The most likely statement is:
The proportion of long-tailed squirrels may have increased due to increased food availability.
This is because long-tailed squirrels have an advantage in their environment (jumping and balancing in trees), and if food availability increased, it could support a larger population of long-tailed squirrels, allowing their proportion to increase over time. The other options do not align as well with the observed trend of increasing long-tailed squirrels.