Download ppt - Kyle Harms

Diversity-Productivity, Diversity-Invasibility,
& Diversity-Stability Relationships
Warmer sea-surface temperatures
(indicated by warmer colors)
= higher productivity
Image from Committee on Earth Observation Satellites (CEOS): http://www.ceos.org/
Diversity-Productivity Relationship
Many shapes for this relationship have been observed in nature
Rosenzweig & Abramsky (1993)
Diversity-Productivity Relationship
Many shapes for this relationship have been observed in nature
Rosenzweig & Abramsky (1993)
Diversity-Productivity Relationship
Many shapes for this relationship have been observed in nature
S or D
Productivity
II. b.
II. a.
S or D
Sometimes curves like
I and III may arise
from sampling opposite
ends of productivity
gradients in which
curve II is the overall
relationship
Productivity
S or D
S or D
Productivity
III.
S or D
II.
I.
Productivity
Productivity
Diversity-Productivity Relationship
Many shapes for this relationship have been observed in nature
Mittelbach et al. (2001)
Methods:
Examined the relationship between productivity & diversity for 171 studies
Observations:
Even though many researchers are enamored of hump-shaped curves,
the curves vary dramatically from site-to-site, as well as within & among
taxonomic groups
Suggestions and conclusions:
Don’t assume a particular relationship – measure it
Be wary of the independent variable used as a surrogate for “productivity”
Diversity-Productivity Relationship
Many shapes, but what are the mechanisms?
Tilman (1982, 1988), Tilman et al. (1996), etc.
Explained hump-shaped curves by the changes in heterogeneity that sometimes
accompany changes in resource availability
A
A
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A
A A
A A
A A
B
A
A
A A
A
A
B
B
B
B
A A
A A
B
B
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B B
B
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B B
B B
B
B
E.g., soil fertility / productivity gradient
= poorest soil;
species A outcompetes
species B
= richest soil;
species B outcompetes
species A
B
Diversity-Productivity Relationship
Rosenzweig & Abramsky (1993)
Summarized several mechanistic hypotheses for hump-shaped curves
Suggested that separate mechanisms account for the rising vs. falling portions
Preferred mechanism for the rising portion:
“A poor environment supplies too meager a resource base for its would-be
rarest species, and they become extinct”
In other words “poor environments support lower population sizes, and
population size is inversely related to extinction probability”
No well-supported mechanism for the falling portion:
Provided several potential mechanisms, but claimed that none are
well-supported by observations or experiments; even so, Tilman’s
heterogeneity hypothesis has some empirical support
Diversity-Productivity Relationship
Stevens & Carson (2001)
Declining curves could result simply from size differences; if sites are sampled with
the same sized plots, more productive sites may have fewer species because they
have fewer individuals, especially owing to the ubiquitous clumping that occurs in
natural populations
A
A
A
B
B A
A
A B
B
A
B
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A
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A
E.g., cloudiness-induced productivity gradient
= lowest light availability
How might this problem be avoided?
= highest light availability
Use an index that is insensitive to sample size
Diversity-Productivity Relationship
Kyle’s conjecture…
If disturbance, predation, competitive equivalence, or dispersal limitation occur
alone or in combination such that competitive exclusion does not occur among the
recruits of species within a guild (especially plants), then sites with conditions in
which more species are capable of surviving and reproducing will contain more
species, i.e., diversity will increase up the resource (e.g., fertility) gradient
Species-poor
community at low
end of resource
gradient
Species 1
Species 2
Species 3
Species 4
Species 5
Species 6
Resource availability, e.g., soil fertility
Species-rich
community at high
end of resource
gradient
Diversity-Productivity Relationship
A hump-shaped diversity-productivity relationship could result
in the “Paradox of Enrichment” within trophic levels…
Community sampled before fertilization
Species diversity
Community sampled after moderate-level fertilization
Community sampled after high-level fertilization
Productivity
Diversity-Productivity Relationship
A hump-shaped diversity-productivity relationship could result
in the “Paradox of Enrichment” within trophic levels…
Gough et al. (2001)
Methods:
Examined long-term experiments from 7 Long-Term Ecological Research
(LTER) sites in North America
Observations:
Nearly all demonstrated a decline in diversity after fertilization
Suggestions & conclusions:
The results have utility for similar situations, but little relevance to natural
productivity gradients, since species distributions along natural gradients
are influenced by long-term ecological & evolutionary processes, e.g.,
species may preferentially colonize or originate within sites of high
productivity, giving rise to a positive relationship
Diversity-Productivity Relationship
(Productivity-Diversity)
So far we have considered productivity gradients due to gradients
in resource availability, e.g., physical gradients
What happens when we reverse the axes, and ask how diversity in a given
site, i.e., one set of physical conditions, influences productivity?
