How new species establish in ecosystems and what this reveals about nature's assembly rules
Imagine a bustling city where every niche is filled—shops, parks, and homes all occupied. Now, what happens when a new business tries to move in? Its success doesn't just depend on its own strengths but also on how much space is available, how similar it is to existing ones, and how well the current community resists change. Similarly, in natural ecosystems, the ability of a new species to invade and establish itself—a concept ecologists call invasibility—reveals fundamental rules about how communities are assembled and why some are more diverse than others.
Invasibility isn't just about aggressive invaders like kudzu or zebra mussels; it's a window into the very mechanisms that maintain biodiversity. Recent research shows that invasibility is shaped by a complex interplay of factors: the traits of invaders, the traits of resident species, environmental conditions, and even human activities. Understanding these dynamics isn't just academic—it's critical for conservation, restoration, and managing ecosystems in an era of rapid global change 2 7 9 .
In this article, we'll explore the fascinating science behind invasibility, from key theories and concepts to groundbreaking experiments that are unraveling nature's assembly rules.
Two competing hypotheses explain how resident communities respond to invaders:
Abiotic conditions like climate, soil moisture, or nutrient availability act as filters that determine which species can establish. For instance, in Southern California, slope aspect (north vs. south-facing) influenced invasibility because it affected soil moisture and stress levels 2 .
The Fluctuating Resource Availability Theory suggests that invasions are more successful when resources like light, water, or nutrients become suddenly available, often due to disturbances 9 .
Surprisingly, genetic diversity within resident or invading species has minimal impact on invasibility. A meta-analysis found that resident genetic diversity had a very small, non-significant effect on invasion success 1 .
Recent microbial studies show that communities with fluctuating abundances (e.g., chaotic dynamics or limit cycles) are more invasible than stable communities, even if they are more diverse. This challenges traditional views that diversity always confers resistance 5 .
| Factor | Effect on Invasibility | Example |
|---|---|---|
| High Functional Diversity | Generally decreases invasibility | Prairie grasslands with multiple functional guilds resist invaders |
| Resource Availability | Increases invasibility | Disturbances or pulses of nutrients facilitate invasion 9 |
| Environmental Stress | Varies by system | Abiotic filters stronger on south-facing slopes in California 2 |
| Invader Fecundity | Increases invasiveness | High egg count in fish species 7 |
| Community Dynamics | Fluctuating states more invasible | Microbial communities with chaotic abundance fluctuations 5 |
One of the most illuminating recent experiments on invasibility comes from microbial ecology, where researchers manipulated synthetic communities of bacteria to test how diversity, interaction strength, and community dynamics affect invasion outcomes 5 .
| Step | Description | Purpose |
|---|---|---|
| 1. Assembly | 17 communities of 20 species each | Create varied resident communities with different initial traits |
| 2. Culturing | Daily growth-dilution cycles | Simulate natural environmental conditions and dispersal |
| 3. Invasion | Introduction of 7-9 random invader species per community on day 6 | Test how resident communities resist novel invaders |
| 4. Monitoring | 16S sequencing to track species abundances | Quantify invasion success and community changes |
| 5. Classification | Categorize communities as stable or fluctuating based on pre-invasion dynamics | Link community dynamics to invasibility |
| Community Type | Diversity (Species Surviving) | Invasion Success Rate | Key Dynamics |
|---|---|---|---|
| Stable Communities | Low (2-5 species) | 3% ± 2% | Constant abundances; strong inhibition of invaders |
| Fluctuating Communities | High (6-9 species) | 13% ± 5% | Deterministic fluctuations (chaos or limit cycles); higher invasibility |
| Strong Interaction Communities | Variable | Lower invasion probability | Priority effects; greater impact if invasion succeeds |
This experiment provides a unified perspective on the diversity-invasibility debate. It shows that invasibility is an emergent property of community dynamics, shaped by interaction strength and species pool size. The findings help explain why some studies support biotic resistance while others show positive diversity-invasibility relationships—the outcome depends on whether communities are stable or fluctuating 5 .
Understanding invasibility requires sophisticated tools to monitor species, track resources, and analyze complex interactions. Here are some essential methods used in modern invasion ecology:
| Tool or Method | Function | Example Use |
|---|---|---|
| DNA Sequencing (e.g., 16S rRNA) | Identify and quantify species in a community | Tracking invader and resident species abundances in microbial communities 5 |
| Stable Isotopes (e.g., ¹⁵N, ¹³C) | Trace nutrient flows and trophic interactions | Revealing how invaders exploit resources or disrupt food webs 4 |
| Functional Trait Metrics | Quantify community functional diversity and niche space | Measuring functional dispersion (FDis) or community-weighted means (CWM) to predict invasibility 2 |
| Remote Sensing & GIS | Map species distributions and environmental variables | Assessing invasibility across landscapes 7 |
| Network Analysis | Model species interactions and impacts | Predicting how invaders integrate into food webs or mutualistic networks 4 |
| Environmental DNA (eDNA) | Detect rare or cryptic species from soil or water samples | Monitoring invader presence without direct observation 4 |
| Experimental Mesocosms | Simulate natural communities under controlled conditions | Testing invasion mechanisms in microbial or plant assemblages 5 |
Invasibility is more than just a measure of how easily communities are invaded—it's a fundamental lens for understanding the mechanisms that govern community assembly and biodiversity. From trait-based filtering to dynamic fluctuations, the factors that determine invasibility reveal how nature balances competition, niche space, and environmental change.
As global change accelerates species introductions and alters ecosystems, understanding invasibility becomes increasingly urgent. Future research—like that highlighted in Oikos' upcoming special issue on Biological Invasions in the Context of Global Environmental Change—will need to integrate multiple factors, from climate variability to novel stressors like light pollution or microplastics 8 .
Ultimately, the study of invasibility reminds us that ecosystems are not static collections of species but dynamic, interacting networks. Whether in a prairie grassland, a freshwater stream, or a synthetic microbial community, the same rules apply: success depends on the match between invader traits, community structure, and environmental conditions. By deciphering these rules, we can better protect and restore the ecosystems that sustain us all.