This graph shows the changes in two populations of herbivores in a grassy field.

In the y axis it says number of organisms
in the x- axis it says Time

In the graph A is decreasing , B is increasing, At first they were steady then slightly A decreased and B increased .

A possible reason for these changes is that
(1) all of the plant populations in this habitat decreased
(2)population B competed more successfully for food than population A
(3) population A produced more offspring than population B
(4) population A consumed members of population B

4 answers

From your data, I would pick 2. With B increasing, that eliminates 3 and 4. Since the type of organism is not designated, I am uncertain about 1, but 2 is more likely.
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(2)population B competed more successfully for food than population A
All of the plant populations in this habitat decreased