IB 153 Alternate Stable States and 11/7/2006 Maintenance of Species Diversity
Alternate Stable States Can there be more than one stable • Alternative stable states (the • Regime shifts (the ecosystem community composition in a given habitat? community perspective) perspective) – Parameters are constant – “State” variables are constant Classical succession theory assumes that the changes – “State” variables change – Parameters change through in community structure following a disturbance follow a – Different community states time repeatable sequence that ends in a single, self- result from: – Different regimes are caused replacing, climax state • Differential recruitment or by: initial conditions (“history • New interactions among state Lewontin (1969) suggested that during the course of matters”) variables community development, “pulse” perturbations might • An acute perturbation • A chronic (directional) shift the same assemblage of species into alternate sufficiently large perturbation to the stable states under the same environmental conditions. to move the system changes the The states might then maintained by positive system to a new landscape of the (local) basin of feedbacks (e.g. adults create environmental conditions attractor(s) attraction that favor their own offspring)
Figure from Beisner et al. (2003) Frontiers Ecol. Env. 1: 376-382
Minimum criteria for demonstrating the existence Shortcoming of most of multiple stable states Alternative Stable State examples 1. States must be shown to occur in the same environment 1. Evidence is inapplicable since physical environment is different in the alternative 2. Disturbance or experimental manipulation states must be a “pulse” (one-time) perturbation 2. One or both states persist only when artificial 3. Observations or experiments must be carried controls are applied out over a sufficiently long time and over a large enough area to ensure that the alternative states 3. Evidence is simply inadequate (e.g. records are self-sustaining not sufficient to demonstrate turnover of component populations)
Nonequilibrium theories Equilibrium hypotheses
1. Intermediate disturbance hypothesis 4. Niche diversification 2. Equal chance hypothesis 5. Circular networks 3. Gradual change hypothesis 6. Compensatory mortality
1 IB 153 Alternate Stable States and 11/7/2006 Maintenance of Species Diversity
Intermediate disturbance hypothesis Heron Island, Great Barrier Reef, Australia
Equal chance hypothesis An intermediate rate of disturbance maintains diversity
Force to move Surface area Mean percent Mean species Mean Variance boulder (N) (cm2) disturbed / mo richness eH’ eH’
= 49 = 139 42 1.7 1.5 0.41
50 - 294 144 - 1364 9 3.8 2.5 1.05
> 294 > 1364 0.1 2.9 1.6 0.40 Peter Sale
1. Species have very similar resource requirements 2. Species have very similar life history characteristics (recruitment and longevity) 3. “First-come, first-served” competitive interactions (first individual to occupy space, holds it until it dies) 4. Recruitment rates not tied to local adult abundance (long-distance dispersal of larvae or seeds)
Gradual change hypothesis Niche diversification hypothesis (Hutchinson 1961, Paradox of the Plankton)
Hypothesis developed to explain the “paradox” of multiple phytoplankton species coexisting in an apparently homogeneous environment (water column of a lake)
If the competitive abilities of the species differ with environmental conditions, and Diversity increases with: such conditions shift back-and-forth at a rate • More kinds of resources that is faster than the rate of competitive • More specialization exclusion, the species can coexist (e.g. • Greater allowable overlap seasonal turnover in lakes).
2 IB 153 Alternate Stable States and 11/7/2006 Maintenance of Species Diversity
Compensatory mortality hypothesis (Selective consumption of dominant competitor) The Janzen-Connell hypothesis for maintenance of rain forest diversity
Circular networks hypothesis (Buss and Jackson 1979) Networks in crytpic coral reef communities
Usually assume interpecific interactions to be transitive (A > B > C, and A > C)
But they may sometimes be non-transitive or circular (A > B > C, but C > A). May occur when mechanisms of competition differ among species pairs, especially in interphyletic interactions
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