Forest Highways: How the Amazon Kept Seeding the Atlantic Rainforest

The Amazon and the Atlantic Forest are two of South America's most iconic tropical ecosystems — teeming with biodiversity and separated by what seems like an impassable dry corridor. Known as the “dry diagonal,” this region of savannas, scrublands, and semi-arid plains was long assumed to be a formidable biogeographical barrier. Yet, species from both sides mysteriously appear in each other’s domains.

A new study published in Proceedings of the Royal Society B challenges the idea that these rainforest biomes are disconnected relics of a wetter past. Instead, it reveals a continuous, long-term flow of tree species from the Amazon to the Atlantic Forest, raising major implications for biodiversity conservation and the ecological history of tropical South America.

Led by James A. Nicholls, Jens J. Ringelberg, and Kyle G. Dexter, the international team used advanced genetic tools to trace the evolutionary journey of Inga, a diverse and ecologically important genus of trees, across the continent.

A Persistent Ecological Bridge, Not a Broken Link

Conventional wisdom suggests that species exchange between Amazonia and the Atlantic Forest only occurred during brief, wetter climatic periods — like glacial interludes — when forests temporarily expanded across drier regions.

But the study turns this idea on its head.

“We found evidence for 16 to 20 separate colonization events from Amazonia to the Atlantic Forest over evolutionary timescales,” the authors report, “with little or no clustering around known humid periods.”

Using genome sequencing of 453 samples representing 164 Inga species, the researchers constructed a detailed phylogenetic tree to date the timing and direction of dispersal events. The findings reveal a steady and directional flow — mostly from the Amazon into the Atlantic biome, with only one or two instances of movement in the opposite direction.

River Forests as Green Highways Across a Dry Land

So how did Amazonian trees cross the 1,000-kilometer-wide dry diagonal?

Not through vast climatic shifts, the study argues, but rather via gallery forests — narrow bands of riparian vegetation lining rivers and streams that cut through the dry matrix.

These river-connected forests likely acted as stepping stones, allowing tropical species to gradually migrate over generations.

“This supports the idea of functional connectivity through riparian corridors,” explains co-author Erik J. M. Koenen. “Even a fragmented landscape can facilitate gene flow if key forest patches are preserved.”

The study’s biogeographical modeling also showed that this pattern of continuous dispersal was not random. It defied null expectations, strongly suggesting that Inga trees systematically expanded their range through existing natural corridors, rather than by chance.

Amazonia: A Source, Not a Sink

One of the most striking findings is the unidirectional nature of the species flow. The Atlantic Forest appears to have been a recipient, not a contributor, to Inga diversity.

This aligns with broader ecological theories positioning the Amazon as a “cradle of diversity” — a vast, stable, and complex ecosystem that continuously generates new species and exports them to other biomes.

Such directional dispersal patterns suggest that the Amazon played a critical role in shaping the biodiversity of neighboring ecosystems, challenging the notion that the Atlantic Forest developed its flora in isolation.

“Our results suggest that the Atlantic Forest owes much of its plant diversity to continuous colonization from Amazonia,” the study concludes.

What This Means for Conservation Policy

The findings carry major implications for landscape management and conservation planning across Brazil and South America.

1. Protecting riparian corridors is vital.
These gallery forests may look narrow and isolated, but they are ecological lifelines that support the movement and survival of tree species across vast regions. Their degradation — often due to agriculture or cattle ranching — could sever critical evolutionary pathways.

2. Connectivity matters more than contiguity.
The focus should shift from preserving large, unbroken forest blocks to ensuring functional ecological connectivity. A fragmented landscape can still be biologically connected if key habitat bridges — like river forests — are maintained or restored.

3. Conservation strategies must be regional and dynamic.
Efforts to preserve tropical biodiversity should incorporate evolutionary and biogeographic knowledge, adapting to the unique history and structure of each biome. This means prioritizing areas where natural species flow has historically occurred.

Conclusion: Rivers Remember the Way

This study redefines how we understand forest connectivity in South America. Far from being isolated refuges, the Amazon and Atlantic Forest are part of a living network, linked by rivers and evolution over millions of years.

By recognizing the role of hidden green corridors, we can better plan for the future — preserving not just what’s left, but what still connects.

“If we lose these riparian forests,” warns co-author Catherine A. Kidner, “we’re not just losing habitat — we’re losing the pathways that sustain tropical diversity.”

Topics of interest

Biodiversity