Competition between diatoms

The final example is from a laboratory investigation of two species of freshwater diatom: Asterionella formosa and Synedra ulna (Tilman et al., 1981). Both these algal species require silicate in the construction of their cell walls. The investigation was

... and between diatoms

Figure 8.4 (right) Percentage difference in feeding rates (mean ± SE) at orange-crowned warbler and virginia's warbler nests on plots where the other species had been experimentally removed. Feeding rates (visits per hour to the nest with food) were measured during incubation (inc) (rates of male feeding of incubating females on the nest) and during the nestling period (nstl) (nestling feeding rates by both parents combined). P values are from t-tests of the hypothesis that each species fed at higher rates on plots from which the other had been removed. This hypothesis was supported for virginia's warblers but not orange-crowned warblers. (After Martin & Martin, 2001.)

Asterionella alone

105r

Asterionella alone

105r

Competition Between Organisms

(b) Synedra alone

105r

(c) Interspecific competition

(c) Interspecific competition

Asterionella Synedra Competition

Asterionella -O Synedra

Silicate

Figure 8.5 Competition between diatoms. (a) Asterionella formosa, when grown alone in a culture flask, establishes a stable population and maintains a resource, silicate, at a constant low level. (b) When Synedra ulna is grown alone it does the same, but maintains silicate at an even lower level. (c) When grown together, in two replicates, Synedra drives Asterionella to extinction. (After Tilman et al., 1981.)

unusual because at the same time as population densities were being monitored, the impact of the species on their limiting resource (silicate) was being recorded. When either species was cultured alone in a liquid medium to which resources were continuously being added, it reached a stable carrying capacity whilst maintaining the silicate at a constant low concentration

(Figure 8.5a, b). However, in exploiting this resource, Synedra reduced the silicate concentration to a lower level than did Asterionella. Hence, when the two species were grown together, Synedra maintained the concentration at a level that was too low for the survival and reproduction of Asterionella. Synedra therefore competitively excluded Asterionella from mixed cultures (Figure 8.5c).

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