The sampling effect hypothesis is an explanation for why productivity increases when plant species diversity increases. which of these predictions follows from this hypothesis? a) productivity should be highest in the tropics, where species diversity is highest. ' b) the more mutualists are present (e. g. beneficial fungi, nitrogen-fixing plants), the higher the biomass. c) if a certain "big-producer" is present, then biomass in a plot is high. if the big producer is absent, biomass is low. d) there should be a strong positive relationship between the number of functional groups in an experimental plot and biomass.