Man-made Ponds

Aside from the rare natural pond, ponds occur on farms because they are constructed for irrigation, watering or drainage. Small natural ponds are relatively unusual in our landscape, probably in part because they rapidly fill in with vegetation and debris, as many pond owners know. More recently, perhaps in part to emulate this on-farm tradition, ponds have become a frequent component of semi-rural landscaping.

Fig. 1. The biodiversity value of a pond is affected not only by how kempt the margins are (with messier tending to be better) but also by how the aquatic portion is treated - are fish introduced? is aquatic vegetation discouraged through fish or chemicals?

Ponds can vary in their value for nature (Fig. 1). At one end of the spectrum, heavily landscaped ponds with close-mown edges, aquatic vegetation controlled by chemicals and/or carp, and stocked with predatory fish like Largemouth Bass, can be ecological traps for native organisms. For example, amphibians or dragonflies attracted to such ponds may quickly find that they or their progeny have little to shelter them from the hungry predators. At the other end of the spectrum, ponds which are similar to our shallow vernal pools - they possess natural aquatic vegetation, have gradual banks populated by wetland plants, and are not stocked with predatory fish - can offer functional ecological analogies for some of our vernal pool amphibians or fish-sensitive dragonflies.

Fig. 2. Seasonal (or vernal) pools can be unique and required habitat for some of our amphibians who reside in forested uplands during much of the year but return to such ponds to breed. These include the Spotted Salamander (lower left) and Wood Frog (lower right).

Vernal pools (Fig. 2) are temporary ponds where periodic drying, a solid freeze and/or low-oxygen conditions eliminate predatory fish and make the establishment of predatory amphibian populations (such as Bullfrogs or Red-spotted Newts) less likely. A certain set of amphibians (including, in our area, Wood Frogs and Spotted & Jefferson Salamanders) have evolved to reproduce in these pools. Apparently, at least one crucial characteristic is the absence of predators. Shelter, in the form of aquatic vegetation or a thick carpet of leaf litter, is probably also important. We know several permanent ponds which do not have predatory fish and where these sensitive amphibians reproduce every year.

While we do not seem to have specialized vernal pool dragonflies and damselflies, the aquatic larvae of some of our species are especially susceptible to fish predation (at least in part, the difference seems to be behavioral: the larvae who freeze at the approach of a predator seem better off than those who shoot away). The presence of shelter and the absence of fish seems to benefit these susceptible creatures. As one might predict, our data suggested that shelter was especially important in the presence of fish.

Man-made ponds can also serve as analogues for beaver ponds and other water bodies. Several of our native reptiles and amphibians (such as Tree Frogs, Spring Peepers, and Painted Turtles; Fig. 3) will readily adopt them. Although these are not strictly vernal pool species, shelter (in the form of aquatic vegetation) and lack of predatory fish seem to also benefit these species.

Fig. 3. Ponds can also provide habitat for other amphibians.

Perhaps the first question of pond management is whether one really needs a pond and, if so, where it is created. In most agricultural situations, the practical needs are evident. Recreation can also be an important reason. In any case, the net impact of pond construction depends largely on where it is installed: installing a pond in an upland field (when hydrology allows it) can bring ecologically useful wetland habitats to an old field; on the other hand, installing such a pond in an existing wetland can mean replacing a diverse natural wetland community with a biologically poorer substitute (Fig. 4).

Fig. 4. Where ponds are built can be an important determinant of their ecological impact. Constructing a pond in a pre-existing wetland tends to replace a more ecologically valuable habitat with a lesser one.

When dealing with an existing pond, it is useful to think of the ecological analogy you are seeking to emulate. In most cases, the more shelter you allow to develop and the fewer aquatic predators you introduce, the better from the perspective of native biodiversity (Fig. 5). The pond habitat really includes not only the waterbody itself, but also its immediate surroundings. We were initially perplexed, for example, when we found a correlation between wetland-specialist dragonflies and the amount of pasture surrounding the ponds (Fig. 6). We then realized that, in contrast to lawns, the light cattle trampling and browsing (none of these were loafing yard ponds) of the shoreline created a mosaic of wet meadow around the ponds edges. This not only provided habitat for native wetland plants but also the diverse structural texture that adult dragonflies seem to appreciate during their mating and territorial patrol.

Fig. 5. During our study of upland open ponds, we found that lawn ponds were the least diverse group of ponds we studied. Looser management tended to promote more native biodiversity.

Fig. 6. We found that, among the ponds we studied, the more specialized dragonflies favored ponds margined by pasture. In other words, in contrast to lawn or hay field, pasture seemed to create a semi-wetland margin that was more conducive to dragonflies, perhaps because it offered more perches for the adults.

For more on ponds, see our general Ponds page.