Before Dr. Fridley and Dr. Sax met in 2007, each had become convinced that Darwin might be right. When they discovered they shared the same belief, they joined forces to test Darwin’s idea. Their approach was two-pronged.

First, they looked at the places where invasive plant species tend to originate, examining the number of plant families in various regions. Invasive plants, they found, were more likely to have evolved in habitats with a great diversity of competing species. Darwin was right: Some plants have evolved to be fighters.

“We were both kind of gobsmacked,” said Dr. Fridley.

In the second test, Dr. Fridley and Dr. Sax looked at the role that canals have played in helping some species become invasive. Sometimes, humans have dug canals that linked ecosystems with an equal diversity of species. But in other instances, canals have connected regions with low diversity to those with great variety of species.

The scientists predicted that invasive fish and mollusks would tend to come from places with high diversity and would have used canals to establish themselves in habitats with low diversity.

“It’s not a perfect experiment,” said Dr. Sax. “But it’s still a pretty good unplanned experiment.”

In 1825, the Erie Canal joined the Great Lakes to the Hudson River. The two ecosystems orginally had about the same diversity of fish and mollusk species, the ecologists found, and species from each side became invasive on the opposite side in roughly the same proportions.

The story of the Suez Canal was very different. On one side was the Red Sea and the Indian Ocean, a huge, stable ecosystem with a deep diversity of fish and mollusks. On the other side was the Mediterranean, a relatively young habitat without nearly as much species diversity.

Dr. Fridley and Dr. Sax found the Mediterranean was overwhelmed with invasive species from the other side of the canal, while hardly any from the Mediterranean took up residence in the Red Sea.