When herbivores consume their food plants, sometimes they just nibble a bit and there is little impact on the plant or on the consumer. Aside from that trivial outcome, there are two possibilities. One: the herbivore consumes so much plant material that the remaining plants are very badly damaged (think of overgrazed pastures, for instance) or are stimulated to produce defensive chemicals that deter further consumption. For example, browsing by snowshoe hares induces the production of chemical defenses in feltleaf willows, and the hares then eat less of plants with increased defenses. In both cases, the food supply for consumers is markedly reduced.
The second possible outcome is that consumption by the herbivore increases the future supply of the food resource. This sounds crazy—how could damage to the plants increase the resource and ultimately benefit later consumers? It is not entirely crazy; in certain circumstances, it has been documented to happen.
The classic example comes from studies of the grazing herds of the Serengeti in Africa. As the herds move across the plains, they crop the grasses. This stimulates the grasses to grow (in order to produce seeds eventually), helped along by fertilization from the animals’ waste products. So when the next bunch of grazers passes by, the supply of grasses has recovered and even improved. A similar effect is achieved when humans mow their lawns. Ecologists call this an increase of primary productivity, because the basic producers of energy and nutrients for the food web (namely, the plants) have increased. This kind of response to grazing can happen when the system is rich enough to support the continued growth of the plants; it doesn’t work in nutrient-poor or water-limited systems.
A neat example of herbivore-induced increase of resources comes from an Arizona study of the effects of a stem-galling sawfly that parasitizes arroyo willows; the gall is produced by the plant in response to the irritation by the sawfly. Female sawflies insert their eggs into young shoots and the larva feeds on the resulting gall tissue. When there is little egg-laying by sawflies, the willow branches naturally become more resistant to galling as they age. However, when galling is heavy, something else happens. Heavy galling kills the end of the shoot, and this allows sprouting of dormant buds near the base of the shoot. These buds make new, young shoots that are susceptible to the galling action of the sawflies. In effect, severe galling activity can thus renew and increase the food supply for later sawfly larvae (and anyone else that likes young willow shoots).
Closer to home, the browsing of ptarmigan (and moose) on feltleaf willows in northern Alaska affects the growth patterns of the willows: removal of the terminal buds and shoots kills the twig and allows the buds and shoots lower on the branch to sprout. The new shoots are more numerous and have more buds on browsed branches than unbrowsed shoots. Repeated browsing produces a ‘broom’ architecture and eventually reduces the height of the willow shrub. Thus, not only are there more shoots with more buds for ptarmigan to eat, but also the buds on shorter shrubs are just at a height where ptarmigan like to forage. So the bud supply for ptarmigan in the future is increased. However, the effect on the willows is negative—production of flowers and seeds is much reduced.
The take-home lesson is that the interactions between the eater and the eaten are not necessarily simple! The plants are not merely inert victims of munching animals. A close look is needed to understand what is going on and then explore the ramifying consequences.