A new angle on flowers: fish are players in pollination

October 5, 2005

GAINESVILLE, Fla. — Flowering plants near ponds may owe their pollination not only to the winged creatures of the air, but also to the finned ones of the deep.

Scientists have discovered that fish indirectly help spread pollen among flowers near the water. That’s because they eat dragonfly larva, which live in freshwater ponds and other water bodies. Adult dragonflies are major predators of bees, butterflies and other insect pollinators. The result is a simple but unexpected cascade: The more fish, the fewer dragonflies, the more bees and butterflies, the more plant pollination, reproduction and seeds.

A paper about the discovery, co-authored by scientists at the University of Florida and Washington University in St. Louis, will appear Thursday in Nature.

“Much of the science of ecology is elucidating the surprising and counterintuitive connections among species,” said Robert Holt, a UF professor of zoology and one of five authors. “What we’ve done here is elucidate a heretofore unsuspected connection of that sort.”

The research is significant in part because it shows how organisms from one ecosystem can change the lives of those in a seemingly separate one – with the help of something, in this case the dragonfly, tying them together.

But it’s also notable because it highlights an unusual example of how humans can potentially shape nature. For example, it’s well known that habitat destruction and overfishing of salmon have ecological consequences from limiting food for grizzlies and other predators to reducing mountain stream nutrient levels. The fish-flower research shows the same potential for human impacts at a far more local and everyday level. People may never notice, but when they create, fill or drain a pond, or stock it, the ecological ripples lap onto shore.

“The presence or absence of fish can have an influence, we showed, on terrestrial plant reproduction,” Holt said. “What often determines the presence or abundance of fish — people!”

The paper is based on field research from 2003 to2005 at UF’s Katherine Ordway Preserve, a 9,600-acre biological field station east of Gainesville. There, a crew of faculty members, postdoctoral associates and graduate students spent weeks observing the creatures at eight ponds, four with fish and four without.

The researchers counted and compared both the dragonfly larvae in the ponds and the dragonflies buzzing above them. They also compared the number of bees, butterflies and other pollinators visiting selected plants near the ponds. And they investigated the pollination of certain plants near the ponds.

Michael McCoy, a zoology doctoral student and another author of the paper, said the researchers chose dragonflies because they wanted to probe how predators affect pollination.

Dragonflies, he said, are known to attack and eat insects such as mosquitoes, biting flies and pollinators such as bees and butterflies — and they can even eat insects as large as they are, including other dragonflies. Stinging insects do not daunt them – the common name for one species, in fact, is “bee butcher.”

For the fieldwork, the researchers tapped methods used by both aquatic and bird biologists. To get an accurate estimate of the dragonfly larvae, they placed a metal box at random areas in each pond and then dip-netted and counted larva. To gauge dragonfly numbers, they observed designated spots, counting dragonfly visits with hand clickers.

The result was “much stronger than we originally had anticipated,” McCoy said.

In one example, the scientists counted an average of nearly 70 large adult dragonflies every five minutes around ponds without fish, more than double that of ponds with fish. And plants near fish-free ponds were significantly more likely to be “pollen starved” — or lack sufficient pollen to produce seeds — than those near ponds with fish.

It would be natural to conclude from the research that ponds with fish have more plants around them. But that’s not necessarily the case, the researchers said. Many factors play a role in plant abundance. Dragonflies can consume some plant-eating insects, as well as pollinators, so may at times benefit plants. Moreover, many plants produce both sexually through pollination and asexually through deploying underground runners or other means. Some perennials, meanwhile, may need only one successful season of pollination for several years of growth, which could limit power of the “fish effect” in any one year.

Krista McCoy, a UF doctoral zoology student, noted the five researchers had different specialty areas. “I think this research shows how important it is to mull around ideas that other people have been thinking about, and work on each other’s strengths,” she said.