Final answer:
To find the percentage by which the bird population has decreased, we compare the population three years ago to the population this year. The bird population in the biologist's sample has decreased by about 27.7% over the last 3 years.
Step-by-step explanation:
To find the percentage by which the bird population has decreased, we need to compare the population three years ago to the population this year. Three years ago, the population was 20 birds per acre. This year, the biologist recorded a total of 47 birds in an area equal to 3.25 acres.
To calculate the percentage decrease, we first need to find the initial population. We can do this by multiplying the number of birds per acre by the number of acres: 20 birds/acre * 3.25 acres = 65 birds.
Next, we can calculate the decrease in population: 65 birds - 47 birds = 18 birds.
Finally, we can calculate the percentage decrease by dividing the decrease in population by the initial population and multiplying by 100: (18 birds / 65 birds) * 100 ≈ 27.7%.
Therefore, to the nearest tenth, the bird population in the biologist's sample has decreased by about 27.7% over the last 3 years.