There were two original occurrences in the text of “the effects” that took the singular verb form is (here in Alma 30:16 and earlier in Mosiah 7:30). In both cases, the plural effects was eventually edited to singular effect in the printed editions (in the 1840 edition for Mosiah 7:30, in the 1837 edition for Alma 30:16). The critical text will, of course, restore the original plural effects (it will also maintain the singular verb is). Here in Alma 30:16, this unusual reading is extant in the original manuscript. See under Mosiah 7:30 for further discussion.
Summary: Restore the original plural effects in Alma 30:16 (as well as in Mosiah 7:30).