Before the bulldozers rolled into the Ivanpah Valley, in the California Mojave just short of the Nevada line, the biologists walked the site and counted. Their first survey of the 3,500-acre footprint found 16 desert tortoises. Refined estimates pushed the likely total to a few dozen, with 32 commonly cited as the upper bound. Federal regulators wrote the permits accordingly: the $2.2 billion BrightSource solar project could relocate a maximum of 38 adult tortoises, with a hard cap of three accidental deaths per year. Exceed either number and the whole project could be shut down.

Then construction started, and the desert began handing up tortoises.

The count that would not stop

Desert tortoises are almost comically hard to survey. They spend up to 95 percent of their lives underground in burrows, and a juvenile is the size of a cookie, the color of the dirt, and silent. A walked survey samples the surface of a landscape whose inhabitants are almost all beneath it.

At Ivanpah, that sampling error detonated. As crews cleared land through 2010 and 2011, workers and monitors kept finding animals the surveys had missed, until biologists’ estimates of the site’s population reached 162 adults and 608 juveniles, roughly 770 tortoises on ground certified to hold a few dozen, and federal reviewers concluded that more than 2,300 additional juveniles lived within a couple of miles of the fences. Every discovery pushed the company closer to its federal thresholds, and in 2012 the tortoise did what almost nothing else could do to a flagship energy project: excavation work found so many animals that construction stood still for three months.

The most expensive reptiles in America

What followed became the most elaborate, and costly, tortoise rescue ever mounted. BrightSource hired teams of consulting biologists, some billed at up to $1,000 a day, to locate, excavate and move the animals under protocols negotiated line by line with state and federal agencies. The company built a tortoise nursery on site, agreed to five years of post-move monitoring, and strung some 50 miles of buried, tortoise-proof fencing at up to $50,000 per mile to keep relocated animals from burrowing straight back.

By the time the plant’s mirrors were rising, the company had spent $56 million on protecting and relocating the tortoises. Divide that by the animals actually handled and Ivanpah’s tortoises rank among the most expensive wild animals ever moved, tens of thousands of dollars a head.

The money could not buy a clean outcome. The Los Angeles Times, which chronicled the effort, catalogued the mishaps: tortoises crushed under vehicle tires, army ants swarming hatchlings in the makeshift nursery, and one small tortoise plucked from the desert and carried off to a golden eagle’s nest, the microchip embedded in its shell still pinging faintly as it receded out of range. Relocation itself is hard on the species; moved tortoises often try to walk home, and studies of past translocations have found elevated death rates from predators, exposure and stress.

Why the surveys failed

The Ivanpah undercount was not incompetence so much as the nature of the animal colliding with the nature of a deadline. Tortoise numbers fluctuate with rainfall: in dry years the animals stay sealed in their burrows and are effectively invisible, and the pre-construction surveys were walked in drought. Wetter seasons during construction brought the population to the surface just as heavy machinery arrived to meet it.

The lesson was expensive enough that it changed the industry. Later Mojave solar projects went in with multi-season surveys, bigger relocation budgets priced in from the start, and in some cases redesigned footprints. BrightSource itself had already shrunk Ivanpah by 12 percent and cut its planned power towers from seven to three during permitting, sacrificing about a tenth of its output to reduce the tortoise toll before a single mirror was installed.

The desert’s stubborn survivor

There is an irony in which species stopped the bulldozers. The desert tortoise is one of evolution’s great endurers, a lineage that watched the dinosaurs go and sat out the ice ages, capable of living a year on the water in one spring meal. What it cannot easily survive is the pace of the modern Mojave: highways, ravens subsidized by landfills, off-road traffic, and now the utility-scale build-out of the sunniest land in America, much of it prime tortoise habitat.

Ivanpah became the case study both sides cite. To solar’s critics, it is proof that renewable energy has an ecological price tag that gets paid in bulldozed habitat and six-figure animal removals. To its defenders, it is proof the system worked: the tortoises were found, the project paid, the species’ presence reshaped a two-billion-dollar build. Both readings agree on the underlying fact, which is the one the surveys got wrong. The empty-looking desert was never empty. It was full of tortoises, underground, waiting out the dry years, right up until the day the ground began to move.