The steady rise in alien invasions is troubling scientists, who are concerned about the innocent Earth creatures that could be threatened.

A group of researchers reported that the number of invasive plant and animal species continues to increase around the world and constitutes a threat to biosecurity. Their study, in the journal Proceedings of the National Academy of Sciences, analyzes how many of those plants and animals are being encountered as invasive species for the first time — referring to them as “emerging alien species” — and says that understanding how they operate will be crucial for understanding and mitigating their effect on native species and on the rest of the environment.

Trends show that “even after many centuries of invasions the rate of emergence of new alien species is still high,” according to the study.

Up to 16 percent of Earth’s species could be classified as alien species, based on the analysis, which used data and accounts of invasive species going all the way back to the year 1500.

“These results suggest that there remains a high proportion of emerging alien species we have yet to encounter, with future impacts that are difficult to predict,” the paper says.

The study points to “expanding trade networks and environmental change” as drivers of these alien invasions.

Humans have introduced non-native, invasive species to many places around the globe as they have explored and expanded their domains, but it’s not the only way invasive species spread. In Antarctica, for example, some scientists have asserted that climate change could allow invasive animals to threaten others, as ice melts and leaves more ground open for those species to cover.

“Humans have been moving species to new places for thousands of years, so we might have expected that most species that have the potential to become aliens would already have done so,” study co-author Tim Blackburn said in a statement from University College London. “Instead, it seems the pool of new aliens is far from dry.”

Invertebrates like insects and mollusks are most at risk, the university notes, with the highest proportion of species operating as emerging aliens.

“With measures being taken to prevent alien species introductions and spread, there has been a decline in the proportions of newly emerging alien species from established sources, such as historical European colonies,” co-author Ellie Dyer said in the statement. “However, this decrease has been offset by newly emerging alien species elsewhere and it is likely that we can expect many more new invasions starting to appear from regions with large and growing economies. These findings will be extremely helpful for horizon scanning studies that aim to identify ‘door knocker’ species, which are those not yet recorded but are suspected of presenting a high risk of arrival and detrimental impacts.”