A question that has puzzled biologists for more than 150 years may finally have a simpler answer.
Climate appears to influence whether invasive plants will thrive, blending in with native species or standing out from them.
This can help local communities identify environmental threats early, before landscapes, wildlife habitats, and even human health are affected.
Researchers analyzed millions of preserved plant specimens from across the continental United States, some dating back more than 200 years, and found that successful invasive species tend to obey different rules depending on the environment.
A study published in the journal Proceedings of the National Academy of Sciences examines what is often called Darwin's naturalization puzzle.
Charles Darwin proposed two possibilities: new species might become established because they resemble native plants and adapt to local conditions, or because their differences allow them to avoid direct competition.
«"These results suggest that Darwin's naturalization puzzle may not be a puzzle at all, but a predictable range of outcomes," said lead author Tadeo Ramirez-Parada.
To reach this conclusion, the team used herbarium specimens—dried plants collected by scientists over generations—as well as historical weather data to determine the bloom periods of thousands of native and invasive species.
In hot, cold, or arid climates, successful invasive species typically resembled native plants more closely. In milder regions, they often thrived better, exhibiting different behavior, especially flowering earlier.
Invasive plants don't just crowd out native flowers. They can alter entire ecosystems in ways that impact the lives of both humans and wildlife.
California provides a prime example. Many native meadows and wildflower habitats there have been overrun by fast-growing invasive annual grasses that emerge early, occupy space, and crowd out slower-growing native species.
This can reduce food sources and habitat for pollinators, hinder the recovery of native plants, and even increase the risk of wildfires.
There are also direct human costs. Some invasive grasses are wind-pollinated, producing massive amounts of pollen. Susan Meiser has stated that pollen-related respiratory allergies cost the United States over $3 billion annually in medical expenses.
When native plants that attract pollinators disappear, insects that help sustain crops also come under greater pressure.
The researchers said they were struck by the clarity of the pattern they discovered.
«"I was surprised and delighted to see such a clear pattern and such a strong match with theoretical expectations," Ramirez-Parada said. "Ecology is typically complex—after all, it deals with incredibly intricate systems—so signals are rarely so strong and clear.".
The findings could help land managers focus on controlling the most dangerous invasive species before they spread.
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