecosystem function/process
      Species Richness and
      Ecosystem Function:
             Theory

• If niches are complementary, adding species
  could increase process rates linearly
• As niches overlap the response should
  saturate
Niche differentiation and productivity. a,
A simple model — the 'snowballs on the
barn' model — of niche differentiation
and coexistence. The range of conditions
in which each species can exist is shown
with a circle, the position of which is
defined by its centre. By randomly
choosing locations for various numbers
of circles (species), it is possible to
calculate the effect of diversity on the
'coverage' of the heterogeneous habitat.
The amount of such coverage is
proportional to community biomass. b,
Results of simulations (triangles) and of
an analytical solution (solid curve) to the
effects of diversity on community
productivity for the snowballs on the
barn model

From: Tilman (2000), Nature.
Tilman‘s biodiversity experiment
(A) Dependence of
    1996 aboveground plant biomass
    (that is, productivity) (mean and
    SE) on the number of plant
    species seeded into the 289 plots.



(B) Dependence of 1996 above-
    ground plant biomass on the
    number of functional groups
    seeded into each plot. Curves
    shown are simple asymptotic
    functions fitted to treatment
    means. More complex curves did
    not provide significantly better
    fits

From: Tilman et al. (1997) Science
     Problems With Richness
           Experiments
• Disentangling interactions in natural systems is
  difficult
• Measuring productivity (below ground)
• Scale (too short and too small)
• ‗Sampling effect‘ problem in constructed
  communities
   – Sampling effect may be an important biological
     property or an experimental artefact if natural
     community assembly rules are broken
Hypothesized mechanisms
involved in biodiversity
experiments using synthetic
communities. Sampling effects
are involved in community
assembly, such that communities
that have more species have a
greater probability of containing a
higher phenotypic trait diversity.
Phenotypic diversity then maps
onto ecosystem processes through
two main mechanisms:
dominance of species with
particular traits, and
complementarity among species
with different traits. Intermediate
scenarios involve
complementarity among
particular species or functional
groups or, equivalently,
dominance of particular subsets
of complementary species.

From: Loreau et al (2001)
Science
      Richness Conclusions
• With our knowledge now, we cannot reject
  the hypothesis that a few dominant species
  suffice to provide the functional diversity
  that is necessary to explain the level of
  primary production observed in grassland
  ecosystems at the small spatial and temporal
  scales considered in recent experiments.
         Species Evenness
• Human effects on species more commonly
  involve alteration of relative abundance
  than extinction
• Little research on importance of evenness of
  function so far
• Future richness experiments should include
  evenness effects
      Species Composition:
• Species mediate pathways of energy
  and material flow
• Examples: Introduced species can alter
  patterns of ecosystem processes
    Introduced Species Can Alter
  Patterns of Ecosystem Processes I

• Introduction of N-fixing tree Myrica faya to
  N-limited Hawaiian forests led to 5-fold
  increase in N inputs
• Significant impacts on forest structure and
  function


                        Vitousek et al. (1987) Science
    Introduced Species Can Alter
 Patterns of Ecosystem Processes II
• Introduction of deep-rooted salt cedar
  (Tamarix sp.) to Mojave and Sonaran
  deserts resulted in:
  – Increased water accessed by vegetation
  – Increased surface litter and salts
  – Inhibited many native species, reduced
    biodiversity

                                 Berry (1970)
   Introduced Species Can Alter
Patterns of Ecosystem Processes III

• Introduction of Agropyron cristatum,
  tussock grass, to US Great Plains
  – Reduced allocation to roots compared to
    native grasses
  – Soil N levels reduced, and 25% less total
    soil C compared to native prairie soil


                           •Christian & Wilson (1999)
   Introduced Species Can Alter
Patterns of Ecosystem Processes IV
• Introduction of Bromus tectorum,
  cheatgrass, to western US
  – Fire frequency increased by a factor of 10 in
    the >40 million ha it now dominates




                             •Whisenant (1990)
     CASE STUDY:
Diversity of tropical trees
and carbon water relations
   Exploring compositional effects on
          ecosystem function

• Are Amazonian trees
  water stressed?
• Does water stress
  depend on soil texture?
• Does water stress
  depend on species?
Ecological
Diversity in
Amazonian
Rain Forest
Diversity in Rooting Depths
                                                              Dawn water potential along
                                       2.0
                                                                a soil texture gradient
  Dawn leaf water potential (MPa -1)

