Instant Notes: Plant Biology

For an iteroparous perennial:

λp=cmp+p

wherepis the adult survival.

We can derive from this that an ephemeral plant will reproduce faster if

me>mp+p/c. This indicates that one of the important terms is the ratio between

adult survival and seedling survival (p/c): if there is good seedling survival it

will be advantageous to be ephemeral; if seedling survival is poor an

iteroparous perennial will be at an advantage. Studies have confirmed that this

is largely true.

There is often high mortality of the seeds, and there may be many natural

hazards before they can germinate, such as seed-eating animals, rain and wind.

After germination, the main competition occurs during active growth to

flowering, and this is likely to involve competition with associated plants as

well as members of the same species. After this an established plant faces many

fewer hazards, so the greatest selection is likely to be in the seedling stage. In

general, plants live in populations in the same way as animals, with birth rates,

death rates, immigration and emigration. Any change in that population must

take these into account and this can be expressed by this basic equationof

population change:

Nt+1=Nt+B–D+I–E

whereNtis the number at time t,Bis number of births, Dnumber of deaths, Iis

immigration,Eis emigration. When B+I=D+Ethe population is stable, but

whenB+I>D+Ethe population will increase exponentially, i.e. increasing by

the same factor each year, giving a logarithmic growth curve (Fig. 1). This has

been recorded when a plant is colonizing a new area. Clearly this rate of increase

can only continue for a short period after which the population will be regulated

by some finite resource such as germination sites or nutrient availability.

Population
dynamics

K4 – Populations 177

50 000

25 000

0

9500 9250

Years before present

9000

Pollen density

Fig. 1. Exponential increase in the population of the scots pine, Pinus sylvestris, in Norfolk,

UK, from pollen records in peat deposits (Redrawn with permission from Bennett, K.D., Nature

1983; 303 ,164–167. Copyright 1983 Macmillan Magazines Limited).