Scientists plan to use satellite photos to count giant kangaroo rats, the first-ever monitoring of an endangered species from outer space.

Scientists at the University of California, Berkeley, and the California branch of the Nature Conservancy will examine images taken from the same satellite used by Israeli Defence Forces to find the circular patches of earth denuded by the rats as they gather food around their burrows. From that they plan to get the first-ever accurate population count of the rodents, a bellwether for the health of a parched plains environment.

By comparing the photos to 30 years of satellite images being released this month by the U.S. Geological Survey, researchers hope to better understand how the population has fluctuated in response to climate change and as the arrival of state and federal canal water turned the arid San Joaquin Valley into a patchwork of intensely cultivated farms and forced giant kangaroo rats to concentrate on higher ground.

The information will help the scientists determine when cattle might be used to reduce non-native grasses, allowing the rats to more easily find food.

This study using satellite technology is taking place on the vast Carrizo Plain, a 1,000-square-kilometre desert grassland 240 kilometres southwest of Fresno that is home to the most concentrated remaining populations of kangaroo rats.

The technology replaces trapping and tedious airplane flyovers as a means of taking census.

"It allows us to more quickly recognize whether populations are declining where we want them to exist," said Scott Butterfield, a biologist with the Nature Conservancy. "If they go below a threshold, that is when we would consider intervening."

Giant kangaroo rats, nocturnal rodents so named because they hop on back legs, adapted to their desert environment by extracting moisture from seeds and in their nasal passages from the humid air they exhale.

For food, they pile seeds from native grasses in circles outside their burrows, which provide shelter for the endangered San Joaquin antelope squirrel and blunt-nosed lizards. Their fat 13-cm bodies are a favoured source of food for the endangered kit fox.

High rainfall encourages the growth of taller non-native grasses, which overrun the shorter grasses that kangaroo rats depend on for food. Less food means fewer offspring. When kangaroo rats decline, so do the endangered native plant and animal species that depend on them for survival, the researchers say.

Determining at what point rainfall affects foraging will help the U.S. Bureau of Land Management establish grazing policy to control non-native grasses and encourage a healthy kangaroo rat population.

"Without them the entire ecosystem would go out of whack," said Tim Bean, a doctoral student with the department of environmental policy and management at the University of California, Berkeley. "It's fairly rare for something so small to be a keystone species. It's easier to track, say, bison."

Farming has taken 90 per cent of the kangaroo rat habitat since the middle of the last century.

The Carrizo Plain National Monument is California's largest remaining undisturbed tract of grasslands similar in biology and geography to the San Joaquin Valley, and it supports many plant and animal species that once thrived on the valley floor.

"Carrizo is like a Yosemite for grasslands, and there are decisions people are learning to make to manage it in a way that preserves its natural state," Bean said. "Since the kangaroo rat is so important to its function, we've got to get a handle on it."