Answer:
These species form part of the Trophic chain - Trophic web. By the "predator-prey" relationship, they get to balance the ecosystem, keeping their populations in a controlled size.
Step-by-step explanation:
White-tailed deer consume many different plant species. Deers are herbivores that seem to easily overpopulate different areas in the absence of their predators, reaching the carrying capacity. The population increase causes too much damages, affecting different plant species survival, structure, and composition. And by doing this, they also end affecting other wild animals and invertebrate species that also depend on the same plant species, not only for food but for refuge and nesting sites.
Natural controllers of deer populations are the coyote and the cougars, considered key-species, important in keeping healthy natural areas. Their presence is an indicator of the good state of an ecosystem. These animals act as a major limiting factor, decreasing deep population or keeping it below forage carrying capacity. This natural control increases and maintain the population of other species affected by deers, mainly plants.
The deer is an important item for both the coyote and the cougar, being probably a reason for interspecific competition between these two species.
The equilibrium is very delicate. When key-species populations fluctuate, it also does the deep population, and hence, plants populations. By the cascade effect in the trophic chain, the fluctuation in one species population affects the other species too. A small reduction in key-species such as the coyote might implicate an automatic increase in the deer population. Although a sharp increase in coyote or cougars population might result in severe damage to the deer population. These opposite effects have been described in many different studies.