Mammal bones are visible in this owl pellet, which is tangled in cheatgrass and buried in sediment. Credit: Rebecca Terry.
In Homestead Cave near Utah’s Great Salt Lake, owls have been regurgitating pellets containing the undigested bones and hair of prey — typically small mammals like rodents — at a relatively constant rate since the end of the Pleistocene glaciations about 13,000 years ago. Those pellets have stacked up and fossilized in the cave to present a near-continuous glimpse into how mammal communities in this part of the Great Basin region have changed over time. Now, paleontologists examining bones in the pellets have found that, although small mammals in the region have generally been able to adapt to shifting ecosystems in the past, today, in the face of landscape-altering human activity, the mammal population is changing in unprecedented ways.
“Homestead Cave is a fairly protected and dry place, which is one of the reasons its owl pellet fossil record is so complete,” says Rebecca Terry
, a paleoecologist at Oregon State University and lead author of the new study
, published in Proceedings of the National Academy of Sciences. “We were really interested in how the small mammal communities in the Great Basin have responded to both natural changes — such as how the climate has changed over the past 13,000 years — and anthropogenic environmental change that started about 150 years ago.”
To study how mammals responded to natural change, Terry and her co-author, Rebecca Rowe
of the University of New Hampshire, measured the energy flow in the ecosystem, which combines body size, mammal population abundance and metabolic rate (which is a function of body size) for all the animals in the community. “Energy flow is one metric that summarizes how much energy is needed to sustain the biomass of that group of animals per unit time,” Terry says. Species were also divided into different functional groups based on body mass, diet (such as carnivore or herbivore) and habitat (open or closed vegetation, for instance).
After the Pleistocene glaciations ended and the planet began warming, “the overall energy flow that the mammal communities represented stayed relatively constant,” Terry says. However, energy flow decreased among the largest and the smallest animal groups, and shifted more to mid-sized mammals. In this way, she says, the mammal community compensated and conserved its total energy flow over time.
That started changing just over a century ago. At the turn of the 20th century, the landscape around Homestead Cave was shrub-dominated; today, it is grass-dominated, with few shrubs in sight. That’s because a European grass known as cheatgrass — brought over accidentally in ship ballasts and shipments of crop seed — has replaced much of the natural shrublands.
In North America, cheatgrass’ invasion is “happening on a grand scale,” says Julio Betancourt
, an environmental scientist with the U.S. Geological Survey in Reston, Va., who was not involved in the new study. “All of North American deserts — including the Sonoran, Mojave and Chihuahuan deserts — are seeing the same grass invade.” Cheatgrass outcompetes native plants, especially those in the Great Basin, because it requires less water to survive and proliferate than native shrubs.
“Structurally, cheatgrass is closing off the native habitat,” Terry says. The landscape “is going from being very open between shrubs, to being one that’s closed, with no space between vegetation.” Cheatgrass seeds are harder for small mammals to eat, and “a lot of the small mammals are desert-adapted animals that don’t drink — they get water from their food. So cheatgrass being a dry seed is also detrimental.”
Looking at energy flow over the last century — both from owl pellet remains at Homestead Cave as well as from modern mammals — Terry and Rowe found overall that energy flow and body size have decreased, and that the community has not been able to compensate. “Energetically, the modern is distinct from the 13,000 years that have preceded it in terms of how much energy those small mammals represent,” Terry says. “Without the fossil record we wouldn’t be able to see that [discrepancy] because our modern ecological data don’t go back far enough.”
Using data from the fossil record to inform how animal populations today are changing is an emerging field, Betancourt says. He says paleontological information will, more and more, be used to inform decisions in fields like conservation biology because understanding change in ancient ecosystems can help us better understand the severity of changes happening today. Such studies are also “going to get really interesting in the future as people become more capable of extracting and analyzing ancient DNA,” Betancourt adds, which could reveal evolutionary changes that have occurred in concert with changes in energy flow brought about by things like climate change and invasive species.