AboutZoos, Since 2008


Dar­win was right – again — about inva­sive species

pub­lished 04 Octo­ber 2014 | mod­i­fied 18 Octo­ber 2014

Based on insights first artic­u­lated by Charles Dar­win, pro­fes­sors at Brown Uni­ver­sity and Syra­cuse Uni­ver­sity have devel­oped and tested the “evo­lu­tion­ary imbal­ance hypoth­e­sis” to help pre­dict species inva­sive­ness in ecosys­tems. The results sug­gest the impor­tance of account­ing for the evo­lu­tion­ary his­to­ries of the donor and recip­i­ent regions in invasions.

Dov Sax of Brown Uni­ver­sity and Jason Fri­d­ley of Syra­cuse Uni­ver­sity aren’t propos­ing a novel idea to explain species inva­sive­ness. In fact, Charles Dar­win artic­u­lated it first. What’s new about Sax and Fridley’s “Evo­lu­tion­ary Imbal­ance Hypoth­e­sis” is that they’ve tested it using quan­tifi­able evi­dence and proved that it works well. Their study is pub­lished in Global Ecol­ogy and Bio­geog­ra­phy on 1 October.

The Evo­lu­tion­ary Imbal­ance Hypoth­e­sis

The Evo­lu­tion­ary Imbal­ance Hypoth­e­sis (EIH) pro­poses the fol­low­ing: Species from regions with deep and diverse evo­lu­tion­ary his­to­ries are more likely to become suc­cess­ful invaders in regions with less deep, less diverse evo­lu­tion­ary his­to­ries. To pre­dict the prob­a­bil­ity of inva­sive­ness, ecol­o­gists can quan­tify the imbal­ance between the evo­lu­tion­ary his­to­ries of “donor” and “recip­i­ent” regions as Sax and Fri­d­ley demon­strate in sev­eral exam­ples.

Darwin’s orig­i­nal insight 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 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 add his­tor­i­cal evo­lu­tion­ary imbal­ance to the other fac­tors they con­sider.

Inva­sion biol­ogy is well-​studied now, but this [his­tor­i­cal evo­lu­tion­ary imbal­ance] is never listed there even though Dar­win basi­cally spelled it out
Dov Sax, Depart­ment of Ecol­ogy and Evo­lu­tion­ary Biol­ogy, Brown Uni­ver­sity, USA »

“It cer­tainly hasn’t been tested before. We think this is a really impor­tant part of the story,” Sax adds.

Evi­dence for EIH
Advanc­ing Darwin’s insight from idea to hypoth­e­sis required deter­min­ing a way to test it against mea­sur­able evi­dence. The ideal data would encap­su­late 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: “phy­lo­ge­netic diver­sity” (PD), an index of how many unique lin­eages have devel­oped in a region over the time of their evolution.

“All else equal, our expec­ta­tion 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 this regional dis­par­ity, approx­i­mated by PD, that allows pre­dic­tions of global inva­sion pat­terns,” they wrote.

With a can­di­date mea­sure, they put EIH to the test.

Invasive plant speciesAn exam­ple in plants
Using detailed data­bases on plant 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.

They found that in all three regions, the higher the PD of a species’ native region, the more likely it was to become inva­sive in its new home. The size of the effect var­ied among the three regions, which have dif­fer­ent evo­lu­tion­ary his­to­ries, but it was sta­tis­ti­cally clear that plants forged in rough neigh­bour­hoods were bet­ter able to bully their way into a new region than those from evo­lu­tion­ar­ily more “naïve” areas.

An exam­ple in ani­mals
Sax and Fri­d­ley con­ducted 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 an imbal­ance of evo­lu­tion­ary robust­ness between the sides, would allow a species-​rich region to dom­i­nate a less diverse one on the other side of the canal by even more than a mere ran­dom mix­ing would suggest.

The idea has a pale­on­to­log­i­cal prece­dent. When the Bering land bridge became the Bering Strait, it offered marine mol­luscs a new polar path between the Atlantic and Pacific Oceans. Pre­vi­ous research has shown that more kinds of mol­luscs suc­cess­fully migrated from the diverse Pacific to the less diverse Atlantic than vice-​versa, and by more so than by their rel­a­tive abundance.

Other exam­ples
In the new paper, Sax and Fri­d­ley exam­ined what has hap­pened 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 an over­whelm­ing flow of species north through the Suez.

But evo­lu­tion­ary imbal­ances across the Erie and Panama Canals were fairly 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.

Applic­a­ble pre­dic­tions
Sax and Fri­d­ley acknowl­edge in the paper that the EIH does not sin­gle­hand­edly pre­dict the suc­cess of indi­vid­ual species in spe­cific inva­sions. Instead it allows for ecosys­tem man­agers to assess a rel­a­tive inva­sive­ness risk based on the evo­lu­tion­ary his­tory of their ecosys­tem and that of other regions. Take, for instance, a wildlife offi­cial in a his­tor­i­cally iso­lated ecosys­tem such as 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 noted. New­com­ers can pro­vide some ecosys­tem ser­vices — such as ero­sion con­trol — more capa­bly if they can become estab­lished. The EIH can help in assess­ments of whether a new wave of poten­tial inva­sion is likely to change the way an ecosys­tem will pro­vide its ser­vices, for bet­ter or worse.

“It might help to explain why non-​natives in some cases might improve ecosys­tem func­tion­ing,” Sax said.

But per­haps Dar­win already knew all that.

(Source: Brown Uni­ver­sity news release, 02.10.2014)

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