Ecologist David Tilman grew laboratory populations of two species of diatoms namely Asterionella Formosa (Af) and Synedra ulna (Su) in liquid media supplemented with silica. The researcher monitored not only the population growth but also the level of silica in the medium. The results of the experiments are shown in the graphs (P, Q, and R). P and Q respectively are graphs obtained when Af and Su are grown respectively. Graph Q is obtained when Af and Su are grown together. The correct conclusion(s) that can be drawn from the result is/are.

(a) When grown together, the presence of Af in the medium has a positive effect on the growth of Su.
(b) When grown together, Su reduces silica concentration to a level that does not allow Af to multiply.
(c) Af reduces resource availability at a faster rate than Su.
(d) There is competition for the resource between Af and Su.

Hint: The graph shows the rate of growth of both the species of the diatoms. One species requires a very less amount of silica for its growth while the other species require a little more amount of silica which is not fulfilled when both are grown together.

Complete answer:
The graphs show the growth of both the species of diatoms in the liquid medium namely, Asterionella Formosa (Af) and Synedra ulna (Su). The liquid media is provided with the silica. The growth of both the diatomic species depends upon the amount of silica present in the liquid media and how much they require for their growth.
In the graph P, the growth of Asterionella Formosa (Af) is shown. Af maintains the rate of growth at a steady and low level due to the maintenance of the low amount of silica level in the liquid medium constantly. This will maintain steady growth in Af.
In the case of graph Q, the growth Synedra ulna (Su) is shown which is quite similar to the growth of Asterionella Formosa (Af). Here the growth of Su is also maintained at a steady-state but the amount of silica in the liquid medium is much lower. It is lower than the amount of silica present in the growth of Asterionella Formosa (Af).
Lastly, in the case of the graph R both the species of diatoms, Asterionella Formosa (Af) and Synedra ulna (Su) are grown together. In this case, Synedra ulna (Su) requires a very less amount of silica in the liquid medium while in the case of Asterionella Formosa (Af) requires a little larger amount of silica which is not possible due to the growth of Su. Su will rescue the concentration level of silica such that it will not allow the growth of Af and thus result in the prevention of the multiplication of Af, thus resulting in its extinction. Hence, Synedra ulna (Su) is responsible for the extinction of the Asterionella Formosa (Af).

So, the correct answer is ‘When grown together, Su reduces silica concentration to a level that does not allow Af to multiply’.

Note:
The type of competition as shown in the case of the above two diatomic species Asterionella Formosa (Af) and Synedra ulna (Su) is called interspecific competition. In this competition, the individuals that belong to the different species will compete with each other for the same resources that are found in the ecosystem.