Answer:
The correct answer is -
Gar population: increase
crabs: decrease
diving duck: decrease.
Step-by-step explanation:
As it is known that alligators eat gar in any aquatic ecosystem and gar depends on the crabs for their food and nutrition and share the crab population as food with diving ducks in a particular ecosystem. So if the alligator population decrease in the ecosystem then The gar population will increase as there are fewer or no alligators eating them.
And as the population of gar will increase then there would be a decrease in the population in crab population as there will be more gars to eat them and the population of diving duck will reduce due to the increase in the gar population which results in competition for the crabs as food.