  The mass evolution of protoplanetary disks is driven by both internal
processes and external factors, such as photoevaporation. Disentangling these
two effects, however, has remained difficult. We measure the dust masses of a
sample of 132 disks in the Orion Molecular Cloud (OMC)-2 region, and compare
them to (i) externally photoevaporated disks in the Trapezium cluster, and (ii)
disks in nearby low-mass star forming regions (SFRs). This allows us to test if
initial disk properties are the same in high- and low-mass SFRs, and enables a
direct measurement of the effect of external photoevaporation on disks. A ~
$20' \times 4'$ mosaic of 3 mm continuum observations from the Atacama Large
Millimeter/submillimeter Array (ALMA) was used to measure the fluxes of 132
disks and 35 protostars >0.5 pc away from the Trapezium. We identify and
characterize a sample of 34 point sources not included in the Spitzer catalog
on which the sample is based. Of the disks, 37 (28%) are detected, with masses
ranging from 7-270 M_e. The detection rate for protostars is higher at 69%.
Disks near the Trapezium are found to be less massive by a factor
$0.18^{+0.18}_{-0.11}$, implying a mass loss rate of $8\times10^{-8}$ M_sun/yr.
Our observations allow us to distinguish the impact of time and environment on
disk evolution in a single SFR. The disk mass distribution in OMC-2 is
statistically indistinguishable from that in nearby low-mass SFRs, like Lupus
and Taurus. We conclude that age is the main factor determining the evolution
of these disks. This result is robust with respect to assumptions of dust
temperature, sample incompleteness and biases. The difference between the OMC-2
and Trapezium cluster samples is consistent with mass loss driven by
far-ultraviolet radiation near the Trapezium. Together, this implies that in
isolation, disk formation and evolution proceed similarly, regardless of cloud
mass.
