This is an example of "Population Cycle"
In a population cycle, there is drop and rise in the population of species with time. Also this process of population drop and rise is repetitive in nature. Here in this case the series of events taking place are as follows:
a) Oak trees have large acorn production
b) With rise of acorn production, squirrel population increases owing to availability of food
c) Oak trees have lower acorn production
d) With decrease in acorn production, squirrel population decreases owing to reduction in food availability
If we plot the population cycle of two species i.e oak tree and squirrel on a graph, it results into two parallel sinusoidal graphs. The population of one species changes with the change in population of other species