The Ragwort and its Seeds: 3 Exercises
In a text on Population Ecology, Begon
(1996, p. 7) describes the Ragwort, Senecio jacobaea, a biennial plant which
generates around 5,000 seeds per year when in the flowering stage. When
a seedling is established, the plant spends about one year forming a rosette
of leaves. In the second year, the adult plant forms a flowering stem. Observations
in a sand dune environment in the Netherlands provide a numerical example
with around 1 adult plant and 4 young plants spread over ten square meters.
The Model
An equilibrium diagram of a ragwort model is shown below. Some of the assumptions
are close to the numerical example by Begon, but information on the fate
of the seeds is quite limited. This model extends Begon's example by assuming
that system is in equilibrium with 20,000 seeds in the surface bank and
an equal number in the buried bank. Begon suggests that around 1% of the
surface seeds germinate each year. Let's assume that 80% of the surface
seeds become buried each year, and the remainder is lost due to wind, animals
and decay. Seeds in the buried bank may return to the surface or decay.
Resurfacing occurs at the rate of 60%/yr; decay is based on a 5 year persistence
time, so it occurs at the rate of 20%/yr.
The equilibrium diagram shows 1 adult plant which produces
5,000 seeds per year. If 60% land in the test area, we would get 3,000 local
seeds. The flow "seed rain on the soil" is the combination of
the local seeds and 5,000 invading seeds which also land in the test area.
(Begon does not comment on the number of invading seeds, so let's simply
assume that the test area is well positioned to receive seeds from upwind
plants.)
The Ragwort Exercises
1. Verification:
Build the model and verify the results shown in the equilibrium
diagram.
2. Stability Test:
Introduce a disturbance flow to add additional young plants
in a test year. Does the test reveal a stable, unstable or neutral equilibrium?
3. Importance of the "Buried Seeds Persistence Time"
Begon does not describe the persistence time for the buried
seeds. Conduct a sensitivity analysis with values of the persistence time
ranging from a low of 1 year to a high of 9 years. With a one-year persistence
time, the number of flowering adults should decline to around 0.5 adults
per ten square meters within a decade. With the 9-year persistence time,
they should grow to around 1.4 adults per ten square meters. (With the 9-year
persistence time, let the simulation run 30 or 40 years to allow the system
to reach equilibrium.)