Lecture 25. Alternate Stable States
/Grammatical Rant
There is a difference between alternate (adjective) and alternative. One switches back and
forth (alternate, verb) between alternates; one chooses among alternatives. Hence, when Mr.
Spock says, “There are always alternatives, Dr. McCoy”, he is speaking correctly. We have
choices, even if we don’t like them. “I didn’t want to press charges but I saw no alternative” is
also correct. When a road sign says “Road closed. Use alternate route” it makes a mistake
(probably to save space) because you are free to choose which other route you take. To take an
alternate route would be to take road A this time, road B next time, then road A again, then road
B, and so on. I mention this apparent minutiae because of my conviction that precision of
thought and precision of speech are inseparable. Taking time to choose the right word allows us
to express subtle or complicated ideas exactly.
/End Rant
-recall that succession does not necessarily lead to the same end community every time
-may be more than one climax for any given ecosystem
-depending on nature of the disturbance and stochastic events
-logical extreme of this flexibility is known as alternate stable states
-in which an ecosystem can succeed to either of two very different “climax” communities
-both of which are stable (persistent) through time
-key attribute of alternate states is that they arise from identical initial conditions
-random chance pushes developing community in one way or the other
-as usual, aquatic ecosystems provide best examples
-alternate stable states were first observed in shallow (< 3 m), productive lakes in England
-by J.I. Jones and C.D. Sayer (2003, Ecology 84(8): 2155-2167)
-strong competition in these lakes between:
rooted plants (macrophytes) growing from the bottom
and phytoplankton in the water column
Wolterton Lake (with Wolterton Hall in background), one of 17 English lakes in which Jones and
Sayer observed alternate stable states. The caption says: “The lake was created in the 1720s from
a dammed a stream, according to a design by Thomas Ripley. Photograph by Iwan Jones, with
the kind permission of Lord and Lady Walpole.” I love England.
-because the lakes are so shallow, macrophytes cover most of the lake bottom
-simultaneously controlled by light and phosphorus
-relatively low P in water favours macrophytes, which can obtain P through roots
-leading to a clearwater lake
-high P in water, as from pollution, favours phytoplankton, which can shade out the plants
-leading to a turbid lake
** in lakes with the right P balance, ecosystem can switch between turbid and clear
-Example: a lake that has been clear, macrophyte-dominated, for years receives P influx
-results in classic eutrophication: algal blooms, loss of the macrophytes
-so far, nothing unusual; we know lakes respond like this to P enrichment
** but turbid, algae-dominated system persists when P enrichment is removed
-therefore, algal domination is an alternate stable state for the ecosystem
-we discovered this problem by accident while trying to recover polluted small lakes
-reducing P income often is not sufficient to return the lake to the clearwater state
-some sort of biomanipulation may be required to push lake back in the other direction
-for example, introducing an algae-feeding fish,
-or a top predator that releases zooplankton grazing through a trophic cascade
-these additional manipulations may push the system back to the other state
-that state, a clearwater lake, is also stable:
-once there, it persists, even if the controlling fish die
** alternate stable states often represent catastrophic changes to the community
-these systems are prone to abrupt and unannounced changes between states
-often in response to some random disturbance, such as a storm or a change in water level
**different states are due to dominance of organisms with different life forms
Examples: collapse of fishery stocks off the East Coast of Canada,
-system persisted in the face of heavy fishing pressure until a critical point was reached
-then fish populations dropped drastically
** fish populations have not recovered despite >10 yr of fishing moratorium
-appears that communities have entered a stable, low-fish state
Example: decline of acidifying lakes
-there is no obvious change in the ecosystem as the pH lowers
-until a sudden and unexpected switch to a barren, fishless lake
Example: grasslands can be kept open by herbivores
-which easily control the seedlings of trees
-sometimes in connection with wildfire
-but woodlands, once established, are stable
-because herbivores cannot eat adult trees
-and trees cool the microclimate, preventing severe fires
-we have driven many of these changes ourselves, by accident
Example: Cedars of Lebanon (Cedris libani)
-these majestic conifers once covered large areas at high elevation in Middle East
-in semi-arid lands, adult trees draw up deep water (hydraulic lift)
-and maintain moisture near the surface
-moist soil supports seedlings, which do not yet have deep root system
-forests were extensively cleared to make buildings (including King Solomon’s temples)
-and, tragically, the wood was used to fuel steam locomotives during the Ottoman empire
-once adult trees are cut, climate is too desiccating for seedlings to grow
-therefore clear-cut forests do not regrow
-historical Cedar of Lebanon forests are now permanent scrub desert
King Solomon of Old Testament fame. His great
temples were built of Cedris libani and contributed
to the decline of those forests. Here, he is depicted
making the famous decision: “Cut the baby, and
take a blood sample for DNA testing.” Wise man.
