Deer and elk are closely related species in the Deer family (Cervidae). They often inhabit similar habitat types in locations where they co-occur. They are also active during the same parts of the day, mainly in the early morning and in the evening. It is not at all uncommon to see them in close proximity, exhibiting no signs of competition and apparently coexisting peacefully.
Deer are browsers. Most of the time they eat leaves, stems, twigs, and bark. Shrubs constitute about 75% of their diet, and about 25% comes from forbs (non-woody herbs). In contrast, elk tend to graze (eat grasses and forbs) whenever possible. They get 85% of their food from grazing. They often take the most rapidly growing, thus frequently the softest and most nutritious, plants. During most of the year there is no competition for food between deer and elk, because they are using different eating strategies which include different plants or different parts of the same plant.
However, in winter, the peaceful coexistence scenario shifts. In winter, elk become browsers. This means that in the coldest time of the year, when food may become scarce, deer and elk compete.
It is not an even battle, and it is waged very subtly. Elk are about two to three times the size of their smaller cousins, the deer. They can easily push deer off prime browsing areas, although direct aggression is exceptionally rare. In good range, in mild winters, when numbers of animals are low to moderate, the competition may not be intense. In areas where food is not abundant, in hard winters, or when animal numbers are high, competition can be severe. In such situations, it is not uncommon to see deer numbers decline.