Monocausal Decay

Monocausal decay in regards to the Roman Empire is an idea that states that the fall of the Roman Empire can be traced down to one cause.  On Wikipedia, the article has three different theories as to what was this one cause was: epidemics, deforestation, and lead poisoning.  I believe that the epidemic theory is plausible.  The Roman Empire was massive, and we discussed how there were bound to be issues even while the conditions of the empire were optimal.  Now imagine what would happen when half of the population within this empire were killed; there must have been intense paranoia coupled with the fact that people needed to be taxed twice as much to make up for the deaths of half of the population.  McNeill asserts that  "the severe fall in population left the state apparatus and army too large for the population to support, leading to further economic and social decline."

Would a sparser population of people even make a difference if epidemics were so widespread?

It would indeed make a difference.  The barbarians located across the Danube and Rhine River had population increases.  They lived in isolated villages that were not conducting as much trade, and they did not use public facilities.  This changing demographic lead to the fall of the Roman Empire because the Roman Empire got weaker as the barbarians grew stronger.

Environmental issues would affect the empire, but only specific portions of it.  If a certain portion of the empire could not feed themselves, their population would drop, leaving them weakened for others to conquer.  Environmental issues couldn't have plagued the entirety of the Roman Empire.

Lead poisoning also seems a bit far fetched.  Lead poisoning affected the entire empire and caused the decay of it because defrutum "was used to some degree to sweeten wine and food" after being boiled in lead pots.