Diversity-Productivity Relationship
(Productivity-Diversity)
Examples from artificial communities…
Loreau et al. (2001)
“Biodiversity and Ecosystem Functioning…”
Methods:
Compiled data from a variety of field, Ecotron & other mesocosm
experiments in which S or D were varied experimentally
Observations:
Productivity
Sites of high intrinsic
resource availability
Sites of low intrinsic
resource availability
S or D
Diversity-Productivity Relationship
(Productivity-Diversity)
Examples from artificial communities…
Loreau et al. (2001)
“Biodiversity and Ecosystem Functioning…”
Conclusions:
A monotonic or saturating curve almost always results from experimental
settings examining the influence of diversity on productivity
At least two mechanisms can create a positive relationship between
diversity and productivity:
1. Complementarity – species use complementary niche space
2. Sampling – random sampling a large species pool is more likely to
select a key (highly productive) species than sampling a small pool
How might these two mechanisms differ in their implications for
conservation, global change, etc., especially with respect to redundancy?
Diversity-Productivity Relationship
Diversity
(Productivity-Diversity)
Latitudinal gradient
Diversity
Productivity
Biomass gradient
Diversity
Experimental diversity gradient
Productivity
Productivity
Diversity
Productivity
For plants, the relationship may change with scale
(see Mittelbach et al. 2001)
Experimental manipulations of plant diversity within habitats
generally yield positive relationships
Figure from Purvis & Hector (2000)
Diversity-Invasibility Relationship
This is especially germane in today’s world of rampant spread of
exotic species, i.e., the “homogenization of biodiversity”
Charles Elton first proposed that more diverse
communities should be less invasible
Photo of Charles Elton from http://www.wku.edu/~smithch/chronob/ELTO1900.htm
Diversity-Invasibility Relationship
This is especially germane in today’s world of rampant spread of
exotic species, i.e., the “homogenization of biodiversity”
Fargione et al. (2003)
Methods:
Experimental, grassland plots containing mixtures of plants from
four “functional guilds”: C3 (cool-season) grasses, C4 (warm-season)
grasses, legumes, non-N-fixing forbs
Experimentally introduced seeds of representatives of each guild
Results and conclusion:
C4 grasses exhibited the greatest inhibitory effect on introduced species
(i.e., they were competitive dominants); established species from each
functional guild most strongly inhibited species from its own guild
Diversity reduces invasibility, both by increasing the chances of
encountering established plants of the same guild (“close competitors
cannot invade”), as well as established plants of the dominant guild (a
“sampling” effect)
Diversity-Invasibility Relationship
This is especially germane in today’s world of rampant spread of
exotic species, i.e., the “homogenization of biodiversity”
Levine (2000)
Methods:
Removed non-Carex species from sedge tussocks along streams in
California, and subsequently added 1, 3, 5, 7, 9 native species, but kept the
total cover of plants identical
After one year, added 200 seeds of 3 exotic species to each tussock
Results and conclusion:
Fewer exotic seedlings established on more species-rich tussocks
Increased diversity provides increased “immunity” to invasion
Levine also found that diversity of native species was positively related to
diversity of exotics in unmanipulated tussocks
In light of his experimental results, how could this happen?
