Assuming that the traps stay the same, we know that the number of moths they catch will be relative to the size of the population of moths. From this, we can see that the trend in the population of light moths is a steady decrease from year to year, and the trend in the population of the dark moths is a steady increase over the same time period.
Since the trees have been darkened by industrial pollution (they have black smoke on them), we know that the dark moths camouflage better than the light moths. Because of this, the white moths get eaten by predators more often and their population goes down gradually; the dark moths, however, camouflage easily, and their population goes up since they get eaten less often.
If the environment were cleaned up, the light moths' population would increase (since they camouflage better on clean trees), while the dark moths' population would decrease (since they camouflage better on dirty, polluted trees).
