Biodiversity and ecosystem function

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Biodiversity and ecosystem function
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Biodiversity

• BiodiversityThe variety of organisms considered
  at all levels, from genetic variants belonging to
  the same species through arrays of species to
  arrays of genera, families, and still higher
  taxonomic levels
• E.O. Wilson. The diversity of life. 1992.

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• Often biodiversity is measured by numbers of
  species. I will use that accounting of
  biodiversity today, keeping in mind that there
  are other ways to quantify biodiversity.
• The number of species is typically called species
  richness

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Patrones de Biodiversidad--más especies cerca del
ecuador
Números aproximados de especies residentes (que
reproducen en el lugar)
Para más información sobre el proyecto de aves en
parcelas de restauración comuniquese con
Catherine Lindell, Michigan State University,
lindellc_at_msu.edu, o con Zak Zahawi, 773-4004
Karen Holl, kholl_at_ucsc.edu
Cara Blanca
Trepador
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Patterns of species richness (there are often
exceptions)

• More species at lower latitudes
• More species at lower altitudes
• More species in wetter areas
• More species in areas with high habitat
  heterogeneity (for example a forest compared to a
  grassland)
• More species in less disturbed areas

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Pressing question in ecology today

• Does greater species richness mean there is a
  higher level of ecosystem function?
• In other words, would a tropical rainforest
  function the same with1000 individuals of 3
  different tree species as it does with 1000
  individuals of 200 different tree species?

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• If we lose species, does that matter in the grand
  functional scheme of things?

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• This is a particularly important question because
  so many of the worlds species are in danger of
  extinction (25 of the worlds mammal species are
  endangered)
• Will less species-rich communities function the
  same as the species-rich communities they
  originally were?

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Notes Data refer to globally extinct species,
not national or geographic populations. 91 fish
species were listed as extinct in the wild in
1996 this table includes 50 Lake Victoria
cichlids all treated here as becoming extinct
during the 1980s, and 31 other species for which
estimated dates are available. A further 10
species could not be assigned to a decade.
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What are ecosystem functions?

• Can be measured in different ways
• 1. Productivityamount of biomass produced per
  unit area per unit time by any class of organisms
• Higher productivity is considered to be evidence
  of higher ecosystem functioning

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What are ecosystem functions?

• 2. Decomposition rates
• 3. Species interactions, including pollination,
  seed dispersal, regulation of herbivorous insects
• 4. C02 consumption

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What are ecosystem functions?

• 5. Resistance to disturbances
• 6. Resilience (rate of recovery from
  disturbances)
• There are others

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Null hypothesis increased species richness does
not affect ecosystem function Rivet hypothesis
increased species richness leads to increased
ecosystem function Redundancy hypothesis a
minimum number of species is necessary for
maximum ecosystem function Idiosyncratic
hypothesis the relationship between species
richness and ecosystem function is
unpredictable Naeem et al. 1995
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Early work

• Study 1--Tilman and Downing 1994field experiment
• Tested idea that more species-rich plant
  communities would be more resistant to drought
  that less species-rich communities
• Reasoning is that with more species, at least a
  few are likely to thrive during drought,
  compensating for species that dont thrive

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Tilman and Downing (1994)

• Minnesota grasslands
• Experimental plots have different plant species
  richness because of different levels of nitrogen
  addition (nitrogen is a limiting nutrient)

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• Investigators compared pre-drought plant
  community biomass with peak-drought community
  biomass

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Results

• Most species-rich plots had about ½ the
  pre-drought biomass during peak drought
• Least species-rich plots had about 1/8 the
  pre-drought biomass during peak drought

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• Species-rich plots were more likely to contain
  some drought-resistant species than species-poor
  plots

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• Species-rich plots also returned to their
  pre-drought biomass more quickly than
  species-poor plots

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Conclusion

• Species-rich communities are more resistant and
  resilient to disturbance than species-poor
  communities

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Caveat

• One criticism of this work is that nitrogen
  levels varied with species richness levels (to
  induce species richness differences in the first
  place)so is increased function in high species
  richness communities due to species richness or
  nitrogen?

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Study 2--Naeem et al. 1995mecocosm experiment

• Worked with experimental mesocosms in the Ecotron
• Mesocosms are experimental chambers with quite
  tight control over environmental variables. In
  this experiment chambers were 2m x 2m x 2m

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• Tested idea that experimental mesocosms with
  greater species richness had greater
• community respiration
• productivity
• decomposition
• nutrient retention
• water retention

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• Communities composed of
• herbaceous plants (producers)
• herbivorous molluscs and phloem-sucking insects
  (primary consumers)
• parasitoids (secondary consumers)
• earthworms, springtails, microbes (decomposers)

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• Communities had 9, 16, or 31 plant and animals
  species
• Monitored for 206 days

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• community respiration (C02 consumption)
• productivity (leaf area index)
• decomposition (change in litter on soil surface
  and sticks below ground)
• nutrient retention (analysis of N, P, K in soil)
• water retention (water outflow)

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Results for high species-richness treatments
compared to lower species-richness treatments

• community respiration
• productivity--
• decomposition-no pattern
• nutrient retention--P and K in high-richness,
  patterns for other nutrients variable
• water retentionno pattern

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• Greater C02 consumption and productivity likely
  result from greater light interception in high
  species richness treatments because of a greater
  variety of leaf forms

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West Sumatran forest
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Conclusion

• Declining species richness may alter some
  ecosystem processes

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Caveats

• Experimental systems with limited number of
  species
• Run for less than a year

