Given below are four niche-oriented models proposed to explain species-abundance distributions in ecological communities.
A. The dominance-preemption model involves successive species preempting a dominant portion (50% or more) of the remaining niche-space.
B. In the random fraction model, successive species invade and take over an arbitrary portion of the niche space of any species previously present. Here, irrespective of their dominance status, all species are subjected to niche division with equal probability.
C. The MacArthur fraction model assumes that larger niches are more likely to be invaded by new species.
D. The dominance-decay model postulates that the largest niche in an existing assemblage is always subject to a subsequent (random) division.
Assuming a process of community assembly that involves successive species invading and establishing in the community, what would be the correct order of evenness in community structure produced by these mechanisms?

Correct option is B

Rationale (evenness from lowest → highest):

A. Dominance–preemption produces the lowest evenness — early colonists preempt large fractions (≥50%) of available niche space, creating very skewed (dominant) abundance distributions.

B. Random-fraction yields intermediate evenness — niche space is partitioned randomly, so abundances are more mixed than in dominance–preemption but still variable.

C. MacArthur-fraction gives higher evenness than random-fraction because larger niches are preferentially invaded and split, which reduces extreme dominance.

D. Dominance-decay produces the highest evenness because the largest niche is repeatedly targeted for division, actively eroding dominance and driving the community toward more even partitioning.

So ordering by increasing evenness is A < B < C < D, i.e. option 2.