Elk herds in yellowstone national park are herbivores that browse on the available vegetation. which factor is critical to ensuring stability in this ecosystem?

Elk obviously need the vegetation they eat to be abundant and unlike believed in the past they need to be prayed on by predators. Without predators, the elk overpopulate and eat all of the plants and cause other animals like beavers to die off. 

Let x = time, weeks.
Let y =  length of Rip van Winkle's beard, mm

Initially, his beard was 888 mm long, therefore
When x = 0, y = 888.

For every week, y increases by 222 mm.
Therefore
y = 222x + 888

This is an equation for a straight line with 
slope = 222,
y-intercept = 888

When x = 1, y = 222 + 888 = 1110 mm
To draw the straight line, choose two points:
(0, 888)  from the y-intercept
(1, 1110)

The graph of the straight line is shown below. The two points are shown on the line.