Four species of salamanders in the genus Desmognathus form a streamside salamander guild in the mountains of North Carolina. The four species are found at different distances from the stream, as listed in the table

Question

Suggest one or more hypotheses to explain the relationship between the body sizes of these species and the distance each is found from the water. What prediction can you make for each hypothesis, and how could you test that prediction? Species D. quadramaculatus D. monticola D. ochrophaeus D. anaeus Adult body length 100-175 mm 83-125 mm 70-100 mm 44-57 mm Distance from stream Less than 10 cm Less than 1 m 1-4 m 4-7 m

Answer 1

Of course there are many plausible and mutually compatible hypotheses; here are three.
Hypothesis 1: The terrestrial salamanders require microhabitats with moist soil to extract water. As the distance from the stream increases, the habitat grows drier and the size of moist microhabitats becomes smaller. Thus, species of salamanders that live progressively farther from the stream must be progressively smaller.
Prediction for hypothesis 1: The prediction—the size of moist microhabitats becomes smaller as the distance from the stream increases—is incorporated into the hypothesis, and the water potential of soil can be measured, although measuring the water potential of a tiny sample of soil is technically difficult.
Hypothesis 2: The large species require larger prey than the smaller species, and the salamanders' prey organisms respond to the soil moisture gradient.
Prediction for hypothesis 2: Sites distant from the stream have prey that are too small for the larger species of Desmognathus to consume. Prey size can be measured directly; the difficulty lies in obtaining an unbiased representation of the range of potential prey species.
Hypothesis 3: Closer to the stream is better, and either the larger species exclude the smaller species by direct or indirect competition, or the larger species prey on the smaller species.
Prediction for hypothesis 3: Removal of any one of the larger species should allow the next smaller species to expand into the area that had been occupied by the larger species.