Community Ecology
Importance of diversity - indicator species & keystones
Outline:
1. Why should we care about maintaining biodiversity, anyway?
A. Utilitarian vs. intrinsic reasons
B. Diversity is related to overall system resistance and resilience to disturbance
1. Paradigm shifts in action: positive relationship first proposed by Elton, then
refuted by May’s models, then upheld by Tilman’s experiments
a. Tilman and Downing (1994) experiments - concept of functional
redundancy
b. Redundancy hypothesis vs. rivet hypothesis vs. idiosyncratic
hypothesis
2. Possible mechanism: sampling effect
2. Are all species equal?
A. Paine’s species-removal experiments, which resulted in surprisingly large community
changes
i. One species (predatory starfish Pisaster) was acting as a "keystone" species
B. Definitions and examples of keystone species and processes
C. Conservation of keystones
D. Problems with the keystone species concept
E. Remedies and suggestions with the keystone species concept
Terms/people:
indicator species
miner’s canary
functional redundancy
stewardship
redundancy hypothesis (Walker)
idiosyncratic hypothesis (Naeem)
rivet hypothesis (Ehrlich and Ehrlich)
D. Ehrenfeld
Chas. Elton
biophilia (Wilson)
N. Eldredge
intrinsic
Robert May
utilitarian
keystone (Paine)
ecosystem engineer
David Tilman
land ethic (Leopold)
Costanza et al. (ecological economics)
umbrella species
dominant species
trophic cascade
community importance
portfolio effect
Regarding the biodiversity crisis: “This is the folly that our descendants are least likely to forgive
us.” - E.O. Wilson, 1985
Importance of diversity: (utilitarian and intrinsic reasons)
1. Esthetics
2. Information
Indicator species / miners’ canaries
And problems…
3. Economic value
And problems…
4. Ethics & stewardship
So is high biodiversity indicative of community health or stability? (see Fig. 3.10 in
Mittelbach text)
Charles Elton (1920s)
Robert May (1972, 1973, 1974)
David Tilman and colleagues:
Tilman and Downing 1994 (see Fig. 3.2 in Mittelbach text)
“portfolio effect”
Are species within a community interchangeable?
The answer depends on which of 3 hypotheses you believe (three alternatives to role biodiv.
plays in overall system functioning/health):
- functional redundancy / redundancy hypothesis (Walker 1992)
- Ehrlich and Ehrlich’s (1981) “rivet hypothesis”
- idiosyncratic hypothesis (Naeem et al. 1994, 1995) - see Fig. 3.3 in Mittelbach text
So, which to believe? Is biodiversity positively or negatively associated with community
stability?
Some work has shown a positive relationship between biodiversity (usually spp. richness) and
community stability:
- Elton’s ideas
- Tilman’s work
- rivet and redundancy hypotheses
But other studies (primarily models) have shown a negative relationship:
- May’s models (1972, 1974)
- other food web models (topic of a future lecture) have shown that connectance
decreases as species richness increases
The idiosyncratic hypothesis does not fall into either camp because it considers not all species to
be equivalent. Is this assumption true?
Species equivalency (or lack thereof)
Paine 1969 - Pacific intertidal communities
keystone species
cf. dominant species (see Fig. 10.7 in Mittelbach text)
removal of keystone species results in trophic cascade
one example - ecosystem engineers (Jones et al. 1994)
Problems with the keystone species concept (Mills et al. 1993, Simberloff 1998):
-"poorly defined" (although one rule of thumb in identifying keystones is the "50% loss
rule")
-is a posteriori, not a priori
-removal of combinations of non-keystones may have just as large an effect as removing
a keystone (remember the rivet hypothesis?)
-seems like everybody’s favorite organism is a keystone
-conflicting identification of species as keystones – e.g. sea urchins have been called
keystones because they graze algae and keep it from dominating; but sea otters have been called
keystones because they keep urchins from dominating  so which is it?
So what should we do?
Power et al. (1996) refined the keystone species concept to take community importance
(CI) into account:
(T  T ) 1
CI  [ n d ][ ]
Tn
Pi
where Tn is a quantitative measure of a community trait (e.g. diversity)
Td is the measure of the trait when the species is deleted
Pi is the abundance of species i
when CI > 1, the species if a keystone species (e.g. bison CI ranges from 6 to 25)
-remove dualistic thinking (of keystones vs. nonkeystones)
-focus on interaction strengths among spp. and how these interactions vary with
abundance and under diverse ecological conditions (keystone concept has been useful
in showing how some species have particularly strong interactions)
Kotliar (2000) added one more element:
-incorporate an additional criterion to the keystone species definition: keystones
perform roles not performed by other species or processes
For a comprehensive review, see Hooper et al. (2005).
A video on the importance of biodiversity:
https://www.youtube.com/watch?v=TartoYpK1yI&NR=1
Next lecture: species-area relationship
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