According to IDH, at low disturbance frequency, diversity is low because of:
At low disturbance frequency, long intervals allow biomass to accumulate and competitive hierarchies to develop. Species that are highly efficient at capturing light, nutrients, space, or another limiting resource can suppress and eventually eliminate weaker competitors. This competitive exclusion reduces local diversity, especially when no spatial refuges or temporal fluctuations interrupt dominance. Intermediate disturbance may periodically remove dominant biomass, open recruitment sites, and permit colonizers and slower competitors to coexist. At the high-disturbance extreme, only tolerant
Ref: NCERT Biology Class 12, Ch. 15 Biodiversity and Conservation