As lower treshhold entry in learning system dynamics (SD) for me, there are to things are presented here, causel lood diagram (CLD) and differential equation (DE). And as examples general population model and chicken system are discussed in brief.

cld

There are two kinds of CLD, which are balancing loop and reinforcing loop ¹.

Figure 1. Reinforcing loop (left) and balancing loop (right).

Influence in the same direction (s) has + sign, where influence in opposite direction (o) has - sign, as both shown in Figure 1. From this point forward only + and - signs are used instead of s and o.

And there is also another example as folows ¹.

Figure 2. Reinforcing loop (left) and balancing loop (right).

CLD for chickens, eggs, and road crossing dynamics is shown in Figure 2. There is new symbol || representing delay, e.g. in this case an egg becoming a chicken requires time and not instantenous.

de

Following are DEs of second system (chickens $C$, eggs, $E$, and road crossings $R = rC$) ²

$$\tag{1} \frac{dC}{dt} = hE - rC $$

and

$$\tag{2} \frac{dE}{dt} = eC - hE, $$

where $h$ is hatching rate per egg, $e$ is egg-laying rate per chicken, and $r$ is road-crossing mortality rate per chicken with assumption that every road-crossing is fatal.

For the first system following are the DEs [^gpt52_2026b]

$$\tag{3} B = bP $$

where $B$ is number of births per unit time, $b$ per-capita birth rate (births per person per unit time), and $P$ is population, and

$$\tag{4} D = dP $$

where $D$ is number of deaths per unit time, $r$ per-capita date rate (deaths per person per unit time), which makes

$$\tag{5} \frac{dP}{dt} = (b- d) P $$

as the final equation for the system.

notes

• At home, about 1754, this note is continued.
• It is still empty at about 1622 while at work, but will be further written at home.
• DEs for both systems are added a day after the creation day.
• It ends at 1132 in the second day as the first nonte in sailing/sd category.

refs