Final answer:
As reindeer consume more lichens with the increasing size of lichen populations, this is termed a functional response in predator-prey dynamics, where predators adjust their consumption rates according to prey density.
Step-by-step explanation:
When each reindeer in a herd consumes a greater mass of lichens each day as the size of lichen populations increases, this is considered a functional response to changes in prey population size. A functional response involves a change in the rate of prey consumption by predators in relation to prey density. In contrast, a numerical response would imply a change in the number of predators in response to prey density.
Adaptations to predation and herbivory are driven by evolutionary pressures that lead species to develop mechanisms to escape predation, like mechanical, chemical, physical, or behavioral defenses. Conversely, characteristics that improve the ability of predators to capture prey can, over generations, become more common within the predator population. This is evident in cycles observed in predator-prey dynamics, such as the classic case of the lynx and snowshoe hare populations.