Frog Pond

Frogs, toads and dragonflies begin life in water and spend the rest of it on land, hunting insects. A frog pond is a small basin built to supply the water half, so the hunting half happens near the crops. It is not a fish pond and not a water store. Fish eat amphibian eggs and larvae, so one introduction ends the point of it, and the water is never drawn off for other uses. What the pond holds is a hydroperiod, a stretch of weeks that never dries, because eggs must stay submerged and larvae must grow old enough to leave the water. A basin that dries halfway through produces nothing rather than less.

No standard covers a pond this small, so its shape follows what the animals need. The edges slope very gently, about one in ten or flatter, since a steep bank stops them getting in and out. Somewhere in the basin is 75 cm (30 in) of depth so it cannot freeze solid. Roughly a third to two fifths of the surface stays open water and the rest is planted, with rocks and anchored logs for shade, damp and basking. A flexible liner or a preformed shell stops the water soaking away. The ground around the pond counts as much as the water, since the adults live most of the year in the soil and leaf litter nearby, so that buffer is left unmown, undug and unsprayed.

Everything a manager would normally reach for harms this pond. Fertiliser, nutrient-rich runoff, algaecide, aquatic herbicide and pond dye all strip out the vegetation the animals depend on. Left alone it closes in: cattail and other vigorous marginal plants take over the open water, so the emergent growth is thinned each year and a small basin is emptied and reset every two or three years. It goes close enough to beds and orchard for the predation to reach them, near a bird bath or field margin, and well away from any pond holding fish. The pest control is real, and how much of it there is has never been measured.