SYDNEY: History, it seems, rules: a seven-year experiment shows that pond communities bear a lasting imprint of random events in their past. Frustrated by trying to decode nature’s experiments on pond ecosystems, researcher Jonathan Chase set up his own in a group of Rubbermaid cattle tanks.

After monitoring 45 mini-ponds for seven years, the biologist learned that the eventual make-up of species in the tiny ecosystems came down to history. Chase created different pond ecosystems 1,100-litre tanks in the summer of 2002 at the Tyson Research Centre, a field station owned by Washington University in St. Louis, Missouri, where he is a professor.

“After seven years of essentially doing nothing but varying the colonisation in the first few years, they remained very different,” said Chase.

The tank experiment addresses a recent idea in ecology proposed by Steven Hubbell, known as the unified neutral theory.

Random fluctuations

Hubbell proposed that random fluctuations in births, deaths, and introduction of new species play a larger role than adaptations and competition among species in determining the make-up of a community. To test this, Chase fertilised his cattle tank ponds with different amounts of nitrogen and phosphorous, creating groups of low, medium, and high-productivity systems. He included the same set of species in each pond, but varied the time at which he introduced them.

In each of the first three years of the experiment, he added a different and randomly selected group of species.

“Then we let nature take over,” said Chase, who along with his students studied the species diversity and prevalence in the ponds for a total of seven years. In the end, the low-productivity tanks all ended up looking very similar. “But each high productivity pond looked different,” said Chase. “In that case, where you started determined where you ended up.”

History plays a significant role

Chase concluded that random processes, as Hubbell described, were important in high productivity pools, but couldn’t fully explain his results: history played a significant role. In lower productivity cases, species’ abilities to adapt and compete drove the outcomes, since their effectiveness in using resources in a nutrient-poor environment plays a much larger role in colonising.

By isolating the effects of history on ecological communities, the tub experiment results suggest a new way to approach restoration efforts.

“Restoration projects rarely consider history as an important component of biodiversity (and often eliminate it),” said Chase. The amount of variability he saw in high-productivity vats could explain why restored tropical rainforest or coral reef systems often don’t end up looking as they did originally.

“While the experimental ponds are only semi-realistic, they allow us to test ideas that couldn’t be tested in more natural (and variable) ecosystems,” said Chase.

Novel approach

Chase’s efforts to unravel influences on ecosystem development did not go unnoticed by Margie Mayfield, an ecologist at the University of Queensland.

“It does take several years to do the appropriate test,” said Mayfield. “He’s been able to do that, so that’s really quite novel.”

The results might help focus habitat restoration efforts. “If you’re trying to restore something in low-productivity versus high-productivity environments, you can take much different approaches,” said Mayfield. Low-nutrient environments require careful selection of species, while nutrient-rich habitats can sustain many different combinations of species.

“Restoration is a very new field, so basically everything is helpful when we learn something about how communities form,” said Mayfield, and lessons learned from the pond example should be applicable to other types of ecosystems.