Invasion histories reveal most North American introduced plants have not yet reached climatic stasis.

An analysis of 258 invasive plant species in North America reveals that the majority have not yet reached climatic stasis, challenging the common assumption of environmental equilibrium in species distribution models and highlighting the unrealized expansion potential of these invaders.

The Gist

Imagine you are watching a movie about a new character moving into a neighborhood. You want to know: Has this character finished exploring the whole neighborhood, or are they still discovering new streets?

This paper is essentially a massive investigation into 258 different "invader" plants that have moved into North America. The scientists wanted to answer a critical question: Have these plants finished spreading out, or are they still on the move?

Here is the breakdown using simple analogies:

1. The Big Problem: The "Snapshot" Trap

For a long time, scientists tried to predict where invasive plants would go next by taking a single "snapshot" of where they are right now. They assumed that if a plant is growing in a hot, dry place, it has already found all the hot, dry places it could possibly live in.

The Analogy: Imagine you see a person eating a sandwich in a park. A "snapshot" scientist would assume, "Okay, they have eaten every sandwich in the park." But in reality, they might just be starting their lunch and haven't walked to the other side of the park yet.

The problem is that invasive plants are like hungry explorers. They often haven't finished exploring their new home. If we think they've stopped moving when they haven't, we might be caught off guard when they suddenly show up in a new area.

2. The New Idea: "Climatic Stasis" (The "Parking Lot" Test)

Instead of guessing, the authors came up with a new way to check if a plant has "parked" or is still "driving." They call this Climatic Stasis.

The Analogy: Think of a plant's spread like a car driving through different weather zones (hot deserts, cold mountains, rainy forests).

• Not at Stasis: The car is still driving. Every year, it enters a new weather zone it hasn't seen before. It's still exploring.
• At Stasis: The car has been parked in the same weather zone for a long time (at least 30 years). It isn't finding any new types of weather to live in. It has likely finished its journey.

The scientists looked at 258 plants over 400 years of history (using old museum records and modern data) to see which cars were still driving and which were parked.

3. The Results: Most Are Still Driving!

The findings were surprising and important:

• The "Parked" Group: Only about 44% of the plants had reached "stasis." They had stopped expanding into new climates. They were essentially "parked."
• The "Driving" Group: The other 56% were still actively exploring. They hadn't finished finding all the places they could live.
• The Native Comparison: When the scientists looked at where these plants originally came from, 85% were already "parked." This makes sense; they've lived there for thousands of years. But in North America, they are still new and still exploring.

The Time Factor: For the plants that did stop moving, it took them an average of 90 years to finish their journey. That's almost a human lifetime!

4. Why This Matters: The "Unrealized Threat"

This study changes how we manage invasive species.

• If a plant is "Parked" (Stasis): We know it has likely found all the suitable climates. We can use standard maps to predict where it is safe and where it isn't.
• If a plant is "Driving" (No Stasis): We are in the danger zone. Just because a plant isn't in a specific city today doesn't mean it won't be there tomorrow. It might just be waiting to spread.

The Analogy: Imagine a fire.

• If the fire has stopped spreading and is just burning in one spot (Stasis), you know exactly where the danger is.
• If the fire is still jumping from tree to tree (No Stasis), you can't just look at where the flames are right now. You have to assume it could jump to the next tree any second.

5. The Takeaway

The main lesson is: Don't assume the invasion is over just because it looks quiet right now.

Most of the invasive plants in North America are still in the "exploration phase." They haven't reached their final destination yet. This means there is a huge "unrealized threat"—areas that are currently safe but could become infested in the next few decades.

By tracking how long it takes for these plants to stop expanding, scientists can better predict future risks and help managers decide where to focus their efforts:

• Stop the drivers: Focus on the plants that are still spreading fast.
• Monitor the parked: Keep an eye on the ones that have stopped, just in case they start moving again.

In short, nature is a marathon, not a sprint, and most of these invasive plants are still running.

Technical Summary

1. Problem Statement

Species Distribution Models (SDMs) are widely used to predict the range expansion of invasive species. However, these models rely on a fundamental assumption: environmental equilibrium. This assumption posits that a species currently occupies all suitable environmental conditions within its range.

• The Gap: Introduced invasive species are often in a state of rapid expansion and may undergo niche shifts relative to their native ranges. Consequently, they frequently violate the equilibrium assumption, leading to inaccurate predictions of future spread.
• The Challenge: While the lack of equilibrium is theoretically acknowledged, it is rarely empirically tested at broad scales. Existing methods often rely on "snapshots" comparing current distributions to potential native niches, which can be flawed due to niche shifts or incomplete native range data.
• The Objective: The authors aim to move beyond static comparisons by developing a temporal metric to assess whether invasive plants in North America have reached a state of climatic stasis (a prolonged period without expansion into new climatic space), serving as a proxy for equilibrium.

