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Community Ecology Importance of diversity - indicator species & keystones Click here for supplemental materials for today (PDF) 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 “Biodiversity” is a buzzword and a key focus of biology (and THE key focus of community ecology): but what is the importance of having high biodiversity and why is it important to conserve biodiversity? Why prevent extinctions? Why care? (Some good reviews may be found in Kinzig et al. 2001.) Importance of diversity: (utilitarian and intrinsic reasons) 1. Esthetics 2. Information Indicator species / miner’s canary analogy And problems… 3. Economic value (goods and services) Ecological economics And problems… 4. Ethics & stewardship Leopold’s “land ethic” Wilson’s “biophilia” Is high biodiversity indicative of community health or stability? Charles Elton (1920s) Robert May (1972, 1973, 1974) David Tilman and colleagues: Tilman and Downing 1994 “portfolio effect” Mechanisms somewhat unclear: what do species actually do? The simplest functional community can be made from only 2 functional groups: decomposers (microbes, bacteria, fungi) and primary producers (plants). Increasing diversity (especially diversity of other functional groups/trophic levels) serves 2 purposes: (1) additional species increase the rate of interactions between decomposers and primary producers (e.g. herbivores eat plants, then produce dung that feeds decomposers that would otherwise have had to wait for the entire plant to die), and (2) additional species smooth out what would otherwise be pulsed interactions. So does that mean that species within a functional group are 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) 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 removal of keystone species results in trophic cascade many types e.g. ecosystem engineers (Jones et al. 1994) The rationale behind focusing conservation efforts on keystones is because if you preserve them, you incidentally preserve lots of other spp. (Managing for a keystone = managing for a whole community.) keystone in this instance acts as an umbrella sp. Conservation, however, takes $. Re: keystones--is this money well spent? 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 Regarding the biodiversity crisis: “This is the folly that our descendants are least likely to forgive us.” - E.O. Wilson, 1985 Next lecture: species-area relationship References: Andersen, A.N. 1997. Using ants as bioindicators: multiscale issues in ant community ecology. Conserv. Ecol. 1:8 [online: http://www.consecol.org/Journal/vol1/iss1/art8/index.html]. Andersen, A.N. 1999. My bioindicator or yours? Making the selection. J. Insect Conserv. 3:6164. Bond, W.J. 1994. Keystone species. Pp. 237-253 in: Biodiversity and Ecosystem Function (E.D. Schulze and H.A. Mooney, eds.). Springer-Verlag, Berlin. Brown, J.H., and E.J. Heske. 1990. Control of a desert-grassland transition by a keystone rodent guild. Science 250:1705-1707. Costanza, R., R. d’Arge, R. de Groot, S. Farber, M. Grasso, B. Hannon, K. Limburg, S. Naeem, R.V. O’Neill, J. Paruelo, R.G. Raskin, P. Sutton, and M. van den Belt. 1997. 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