Diversity-Stability Relationship
Initial empirical guess…
MacArthur (1955)
a.k.a. “complexity-stability” relationship
Alternative energy pathways in complex food webs might favor more constant
population sizes with reduced fluctuations, thus promoting stability
Diversity-Stability Relationship
Early modeling results…
May (1973)
Challenged MacArthur’s intuition & verbal arguments with mathematical
models that showed no theoretical basis for the relationship to necessarily
be in any particular direction (all possibilities could be obtained)
Diversity-Stability Relationship
Back to empiricism, with potential reasons for differences of opinion…
Pimm (1984; 1991) – Three levels of organization at which to measure “stability”
Population
Community (especially community composition)
Ecosystem (especially biomass, energy flux, or the flux of matter,
e.g., C, N, etc.)
Diversity-Stability Relationship
Back to empiricism, with potential reasons for differences of opinion…
Pimm (1984; 1991) – Five definitions for “stability”
Stability (in the strict mathematical sense) – a system is stable if, and only if,
the variables all return to equilibrium conditions after displacement from them
Resilience – the rapidity with which a variable that has been displaced from
equilibrium returns to it
Persistence – the duration that a variable maintains a given value until it
changes to a new value
Resistance – the degree to which other variables change when a given variable
is permanently changed to a new value
Variability – the degree to which a variable varies over time
Diversity-Stability Relationship
Back to empiricism, with potential reasons for differences of opinion…
Pimm (1984; 1991) – At least three definitions for “complexity”
Species richness – S
Connectance – the degree to which all nodes interconnect with other
nodes in a food web
Relative abundance – D
…and this isn’t an exhaustive list!
(3 levels of ecological organization) x (5 definitions of “stability”) x
(3 definitions of “complexity”) = at least 45 different questions that could be
asked about the relationship between community complexity & stability!
Diversity-Stability Relationship
Mechanisms that could generate a positive relationship between species
diversity & ecosystem-level stability…
McCann (2000) –
Averaging effect – “Assume covariances between species are zero and
variance (si2) in abundance of individual species i in a plant community is
equal to cmiz, where c and z are constants and mi is the mean density of
species i. Given that all k species in a community are equal in abundance
and sum to m (that is, mi=m/k), then the coefficient of variation (CV) of
community abundance can be determined as:
CV = 100s/m = 100(c/k)1/z
For the case z > 1, increasing k (species number) decreases the variation in
biomass for the plant community”
Diversity-Stability Relationship
Mechanisms that could generate a positive relationship between species
diversity & ecosystem-level stability…
McCann (2000) –
Negative-covariance effect – “If covariances between species (say,
species a and b) are negative (that is, cov(a,b)<0), then the variance in the
abundance of two species:
s2(a+b) = sa2 + sb2 + 2cov(a,b)
will be less then the sum of the individual variances (that is, sa2 + sb2), and
so will decrease overall biomass variance in the plant community”
Diversity-Stability Relationship
Mechanisms that could generate a positive relationship between species
diversity & ecosystem-level stability…
McCann (2000) –
Insurance effect – “An ecosystem’s ability to buffer perturbations, loss in
species and species invasions is dependent on the redundancy of the
species having important stabilizing roles, as well as on the ability of the
species in the community to respond differently to perturbations. Increasing
diversity increases the odds that such species exist in an ecosystem. This
idea has been extended to suggest that the greater the variance of species’
responses in a community then the lower the species richness required to
buffer an ecosystem.
…increasing diversity increases the odds that at least some species will
respond differentially to variable conditions and perturbations…
greater diversity increases the odds that an ecosystem has functional
redundancy by containing species that are capable of functionally replacing
important species… taken together, these two notions have been called the
insurance hypothesis”
Diversity-Stability Relationship
Mechanisms that could generate a positive relationship between species
diversity & ecosystem-level stability…
McCann (2000) –
Weak-interaction effect – “Weak interactions serve to limit energy flow in a
potentially strong consumer-resource interaction and, therefore, to inhibit
runaway consumption that destabilizes the dynamics of food webs. In
addition, the weak interactions serve to generate negative covariances
between resources that enable a stabilizing effect at the population &
community level. The negative covariances ensure that consumers have
weak consumptive influences on a resource when the resource is at low
densities.”