2. Methodology

The study utilized a large-scale, temporal analysis of historical occurrence data to quantify spread dynamics in climate space.

• Data Collection:

  • Species: 258 invasive terrestrial plant species (trees, shrubs, forbs) in North America.
  • Sources: Georeferenced specimen records from the Global Biodiversity Information Facility (GBIF) spanning 1614–2020.
  • Selection Criteria: Species required at least one record prior to 1990 and a minimum of 20 records in both their native and introduced ranges.
  • Climate Data: WorldClim version 2.1 (1970–2000, 19 bioclimatic variables) at 10 km resolution.

• Climate Space Quantification:

  • Climate space was defined using the first two axes of a Principal Components Analysis (PCA) combining native and invaded range climate data.
  • The space was discretized into a 10,000-cell grid (100x100).
  • Metric Definition: Using the ecospat package, the authors calculated "spread" as $1 - \text{unfilling}$.
    • Unfilling: Conditions suitable in the native range but not yet occupied in the invaded range.
    • Spread: The proportion of the native climatic niche currently occupied in the invaded range.

• Analytical Framework:

  • Temporal Resolution: Spread was calculated at 5-year intervals.
  • Trajectory Modeling: Species trajectories were fitted to three models:
    1. Linear: Constant rate of expansion.
    2. Two-phase (Asymptotic): Rapid expansion followed by a decline in rate.
    3. Three-phase (Sigmoidal): Lag phase $\rightarrow$ Rapid expansion $\rightarrow$ Decline/Equilibrium.
  • Model Selection: Best-fit models were determined using Akaike Information Criterion (AIC) and $\Delta$AIC > 2.
  • Stasis Definition: A species was considered to have reached climatic stasis if it exhibited a period of at least 30 years (matching climate normal standards) with no significant increase in spread (slope $\beta < 0.6\%$/year).

3. Key Contributions

• Conceptual Shift: The paper operationalizes "environmental stasis" as a necessary (though not sufficient) condition for equilibrium, providing a tractable proxy for a concept that is difficult to measure directly.
• Temporal Approach: Unlike previous studies relying on static snapshots, this study analyzes the history of invasion to determine when and if species stabilize.
• Large-Scale Empirical Test: It provides one of the first broad-scale assessments of equilibrium status across hundreds of invasive species, moving beyond single-species case studies.
• Management Framework: It offers a classification system (Lag, Active Spread, Approaching Stasis, Stasis) to guide management priorities based on the invasion stage.

4. Key Results

• Prevalence of Stasis: Only 43.8% (113/258) of the studied invasive species have reached climatic stasis in North America. In contrast, 84.5% of these species are in stasis within their native ranges.
• Invasion Trajectories:
  • 85.3% of species followed non-linear patterns (two-phase or three-phase), indicating a declining rate of expansion.
  • 12.4% are currently in a phase of continuous linear expansion.
  • 47.7% of species showed no detectable lag phase; they began expanding immediately upon introduction.
• Time to Stasis: For species that reached stasis, the time from first introduction to stasis ranged from 30 to 145 years, with a mean of 90 years.
• Niche Occupancy: Species that reached stasis occupied, on average, 97% of the climatic space available to them in their native range.
• Current Status:
  • 41.5% of species are "approaching stasis" (slowing expansion but not yet stable for 30 years).
  • 12.4% are actively expanding.

5. Significance and Implications

• Validity of SDMs: The findings suggest that SDMs for the majority (approx. 56%) of North American invasive plants are likely unreliable for predicting future range expansion because these species have not yet reached equilibrium. Models built on non-equilibrium data will underestimate potential distributions.
• Management Prioritization:
  • High Priority: Species in linear expansion phases (12%) require immediate early detection and rapid response, as they are actively filling unoccupied suitable space.
  • Medium Priority: Species "approaching stasis" may be suitable for containment or pathway management.
  • Research Opportunity: Species in stasis with significant remaining unoccupied climate space (>15%) offer a unique opportunity to study dispersal barriers, biotic interactions, or niche shifts that prevent full equilibrium.
• Ecological Insight: The long time required to reach stasis (mean 90 years) highlights that invasions are multi-generational processes. The lack of a lag phase in nearly half the species suggests that many invaders adapt or disperse rapidly, challenging the assumption that a lag phase is a universal feature of invasion.

Conclusion: The study concludes that most North American introduced plants are not yet at equilibrium. Recognizing the "unrealized threat" posed by species not yet in stasis is critical for effective biosecurity and conservation planning under accelerating climate change.