Researchers are predicting that the number of non-native species will rise by about 36 percent worldwide by 2050.

Non-native, or alien species, are plants or animals that humans have moved around the world to places where they are not naturally found. Sometimes the introduction of an alien species to an area is accidental. Sometimes, it is done on purpose.

Research shows the movement of plants and animals rose across the planet over the last century as human trade and travel opened up new worldwide pathways.

A new study predicts such movements are likely to continue, with the largest increases expected to be big insects, birds and small creatures such as mollusks and crustaceans.

Researchers are calling for more rules and better observation methods to help reduce the spread of alien species.

The findings were recently reported in the publication Global Change Biology.

Scientists involved in the study say more than 35,000 alien species had been identified in the most recent report on the subject in 2005. Some of the species can go on to become invasive, meaning they spread quickly in undesirable and harmful ways.

The study suggests alien species introductions will increase on every world continent. But the largest increases are predicted to be in Europe. The researchers estimate such species will increase 64 percent across Europe by around 2050.

Hanno Seebens is an ecologist at Germany's Senckenberg Biodiversity and Climate Research Centre. He was the lead writer of the study. "Together with climate change and land use change, invasive alien species are posing one of the greatest threats to biodiversity," Seebens said.

He added that a species can only arrive in a new area when human activity connects different areas. "When we extended our trade networks, we connected more and more (areas), which allowed more and more species to come."

The researchers developed a mathematical model to predict alien species introductions for each continent between 2005 and 2050. The model was based on past records of alien species introductions, as well as estimates of species that could end up becoming invasive if current movements continue.

Cascade Sorte is a professor of ecology and evolutionary biology at the University of California Irvine. She was not part of the research. "We know that a certain proportion of alien species will be problematic, so the more of them that there are, the higher the likelihood that we'll have problems," she said.

Sorte described the latest predictions as "shocking" because even with the past rises in alien species, "there's even a possibility that things can get worse."

However, Hanno Seebens said it is possible the number of species could fall in the future based on continued rising movements. "We may just run out of species to be transported, because at some point, all species may have been transported already," he said.

I'm Bryan Lynn.

Asher Jones wrote this story for VOA News. Bryan Lynn adapted the report for Learning English, with additional information from University College London. Caty Weaver was the editor.