Lecture 5 background

Reading notes from Ricklefs chapter 19: "Ecological Succession and Community Development"

Succession refers to the changes in a habitat after it has been disturbed by events such as fire, volcano, hurricane, etc (including man-made changes like a forest being cleared). After a period of changes (in a time frame varying from ~20 to as much as 1,000 years), the habitat will reach a more or less stable situation where the species found there do not change dramatically (although they're certainly not completely static - and human impacts often bring about more changes than would otherwise occur naturally). This mature state is called the climax community. The sequence of all these changes, which differs depending on the starting conditions and the type of environment, is called a sere. Different seres can lead to the same climax community.

  • Example: a bare sand dune begins growing grasses, which help hold the dune together and, as they decompose, add organic matter to the sand, which allows herbs to begin growing. The herbs are followed by shrubs, and then by pine trees, which then give way to oak, beech, maple, and other trees that make up the climax community.

There are two types of succession. Primary succession describes new communities forming in areas that start with virtually no life (e.g. after a lava flow covers an island and kills all life there) - basically it is a community starting from scratch. Secondary succession refers to the regrowth in a community that has been badly disturbed (e.g. by a fire) but not totally destroyed, and where some basic life (and support for life) remains, like seeds and nutrients in the soil.

Which species are present at which stage of a sere depends on how quickly they grow and disperse, and other similar factors. Factors that influence the species in a sere are grouped into 3 categories:

  • Facilitation is when one species makes it easier for the next one to follow (e.g. a plant that enriches the soil around it with nutrients will make it easier for another species which needs those nutrients to establish itself)
  • Inhibition is when one species makes it harder for another one to survive or grow (e.g. by eating it). Sometimes multiple different species could be similarly likely to be established in a sere but a "priority effect" occurs where, because of inhibition, whichever one gets there first is the "winner" because it can then prevent the others from getting established. Invasive species can often enter habitats during the early phase of succession and take over because the normally inhibiting species from their native habitats aren't there to inhibit them.
  • Tolerance refers to a species' ability to tolerate the abiotic environment and to disperse / grow within it. Regardless of facilitation or inhibition from other organisms, a species' ability to establish itself in a sere will depend on how well it can tolerate the physical conditions in the habitat. Sometimes tolerance factors will determine the early stages of succession but then stronger competitors will come in and inhibition will take over in shaping the rest of the succession.

Species that tend to enter at different stages of succession, as expected, exhibit different adaptations that make them best suited for that period. Early arrivers typically have seeds that disperse quickly (sometimes through seed banks where the seeds are stored in an environment for many years until conditions are right for them to germinate), and they may be more tolerant to stressful conditions. Later species tend to be slower-growing, but they must be adapted to growing under the shade of the existing plants, and use more nutrients to eventually grow bigger than the earlier species. The earlier species will change their environment (light, soil composition, temperature etc.) quite dramatically leading to more rapid change, and once the more mature species are established, the rate of those changes slows down leading to the climax community's stability.

Not all climax communities are stable / steady-state. Transient climaxes occur in areas with dramatic climate changes (e.g. a pond can have a different climax community in the summer when it is dried up than in the rest of the year when it is wet, and succession begins again each year). A few places - typically with stressful environments - have a cyclic climax that rotates between different climaxes in a cycle.

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