Super rodents build super habitat

  Faster than a speeding coyote, able to leap small cacti in a single bound -- two “superhero” rodents, the kangaroo rat and the prairie dog, thrive amid the heat and dry sand of the desert Southwest. Each creature influences its environment to an extent that far outweighs its size – a real-life version of Mighty Mouse. In the Chihuahuan Desert, banner-tailed kangaroo rats and Gunnison and black-tailed prairie dogs fill the role of keystone species, animals whose presence significantly affects the overall health of an ecosystem. Now, a groundbreaking new study has determined that where one keystone species is good, two may be even better. For with their powers combined, these “superhero” rodents create super habitat.

The Chihuahuan Desert is North America’s second largest, 140,000 square miles spanning Arizona to Texas and down into Mexico, encompassing an area almost as large as California. In this desert, “sky islands” rise from a brittle and windblown basin, stubbly with prickly pear, agave and grasses. These mountains support pockets of temperate climate and create one of the most diverse desert habitats in the world.

Here, the presence of ecosystem engineers, such as kangaroo rats and prairie dogs, adds to the desert’s diversity. These rodents dig burrows and disperse seeds, providing shelter and food for hundreds of other animals. The changes trigger ripple effects that cascade throughout all levels of the ecosystem, influencing everything from top predators, such as sharp-eyed eagles, to chemical processes, such as nutrient cycling. And while a solitary kangaroo rat burrow or a prairie dog colony can harbor a Noah’s Ark of other critters, researchers at the University of New Mexico have found that a combination of the two types of burrowers boosts the overall health of the environment by significantly increasing the diversity of habitat and wildlife. “We’re looking at two ecosystem engineers that have really large transformation effects on the habitat,” says Ana Davidson, who completed the study for her doctoral thesis at the University of New Mexico.

Between 2000 and 2002, Davidson and associate professor David C. Lightfoot monitored invertebrates around kangaroo rat and prairie dog colonies. They collected and identified more than 250 species of insects, spiders and other invertebrates relying on the habitat created by these small rodents. “I wanted to know, is a mound just a mound and is a burrowing rodent just a burrowing rodent?” says Davidson.

What she found was that while kangaroo rats and prairie dogs are similar in some ways -- burrowing, creating mounds of dirt at tunnel openings and feeding primarily on grasses, seeds and wildflowers -- they nevertheless play distinct roles within their environment.

Not only are those roles distinct, they’re synergistic; where prairie dogs and kangaroo rats co-exist, a healthier environment prevails. For instance, the researchers found that in central New Mexico’s Sevilleta National Wildlife Refuge and Mexico’s Janos-Casas Grandes region, insects, spiders and other invertebrate populations have surged in areas where both rodents live. A plethora of insects backpack on the hard work of the two rodents as they diversify plant communities and excavate their subterranean cities.

“It was a surprisingly strong pattern,” says Davidson, referring to the increase in abundance and variety of bugs connected with the multi-rodent metropolis. Her surveys indicate a greater than 95 percent certainty that the relationship between the rodents and invertebrate communities is not due to random chance.

Differences in prairie dog and kangaroo rat behavior create the bonanza of habitat and food choices for other animals.

Kangaroo rats selectively harvest and store seeds underground, a ready food cache for raiding beetles and a rare burrowing ant. The rodents scatter seeds across the desert when they gather and hoard, shaping plant communities as they scurry about. Prairie dogs, on the other hand, rarely bring food into the burrow, grazing instead on open prairie. They eat different plants than the kangaroo rat does and act almost like a natural lawn-care service for the grasses and wildflowers around their mounds. This “gardening” supports a variety of grasshoppers and crickets, which rely on everything from barren soil to dense patches of grass.

The rodents’ subterranean homes offer a variety of shelter for insects, spiders and other invertebrates. Kangaroo rats excavate intricate mazes, in which the temperature fluctuates only moderately throughout the day, creating a more stable habitat than at the desert surface. These tunnels are particularly appealing to ground-dwelling invertebrates, such as beetles and spiders. Prairie dogs dig deep, long tunnels that are damp and cool. It’s the antithesis of the desert -- just right for camel crickets and burrowing ants, elusive inhabitants of desert environments. Keystone species create biological “hotspots,” says Davidson, “and you find these rare species that would not have homes if it weren’t for these engineering rodents.”

The increased diversity of insects supported by the rodents means more scavengers are working to decompose plant matter and dead animals, cycling nutrients through the system. Insect-eating birds feed on the bugs, and other predators, such as rattlesnakes and the Mexican gray wolf, prey on the birds, kangaroo rats and prairie dogs. It’s a cycle anchored by the changes wrought by these dynamic rodents.

Davidson and Lightfoot’s project ranks high on the “cool” meter, according to other scientists. Their research verifies what ecologists have suspected would happen should two keystone animals share the habitat. Scott Mills, a population ecologist with the University of Montana, says the majority of wildlife species in most environments are not considered keystone species and play only moderate roles in shaping the ecosystem. But when two or more keystone species are present, their effects are likely to cumulate, with each species strongly influencing the ecosystem in distinct ways. “It definitely gives a good, strong message for conservation,” says Mills. “Losing one (of these keystone) species could start popping the proverbial rivets off the airplane.”
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