Final answer:
The current umbrella tree density is likely less than the mean historical density due to the dispersal of boreblasters, which would reduce seed dispersal and, consequently, tree density.
Step-by-step explanation:
If a change in the density of umbrella trees caused the bore-blasters to disperse, we would expect the current umbrella tree density to be less than the mean historical density. This assumption is based on ecological dynamics where changes in species interactions, such as those between plants and their dispersers, can influence population densities.
Considering the principles of ecology and evolutionary biology, animal species often depend on plants for food and shelter. When a primary food source like the umbrella tree changes in density, it can cause the animals that depend on them, like boreblasters, to disperse in search of other habitats with more abundant resources. This dispersion can lead to a decrease in the local population density of the umbrella trees, as fewer bore-blasters remain to eat seeds or fruits, which in turn were likely helping in seed dissemination.
Based on studies that highlight the consequences of increased hunting pressure on plant regeneration, we understand that the removal of large-vertebrate dispersers, like boreblasters, could decrease the effectiveness of seed dispersal. Therefore, this would likely result in a reduced population density of the umbrella trees in areas affected by the change in boreblaster behavior or density. Ecological studies confirm that plant species dispersed by certain animals decrease in abundance when their dispersers are reduced in number due to external pressures like hunting.
In summary, the importance of animal dispersers in maintaining plant populations implies that a significant change in their behaviors or densities can result in a demographic shift in the plant populations they help to propagate. This aligns with the concept of dispersal-limitation and its impact on plant populations within ecological systems.