A question that has gone unsolved for biologists for more than 150 years may finally have a simpler answer. 

Climate appears to shape whether invasive plants thrive by blending in with native species or by standing out from them.

That could help communities identify ecological threats earlier, before landscapes, wildlife habitat, and even human health take a hit.

Researchers analyzed millions of preserved plant records from across the continental United States, some dating back more than 200 years, and found that successful invasive species tend to follow different rules depending on the environment.

The study, published in Proceedings of the National Academy of Sciences, tackles what is often called Darwin's naturalization conundrum. 

Charles Darwin laid out two possibilities: New species might do well because they resemble native plants and fit local conditions, or because their differences allow them to avoid direct competition.

"These results imply that Darwin's Naturalization Conundrum might not be a conundrum at all, but a predictable range of outcomes," study lead author Tadeo Ramirez-Parada said.

To reach that conclusion, the team used herbarium specimens, dried plants collected by scientists over generations, along with historical weather records to infer flowering periods for thousands of native and invasive species.

In climates that are hot, cold, or dry, successful invaders usually looked more like native plants. In milder regions, they often fared better by behaving differently, especially by flowering earlier.

Invasive plants do more than crowd out native flowers. They can reshape entire ecosystems in ways that make life worse for both people and wildlife.

California offers a clear example. Many native grasslands and wildflower habitats there have been overtaken by fast-growing invasive annual grasses that germinate early, claim space, and shut out slower native species. 

That can reduce food and habitat for pollinators, make it harder for native plants to recover, and even increase wildfire risk.

There are direct human costs as well. Some invasive grasses are wind-pollinated, meaning they release huge amounts of pollen. Susan Mazer said respiratory allergies linked to pollen account for more than $3 billion a year in U.S. medical costs.

When native, pollinator-friendly plants disappear, insects that help support food crops also come under greater pressure.

The researchers said they were struck by how clear the pattern was.

"I was surprised and excited to see such a strong pattern and clean agreement with theoretical expectations," Ramirez-Parada said. "Ecology is typically messy — it deals with incredibly complex systems, after all — so signals are rarely this strong and clear."

This insight could help land managers focus on the riskiest invaders before they spread.

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