In 1917, biologist A. G. Tansley performed a famous series of studies on two species of bedstraw, a plant that was used to stuff beds. In nature, heath bedstraw grows on peat soil, and slender bedstraw grows on limestone soil. Tansley discovered both species can grow on both types of soil when the other species is absent. He hypothesized competition explains why, in nature, the two species do not coexist. Which experimental approach would best test his competition hypothesis?

Answer and Explanation:

The competitive exclusion principle establishes that two or more species that share the same niche requirements and are in biological competition for the same resources, won´t be able to coexist indefinitely in the same space if the other ecological factors are constant. When the superior competitor occupies entirely the inferior competitor´s fundamental niche, the superior species has advantages over the other species and monopolizes all the resources. At this point, the inferior species is displaced and must adapt to use other resources changing its behavior, or must migrate to another area where it can get access to its needs, or get extinct.    

In the exposed example, heath bedstraw grows on peat soil (acidic), and slender bedstraw grows on limestone soil (calcified). But both species can grow on both types of soil when the other species is absent. Probably, even though both species can live on both soils, each species got to adapt better to the ground where they are used to be, so when growing together, the species that naturally grow in that soil is more adapted to living there and hence turns to be the superior competitor occupying entirely the area and displacing the other species, that becomes the inferior competitor. This is,

A proper experiment to test the hypothesis would be:

Having three growth-boxes of each type of soil (three growth-boxes with acidic soil three growth-boxes with calcified soil), and keeping other variables constant and equal, measure the growth of both species on each soil with no interaction among each other (1a, 1b, 2a, 2b), and with interaction (3a, 3b). This is:

1a) heath bedstraw growing alone in a growth-box with acidic soil

1b) heath bedstraw growing alone in a growth-box with calcified soil

2a) slender bedstraw growing alone in a growth-box with acidic soil

2b) slender bedstraw growing alone in a growth-box with calcified soil

3a) heath and slender bedstraw growing alone in a growth-box with acidic soil

3b) heath and slender bedstraw growing alone in a growth-box with calcified soil

Plants should be planted and grown for a certain period of time, measuring their survival rate and development and reproductive rate.

1a) More Heath bedstraw individuals are expected to survive and live better on acidic soil

1b) On calcified soil, Heath bedstraw is expected to express a lower survival rate, with few individuals getting to grow and other dying.

2a) Slender bedstraw are expected to survive on acidic soil but expressing a low survival rate

2b) Many individuals of Slender bedstraw are expected to survive and grow properly on calcified soil

3a) When growing together on acidic soil, Heath bedstraw is expected to grow successfully, outcompeting Slender bedstraw

3b) When growing together on calcified soil, Slender bedstraw is expected to grow successfully, outcompeting Heath bedstraw.

i am going to guess that it is co2.