Grazing increases the productivity of some plants

Herbivores are predators that prey on plants, but they often do not kill their prey. In grazing, an herbivore eats part of a plant, such as the leaves, without killing the plant, which then has the potential to grow back. What are the consequences of grazing? Is it always detrimental to plants, or are they somehow adapted to their place in the food chain? Certain plants and their predators have evolved together, each acting as the agent of natural selection on the other. Because of this coevolution, grazing actually increases photosynthetic production in some plant species.

Removing some leaves from a plant may increase the rate of photosynthesis in the remaining leaves. This phenomenon probably is the result of several factors. First, nitrogen obtained from the soil by the roots no longer needs to be divided among so many leaves. Second, the export of sugars and other photosynthetic products from the leaves may be enhanced because the demand for those products in the roots is undiminished, while the sources for those products— leaves—have been decreased. The remaining leaves may compensate by photosynthesizing more rapidly.

A third and particularly significant factor increasing photosynthesis, especially in grasses, is an increase in the availability of light to the younger, more active leaves or leaf parts. The removal of older or dead leaves by a grazer decreases the shading of younger leaves. Unlike most other plants, which grow from their shoot and leaf tips, grasses grow from the base of the shoot and leaf, so their growth is not cut short by grazing.

Mule deer and elk graze many plants, including one called scarlet gilia. Although grazing removes about 95 percent of the aboveground plant, the scarlet gilia quickly regrows not one but four replacement stems (Figure 40.4). Grazed plants produce three times as many fruits by the end of the growing season as do ungrazed plants.

Question: Is grazing by herbivores always detrimental to a plant?

The cropped plant grew four new stems and produced four times as many offspring...

..as did uncropped control plants.

A scarlet gilia was cropped, triggering the emergence of buds, thus producing more shoots.

Question: Is grazing by herbivores always detrimental to a plant?

The cropped plant grew four new stems and produced four times as many offspring...

..as did uncropped control plants.

A scarlet gilia was cropped, triggering the emergence of buds, thus producing more shoots.

Conclusion: Grazing can lead to increased growth.

40.4 Overcompensation for Being Eaten Experiments confirm that some plants benefit from the effects of grazing.

Some grazed trees and shrubs continue to grow until much later in the season than do ungrazed but otherwise similar plants. This longer growing season results in part because the removal of apical buds by the grazers stimulates lateral buds to become active, producing a more heavily branched plant. Leaves on ungrazed plants may also die earlier in the growing season than leaves on grazed plants.

A plant may benefit from moderate herbivory by attracting animals that spread its pollen or that eat its fruit and thus disperse its seeds. Nevertheless, resisting attack by herbivores is often to the advantage of a plant.

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Responses

  • TEIGAN
    Do grazed or ungrazed plants leave more seeds?
    8 years ago
  • hamid
    How does grazing increase plant productivity?
    7 years ago

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