Dov Sax of Brown University and Jason Fridley from Syracuse University aren’t offering a new idea to explain species invasiveness. In fact, Charles Darwin suggested it first. What’s new about Sax and Fridley’s “Evolutionary Imbalance Hypothesis” (EIH) is that they’ve tested it using quantifiable evidence and report in Global Ecology and Biogeography that the EIH works well.
The EIH idea is as follows: Species from regions with deep and diverse evolutionary histories have a higher chance of becoming successful invaders in regions with less deep, less diverse invasiveness, ecologists can quantify the imbalance between the evolutionary histories of “donor” and “recipient” regions as Sax and Fridley show in several examples.
Darwin’s original understanding was that the more challenges a region’s species have faced in their evolution, the more robust they’ll be in new environments.
“As natural selection acts by competition, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates,” Darwin wrote in 1859. Better tested species, such as those originating from larger regions, he reasoned, have “consequently been advanced through natural selection and competition to a higher stage of perfection or dominating power.”
To Sax and Fridley the explanatory power of EIH suggests that when analysing invasiveness, ecologists should include historical evolutionary imbalance to the other factors under consideration.
“is well-studied now, but this is never listed there even though Darwin basically spelled it out,” said Sax, associate of ecology and evolutionary biology. “It certainly hasn’t been tested before. We think this is a really important part of the story.”
Evidence for EIH
Advancing Darwin’s insight from idea to actual hypothesis required determining a way to test it against measurable evidence. The ideal data would summarise a region’s population size and diversity, relative environmental stability and habitat age, and the intensity of competition. Sax and Fridley found a suitable proxy: “phylogenic diversity” (PD), an index of how many unique lineages have developed in a region over the time of their evolution.
“All else equal, our expectations is that biotas represented by lineages of greater number or longer evolutionary history should be more likely to have produced a more optimal solution to a given environmental problem, and it is regional disparity, approximated by PD, that allows predictions of global invasion patterns,” they wrote.
With a candidate measure, they put EIH to the test.
Using comprehensive databases on planet species in 35 regions of the world, they looked at the relative success of those species’ invasiveness in three well-documented destinations: Eastern North America, the Czech Republic, and New Zealand.
The discovered that in all three of the regions, the higher the PD of a species’ native region, the more likely it was to become invasive in its new environment. The size of the effect varied across the three regions, which have different evolutionary histories, but it was statistically clear that plants forged in rough neighborhoods were better to bully their way into a new region than those originating from evolutionarily more “naïve” areas.
Sax and Fridley undertook another test of the EIH in animals by looking at cases where marine animals were suddenly able to mix after they became united by canals. The EIH predicts that the imbalance of evolutionary robustness between the sides would enable a species-rich region to dominate the less diverse one on the other side of the canal by even more than a mere random mixing would imply.
The idea has a palaeontological precedent. When the Bering land bridge turned into the Bering Strait, it gave marine mollusks a new polar path between the Atlantic and Pacific Oceans. Research conducted previously indicated that more types of mollusks successfully migrated from the diverse Pacific to the less diverse Atlantic than vice-versa, and by more so than by their relative abundance.
In the new paper, Sax and Fridley examined what has occurred since the openings of the Suez Canal in Egypt, the Erie Canal In New York, and the Panama Canal. The vastly greater evolutionary diversity in the Red Sea and Indian Ocean compared to the Mediterranean Sea and the Atlantic led to a vast flow of species north through Suez.
However, evolutionary imbalances across the Erie and Panama Canals were quite small (the Panama canal connects freshwater drainages of the Atlantic and Pacific that were much more ecologically similar than the oceans) so as EIH again predicts, there was a more even balance of cross-canal species invasions.
Sax and Fridley recognize in the paper that the EIH does not singlehandedly predict the success of individual species in certain invasions. Instead, it allows for ecosystem managers to assess relative invasiveness risk based on the evolutionary history of their ecosystem and that of other regions. Take, for example, a wildlife official in a historically isolated ecosystem like an island.
“They already know to be worried, but this would suggest they should be more worried about imports from some parts of the world than others,” Sax said.
Not all invasions are bad, Sax illustrated. Newcomers have the potential to provide some ecosystem services- such as erosion control- more capably if they become established. The EIH can aid in assessments of whether a new wave of potential invasion is likely to alter the way the ecosystem will provide its services, for better or worse. “It might help to explain why non-natives in some cases might improve from ecosystem functioning,” Sax said.
But perhaps Darwin already knew all that.
Contributing Source: Brown University
Header Image Source: Wikimedia
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