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Study 3--Snyder et al. 2006agricultural field
experiment

• Does increased predator species richness lead to
  increased control of herbivorous insects?
• Herbivorous insects eat plants

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• Collard plants are eaten by green peach aphids
  and cabbage aphids
• Aphids have predators-- two bugs, two species of
  ladybird beetles, and are parasitized by a wasp

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• 2m x 2m cages were established in fields with
  collard plants in each cage (bottom of cages
  buried in soil)
• 80 aphids were released in each cage

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3 treatments for cages with collard plants and
aphids

• no predators added
• one predator added
• four predators added
• 48 predators added for the two predator treatments

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• number of aphids sampled several times during
  28-day experiment
• collard plants mature after 28 daysbiomass
  determined
• predators counted after 28 days as well

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Results

• Green peach aphid abundance was lower in predator
  treatments compared to controls
• Differences in green peach aphid abundance
  existed between some of the one-predator and
  four-predator treatments

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Results

• However, when one of the ladybird beetles or the
  parasitoid wasp was the one-predator in the
  one-predator treatment, there was no difference
  between green peach aphid abundance in the one-
  and four-predator treatments

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Results

• Cabbage aphid abundance was lower in predator
  treatments compared to controls
• Cabbage aphid abundance lower in four-predator
  vs. one-predator treatments

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Densities of (a) green peach aphids and (b)
cabbage aphids, through time for three predator
diversity treatments no predator control
(control), one predator species in monoculture
(mono), or a diverse polyculture of four predator
species (diverse). Error bars represent mean  1
SE.
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Results

• Collard biomass greater in predator treatments
  than no-predator treatments
• No difference between collard biomass in two
  predator treatments

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Collard yields vs. mean total aphid densities.
Circles are no predator control, triangles are
one-predator treatment, and diamonds are
four-predator treatment.



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Conclusion

• Greater predator species richness provides
  greater control of herbivores in most
  situationsperhaps greater species richness
  provides insurance.
• The insurance idea is that more species-rich
  communities are more likely to have at least some
  species that do well under any given set of
  environmental conditions. When some type of
  disturbance or perturbation arises, e.g. a pest
  outbreak, some species will be able to respond
  and protect the integrity of the community

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Caveats

• Short time-scale
• Simplified prey base (only one species of plant
  examinedmultiple plant species planted together
  may have produced different results)

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Do more diverse bird communities prevent pest
outbreaks? (Does greater species richness lead to
greater ecosystem function?)

• Perfecto et al. 2004. Greater predation in shaded
  coffee farms the role of resident neotropical
  birds. Ecology 852677-2681.

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Table 1. Vegetation characteristics and bird
densities and richness at two large coffee farms
in Tapachula, Chiapas, Mexico
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Fig. 1. Proportion of initially presented
larvae remaining vs. time at two large coffee
farms in Mexico, Finca Irlanda, with diverse
shade ( 200 tree species), and Finca Hamburgo,
with lower monodominant shade cover (dominated by
a few Inga species). Four bird exclosures were
used at each site (see Methods). Data are means,
and error bars represent 95 confidence limits.
Points are slightly offset on the x-axis to show
error bars.
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Insect larvae from shade-grown coffee farm with
more diverse tree canopy were removed more
quickly than larvae from less diverse coffee farm
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More diverse canopies (species richness) may
result in more bird activity and so greater pest
control (ecosystem function).
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Caveats

• Study only took place at two farms
• Bird density and richness correlatedwhich caused
  greater removal of pests at farm with greater
  richness of canopy trees?
• Exclosures excluded bats as well as birds and so
  effects cant be attributed to birds only

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Null hypothesis increased species richness does
not affect ecosystem Function Rivet hypothesis
increased species richness leads to increased
ecosystem function Redundancy hypothesis a
minimum number of species is necessary for
maximum ecosystem function Idiosyncratic
hypothesis the relationship between species
richness and ecosystem function is
unpredictable Naeem et al. 1995
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Nuances and complexities

• Some species-poor ecosystems, like boreal forests
  and some marshes, function just fine.

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Nuances and complexities

• Comparisons of ecosystem function between
  communities with different levels of species
  richness are most likely to be important and
  informative when comparing communities that began
  similarly but are now different, for example
  comparing forests with tree plantations or
  forests with pasture created through the clearing
  of forest

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Nuances and complexities

• Some ecologists argue that it is not the number
  of species in the community that is important but
  the functional characteristics of the dominant
  species. For example, in the aphid experiment,
  when one of the ladybird beetles or the
  parasitoid wasp was the one-predator in the
  one-predator treatment, there was no difference
  between green peach aphid abundance in the one-
  and four-predator treatments.

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Nuances and complexities

• Species used in studies to date tend to be random
  subset of species available or, in some cases,
  species of economic interest. Species going
  extinct are likely not a random subset of
  species. Thus, do results from studies apply to
  real-world situations?

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• The first rule of intelligent tinkering is to
  save all the partsPaul Ehrlich
• One planet, one experimentE. O. Wilson

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References

• Naeem, S. et al. 1995. Empirical evidence that
  declining species diversity may alter the
  performance of terrestrial ecosystems. Phil.
  Trans. R. Soc. Lond. B 347249-262
• Snyder, W. E. et al. 2006. Predator biodiversity
  strengthens herbivore suppression. Ecology
  Letters 9789-796.
• Tilman, D. and J. A. Downing. 1994.
  Biodiversity and stability in grasslands. Nature
  367363-365.