Cedar of Lebanon. A few intact stands remain.
Efforts are now underway to re-create some of
the lost forest, but grazing pressure is a hindrance.
Theory of Alternate Stable States
-envision the concept of stable states using a ball-and-cup diagram
** look at Text Figure 12.5, p. 344
-ball represents state of the ecosystem at any time
-surface, or cup, represents environmental conditions controlling the ecosystem
-alternate stable states arise from a surface with two cups (zones of attraction)
-within either zone, ball is stable,
- ball returns to same state after a minor perturbation
-if a strong disturbance comes along, it displaces ball to the other zone,
-from which it does not return, because the other cup is also stable
How do alternate states arise?
** essential mechanism requires a positive feedback
-between an interactive control and state of the community
-these feedbacks magnify and re-inforce small differences in direction of successional change
-thereby maintain the community in one state or the other
-in the model, these feedback loops create the cups that the ball slides into
Example: in lakes, low P favours macrophytes,
-which can draw P from the sediments,
-but can also remove P from the water too, outcompeting phytoplankton
-this is a positive feedback that maintains the clear-water condition
-in addition, plants protect Daphnia, an algal grazer, from fish predation
-and prevent fish from resuspending the sediments
-perturbation of macrophyte community can switch equilibrium to algal dominance
-once algae are established, they render the water turbid,
-therefore outcompeting macrophytes for light
-fish then control Daphnia, which have no place to hide
-fish are also free to churn up the sediments, maintaining turbidity
-again, positive feedbacks maintain algal dominance
-a switch from one alternate state to the other may arise from two situations:
(1) gradual change in environmental conditions
-change erodes positive feedback mechanism maintaining state 1
-in ball-and-cup model, change makes zone of attraction shallower or less steep-sided
-in shallow lakes, addition of nutrients eventually pushes system into the algal state
-establishment of a species from the other community can sometimes change conditions too
(2) random physical or biological disturbance
-removes dominant members of one community type
-leaves habitat open for colonization by the alternate community
Example: in lakes, violent storms, herbicides, water level fluctuations can trigger state changes
Diagram below shows how each of these mechanisms works
(From Jones and Sayer, again)
-graph shows rooted plant abundance plotted against nutrient concentration
-at low nutrient levels, lake is always macrophyte-dominated (State I)
-at very high nutrient levels, lake is always algae-dominated (State III)
-in between, State II (shaded area), is zone where either community is stable
-follow the solid line as nutrients increase (move along X-axis, left to right):
-as lake enters State II, macrophytes continue to dominate until point (a) is reached
-at which point a catastrophic switch to algal domination is inevitable
-coming back the other way (right to left),
-macrophyte biomass remains low as nutrients decrease
-until point (b) is reached,
-where algae can no longer compete and macrophytes must dominate
-within State II, dotted line is an unstable equilibrium that can switch either way
-shown as (c) and (d)
-sudden, catastrophic switches between states are precipitated by a disturbance
-event which reduces abundance of one species assemblage favours the other
-which is then re-inforced by positive feedbacks
-imagine zones of attraction around the solid line (think grooves instead of cups)
-once State II system has switched, one way or the other, it stays there
-Once system has moved to State III, returning to conditions in State II
is not sufficient to return system to the previous state
-we must go back further, to (b), before a return to State I can be guaranteed
-hence we must reduce P loading far below the point where the lake became turbid
Jones and Sayer did original work on shallow English lakes
-they showed that alternate states were mediated by a trophic cascade
-switch from macrophytes to algae controlled by growth of periphyton on leaves of plants
-periphyton block light and compete for carbon dioxide
-periphyton are controlled by grazers (insects and snails)
-which are fed upon by fish
-molluscivorous fish, in turn, are food for top predator fish
-therefore, random fluctuation in population of top predators can lead to:
(1) population increase of molluscivorous fish
(2) loss of snails
(3) irruption of periphyton
(4) decline in plants and
(5) switch to algal dominance;
-if correct, this is the longest chain of indirect interactions that I have seen
(also demonstrates that such chains are unstable)
-take a look at Jasinksi and Payette (2005, Ecol. Monogr. 75(4): 561-583)
-found alternate stable states in spruce forests of northern Quebec
-northern Quebec dominated by unique spruce-lichen woodland
-widely spaced spruce trees with carpet of reindeer moss (Cladonia spp.) in between
-more southerly boreal regions support spruce-moss forest
-denser forest with moss in forest floor instead of lichen
Open spruce-lichen woodland in northern Canada. The ground
cover is Cladonia lichens (reindeer moss), shown on right
-more southerly boreal regions support spruce-moss forest
-denser forest with moss in forest floor instead of lichen
Spruce-moss forest, typical of more southern Boreal regions.
