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Biological invasions reshape and ultimately reverse biodiversity–stability relationships in riverine fish communities

This study demonstrates that biological invasions fundamentally reorganize the relationship between biodiversity and stability in riverine fish communities by directly reducing stability and reshaping the positive diversity–stability relationship into a negative one, particularly at high invasion intensities, with effects varying across biogeographic regions.

Original authors: Kai Feng, Bernard Hugueny, Tibor Erős

Published 2026-07-07

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CC BY 4.0

Original authors: Kai Feng, Bernard Hugueny, Tibor Erős

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine a river as a bustling orchestra. In a healthy, natural state, the more musicians (fish species) you have, the more stable the music is. If one musician stumbles, another picks up the slack, keeping the rhythm steady. This is the classic idea in ecology: more diversity equals more stability.

However, this new study suggests that when invasive species (uninvited guests who take over the stage) enter the river, they don't just add noise; they fundamentally rewrite the rules of how the orchestra works.

Here is what the paper found, broken down into simple concepts:

1. The "Mixing" Problem: Why the Global Picture Looks Confusing

If you look at all the rivers in the world at once, the link between "many fish" and "stable music" looks very weak. It's almost flat.

The Analogy: Imagine taking a photo of a choir where half the singers are singing a perfect, harmonious song, and the other half are shouting random noises. If you average the sound, it just looks like a messy, flat line.
The Reality: The study found that rivers without invaders still follow the old rule: more fish = more stability. But rivers with invaders have a flat, broken relationship. When you mix these two groups together in a global dataset, the clear pattern of the healthy rivers gets hidden by the chaos of the invaded ones.

2. The "Destabilization" Effect: The Invader's Direct Hit

Invasive species don't just change the mix; they actively make the river less stable, no matter how many fish are there.

The Analogy: Think of a tightrope walker. Adding more people to the net below (biodiversity) usually makes them safer. But if someone starts shaking the tightrope violently (invasion), the net doesn't matter as much. The more violently they shake it (higher invasion intensity), the more likely the walker is to fall.
The Reality: The study shows that as the number of invasive fish increases, the river community becomes less stable. This happens even if the river has a high number of species. The invader acts like a constant disturbance that overrides the natural safety net of diversity.

3. The "Reshaping" Effect: Turning the Rule Upside Down

This is the most surprising finding. In rivers with very heavy invasions, the relationship doesn't just break; it flips.

The Analogy: In a normal orchestra, adding more musicians makes the sound richer and steadier. But in an orchestra where an invader has taken over the conductor's podium, adding more musicians actually makes the performance worse. The more people you add to a chaotic, dominated group, the more synchronized and volatile the chaos becomes.
The Reality: In rivers with low invasion, more diversity still helps a little. But as invasion pressure gets high, the relationship turns negative. In these highly invaded rivers, having more species actually leads to less stability. The invader has reorganized the community so that diversity no longer protects the system; instead, it amplifies the instability.

4. The "Context" Factor: It Depends on Where You Are

The study looked at different parts of the world (biogeographic realms) and found that this "rule-breaking" isn't the same everywhere.

The Nearctic (North America): This region showed the clearest example of the flip. Here, the invader completely rewrote the rules, turning the stability curve from positive to negative in a straight, predictable line.
The Palearctic (Europe/Asia): Here, invaders definitely made rivers less stable and blurred the rules, but they didn't always flip the script completely. It was a messier, more varied situation.
Australasia (Australia/New Zealand): Here, the invaders had the least impact. The rivers mostly kept their original "more diversity = more stability" rule, suggesting that in some places, the local ecosystem is better at resisting the invader's chaos.

The Bottom Line

The paper concludes that biological invasions are not just a minor disturbance; they are a force that can reorganize the fundamental laws of nature in river ecosystems.

In simple terms:

Without invaders: More fish = Steadier river.
With invaders: The rule gets blurry.
With heavy invaders: The rule flips. More fish can actually mean a less steady river because the invader has changed how the fish interact, turning a safety net into a trap.

The study warns that as invasions become more common globally, we can no longer assume that simply having more species will save our ecosystems from instability. The context of who is in the river matters just as much as how many are there.

Technical Summary: Biological Invasions Reshape and Ultimately Reverse Biodiversity–Stability Relationships in Riverine Fish Communities

Problem Statement
While classical ecological theory posits a positive relationship between biodiversity and ecosystem stability (the diversity–stability or D–S relationship), empirical results have been inconsistent, ranging from positive to neutral or even negative. A primary gap in understanding this variability is whether external disturbances, specifically biological invasions, alter the fundamental D–S relationship itself. Previous studies have largely focused on how disturbances directly affect stability or how they modify stability within specific contexts, but few have examined whether invasions reorganize the structural link between species diversity and temporal stability. This paper investigates whether biological invasions act as a pervasive driver that reshapes, flattens, or reverses the D–S relationship in freshwater fish communities.

