Q.The figures given below show the results of a lab experiment in which two microbial species A and B belonging to same genus were grown in three petri dishes having same culture medium. In Petri dish-I, Species-A was grown alone for 8 weeks. In Petri dish-II, Species-B was grown alone for 8 weeks. In Petri dish-III, both the species were grown together with the same number as Petri dish-I and Petri dish-II for 8 weeks. What conclusion will you draw from the graphs in terms of impact of interaction on growth? Explain, giving reason.

Concept: Interspecific competition and competitive exclusion

The graphs show classic Gause's competitive exclusion principle in action. When grown alone (dishes I and II), both species A and B reach their carrying capacity and maintain stable populations—each exploits the resources without interference.

When grown together (dish III), species A's population trajectory mirrors its solo growth, while species B's population crashes to zero. This asymmetry reveals that both species compete for the same limiting resource (same genus, same medium), but species A is the superior competitor. Species B cannot coexist because it is outcompeted at every resource level; A either consumes resources faster, tolerates waste better, or otherwise dominates the niche. …

The experiment reveals how two closely related species interact when forced to share the same environment and resources. Because A and B belong to the same genus, they likely have very similar ecological requirements—same nutrients, same pH optima, same physical space needs. This overlap in resource use sets the stage for competition.

When species live alone (dishes I and II), each grows in a classic logistic fashion: rapid exponential growth initially, then leveling off as the population approaches the environment's carrying capacity. The carrying capacity represents the maximum population size the available resources can sustain. Both species reach stable plateaus, showing they can each thrive independently in this medium.

The dramatic shift occurs in dish III. Here we see competitive exclusion (Gause's principle) in action: when two species compete for identical or very similar resources, the species with even a slight competitive advantage will eventually eliminate the other. The graphs show that species A is the superior competitor.

Let me walk through what the data patterns reveal:

1. Species A's performance under competition

   In dish I (alone), species A reaches a high carrying capacity—let's call it K_A. In dish III (with B), species A still persists and dominates, but reaches a somewhat lower final population size. This reduction happens because initially, before B is excluded, both species are consuming resources. Species A expends energy not just on growth but also on outcompeting B (perhaps through faster nutrient uptake, allelopathic compounds, or simply faster reproduction). The final population is lower than K_A but still substantial.

2. Species B's performance under competition

   In dish II (alone), species B reaches its own carrying capacity K_B, demonstrating it's perfectly viable in isolation. But in dish III, species B shows initial growth—confirming both species started with equal numbers—then a sharp decline to zero. Species B is completely eliminated. This isn't mere suppression; it's local extinction within the petri dish.

3. The mechanism: asymmetric competition

   The interaction is competitive (both species negatively affect each other by depleting shared resources) but asymmetric (the effect is not equal). Species A has a higher competitive ability—perhaps a higher intrinsic growth rate r_A > r_B, or more efficient resource utilization, or production of inhibitory metabolites. The mathematical outcome follows the Lotka-Volterra competition model: when the competition coefficients and carrying capacities satisfy certain inequalities, one species inevitably excludes the other.

4. Why "same genus" matters

   Congeneric species (same genus) typically occupy very similar ecological niches—a concept called niche overlap. High niche overlap intensifies competition because there's little resource partitioning. If these were species from different genera with different resource needs, coexistence would be more likely through niche differentiation. …