Rhetorical: Alma is restating his authority and reason for giving this sermon. In this case, Alma is the shepherd who is watching his flock, and attempting to drive away the wolves from within. As we will see in the next verse, the shepherd becomes the Savior. This is a similar shift in the identity of the shepherd that we saw in verses 37 and 38.
Translation: There is a good deal of sheep/shepherd imagery in this text, and this particular passage appears to appeal to a common knowledge of the problems of shepherds. This is an imagery that is completely at home in the world of the Bible, as attested by several references:
Ezekiel 22:27
27 Her princes in the midst thereof are like wolves ravening the prey, to shed blood, and to destroy souls, to get dishonest gain.
Luke 10:3
3 Go your ways: behold, I send you forth as lambs among wolves.
Acts 20:29
29 For I know this, that after my departing shall grievous wolves enter in among you, not sparing the flock.
All of these verses are effective because the imagery is well known. Even in a modern world where we are much less personally cognizant of the problems of wolves and sheep, the images are still sufficient, and have been reinforced by the Biblical passages.
None of these reasons can easily explain Alma's use of the imagery, however. Neither wolves nor sheep are attested for the New World, in particular the Mesoamerican area that we are considering. It should be noted that while this is generally true, we may not yet have all necessary data. As John L. Sorenson has noted:
What does this mean for Alma's text? It may mean that once again the Bible's imagery and language is influencing the translation given by Joseph Smith. While the Mesoamerican peoples might not have been familiar with sheep and wolves, they were certainly familiar with kept animals and predators. Thus the intent of the imagery is applicable, and the particular language may be more directly due to the Biblical influence than a direct translation of what was on the plates.