Biotic factors involve living organisms, including predators, competitors, food organisms and pathogens. Abiotic factors are non-living conditions such as temperature, sunlight, rainfall and salinity. In a stated food web, less producer growth can reduce food for herbivores. Fewer predators can initially allow prey numbers to rise.
Follow the named link and any assumptions that other factors stay similar. A likely first effect is not a guarantee of unlimited growth: food, disease and other limits still apply. Read population tables by comparing the same organism over time. Subtract the later count from the earlier count to find a decrease. Keep counts of individuals separate from rates of change.
Example 1
In grass → rabbit → fox, fox numbers fall while other factors initially stay similar. Predict the first effect on rabbits.
Rabbit numbers are likely to rise because predation is reduced. Food and other limits can prevent indefinite growth.
Example 2
A population falls from 85 to 60 individuals. Find the decrease.
85−60=25 individuals.
Watch out: Reduced predation can increase prey initially, but food, disease and other limits still apply.