Methodology
The study utilized the RivFishTIME database, a global compilation of long-term freshwater fish community time series.

Data Selection: From an initial pool of over 11,000 series, the authors filtered for 3,695 time series meeting strict criteria: a minimum duration of 10 years, consistent sampling protocols, and consistent abundance units. Sampling was restricted to events within ±45 days of the most frequent sampling date to minimize seasonal noise.
Metrics:
- Temporal Community Stability (TCS): Calculated as the inverse of the coefficient of variation of total community abundance.
- Species Diversity: Derived from Hill diversity metrics (orders $q=0, 1, 2$ ) and summarized via Principal Component Analysis (PCA) into a multivariate diversity axis (DiversityPCA1).
- Invasion State: Binary classification (invaded vs. non-invaded) based on the strict absence of non-native species throughout the time series.
- Invasion Intensity: A continuous gradient (InvasionPCA1) derived from five descriptors: non-native species richness ratio, mean non-native abundance proportion, temporal range and variance of non-native abundance, and the temporal trend of non-native abundance.
Statistical Framework: The authors employed Generalized Additive Models (GAMs) to test four interconnected hypotheses regarding how invasions influence the D–S relationship:
1. Mixing: Does pooling invaded and non-invaded communities obscure a strong underlying positive relationship?
2. Destabilization: Do invasions directly reduce stability independent of diversity?
3. Reshaping: Does invasion intensity modify the slope of the D–S relationship?
4. Context Dependence: Do these effects vary across biogeographic realms (Nearctic, Palearctic, Australasia)?

Key Results
The analysis revealed that biological invasions fundamentally reorganize the D–S relationship through four distinct mechanisms:

Mixing of Regimes: At the global scale, the pooled D–S relationship appeared weak and trendless. However, separating communities by invasion state revealed that non-invaded communities retained the classical positive D–S relationship, whereas invaded communities exhibited a nearly flat relationship. The weak global pattern is thus an artifact of mixing these two distinct regimes, with the invaded pattern dominating the pooled signal.
Direct Destabilization: Invasions exert a direct negative effect on stability. Invaded communities showed significantly lower TCS than non-invaded ones. Furthermore, within invaded communities, stability declined progressively as invasion intensity increased, regardless of the diversity level.
Reshaping and Reversal: Invasion intensity significantly reshaped the D–S slope. At low invasion intensity, the relationship was weakly positive. As intensity increased, the relationship flattened and eventually reversed to become negative. In highly invaded communities, high diversity was associated with the lowest stability. This indicates that invasion intensity can override the stabilizing effects of biodiversity.
Context Dependence: These patterns were not uniform globally:
- Nearctic: Showed the clearest evidence for all three processes (mixing, destabilization, and reshaping). The D–S slope shifted linearly from positive to negative as invasion intensity increased, with a clear reversal point around the 50–60th percentile of invasion intensity.
- Palearctic: Supported mixing and destabilization, but evidence for reshaping (reversal) was limited to specific ecoregions, with the overall relationship remaining broadly unimodal.
- Australasia: Showed limited invasion effects, with D–S relationships remaining broadly positive across invasion contexts.

Significance and Claims
The paper claims that biological invasions do not merely act as additive disturbances that reduce stability; they fundamentally reorganize the ecological relationship between biodiversity and stability.

Theoretical Implication: The study challenges the view that biodiversity universally enhances stability. Instead, it demonstrates that the stabilizing effect of biodiversity is context-dependent and can be weakened, neutralized, or reversed under high invasion pressure.
Mechanistic Insight: The results suggest that invasions alter the underlying components of community stability (population variability and species synchrony). In highly invaded systems, destabilizing components (such as dominance by invasive species or increased synchrony) may increase along the diversity gradient, eventually outweighing the stabilizing "portfolio" or "insurance" effects of biodiversity.
Global Pattern Explanation: The apparent inconsistency of D–S relationships across ecosystems is partly explained by the varying prevalence and intensity of biological invasions. The "weak" global D–S relationship observed in many studies is likely a statistical artifact of mixing invaded and non-invaded communities.

The authors conclude that as biological invasions intensify globally, predicting community stability requires understanding not just species richness, but the specific invasion regime and its capacity to restructure the dynamics linking diversity to stability.