Hydra effect

This article is about the ecological phenomenon. For the creature of Greek myth, see Lernaean Hydra.

The hydra effect is the paradox, in population ecology, in which increasing the rate at which individuals die leads to an increase in the size of the population. It is named after the Lernaean Hydra of Greek myth, which grew two heads for every one that Heracles cut off.¹

The effect occurs when removing individuals relieves the survivors of competition, or releases growth that was being held back, so that the population compensates for its losses — and more than compensates. The idea has a long history in population theory, which has identified several mechanisms that can produce it.¹ It is of practical importance wherever populations are harvested or controlled, because a measure intended to reduce a population can, if it is applied in the wrong way, enlarge it.

In giant hogweed

     .:*:*:*:*:.                                        .:*:*:.
      \\\ | ///                        .:*:*:.            \|/
        \\|//                            \|/               /      .:*:*:.
         \|/                              \               /         \|/
          |                                \              /         _/
          |           decapitation           \       _x_ /         /
          |           ------------>           \       |  /       _/
          |                                     \     | /      _/
          |           mowing, strimming          \    | /    _/
          |           or cutting the              \   |/   _/
          |           flower head                   \ |   /
          |                                          \| _/
          |                                           |/
          |                                           |
        /|\                                          /|\

    before: one head                      after: three, from the side buds

Giant hogweed shows the compensation on which the effect rests, at two levels.

The individual plant. Cutting off the terminal umbel removes the source of apical dominance; buds lower on the stem grow out and produce several smaller umbels in its place. When only the terminal umbel was removed at peak flowering, the plant’s total output of seed did not fall at all.² When every umbel was removed, nearly half of the plants regenerated and set seed again in the same season.³ Being monocarpic, a plant that has begun to flower dies at the end of the season whatever is done to it;⁴ cutting only decides how much seed it leaves behind.

The stand. Before they flower, plants that lose their leaves year after year put off flowering and live longer: on grazed pastures the median age at flowering was five years, against three on unmanaged land.⁵ A stand that is kept down without being uprooted therefore keeps its plants, and its seed, in reserve. Whether regular mowing actually enlarges a stand, as a true hydra effect would require, has not been measured.^{[citation needed]} The measure most often taken against the plant is, at best, the one that preserves it.

Avoiding the effect

The hydra effect arises only where control is partial: where the head is removed but the body remains. It does not arise when the root is destroyed. In the myth, Heracles defeated the Hydra only when his companion Iolaus seared each neck as its head was cut, so that nothing could grow back, and the one immortal head was buried under a rock. The modern equivalent is root cutting.

See also

• Control of giant hogweed
• Apical dominance
• Seed bank