Question

The American toad breeds in the spring, laying eggs in nearby ponds. These eggs hatch into tadpoles, which develop and reach metamorphosis (allowing them to leave the ponds) by later in the summer. Depending on how many adult pairs breed in a given pond, the number of tadpoles at a particular site may vary widely. We wanted to determine whether the population density a tadpole experiences affects its size when it reaches metamorphosis or how long it takes to get there, which can both influence longer-term survival and success.

You have been given a dataset from an experiment where tadpoles were raised in large cattle tanks containing either 100 (high density) or 50 (low density) tadpoles. We monitored these tanks and measured each tadpole’s mass (in grams) at metamorphosis and the number of days it took to reach metamorphosis. For tadpoles, this point is determined by the date when we first noticed both front and back legs.

1. Choose either mass at metamorphosis or time to metamorphosis as your response variable. What pattern do you expect to see in this variable across different density treatments? Do you predict that it will be greater at high or low density? Give a potential explanation for why you might see this pattern, given what you know of biology.

Answer 1

The biological factors, such as density directly affects the growth of an organism. In the case of tadpoles, in a high density environment, they are expected to grow faster ( less time to metamorphosis) but have a lower mass at metamorphosis as compared to the ones developing in a low density environment.

After a minimal size required by the tadpoles to survive is attained, the rate of metamorphosis is affected by the environmental pressure. A tadpole in the high density environment will metamorphosize faster but consequenctly will have low mass, in order to leave the high pressure environment.