Where would you expect to find more stable populations of fish—in a large watershed whose main river has few tributary streams, or a small watershed with lots of tributaries?
Up to now, the prevailing wisdom has held that the stability of a species’ population dynamics—how its population changes in size—rests mostly on the size of the ecosystem in which it lives. But a new study by University of Minnesota and Japanese researchers shows mathematically that having more branches—i.e., tributary streams—in a river system works to stabilize the overall populations of fish species, while the size of the watershed has only a “vague” effect.
After working out this mathematical model, the researchers confirmed it with data from Japanese watersheds. The model, they say, likely applies to any ecosystem that may exhibit a branching structure, such as a field, a forest, or even, in the case of very tiny species, a single tree.
“These results imply that whenever humans alter ecosystems to make them simpler and less branched, it could destabilize large populations of many species and leave them vulnerable to rapid environmental changes,” says Akira Terui, first author of the study.
Terui is a researcher in the University of Minnesota Department of Ecology, Evolution, and Behavior (EEB) within the College of Biological Sciences, and also at Hokkaido University in Sapporo, Japan. The study is published in the Proceedings of the National Academy of Sciences.
Why More Branched is Better
Population dynamics tend to be synchronized in local populations of fish in the same stream because a single stream experiences the same set of environmental signals—for example, flood disturbance—and individuals in a stream easily mix. Synchrony tends to be lower between streams or across the whole system.
Under this scenario, if a short-term disturbance like a flood or pollution event cuts a species’ numbers in one or a few streams, fish from streams that have not suffered will disperse to the affected streams. This helps stabilize the overall population.
To confirm this prediction of the model, the researchers drew from 18 years of data on the populations of four ecologically distinct fishes in 31 watersheds on the island of Hokkaido, Japan.
“These small watersheds make an ideal laboratory,” Terui says. “They are separated by the ocean and vary greatly in branching structure due to geological and climatic differences.”
When the researchers examined the data from the 31 watersheds, it turned out that indeed, fish populations enjoyed greater overall stability in river systems with more complex tributary patterns.
Too Much Togetherness
But when a strong, systemwide ecological force smooths out differences in population dynamics among streams, the pattern reverses. Now, synchrony across the general population can become greater than synchrony within individual streams. Any disturbances to fish in one stream will disturb fish in others, it will be harder to replenish populations, and the whole system’s stability is weakened.
Humans can create this scenario by destroying branches of an ecosystem or by connecting them, which would essentially homogenize the branches. And we have other means of homogenizing ecosystems.
“Humans also make environments more similar across landscapes by, for example, adding nutrients or cutting tree cover,” says EEB professor Jacques Finlay, another author of the study.
Therefore, with the climate rapidly changing, the researchers urge a lessening of actions that would increase pressure on ecosystems by simplifying the complexity of their branching patterns.