According to the narrator in "Mending Wall," there are two ways in which a wall is broken. What are they?

The narrator in Frost's "Mending Wall" describes in the very beginning of the poem two ways a stone wall can be broken.  First, he refers to "something"(line 1), by which he is referring to weather and nature, describing how in winter, the boulders get frozen by the cold, they swell, get dislodged, and cause upheavals in a wall, creating openings so large that "even two can pass abreast" (line 4). Second, the narrator says that hunters can cause a wall to fall down,

I have come after them and made repair

Where they have left not one stone on stone, (lines 6-7)

In the spring, the narrator and his neighbor find the wall in disrepair and have an annual ritual in which they mend it, but the narrator finds this to be a futile and really unneighborly task, since the wall has no purpose except to create a barrier between neighbors, and the wall, of course, will be disrupted once again by nature and hunters.