                                       1.5
                               x




                                       1.0



                                       0.5



                                       0.0



                                       -0.5
                                               3   1      4    5   6   9   10     2   11   12   7   8    13
                                              High clay                                         High sand
                                                                           Site
                                                                                                    Williams et al (2002)
Species Interactions
   • Mutualism
   • Trophic interaction
     – Predation
     – Parasitism
     – Herbivory
   • Competition
                Mutualism
• N-fixation in plant-microbe symbiosis
• Plant-mycorrhizal associations
  – Both increase production and accelerate
    succession
• Decomposition is driven by highly
  integrated consortia of microbes
       CASE STUDY
 Mycorrhizal fungal diversity
determines plant biodiversity,
  ecosystem variability and
        productivity
             Background
• arbuscular mycorrhizal fungi (AMF) form
  mutualistic symbiotic associations with
  the roots of 80% of all terrestrial plant
  species, thereby acting as extensions of
  plant root systems and increasing
  nutrient uptake, especially of
  phosphorus
• Communities vary in AMF biodiversity
      Experimental design
• Expt 1: Greenhouse experiment, 48
  microcosms simulating European
  calcareous grassland
  – Quantified plant growth with no AMF or
    different diversities of AMF
• Expt 2: Field experiment, 70
  macrocosms simulating N. American old
  fields.
  – 15 plant species per plot, random mix of
    different numbers of species (out of 23)
    AMF, quantified primary productivity
Experiment 1   van der Heijden 1998
Experiment 2   van der Heijden 1998
   More diverse microbial
communities are more efficient
                  Soil microbial functional diversity
                  (Shannon index H') and metabolic
                  quotient (qCO2 = soil basal
                  respiration/soil microbial biomass)
                  correlate inversely. A higher
                  diversity in the organic plots is
                  related to a lower qCO2, indicating
                  greater energy efficiency of the more
                  diverse microbial community. The
                  Shannon index is significantly
                  different between both conventional
                  systems (CONFYM, CONMIN) and
                  the BIODYN system, the qCO2,
                  between CONMIN and BIODYN
                  (P < 0.05).
                                     Maeder 2002
        Trophic Interactions
• Modify fluxes of energy and materials
• Influence abundance of species that control
  these fluxes
  – e.g., predator removal can lead to a cascade of
    ecological effects
(A) Changes in sea otter abundance
over time at several islands in the
Aleutian archipelago and concurrent
changes in (B) sea urchin biomass,
(C) grazing intensity, and (D) kelp
density measured from kelp forests at
Adak Island. Error bars in (B) and (C)
indicate 1 SE. The proposed
mechanisms of change are portrayed
in the marginal cartoons--the one on
the left shows how the kelp forest
ecosystem was organized before the
sea otter's decline and the one on the
right shows how this ecosystem
changed with the addition of killer
whales as an apex predator. Heavy
arrows represent strong trophic
interactions; light arrows represent
weak interactions.

        Estes et al. (1998) Science
Experimental Lakes
•Two lakes dominated by
Zooplanktivorous
fishes(minnows)
•And two other lakes
dominated by Piscivorous
fish (bass)
•Of each pair, one was
fertilized with N+P while
the other was left as a
control




Schindler et al (1997) Science
bass + P  minnows, grazers, algae,
lake shifts from C source to sink
                          Comparison of crustacean
                          grazer length, primary
                          production, and DPCO2
                          [PCO2(lake) - PCO2(air)] before,
                          during, and after a period of low
                          minnow abundance in Peter
                          Lake (1994 to 1995). Data are
                          shown as the mean ± SD.
                          Sample sizes are given in
                          parentheses. Dotted line in
                          bottom panel represents
                          dissolved CO2 in equilibrium
                          with the atmosphere
                               Schindler et al (1997) Science
       Trophic Interactions:
           Conclusions
• All types of organisms must be considered
  in understanding biodiversity effects
• Interactions among species must be
  considered
• Changes in interactions can alter traits
  expressed by species, so presence/absence
  of species is insufficient to predict impact
  Biodiversity and Ecosystem
           Services
• Ecosystem services are defined as the processes
  and conditions of natural ecosystems that support
  human activity and sustain human life
• E.g., maintenance of soil fertility, climate
  regulation, natural pest control
• E.g., flows of ecosystem goods such as food,
  timber and freshwater
Attaching Value to Biodiversity
• Techniques used include direct valuation
  based on market prices, and estimates of
  what individuals are willing to pay to
  protect endangered wildlife
• Valuation of marginal losses that
  accompany specific biodiversity changes
  are most relevant to policy decisions
• Predictions are highly uncertain
What you should have learned
           today
• The ways in which biodiversity can affect
  ecosystem function
• Experimental approaches and the sampling
  effect
• Examples of species traits that control
  particular processes
• The concept of ecosystem services and
  valuation
                  References
• Chapin et al (2000) Consequences of changing
  biodiversity. Nature 405: 234-242
• Tilman, Wedin and Knops (1996) Productivity and
  sustainability influenced by biodiversity in grassland
  ecosystems. Nature 379: 718-720
• Naeem & Li (1997) Biodiversity enhances ecosystem
  reliability. Nature 390:507-509
• Van der Heijden et al. (1998) Nature 396: 69-72
• Constanza et al (1997) The value of the world's
  ecosystem services and natural capital. Nature 387:
  253-260
• Maeder et al (2002) Soil fertility and biodiversity in
  organic farming. Science 296: 1694-7
        Reading for next week
• Pfisterer. A.B. & B. Schmid. 2002. Diversity-dependent
  production can decrease the stability of ecosystem
  functioning. Nature 416 84-86
   – what insights does this experiment provide?
   – what are the criticisms of the approach?
• McCann, K., A. Hastings G. R. Huxel. 1998. Weak trophic
  interactions and the balance of nature. Nature 395 794-8
   – what insights does the modelling provide?
   – what are the criticisms of the approach?

								
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