Which invasive plants thrive depends on the climate.

For 150 years, biologists have been perplexed by plants, an issue that even Charles Darwin was unable to adequately address. Why do some people spread quickly after moving to a new location while others barely make it?

For decades, scientists have been researching invasive species. From avoiding predators to locating better pollinators, they have uncovered numerous potential explanations for their success. There was still one issue, though. It was challenging to see the wider picture because the conclusions were frequently derived from particular case studies. According to a recent study, climate may have been the solution all along.

Plant characteristics in various settings

Millions of plant data gathered over the course of more than 200 years throughout the continental United States were examined by researchers. According to the team's research, invasive plants' ability to thrive is largely dependent on the environmental factors they face. Invasive species can mimic native plants in harsh settings. Being unique might be advantageous in milder settings.

A query that goes all the way back to Darwin

Darwin put out two opposing theories regarding successful invaders in On the Origin of Species. One theory is that introduced species thrive because they resemble native plants and are already adapted to the local environment. Another was that species thrive because they are sufficiently distinct from one another to avoid direct competition. Evidence appeared to support both theories for years.

Because they leave illnesses and predators behind, some invasive species thrive. Others proliferate because they exploit fresh seed dispersers or pollinators. Despite the abundance of samples gathered by scientists, the larger pattern remained frustratingly ambiguous.

Tadeo Ramirez-Parada, the study's primary author, carried out the research in Susan Mazer's lab at UC Santa Barbara while pursuing a PhD. According to Ramirez-Parada, "these results suggest that Darwin's Naturalisation Conundrum might not be a conundrum at all, but rather a predictable range of outcomes."

A story is told by millions of dried plants.

The researchers looked to herbaria, which are collections of preserved plant specimens that have been collected over centuries by specialists. Important details are included with every dried plant, such as the location and time of collection and whether or not the plant was in bloom.

Only a snapshot may be obtained from a single specimen. A comprehensive record of plant life throughout generations is produced via the collection of millions of specimens. The group concentrated on flowering time, which is important for plant reproduction. A plant may not generate seeds if it flowers too early or too late.

Therefore, plants that misjudge the timing run the danger of not being able to. The researchers calculated flowering times for thousands of species using specimen records, historical meteorological data, and sophisticated statistical techniques. Next, they contrasted native and invasive species in areas with difficult to moderate environmental conditions.

Success rules are shaped by the climate.

A startling pattern emerged from the data. Successful invasive plants strongly resembled native species in areas with difficult conditions, such as intense heat, cold, or dryness. There isn't much space for exploration in those settings. Plants need to react to the same environmental cues as established species and blossom at the appropriate time.

The story was different in milder areas. Stronger competition results from the increased richness and denser plant populations found in these locations. Because they behaved differently from native plants and occupied less congested ecological niches, invading species frequently benefited in these environments.

Ramirez-Parada remarked, "I was shocked and thrilled to see such a strong pattern and clean agreement with theoretical expectations." "Signals are rarely this strong and clear because ecology is usually messy—it deals with incredibly complex systems after all." Additionally, the researchers found that invasive plants frequently blossomed far earlier than native species in warm climates.

Gaining an advantage

Early flowering has a number of advantages. Pollinators are drawn to plants before rivals. They are able to produce seeds earlier and disperse them more widely. This early start could eventually aid invasive species in taking over areas.

"Exotic grass species, which germinate and grow quickly early in spring, have replaced native wildflower species in many places in California," Mazer remarked. "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, especially for annual grasses."

The results could assist land managers in spotting potential dangers before they become serious issues. Evolutionarily unique invasive plants may need more attention in mild climates. Introduced species that closely resemble native plants may be the most dangerous in harsher conditions.

The increasing issue of invasive grasses

Concerns over invasive annual grasses, which have altered numerous ecosystems, are also highlighted by the study. California is a prime example. Diverse ecosystems of native grasses and wildflowers previously flourished in large areas. Fast-growing invasive grasses already dominate many of those ecosystems.

The effects extend beyond the diversity of plants. Dense grass cover can hinder the return of native species, raise the risk of wildfires, and lower the quality of wildlife habitat. Invasive grasses frequently provide less resources for insects that sustain healthy ecosystems since many of them depend on wind pollination.

Additionally, because so much of their wind-borne pollen is lost, wind-pollinated species have evolved to generate significantly more pollen than insect-pollinated plants, according to Mazer. Mazer claims that pollen-related respiratory allergies cost the US economy more than $3 billion annually.

Insects that aid in pollinating food crops are also greatly aided by native plants that are friendly to pollinators. The loss of native habitats puts more strain on those pollinators.