Modeling the tortoises surveyors missed puts Aldabra's population near 180,000

The new best estimate is about 1.83 times the widely cited figure of 100,000 from the late 1990s ^[19][3]. The authors ask readers not to treat that as an 83 percent rise. Earlier surveys did not account for tortoises that were present but unseen in dense scrub or shade, and the researchers caution that the new figure does not mean the population has nearly doubled since 1999 ^[7]. In their view the older figures were probably underestimates, and the population has still genuinely grown ^[9]. The published account does not say how much of the gap comes from each cause.

The two numbers come from different ways of counting. The team used Bayesian hierarchical distance sampling, which corrects for animals the surveyors missed and for visibility that differs from one habitat to another ^[8]. "Aldabra's giant tortoise monitoring program is one of the longest-running in the region, but the atoll's dense vegetation makes counting tortoises genuinely difficult in places," said Nancy Bunbury, a co-author ^[13]. Tortoises were densest on the grassland known as "tortoise turf," then on open exposed surfaces and open mixed scrub ^[10]. They were hardest to detect in dense scrub, mangrove and Pemphis, where the modeling helps correct for reduced visibility ^[10].

I think the design suits the question. A tally of only the animals seen would come out lowest in exactly the habitats where tortoises are hardest to see, and those are the habitats the correction targets ^[8][10]. "By modeling what we didn't see, as well as what we did, we've produced the most reliable baseline yet for this iconic Seychelles species, establishing a benchmark we can use to track the population for decades to come," Bunbury said ^[14]. The figure is strong as a baseline. The stability finding behind it covers only the survey period ^[4].

The recovery shows up over the longer record. Against the 19th-century low, today's estimate is more than 180 times larger ^[20][1]. Aldabra became a strict nature reserve in 1981 and a UNESCO site in 1982, and the study's account credits that protection with aiding the recovery ^[6]. The new survey shows where the population stands now. The case that protection drove the recovery rests on that longer history ^[6].

Frauke Fleischer-Dogley, chief executive of the Seychelles Islands Foundation, gave the credit to the field staff. "The monitoring data in this study were collected by SIF staff living and working on the atoll, year after year," she said ^[15]. Aldabra is one of only two places where giant tortoises live in the wild. The other population, in the Galapagos, is much smaller ^[18].