On rat-free D’Arros Island, scientists counted 2,768 wedge-tailed shearwater pairs in 2016, nearly 10 times the 2009 estimate

If you need an example of nature being more resilient than we could ever hope to be, look no further than the land in the Indian Ocean that most Americans may not even have heard of: D'Arros Island, Seychelles. Roughly 0.8 square miles of land surrounded by turquoise waters, this place was completely overrun with invasive rats until 2003, which were slowly decimating an entire seabird population. Then the rats vanished, and the birds did something nobody may have anticipated: they thrived in the newfound peace.

According to a 2024 paper titled “Rapid Increase in Size of Wedge-tailed Shearwater Ardenna pacifica Colony Following Rat Eradication,” by Keys et al., published in the journal Marine Ornithology, wedge-tailed shearwaters that bred on D'Arros went from an estimated 254 pairs in 2009 to 2768 pairs by 2016 (more than an 11-fold increase, and well above the roughly tenfold figure the paper itself reports), and maintained this number (2406 pairs) upon the last count in 2021. For a species that has seen a slow, quiet decline over the years across the globe, that could be a major win for conservation.

Why should you care about rats on a speck of land?


While it's easy to dismiss this as a curiosity that has nothing to do with your everyday life, the interesting part is that it's a pretty good case study in biodiversity and its importance to ecosystems. Rats are not native to any oceanic island, meaning they were likely brought there by humans who traveled across the ocean. These rats then take advantage of the local seabirds' inability to defend themselves or their nests, decimating clutches of eggs and nests.

Jones et al., in their 2008 study titled “Severity of the effects of invasive rats on seabirds: A global review,” published in the journal Conservation Biology, provide a much-needed global perspective on the issue, stating that invasive rats are one of the most significant drivers of decline and extinction in seabird populations, with documented cases across dozens of islands and species. The fact that the shearwaters came back so quickly and decisively shows a degree of resilience not often seen elsewhere in nature, at least not on such a short timescale.

The comeback wasn't just about the birds

Seabirds like wedge-tailed shearwaters act as 'seeders' of nutrients, both in the form of nitrogen and other minerals. What this means is that while out at sea, the birds consume fish and other marine animals, and then defecate while they fly back home, thus fertilizing large swathes of land with nutrient-rich guano deposits. Where there are rats, however, this process is interrupted at every step, from eggs being devoured to full-grown birds being killed outright, and on D'Arros specifically, rats were the predator responsible, until their eradication in 2003 removed that single threat.

According to Benkwitt et al., in their 2021 paper titled “Rat eradication restores nutrient subsidies from seabirds across terrestrial and marine ecosystems,” published in the journal Current Biology, islands that had been cleared of invasive rats had significantly higher concentrations of soil nutrients, vegetation, and fish populations compared to similar islands that still had rats, although it took decades in some cases to see significant change. Thus, while the focus has been on the shearwaters as flagship species for conservation, the sheer breadth of positive change that has occurred on the island since their return bodes well for the area's overall biodiversity, even if the evidence is largely indirect.

A win, but not without nuance

The authors themselves flag a complication: D'Arros sits near Fouquet Island (part of St Joseph Atoll), home to a far larger shearwater colony, and the researchers suspect D'Arros may partly be acting as a "sink" drawing birds from that larger population, rather than growing purely through on-island breeding. That matters, because if birds are simply shifting from one island to another, it's a localized redistribution rather than a region-wide population boom. That said, the paper doesn't rest on the raw count alone: the researchers also measured the physical area the colony occupies (which grew from 3.00 to 3.85 hectares) and the density of nesting burrows within it, both of which increased, evidence they say points to genuine breeding growth on D'Arros, not just birds moving in from elsewhere.

The honest takeaway is that both forces are probably at work. This nuance is important, as it highlights the difference between conservation programs that operate with a specific species in mind versus those that take a wider look at an entire ecosystem. It is also worth noting that wedge-tailed shearwaters as a species still face a bleak future, despite the reprieve in the survival of the D'Arros population. The lesson here is that reducing one major threat factor (in this case, invasive rats) can allow the total biodiversity of an area to grow, sometimes much quicker than the expected timescale. Thus, biodiversity in general is something to be conserved, not just individual species in isolation. While in the case of the shearwaters their survival is not necessarily guaranteed, there is no doubt that removing a single threat factor can do wonders for a struggling population.