BIOL 4120 Principles of Ecology Phil Ganter 320 Harned Hall 963-5782
Florida scrub-pine forest, the climax community on the sandy soils of central southen Florida

Lecture 17 Community Succession

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Overview - Link to Course Objectives

• Succession
  • Clementsian Succession
    • Primary succession
    • Secondary succession
• Problems with Clementsian succession
• Alternatives to Clementsian succession
  • Assembly Rules
  • Inhibition Model
  • Tolerance Model
  • Markov Models of Succession
• Intermediate disturbance hypothesis
• Restoration Ecology
• Gleasonian Assemblages
• Terms

Succession

Idea of community stability usually includes two related concepts:

• Communities have some kind of equilibrium state that is stable over time if the environment does not change
• Communities return to their equilibrium state after a disturbance that alters community structure
  • This chapter deals with the regular return to a stable state
  • Regular change in community composition is called succession
    • One set of species succeeds a previous set

Clementsian Succession

• Idea developed for terrestrial plant communities
• Following a disturbance that alters community composition but not the nature of the habitat, there will be an orderly return to pre-disturbance community composition
  • each of the stages of the sequence are called a sere
  • first sere called the pioneering sere and the species in this sere are called pioneers or pioneering species.
• Community will return to a climax community composition
  • Climax communities are stable and not prone to invasion by non-climax species
  • Climax communities renew themselves
  • Climax community composition is determined by the interaction between plant species and abiotic factors (climate, soil moisture, etc.)
• Earlier communities alter habitat so that plants in succeeding communities can invade
  • Facilitation is the process of paving the way for your successor
    • as species are introduced, they modify the soil, the light, and may alter the conditions that allowed them to get a start. The new conditions, now not so favorable to the pioneering species, are favorable for species that could not have invaded at first.
  • Climax communities have soil requirements that are built by earlier communities (organic material in soil must build up so that moisture is retained, pH must be correct, etc.)
  • Facilitation is not the only process underlying succession
• Studies have shown that replacement of one species by another is sometimes due to:
  • facilitation (as Clements proposed)
  • nodulating species can raise the nitrogen content of the soil to a point that other species can invade
  • on marine hard surfaces, one set of organisms can facilitate the settlement of organisms that replace the original holders of the space
    • in this case, the first species makes its successor more likely to settle on the surface
    • if the situation is more emphatic such that a species cannot invade unless another species has arrived and flourished first, the first species Enables the second species
    • competition -- some species simply compete for a resource such as light, without either species facilitating the invasion of the other
• Thus few present day ecologists accept that Clementsian succession is the only means by which one community can replace another
• Primary Succession - succession that begins on a surface with no preexisting communities present
  • new islands
  • new volcanic rock
  • new lakes formed by glacier retreat
  • many microbial communities normally undergo primary succession
• Secondary Succession - succession that begins after a disturbance that has left some of the previous community intact
  • Allogenic (literally, "other origin"} succession is caused by an abiotic disturbance
    • Fire and storms often lead to secondary succession
    • Human disturbance (agriculture, forestry, development) can lead to secondary succession once the land is abandoned by humans
  • Autogenic (literally, "self origin") succession is caused by a disturbance due to organisms within the system
    • locust plague can cause a disturbance large enough to initiate succession once the grasshopper populations have returned to low levels
    • outbreak herbivores (gypsy moth in oak forests, spruce budworm in spruce forests, army worm, etc.) can lead to widespread loss of the climax tree species and initiate succession
• Succession has been expanded to other situations beyond primary succession and "old field" succession
  • Decomposition of leaf litter or of accumulated materials on lake and ocean bottoms
    • Leaves in streams
    • Leaf litter in forest
    • Carcass decomposition
  • Colonization of marine hard substrates (rocky areas)
  • Regrowth of forests as glaciers retreat
  • Regrowth after devastating natural disasters (volcanic explosions)

Problems with Clementsian succession

• Facilitation does not always occur
• Climax communities vary and particular conditions may prevent the predicted climax community from developing
• Soils may never be correct because the parent material will not support the climax community normally found in the area (example of Lake Michigan sand dunes in book)
• Recurring fire or other disturbance may prevent formation of climax
  • in this case, the outcome may be that which community occurs is not predictable, in which case the community is called Non-equilibrium

Alternatives to Clementsian succession

Assembly Rules

• If most of the species in a community are found because they are either enabled by another species or are missing because they have been competitively excluded by another species, than it may be possible to predict the composition of a community through the use of Assembly Rules
  • Assembly rules - a set of rules used to choose a subset of a larger set of species that can coexist
  • Assembly rules are useful if they work:
  • indicator species can be used to predict community composition
• However, assembly rules are subject to criticism
  • The rules are usually made from collection data by observing which species never or always occur together
    • Because of this, we do not know why they do or do not occur together
  • Hard to tell if the associations between species are significant or just patterns that randomly arise if you have enough data
    • This problem lead to a debate about which was the appropriate null model to use for the statistical tests
      • If you want to know if the pattern of species co-occurrences is due to competition, then the null model must not contain any effects of competition
      • However, both competition that is ongoing and competition that occurred in the past might both have something to do with the pattern observed.
      • Removing the effect of present competition from a model does not make it null with respect to all competition

