Fish In Ponds Benefit Flowering Plants

A team ofresearchers, headed by Tiffany Knight, Ph.D., Washington Universityassistant professor of biology in Arts & Sciences, has shown acorrelation between the presence of fish in ponds and well-pollinatedSt. John's wort (Hypericum fasciculatum, Hypericaceae) at a Floridaresearch station.

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

Novel find

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

"Thework is different from most trophic cascade - food web - studies inthat it incorporates mutualism instead of focusing strictly onpredator-prey relationships. Taking a complex life history into accountalso presents new insights into ecological processes."

Adragonfly's life history is complex, Knight explained, in that, like anamphibian, it occupies two different habitats during its life, thusbecoming a conduit between one habitat, the pond, and another, thelandscape near a pond.

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

"Thestudy illustrates how spatial mobility can lead to surprisingly strongcouplings among disparate habitats in complex landscapes," said RobertD. Holt, professor of biology at the University of Florida and a studyco-author. "A recognition of this fact opens up fresh questions thatneed to be considered by both basic ecologists and natural resourcemanagers."

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

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

Avoiding fish-free ponds

Theresearchers also found that pollinators tend to avoid fish-free pondsbecause of the presence of dragonflies. Likewise, there is evidencethat dragonflies avoid laying eggs in ponds with fish.

Knightnoted that many organisms, for instance, salamanders, with terrestriallife stages also are key aquatic predators, so the reverse - a cascadefrom terrestrial to aquatic ecosystem - also is true.

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

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

Freshwaterfish introductions have the potential to alter competitiverelationships among terrestrial plants, hampering the competitivenessof non-insect pollinated plants. Wetland destruction impacts dragonflypopulations along with terrestrial plants. Polluted ponds and thosethat dry up at certain times of the year and those suffering from anexcess of certain nutrients - a condition called eutrophication - allcan harm fish abundances and insect-pollinated plants.

"Consumerflows across radically disparate ecosystems can affect landscape-levelprocesses and drive local species interactions, " the authors conclude.