Title: Biodiversity and ecosystem function
1Biodiversity and ecosystem function
2Biodiversity
- 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.
3- 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
4Patrones 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
5Patterns 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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8Pressing 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?
9- If we lose species, does that matter in the grand
functional scheme of things?
10- 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?
11Notes 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.
12What 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
13What are ecosystem functions?
- 2. Decomposition rates
- 3. Species interactions, including pollination,
seed dispersal, regulation of herbivorous insects - 4. C02 consumption
14What are ecosystem functions?
- 5. Resistance to disturbances
- 6. Resilience (rate of recovery from
disturbances) - There are others
15Null 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
16Early 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
17Tilman and Downing (1994)
- Minnesota grasslands
- Experimental plots have different plant species
richness because of different levels of nitrogen
addition (nitrogen is a limiting nutrient)
18- Investigators compared pre-drought plant
community biomass with peak-drought community
biomass
19Results
- 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
20- Species-rich plots were more likely to contain
some drought-resistant species than species-poor
plots
21- Species-rich plots also returned to their
pre-drought biomass more quickly than
species-poor plots
22Conclusion
- Species-rich communities are more resistant and
resilient to disturbance than species-poor
communities
23Caveat
- 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?
24Study 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
25- Tested idea that experimental mesocosms with
greater species richness had greater - community respiration
- productivity
- decomposition
- nutrient retention
- water retention
26- Communities composed of
- herbaceous plants (producers)
- herbivorous molluscs and phloem-sucking insects
(primary consumers) - parasitoids (secondary consumers)
- earthworms, springtails, microbes (decomposers)
27- Communities had 9, 16, or 31 plant and animals
species - Monitored for 206 days
28- 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)
29Results 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
30- Greater C02 consumption and productivity likely
result from greater light interception in high
species richness treatments because of a greater
variety of leaf forms
31West Sumatran forest
32Conclusion
- Declining species richness may alter some
ecosystem processes
33Caveats
- Experimental systems with limited number of
species - Run for less than a year
34Study 3--Snyder et al. 2006agricultural field
experiment
- Does increased predator species richness lead to
increased control of herbivorous insects? - Herbivorous insects eat plants
35- 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
36- 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
373 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
38- number of aphids sampled several times during
28-day experiment - collard plants mature after 28 daysbiomass
determined - predators counted after 28 days as well
39Results
- 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
40Results
- 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
41Results
- Cabbage aphid abundance was lower in predator
treatments compared to controls - Cabbage aphid abundance lower in four-predator
vs. one-predator treatments
42 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.
43Results
- Collard biomass greater in predator treatments
than no-predator treatments -
- No difference between collard biomass in two
predator treatments
44Collard yields vs. mean total aphid densities.
Circles are no predator control, triangles are
one-predator treatment, and diamonds are
four-predator treatment.
45Conclusion
- 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
46Caveats
- Short time-scale
- Simplified prey base (only one species of plant
examinedmultiple plant species planted together
may have produced different results)
47Do 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.
48Table 1. Vegetation characteristics and bird
densities and richness at two large coffee farms
in Tapachula, Chiapas, Mexico
49 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.
50Insect larvae from shade-grown coffee farm with
more diverse tree canopy were removed more
quickly than larvae from less diverse coffee farm
51More diverse canopies (species richness) may
result in more bird activity and so greater pest
control (ecosystem function).
52Caveats
- 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
53Null 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
54Nuances and complexities
- Some species-poor ecosystems, like boreal forests
and some marshes, function just fine.
55Nuances 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
56Nuances 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.
57Nuances 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?
58- The first rule of intelligent tinkering is to
save all the partsPaul Ehrlich - One planet, one experimentE. O. Wilson
59References
- 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.