Note the greater size and density of the spruce trees.
-in one region of southern boreal forest, alternate stable states between two forest types
-Figure 12, p 579, illustrates the situation [Ignore third loop at bottom]
-spruce-moss forest reproduces itself after fire or spruce budworm outbreak
-but two disturbances together (any two of: fire, logging, insects)
leads to spruce-lichen woodland
-which is a second stable state
-switches back only if fire is followed by several years of good weather
-so trees regrow densely, eliminating lichens, favouring moss
-farther north, good weather conditions do not occur, spruce-lichen woodland is permanent
Diversity
-appears to be a connection between diversity and alternate states
-in ball-and-cup diagram, depth and steepness of cups indicate resilience
-recall resilience is rapidity with which a community returns to a stable condition
-after being displaced from it by a disturbance
-a community with more positive feedback mechanisms is more resilient (deeper cups)
-not easily displaced from stable community composition
-resilience of an ecosystem, in turn, may be related to diversity
-especially functional diversity and complementarity, redundancy, as discussed earlier
-Figure FEE 1(9), Fig 1, p. 489) illustrates the argument
-again, ball-and-cup diagram showing alternate states of the community
-lowering diversity weakens positive feedbacks (resilience) that keep community in one state
-equivalent, in the figure, to making the cup less steep
-eventually, system become so unstable (low resilience)
that a small disturbance pushes it into other stable state
** also, zones of attraction of alternate states need not be equally deep
-one may be much more stable or persistent than another
-once we push a system into a new configuration, it may be difficult to reverse
-look at examples in figure above to see connection with diversity:
-coral reefs depend on algal grazers, mostly mollusks and fish
-without them, algae grow quickly and eventually over-run and kill the corals
-algae are kept in check by a large variety and number of algal grazers
-removal of grazers, by overfishing and pollution, leads to a loss of resilience
-or, zone of attraction becomes more shallow around coral-dominated state
-eventually, coral state has such poor resilience that a mild disturbance (storm, bleaching,
disease) can push it into alternate, algal-dominated state
-this state has strong resilience;
-resists returning to the other state even if conditions change again
-this pattern has actually happened around Jamaica
-persistent overfishing over decades moved from large predators to smaller herbivores
-until virtually nothing was left
-yet coral state persisted because of large populations of one species of sea urchin
-when urchin was decimated by disease and hit by a hurricane,
-the reefs abruptly switched to the algal state
-problem difficult to reverse
-because large algae are unpalatable to grazers
-and algae prevent new corals from settling
-even the oceans at large may show alternate states
-Figure below (Nature 413: 595, Figure 7) shows “Ecosystem state” of world oceans
-by averaging 31 climatic & 69 biological time series
(fish catches, oyster health, plankton numbers, many others)
-graph shows a classic shift between two states in 1977, and another in 1989
Continuous Transitions
-alternate stables states are not universal
-many ecosystems transition smoothly from one condition to another
-without any abrupt shifts from one control regime to another
-Mittelbach et al. (2006, Ecology 87(2): 312-318) demonstrated smooth transition
-in hyper-productive Wintergreen Lake, Michigan
-they examined change in zooplankton over 16 years
-in response to loss (winterkill) and re-introduction of two planktivorous fish species
-lake was very different without fish (large Daphnia pulicaria dominates),
or with fish (many small zooplankton)
-but did not have two stable states. See Figure 3, p. 316.
This plot shows change in Daphnia
community relative to 1989. Time trend
shown by arrows and dates. The lake
changes smoothly from low Daphnia to
high Daphnia (1992-2001) and back, with
no abrupt changes
Same plot for the entire cladoceran
community, showing essentially the same
pattern of increase and decline with no
evidence of alternate stable states.
-alternate stable states appear under specific conditions
-requires two possible strong controls on ecosystem structure
-continuous response arises where these conditions do not apply
-I don’t think we are at the point of predicting when alternate states do or do not exist
-I wonder as well if Wintergreen lake could be pushed into an alternate stable state
by some other kind of disturbance