Brown tree snake DNA explains Guam invasion

An invasion can begin with only a handful of individuals yet still reshape an entire ecosystem. That is what happened in Guam with the brown tree snake (Boiga irregularis): despite descending from a small, highly inbred founding population, the reptile spread across the island. A genomic study now suggests that hidden diversity in its DNA may help resolve this paradox.

How did so few brown tree snakes take over Guam?

Native to areas including Papua New Guinea, Indonesia, the Solomon Islands and Australia, the species is thought to have reached Guam accidentally shortly after the Second World War. The United States Geological Survey (USGS) identifies the Admiralty Islands as the likely source of the introduction, which was linked to cargo transported to the island.

The setting provided an advantageous combination: plentiful food and no natural controls able to curb the population quickly. A generalist, nocturnal predator, the snake eats lizards, birds, small mammals and eggs. Within a few decades, what began as an accidental introduction had become one of the Pacific's best-known cases of biological invasion.

What did scientists find hidden in the DNA?

A study published in Science Advances in 2026 examined a striking contradiction: how could a population founded by so few animals flourish despite an extreme genetic bottleneck? The researchers used modern methods that can detect major genomic changes which conventional analyses may miss.

The assessment identified around 19,000 structural variants - changes in which sections of DNA may be duplicated, deleted or rearranged. It also found that roughly half of the genome examined consisted of long regions of homozygosity, an indication of intense inbreeding. However, this structural diversity provided a genetic source that was not readily visible through standard analyses.

• Nearly 19,000 structural variants were identified in the genomes of the snakes studied.
• These changes affect a far greater proportion of the genome than variations involving a single DNA letter.
• Genes linked to immunity and smell were among the regions enriched with these variants.
• The findings help explain how a highly inbred population can still retain genetic resources that may be useful for adaptation.

Why might smell and immunity have mattered?

The structural variants were not spread at random. According to the Science Advances study, they occur at high frequency in regions associated with immunity and smell. This may matter because coping with unfamiliar pathogens, interpreting a new environment and finding food are all essential challenges when a species enters unknown territory.

The researchers also consider an intriguing hypothesis concerning the recognition of other snakes. Changes associated with chemical perception could affect how individuals identify relatives, prey or other animals. Further investigation is still needed, and the scientists do not claim that these variants alone account for the invasion's full success.

Watch the video shared by the YouTube channel 1% A DAY, showing how brown tree snakes managed to take over an entire island.

What happened to Guam's native animals?

The snakes' success came at an enormous cost to local wildlife. The USGS states that the species was among the main causes of the loss of nine of Guam's 11 native forest birds, and it also contributed to declines in bats and lizards. Removing birds from the ecosystem created effects that went beyond a simple fall in species numbers.

The consequences also extended into urban areas. As highly capable climbers, these snakes can reach poles, power lines and substations, where they may cause short circuits. USGS research and programmes indicate millions of dollars in damage to electrical infrastructure, alongside the ongoing need to prevent animals concealed in cargo from being carried to other islands.

Why does this discovery change how small populations are viewed?

For a long time, population bottlenecks have been associated chiefly with lost genetic diversity and reduced adaptive capacity. Guam's case does not overturn that principle, but it adds an important element: some populations may retain relevant structural variations even when other indicators reveal worrying levels of inbreeding.

Researchers now want to establish whether these variants existed before the invasion or whether some changed after the snakes arrived in Guam. Comparing island animals with populations from the species' original range may help answer that question. Understanding this mechanism matters both for predicting invasions and for studying threatened species. The snakes' DNA shows why an apparently vulnerable population should never be underestimated.