Fish in ponds benefit flowering plants

Oct. 5, 2005 -- Fish and flowering plants would seem to have as much in common as pigs and beauty soap. But ecologists at Washington University in St. Louis and the University of Florida have found an amazing relationship between the different species that provides a new direction for understanding how ecosystems "hook up."

A team of researchers, headed by Tiffany Knight, Ph.D., Washington University assistant professor of biology in Arts & Sciences, has shown a correlation between the presence of fish in ponds and well-pollinated St. John's wort (Hypericum fasciculatum, Hypericaceae) at a Florida research station.

The team checked out eight ponds at a University of Florida preserve, four containing fish, the other four fish-free. They found that shoreline St. John's wort plants near the fish ponds were far better pollinated than those near the fish-free ponds. The reason? Fish reduce, if not decimate, dragonfly populations when they start their lives in the ponds as larvae. Those dragonflies that can escape the fish grow up to live outside the water environment where their major prey are bees, moths and flies, which live in a synergistic state with the flowering plants — what ecologists call "mutualism." A bee, for instance, gets nourishment from a flowering plant, and the plant is able to reproduce because of the bee's attention; thus, both species benefit mutually.

	Fish in ponds can be a flowering plant's best friend, according to WUSTL ecologists.

Novel find

"This cross-ecosytem linkage is a novel find," said Knight. "We've shown that species interactions can reverberate across two different ecosystems and have major implications for the food web and species' survival.

Tiffany Knight

"The work is different from most trophic cascade — food web — studies in that it incorporates mutualism instead of focusing strictly on predator-prey relationships. Taking a complex life history into account also presents new insights into ecological processes."

A dragonfly's life history is complex, Knight explained, in that, like an amphibian, it occupies two different habitats during its life, thus becoming a conduit between one habitat, the pond, and another, the landscape near a pond.

The results were published in the Oct. 6, 2005 issue of the journal Nature.

"The study illustrates how spatial mobility can lead to surprisingly strong couplings among disparate habitats in complex landscapes," said Robert D. Holt, professor of biology at the University of Florida and a study co-author. "A recognition of this fact opens up fresh questions that need to be considered by both basic ecologists and natural resource managers."

Not only did the team observe more pollinators in landscapes next to ponds with fish, they also saw differences in the kinds of pollinator species. Most visitors near ponds with fish were hymenopterans, — for the most part, bees — compared with most visitors at the fish-free ponds, mainly flies. Hypericaceae have evolved traits that attract bees, and so bees may be better pollinators of Hypericum than flies. The effect of reduced pollinator visits near fish-free ponds might be magnified, Knight said, in part because the few visits pollinators made to the area were from flies and moths, rather than bees.

The researchers made sure that the vegetation structures of each pond were similar. And they also experimented with another flowering plant, Sagittaria latifolia, and came up with similar results to what they found with Hypericaceae.

Avoiding fish-free ponds

The researchers also found that pollinators tend to avoid fish-free ponds because of the presence of dragonflies. Likewise, there is evidence that dragonflies avoid laying eggs in ponds with fish.

Knight noted that many organisms, for instance, salamanders, with terrestrial life stages also are key aquatic predators, so the reverse — a cascade from terrestrial to aquatic ecosystem - also is true.

"This finding will open up many opportunities to examine interactions across ecosystem boundaries," she said.

For one, getting grips on cross-ecosystem "habitat" connectance could be a key component of gauging the effects of human encroachments on nature. Stocking ponds with fish is universal, whether for a fishing hole or to reduce pests, but now urban and rural landscapers and developers can realize that the fish have a wider impact than their original purpose.

Freshwater fish introductions have the potential to alter competitive relationships among terrestrial plants, hampering the competitiveness of non-insect pollinated plants. Wetland destruction impacts dragonfly populations along with terrestrial plants. Polluted ponds and those that dry up at certain times of the year and those suffering from an excess of certain nutrients - a condition called eutrophication - all can harm fish abundances and insect-pollinated plants.

"Consumer flows across radically disparate ecosystems can affect landscape-level processes and drive local species interactions," the authors conclude.