A model succession has six stages: bare rock, lichens, mosses, herbs, shrubs and forest…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A model succession has six stages: bare rock, lichens, mosses, herbs, shrubs and forest. In each decade, the community has a chance of moving on to the next stage. The chances are for bare rock, for lichens, for mosses, for herbs and for shrubs. The mean waiting time in a stage is divided by its chance, in decades.
- What is the expected time from bare rock to forest, in years?
- A fire burns a forest on the same site. The soil is left, and the succession restarts at the herb stage. What is the expected time to forest, in years?
- How many times longer is the first than the second? Give the answer to one decimal.
Need a reminder? The key ideaShow
Succession is a gradual and fairly predictable change in the species composition of an area. It begins with pioneer species and ends in a stable climax community. Primary succession starts without soil. Secondary succession starts where soil is left, so it is faster. The mean waiting time in a stage is divided by its chance of moving on.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Chances , , , , for each decade.
Plan. Add the waiting times in decades. Multiply by for years.
Step 1. (a) decades, which is years.
Step 2. (b) From the herb stage: decades, which is years.
Step 3. (c) times.
Answer: years; years; times.
Check. The program below plays the chain many times, and its mean agrees with years.
Watch out. Add the waiting times, not the chances.
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