Final answer:
The parameter in the Lotka-Volterra competition equations that describes the impact of one plant species outcompeting another in Tansley's experiment is the competition coefficient (a).
Step-by-step explanation:
In Tansley's experiment where one plant species outcompetes another by overshadowing and blocking sunlight, the parameter in the Lotka-Volterra competition equations that describes this impact is the competition coefficient (a). The competition coefficient quantifies the effect of one species on the population growth of another species within the same environment. It describes the intensity of competition, where a higher coefficient indicates that one species has a significant competitive advantage over the other, potentially leading to exclusion of the weaker competitor. This differs from the maximum population growth rate (r), which is the rate of population growth in perfect conditions, and the carrying capacity (K), which is the maximum population size an environment can sustain without degrading resources.