Combined impacts of invasive alien species and fire on ecosystems are complex, mostly negative, and understudied: a global review

This global review of 464 studies synthesizes the complex and predominantly negative impacts of invasion-fire interactions on ecosystems, highlighting that while fire often facilitates invasive species and prescribed fire is more effective for management, significant knowledge gaps remain regarding understudied taxa, geographic regions, and broad-scale ecological consequences.

The Gist

Imagine nature as a giant, complex house. Sometimes, the house gets a little messy (a fire), and sometimes, unwanted guests move in (invasive species). Usually, we think of these two problems separately: "We need to put out the fire!" or "We need to kick out the squatters!"

But this new research paper argues that fire and invasive species are actually a dangerous team-up. They don't just happen at the same time; they often help each other, creating a vicious cycle that can wreck the whole house.

Here is the breakdown of what the scientists found, using some simple analogies:

1. The "Vicious Cycle" (The Invasion-Fire Feedback)

Think of this like a drunk friend and a match.

• Fire helps the Invaders: When a fire burns through a forest, it clears out the native plants (the "good tenants"). This leaves empty space and extra sunlight. Invasive plants (the "bad tenants") are often like weeds that love to grow in empty, sunny spots. The fire essentially clears the table for them to take over.
• Invaders help the Fire: Once the invasive plants take over, they often change the house. Some of them are like dry kindling (very flammable). They might grow thicker, drier, or more frequently than the native plants. This means the next fire will be hotter, faster, and more destructive.
• The Result: The fire brings in the weeds, and the weeds make the next fire worse. It's a loop that keeps spinning out of control.

2. The "One-Way Street" Problem

The researchers looked at hundreds of studies and found a big imbalance in how we study this.

• We know a lot about Fire $\rightarrow$ Invaders: We have plenty of studies on how fire helps invasive plants spread. It's like knowing exactly how a storm breaks a window.
• We know very little about Invaders $\rightarrow$ Fire: We don't know enough about how the invaders change the fire itself. It's like knowing the window broke, but not understanding that the broken glass is now making the whole room catch fire faster.
• The Blind Spot: The study also noted that we mostly look at plants. We barely study how invasive animals (like rats or foxes) or fungi interact with fire. It's like only checking the furniture for fire hazards and ignoring the electrical wiring.

3. The "Management Mistake"

Sometimes, we try to fix the problem with fire, but we accidentally make it worse.

• Prescribed Burns (The Controlled Spark): This is when experts set small, controlled fires to clean up the forest. The study found this is generally better than letting a wildfire rage, but it's not perfect. Even controlled fires sometimes accidentally help the invaders grow back stronger.
• Wildfires (The Uncontrolled Blaze): These are the worst. They almost always help the invaders take over completely.
• The Analogy: Imagine trying to clean a messy room by throwing a match at it. Sometimes, you just burn the trash (good), but often, you just burn the nice furniture and leave the trash untouched (bad).

4. The "House Damage" (Ecosystem Impacts)

When fire and invaders team up, the damage to the ecosystem is usually negative.

• Native Species: The "good tenants" (native plants and animals) get pushed out. They can't handle the new, hotter fires or the aggressive invaders.
• The Soil: The ground itself gets damaged, losing nutrients and becoming less able to support life.
• The Verdict: While there are rare cases where things work out okay, the overall trend is that this team-up destroys biodiversity. It turns a diverse, healthy forest into a monoculture (a field of just one type of weed).

5. What Are We Missing? (The Knowledge Gaps)

The authors point out that our "map" of this problem has huge blank spots:

• Geography: We have lots of data from the US and Australia, but huge fire-prone areas in Europe, Africa, and Asia are being ignored. It's like having a fire safety manual for New York but no manual for London or Tokyo.
• Scale: Most studies look at a single patch of forest. We need to see the whole picture—how this affects entire countries or continents.
• Failure Stories: Scientists tend to only publish when they find a solution. If a fire management strategy failed, they often don't write about it. This means we keep making the same mistakes because we don't have a list of "what not to do."

The Bottom Line

This paper is a wake-up call. We can't treat fire and invasive species as separate problems anymore.

• If you fight the fire but ignore the weeds, the weeds will come back stronger.
• If you fight the weeds but ignore the fire risk, the next fire will wipe out your progress.

The Solution? We need a "whole-house" approach. Future research needs to look at more places, study more types of animals and fungi, and be honest about when management strategies fail. Only then can we break the cycle and save the house.

Technical Summary

1. Problem Statement

The interaction between invasive alien species (IAS) and fire creates a complex feedback loop known as the "invasion–fire cycle." While both disturbances individually threaten ecosystem integrity, their combined effects are often non-additive and can drive regime shifts, biodiversity loss, and compromised ecosystem functioning.

• The Gap: Existing literature often addresses IAS and fire management separately. Policy frameworks (e.g., EU Regulation 1143/2014, US Executive Order 13112) rarely integrate these threats. Furthermore, previous reviews have focused on specific mechanisms (e.g., how invader growth forms affect fire) or specific taxa, lacking a comprehensive synthesis of the combined ecosystem impacts across all directions of interaction (Fire-on-IAS, IAS-on-Fire, and combined effects).
• The Need: There is an urgent need to synthesize current knowledge to design effective management strategies that simultaneously control IAS and mitigate fire risk, while identifying critical knowledge gaps regarding geographic distribution, taxonomic coverage, and management efficacy.

