The Green Bridge
How Asian Plants Keep Invading Australian Rainforests
Introduction: A Botanical Migration Mystery
Picture two ancient continents—Sundaland (Southeast Asia's rainforest realm) and Sahul (the Australia-New Guinea landmass)—separated by ocean deeps. Yet their floras whisper of deep connections: 81 plant families and 225 genera straddle this divide 1 .
This is the Sunda-Sahul floristic exchange, one of Earth's most dramatic biological handshakes. For decades, scientists believed this exchange was a one-time ancient event. But cutting-edge genomics and fossil evidence now reveal a startling truth: this botanical conversation never stopped. Asian plants are still invading Australian rainforests today, riding invisible bridges built by birds, climate change, and shifting landscapes 5 .
The Sunda-Sahul floristic exchange continues to shape rainforest ecosystems today.
The Living Legacy of an Ancient Exchange
Asymmetry in Green Waves
The plant migration between these realms is strikingly lopsided. Genetic studies show >68 migrations since the middle Eocene (45 million years ago), with southward movements (Sunda→Sahul) outnumbering northward journeys 2:1 1 . This asymmetry peaked dramatically after 15 million years ago when continental collisions created stepping-stone islands.
Two Gateways Into Australia
Contrary to past assumptions, plants didn't just flood through New Guinea. Joyce et al.'s 2021 breakthrough revealed twin invasion routes:
The Northeast Track
A "highway" between New Guinea and Australia's Cape York Peninsula enabling ongoing gene flow as recently as 5,000 years ago.
The Northwest Track
A historical corridor linking Timor-Leste with the Kimberley (Western Australia), now genetically dormant but vital for ancient migrations .
These routes transformed northern Australia into a mosaic of evolutionary histories, where Cape York's flora feels Asian influence 3× stronger than the Kimberley's 5 .
Climate: The Selective Doorman
Not all plants could cross Wallacea (the island corridor between Sunda and Sahul). Genomic analysis shows how climate filters shaped invasions:
- Cold-adapted species failed: Sahul migrants dominated Sunda's lowland forests but couldn't penetrate mountains 5
- Fruit size determined success: Species with fruits >30mm diameter rarely crossed open water gaps, while small-fruited plants like Aglaia rode bird vectors across 100km seas 5
- Quaternary climate swings created "expansion-contraction cycles" where species repeatedly advanced and retreated, leaving genetic scars still detectable today
| Time Period | Migration Events | Dominant Direction | Key Drivers |
|---|---|---|---|
| >15 Million Years Ago | 12 | Moderate southward | Early island chains |
| <15 Million Years Ago | 56 | Strong southward | Australian-Sundaland collision |
| Quaternary (2.6 Ma - present) | Ongoing | Northeast & northwest tracks | Sea-level fluctuations, pigeon dispersal 1 |
Spotlight: Decoding an Invasion with DNA
The Aglaia Expedition: Herbaria to Rainforests
When PhD candidate Elizabeth Joyce investigated Aglaia elaeagnoidea—a rainforest tree spanning India to Australia—she turned herbaria into time machines. Her team extracted DNA from 200+ dried specimens, including a 1835 Robert Wight collection from India . Their methodology revealed nature's invasion playbook:
- Used DArT-seq SNP markers on leaf samples from 129 locations
- Key innovation: Identified "green specimens = good DNA" rule, overcoming challenges with alcohol-preserved samples
- Reconstructed Last Glacial Maximum habitats (21,000 years BP) when lowered seas exposed land bridges
- Mapped climate tolerances using 19 bioclimatic variables
- Analyzed population structure with Bayesian clustering (STRUCTURE) and principal components analysis
| Functional Trait | Sunda→Sahul Migrants | Sahul→Sunda Migrants | Impact on Dispersal |
|---|---|---|---|
| Fruit Diameter | Mostly <30mm | Often >40mm | Small fruits dispersed 3× farther |
| Fruit Fleshiness | High (succulent) | Low (dry capsules) | Fleshy fruits favored by birds |
| Seed Protection | Thin exocarp | Thick sclerocarp | Vulnerability during ocean crossings 5 |
Results: Two Paths, One Species
The data painted an unmistakable picture:
- Kimberley populations genetically diverged from Southeast Asian ancestors >200,000 years ago during Pleistocene sea-level lows
- Cape York plants showed ongoing gene flow with New Guinea, with migrations as recent as 5 kya
- Climate fluctuations fragmented populations, driving speciation—A. elaeagnoidea is actually 3+ cryptic species
This explained why Kimberley Aglaia felt like evolutionary islands: isolated outposts of ancient migrations.
The Scientist's Invasion Biology Toolkit
| Research Tool | Function | Key Innovation |
|---|---|---|
| Environmental Niche Modeling | Predicts past/future suitable habitats | Integrates paleoclimate data with species occurrences |
| Phylogenomics | Reconstructs evolutionary timelines | Uses hundreds of nuclear genes from herbarium DNA |
| Landscape Genomics | Maps gene flow across seascapes | SNP analysis identifies dispersal barriers |
| Fruit Trait Databases | Quantifies dispersal potential | Correlates size/fleshiness with ocean-crossing success 1 5 |
Is the Invasion Still On? Evidence for Ongoing Exchange
The Case for "Live" Migrations
Multiple lines of evidence confirm the exchange never stopped:
- Genetic highways remain open: Northeast track populations show <10% genetic differentiation—too low for isolated lineages
- Pigeon express delivery: Pied Imperial Pigeons (Ducula bicolor) transport seeds 500km/night between New Guinea and Cape York during seasonal migrations
- Climate tipping the balance: As Australia's north warms 0.5°C/decade, Sunda species experience reduced cold barriers 5
Pigeon Dispersal Agents
Pied Imperial Pigeons are key vectors for ongoing plant migrations between New Guinea and Australia.
Future Invaders Waiting in the Wings
Genomic models predict rising threats:
Conclusion: The Never-Ending Story of Earth's Green Tides
The Sunda-Sahul exchange isn't fossil history—it's a living, breathing process. As Darren Crayn notes, this floristic dialogue has "shaped rainforest assembly for 15 million years" 1 , and climate change is amplifying its volume. Yet mysteries remain: How do Australian dry-adapted plants like Acacia resist Sunda invasions? Could rising seas sever the New Guinea-Cape York bridge?
One truth emerges: Wallacea isn't just a biogeographic line—it's a living filter in constant flux, sieving species through mesh woven from fruit traits, ocean currents, and pigeon wings. As Joyce puts it: "Plants will cross oceans if we just wait long enough—and we're discovering they've had more time than we imagined" . In this endless green dance between continents, every genome tells an epic migration story waiting to be decoded.