The most likely statement is:
The proportion of short-tailed squirrels may have decreased due to decreased food availability.
This statement aligns with the scenario presented, where the better jumping and balancing abilities of long-tailed squirrels give them a survival advantage in tree-dominant environments, leading to an increase in their proportion over time. If food availability decreased and it affected survival rates, this could have disproportionately impacted short-tailed squirrels, causing their numbers to decline in favor of the more advantageous long-tailed squirrels.