No Sex, Will Travel?
The Association Between Invasiveness and Mating Systems in Introduced Iridaceae to Australia
The Silent Invasion
Picture a quiet Australian landscape gradually being transformed by an uninvited guest—a plant from a distant continent, now spreading relentlessly across its new home.
This silent invasion repeats itself across ecosystems worldwide, with biological invasions consistently ranking among the top five threats to global biodiversity 6 . Among the most successful plant invaders in Australia are members of the Iridaceae family, a group that includes familiar garden plants like irises and freesias.
Reproductive Strategies
What makes these particular plants so adept at conquering new territories may lie not in what they look like, but in how they reproduce.
Mating System Hypothesis
Scientists are increasingly focusing on the hypothesis that a plant's mating system may powerfully predict its potential to become invasive.
The Language of Plant Sex: A Brief Primer
To understand the invasion dynamics of plants, we must first become fluent in the language of plant reproduction. The sexual system of a species describes the distribution of male and female functions across individuals in a population 1 .
| Sexual System | Description | Advantage for Invasion |
|---|---|---|
| Dioecy | Individual plants are either male or female; requires cross-pollination between separate individuals | Poor - requires mates |
| Monoecy | Male and female flowers occur on the same individual; can self-pollinate or outcross | Moderate - flexible mating |
| Self-compatibility | Hermaphroditic flowers that can fertilize themselves; no need for pollen from other plants | Excellent - can found populations alone |
| Andromonoecy | Plants have both male and hermaphroditic flowers | Moderate - can self-pollinate via hermaphroditic flowers |
| Gynodioecy | Populations include females and hermaphrodites | Variable - depends on ratio |
This variation in sexual systems has profound implications for what happens when plants are transported to new environments. A self-compatible plant arriving in Australia without its native pollinators can still reproduce, while a dioecious species would languish in reproductive limbo 1 .
Reproductive Strategies and Invasion Success: The Theories
Why would mating systems be linked to invasion success? Several compelling theories explain this connection.
Reproductive Assurance Hypothesis
For plants establishing in new territories, finding mates or appropriate pollinators can be challenging. Self-compatible species carry a massive advantage—they can produce offspring even when isolated.
Baker's Law Principle
This concept proposes that species capable of uniparental reproduction are more likely to establish successfully after long-distance dispersal. The principle has been aptly summarized as "the best colonizer is a self-fertilizing hermaphrodite" 1 .
Genetic Diversity Paradox
While self-fertilization provides short-term advantages for establishment, it comes at a cost—reduced genetic diversity. Many successful invaders can self-fertilize when necessary but occasionally outcross to generate genetic diversity.
A Key Experiment: Testing the Link Between Reproduction and Invasiveness
To rigorously test whether reproductive traits can predict invasiveness, a research team designed an elegant experiment that compared invasive and non-invasive plants in their native ranges 6 .
Methodology
Species Selection
Identified five pairs of closely related plant species growing sympatrically in their native range 6 .
Invasive Classification
Each pair included one species known to be invasive elsewhere and one non-invasive species.
Trait Measurements
Measured key demographic traits: size, fecundity, and reproductive frequency.
Statistical Analysis
Used phylogenetic comparative methods to account for evolutionary relationships.
Results: Size Comparison
| Species Pair | Family | Size Ratio (Invasive:Non-invasive) |
|---|---|---|
| Cerastium | Caryophyllaceae | 1.80 |
| Silene | Caryophyllaceae | 1.60 |
| Calluna/Erica | Ericaceae | 1.54 |
| Rhinanthus/Pedicularis | Scrophulariaceae | 1.45 |
| Average | 1.60 |
Invasive species were consistently larger—approximately 60% larger on average—than their non-invasive relatives 6 .
Fecundity Comparison
| Species Pair | Fecundity Ratio (Invasive:Non-invasive) |
|---|---|
| Cerastium | 2.74 |
| Silene | 4.86 |
| Calluna/Erica | 0.50 |
| Rhinanthus/Pedicularis | 0.85 |
| Average | 2.24 |
Size-Adjusted Fecundity
| Species Status | Fecundity per Unit Size |
|---|---|
| Invasive | 15.56 |
| Non-invasive | 7.51 |
The data shows that invasive species don't just produce more seeds because they're bigger—they also have constitutively higher fecundity for their size 6 .
Scientific Importance
This research provides compelling evidence that size and fecundity—key components of reproductive strategy—are reliable predictors of invasiveness. From a conservation perspective, these findings are golden—they suggest we can identify potential invaders before they're introduced by screening for these simple traits.
The Australian Context: Iridaceae and Their Mating Systems
The Iridaceae family in Australia provides a fascinating case study for examining these principles. This plant family, best known for ornamental species like irises and gladioli, has representatives with diverse mating systems.
Reproductive Diversity in Iridaceae
Iridaceae species employ the full spectrum of plant mating systems. While many species have hermaphroditic flowers with both male and female organs, their compatibility systems vary dramatically.
Geosiris australiensis
A striking example of reproductive specialization in Australian Iridaceae is the recent discovery of Geosiris australiensis 4 . This fascinating plant represents an extreme case of adaptation—it's an achlorophyllous mycoheterotroph, meaning it lacks chlorophyll and instead obtains nutrients from fungi in the soil.
Invasion Patterns and Reproductive Strategies
Australian ecosystems have received numerous Iridaceae introductions, with varying outcomes. Those species that have become successful invaders typically share reproductive traits that align with the experimental findings—they tend to be self-compatible, produce abundant seeds, and can reproduce without specialized pollinators.
The combination of high fecundity and reproductive assurance appears to create a powerful engine for invasion success.
The Scientist's Toolkit: Researching Plant Invasions
Studying the connection between mating systems and invasiveness requires specialized methods and approaches.
Common Garden Experiments
Grow different species or populations under identical conditions to isolate genetic influences on traits from environmental effects.
Hand Pollination Studies
Control mating by manually transferring pollen to determine compatibility systems and mating flexibility.
Seed Set Quantification
Count and assess viability of seeds produced to measure fecundity and reproductive success.
Microsatellite Markers
Genetically fingerprint individuals to determine parentage and outcrossing rates.
Herbarium Specimen Analysis
Preserve and study plant specimens to document morphological traits and reproductive structures.
Phylogenetic Comparative Methods
Account for evolutionary relationships to make valid comparisons between related species.
Conclusions and Future Directions
The compelling association between plant mating systems and invasiveness provides both insight and utility.
Key Findings
- Reproductive traits—particularly self-compatibility, large size, and high fecundity—powerfully predict which introduced plants are likely to become invasive.
- This knowledge offers a practical tool for biosecurity agencies seeking to assess the risk of new plant introductions.
- The discovery of previously unknown species like Geosiris australiensis in Australian ecosystems 4 reminds us how much we still have to learn about plant diversity.
Future Research Directions
Genomic approaches
Identify genes associated with invasiveness
Comparative studies
Test the generality of these patterns across more plant families
Climate matching analyses
Predict how invasive potential might shift with climate change
Pollinator network studies
Understand how disrupted plant-pollinator relationships affect invasion success
The Silent Invasion Continues
The silent invasion of ecosystems by introduced plants represents one of the significant environmental challenges of our time. By understanding the intimate connection between sex and travel in plants, we gain not only scientific knowledge but also the power to make more informed decisions about which plants we welcome into our landscapes—and which we exclude for the protection of our native ecosystems.