Climate determines which invasive plants become successful

Plants have puzzled biologists for more than 150 years with a question that even Charles Darwin could not fully answer. Why do some arrive in a new place and spread rapidly, while others barely survive?

Scientists have spent decades studying invasive species. They have identified many possible reasons for their success, from escaping predators to finding better pollinators. Yet one problem remained.

The findings often came from individual case studies, making it difficult to see the bigger picture.

A new study suggests that the answer may have been hiding in plain sight all along: climate.

Plant traits in different environments

Researchers analyzed millions of plant records collected across the continental United States over more than 200 years.

The team’s findings show that the traits that help invasive plants succeed depend heavily on the conditions they encounter.

In tough environments, invasive species tend to resemble native plants. In milder environments, being different can be an advantage.

A question that dates back to Darwin

In On the Origin of Species, Darwin proposed two competing ideas about successful invaders.

One possibility was that introduced species succeed because they are similar to native plants and already suited to local conditions.

Another was that species succeed because they are different enough to avoid direct competition. For years, evidence seemed to support both ideas.

Some invasive species flourish because they leave predators and diseases behind. Others spread because they take advantage of new pollinators or seed dispersers.

Scientists collected plenty of examples, but the broader pattern remained frustratingly unclear.

Study lead author Tadeo Ramirez-Parada conducted the research as a doctoral student and postdoc in Susan Mazer’s lab at UC Santa Barbara.

“These results imply that Darwin’s Naturalization Conundrum might not be a conundrum at all, but a predictable range of outcomes,” said Ramirez-Parada.

Millions of dried plants tell a story

The researchers turned to herbaria, collections of preserved plant specimens gathered by scientists over centuries.

Each dried plant comes with valuable information, including where it was collected, when it was collected, and whether it was flowering at the time.

A single specimen provides only a snapshot. Millions of specimens together create a detailed record of plant life across generations.

The team focused on flowering time, a critical factor in plant reproduction. Flower too early or too late, and a plant may fail to produce seeds.

“So, plants that get the timing wrong risk being unable to produce offspring,” Mazer said.

Using specimen records, historical weather data, and advanced statistical methods, the researchers estimated flowering periods for thousands of species.

They then compared invasive and native plants across regions ranging from environmentally harsh to relatively mild.

Climate shapes the rules of success

The results revealed a striking pattern. In regions with harsh conditions, such as extreme heat, cold, or dryness, successful invasive plants closely resembled native species.

Those environments leave little room for experimentation. Plants must flower at the right time and respond to the same environmental cues as established species.

Milder regions told a different story. These areas support greater biodiversity and denser plant populations, creating stronger competition.

In these settings, invasive species often gained an advantage by behaving differently from native plants and occupying ecological niches that were less crowded.

“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.”

The researchers also discovered that invasive plants in mild climates often flowered significantly earlier than native species.

Getting a head start

Flowering early offers several benefits. Plants can attract pollinators before competitors do. They can produce seeds sooner and spread them over a longer period.

Over time, this early start may help invasive species dominate landscapes.

“I’ve seen many places in California where native wildflower species have been replaced by exotic grass species, which germinate and grow rapidly early in spring,” Mazer said.

“Especially for annual grasses, the common pattern for invasives seems to be to get a head start, occupy space, and displace the native competitors that are a little bit later to the game.”

The findings may help land managers identify future threats before they become major problems. In mild climates, evolutionarily distinct invasive plants may deserve closer attention.

In harsher climates, introduced species that closely resemble native plants may pose the greatest risk.

The growing problem of invasive grasses

The research also highlights concerns about invasive annual grasses, which have transformed many ecosystems.

California offers a vivid example. Large areas once supported diverse communities of native grasses and wildflowers. Today, many of those landscapes are dominated by fast-growing invasive grasses.

The consequences go beyond plant diversity. Dense grass cover can reduce habitat quality for wildlife, increase wildfire risk, and limit the return of native species.

Because many invasive grasses rely on wind pollination, they often provide fewer resources for insects that support healthy ecosystems.

“In addition, wind-pollinated species have evolved to produce much more pollen than insect-pollinated plants because so much of their wind-borne pollen is lost,” Mazer explained.

According to Mazer, respiratory allergies linked to pollen result in more than $3 billion per year in medical expenses in the United States.

Pollinator-friendly native plants also play a key role in supporting insects that help pollinate food crops. When native habitats disappear, those pollinators face increasing pressure.

The full study was published in the journal Proceedings of the National Academy of Sciences.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–