Describe three ways in which ecologist can estimate the size of population.

Trap-mark and release method. This is done by setting harmless traps for the animal of your choice. When captured, you can then mark them and then release them. In analyzing these samples, the ecologists will have an easier time analyzing the population size.
Evaluating the resource area. The resources surrounding the are of the chosen animal can provide a lot of information. If the animal you were looking for lives in a small area with a lot of predators surrounding them, you can then give an estimate of how small the possible population could be.
Sample count. If the animals are found in a large area, just take an area, count the number of animals situated there and multiply by the total area to get a rough estimate.