Inhibition Model

• Inhibition is the competitive exclusion of a species from a community by the activity of another species
  • can occur through the production of a secondary chemical
    • some plants secrete allochemicals that prevent other plants from germinating or prevent root growth
    • some yeast, hyphal fungi, and bacteria secrete allelochemicals to kill other yeast, hyphal fungi, and bacteria
  • can occur through the unequal effects of pathogens on two species, as one species may be excluded by a disease tolerated by another (so that the winning species acts as a reservoir for the disease)
• inhibition is the opposite of facilitation - it explains the loss of species as the community changes
  • Facilitation is not the only process that is needed for succession
    • many early succession plants are prevented from invading later successional stages by competition (inhibition), so even if facilitation is important for the forward progress of seres, the next sere must outcompete the previous sere for space in the community
  • Inhibition can lead to communities that become resistant to invasion
    • Expected climax community never reached because some intermediate community is resistant to invasion by the climax species
    • the climax community may depend on who gets there first and subsequently stops others from invading, even though the environment is otherwise suitable for non-climax species

Tolerance Model

• Species enter the community in an order determined by their dispersal capabilities
  • as the community ages, resources are shifted from the environment into the biomass of the community and so the environment becomes more nutrient poor as time goes on
    • species invade if they can Tolerate the lower levels of nutrients available
    • species are lost if they require the higher levels of nutrients available in disturbed areas
  • Competitive exclusion drives the succession sequence through scramble competition

Markov Models of Succession

• A Markov approach to community structure requires that one know the probability that one "state" will change to another "state" in a given time period (similar to the assembly rules approach)
  • States for communities are all of the possible community compositions
  • heart of the modeling process is construction of a transition matrix that has the probability of a state changing into any other state at a given time
    • transition matrix is a square table with all of the species as row and column labels
    • entries are the transition probabilities
    If these transitions are such that any state can be reached from any other (directly or along a path that goes through other states) then the process is a regular Markovian process
• Regular Markovian processes have the advantage that they will reach a "steady state" as time goes on, no matter which state (community composition) is the beginning state
• Useful because one can see succession as a Markovian process
• The steady state is the climax community
• Approach does not require that one know anything about facilitation or other biotic interactions

Horn's Replacement model

• First applied to canopy trees
  • gathered data on the trees under the canopy trees in forests
    • assumed that one of the understory trees would replace the canopy tree
    • also assumed that the choice would be random among the understory trees, so that the species with the most trees under a particular canopy tree would be the most likely species to replace the canopy tree
• Horn ran his model until a steady state was reached and then compared the steady state prediction from the model to the actual composition of the forests around Princeton
  • The model predicted the known succession of tree species in the local forests
  • The forest species composition data agreed with the model's prediction for the climax community

The Intermediate disturbance hypothesis

• Low levels of disturbance (very stable communities in which the species are not under physiological stress) allow the loss of species through:
  • competitive exclusion of some species
  • loss of species due to overexploitation by predators
• Constantly high levels of disturbance prevent an increase in richness
  • stressful habitats lead to low population numbers leads to higher population extinction rates
  • greater extinction rates lead to simpler, less diverse communities
• Intermediate levels of disturbance (stress) prevent competitive exclusion by reducing population sizes such that the resources are never limiting, but do not reduce the population sizes so low that species go extinct by chance

Restoration Ecology

• The science of managing lands or bodies of water so that they return to their previous community composition after a disturbance
  • disturbances of interest are usually those caused by human activity
    • industrial activities such as mining
    • restoring natural water regime to areas that have been drained or where river channels have been altered
    • restoring land after it has been used by farmers or ranchers
  • sometimes natural disturbances are the cause (fire, storm, etc.)
• sometimes simply waiting for natural processes will often restore a community or species bh
  • recovery from a bottleneck
  • reinvasion through migration
• sometimes this means active intervention
  • releasing members of a species from another population in a similar area
  • preventing some species from overgrowing the missing species of interest
  • important to know what normal succession sequence will be so that one can take appropriate action (and not waste effort)
  • Text lists some concerns for restoring terrestrial communities, take a look at them.
• some efforts at clean-up after a disturbance have slowed, instead of sped-up, the rate of succession
  • washing oil off of Alaskan shores covered by oil spills has slowed recovery
  • removal of topsoil contaminated with a chemical slows recovery unless new topsoil is brought in

Gleason, Succession and Communities revisited

• Gleason, a contemporary of Clements, and later Robert Whittaker both view vegetational changes as the outcome of the interactions of individual plants with the environment
  • thus, there is no community and we have an assemblage of plants
  • often, the difference is depicted by viewing graphs of species density along an environmental gradient (in altitude, temperature, latitude, moisture, etc.)
    • Clementsian communities show many species with similar ranges so that you can see where one community begins and another stops
    • Gleasonian assemblages are characterized by species ranges beginning and ending randomly, so that you can not determine where a community begins by looking for a place where one group of species replaces another group

Terms

Succession, Clementsian Succession, Climax Community, Facilitation, Enablement, Assembly Rules, Primary Succession, Secondary Succession, Allogenic, Autogenic, Sere, Pioneering Sere, Disturbance, Inhibition Model, Tolerance Model, Restoration Ecology, Markov Models, Transition Matrix, Regular Markovian processes, Horn's Replacement model, Restoration Ecology, Clementsian Community, Gleasonian Assemablage

Last updated March 9, 2007