2. Methodology

The study employed a systematic literature review following standard guidelines (O'Dea et al., 2021) and utilized the online tool CADIMA for data management.

• Data Collection:
  • Search: Conducted in February 2025 using Scopus and ISI Web of Knowledge (1984–2024).
  • Validation: Compared against Google Scholar results to ensure reliability.
  • Selection Criteria: Studies had to be peer-reviewed, original research, and explicitly address the interaction between IAS and fire.
  • Final Dataset: 464 studies were included after screening titles, abstracts, and full texts.
• Data Extraction & Classification:
  • Data was categorized into four groups: Context (location, habitat, scale), IAS & Fire Traits (taxonomic group, fire regime attributes), Interaction (direction of effect: Fire-on-IAS, IAS-on-Fire, Fire-and-IAS), and Research Approach (methodology, aims, result types).
  • Definitions:
    • Fire-on-IAS: Effects of fire on invasive species.
    • IAS-on-Fire: Effects of invasive species on fire regimes.
    • Fire-and-IAS: Combined effects of both on ecosystems.
• Analysis: Univariate analyses (plots) and statistical tests (Chi-square, Fisher's exact test) were performed using Python (Matplotlib, SciPy) to identify patterns and significant differences.

3. Key Contributions

• First Comprehensive Synthesis: This is the first review to assess invasion-fire interactions across all three directions of interaction, all major taxonomic groups, and with a specific focus on their combined ecosystem impacts.
• Management Insight: It provides a critical evaluation of management outcomes, specifically comparing prescribed fire (management) versus wildfires in suppressing IAS.
• Gap Identification: The review systematically maps knowledge gaps across geography, taxonomy, and methodology, offering a roadmap for future research priorities.

4. Key Results

A. Temporal Trends

• Research began in 1987 but saw a significant peak in 2005, driven by foundational reviews.
• Fire-on-IAS studies dominate the literature, showing a marked increase, particularly in 2016 and reaching a record high in 2024.
• IAS-on-Fire and Fire-and-IAS studies have increased more slowly.
• Recent trends (2024) show a shift toward conservation aims, restoration projects, and addressing "megafires," alongside growing scrutiny of management failures (e.g., prescribed fire inadvertently promoting invasive species).

B. Invasion-Fire Feedbacks

• Fire-on-IAS: Fire generally promotes IAS.
  • Taxa: Herbaceous plants are the most studied; invasive animals are underrepresented (only 16 records); fungi are virtually unstudied.
  • Outcomes: Positive impacts (promotion of IAS) outnumber negative ones for plants. Mechanisms include increased resource availability, reduced competition, and breaking seed dormancy.
  • Management vs. Wildfire: Prescribed burning resulted in significantly fewer IAS promotions compared to wildfires, yet 37% of prescribed burning cases still promoted IAS (e.g., stimulating resprouting in Celastrus orbiculatus).
• IAS-on-Fire: Much less studied than Fire-on-IAS.
  • Most records report changes in fire regimes (intensity, spread, frequency).
  • Mechanisms: Invasive plants can alter fuel loads (quantitative) or fuel types (qualitative). Some grasses (e.g., Bromus tectorum) increase fire frequency/severity, while others (e.g., high-moisture grasses) can dampen fire spread.
  • Taxa Gap: Very few studies examine invasive animals or fungi affecting fire regimes.

C. Combined Impacts on Ecosystems

• Overall Impact: The combined effects of fire and IAS on ecosystems are predominantly negative, driving biodiversity loss, altering community composition, and reducing native cover.
• Habitat Specificity: Impacts are broadly distributed across habitats, though research is biased toward vulnerable ecosystems (e.g., sagebrush, fynbos) rather than resilient ones.
• Synergy: While often synergistic (worsening each other), interactions are not always so. Some studies show fire mitigating IAS effects or vice versa, but these "non-synergistic" cases are underreported due to publication bias.

D. Methodological Trends

• Methods: Experimental studies (38%) and field observations (19%) dominate. Remote sensing (9%) and laboratory experiments (10%) are severely underutilized, despite their potential for large-scale analysis and controlled variable testing.
• Aims: Most studies focus on specific species or ecosystem properties. Restoration outcomes are the second most common aim.
• Predictive Capacity: Only 13% of studies are predictive (mostly modeling), limiting the ability to forecast future invasion-fire dynamics under climate change.

5. Significance and Future Directions

• Policy Relevance: The findings highlight a disconnect between current legislation (which treats fire and IAS separately) and ecological reality (where they are intertwined). Integrated management strategies are essential.
• Management Implications: Prescribed fire is a useful tool but carries risks of promoting certain invaders. Management must be context-specific, considering the target species and ecosystem resilience.
• Critical Knowledge Gaps to Address:
  1. Geography: Underrepresentation of fire-prone regions in Europe, Asia, and most of Africa (outside South Africa).
  2. Taxonomy: Urgent need to study invasive animals and fungi in the context of fire.
  3. Scale: Shift from local studies to broad-scale analyses using remote sensing.
  4. Management Reporting: Overcoming publication bias by reporting "negative" management outcomes (failures) to improve adaptive management.
  5. Predictive Modeling: Expanding scenario-based modeling to anticipate future interactions under climate change.

Conclusion: The paper concludes that while the invasion-fire cycle is a well-established driver of ecosystem degradation, the field suffers from significant biases and gaps. Future research must prioritize multi-scale, integrative approaches that include underrepresented taxa and regions to develop robust, evidence-based management strategies.