As an up-and-coming UC Riverside professor in the early 1980s, Mary Price often served as “poster girl” for the Natural Reserve System. Her colleague, NRS founder Bill Mayhew, regularly took her out to meet landowners who were considering donating property to the reserve system. “When Charlie Motte was thinking of donating the land for the Motte Reserve,” Price modestly recalls, “Bill hauled me over there to convince him that there were energetic young faculty who would do wonderful things, and find out all sorts of terrific information, with his property. In reality, it was Bill’s passion and total dedication that were most convincing, but I was happy to help.”
Today Price is winding up her career at UC Riverside (UCR), having fulfilled Mayhew’s promise by working regularly at both the Motte Rimrock Reserve in Riverside County and the Sweeney Granite Mountains Desert Research Center in San Bernardino County. Her fieldwork at these sites, as well as others, has produced significant contributions to our understanding of community ecology and the coexistence of species.
Price’s relationship with the Granite Mountains is particularly strong and began in the days when various sections of the reserve were still being assembled. It was January 1983 when Price and some colleagues took a memorable research trip to the area. Kevin Heinz, one of her graduate students, was studying the elevation distribution of two species of kangaroo rats, the Merriam’s k-rat (Dipodomys merriami) and the Panamint k-rat (Dipodomys panamintinus). Heinz had set up transects throughout the Mojave, where both species occur. One of his study sites was in the Granites, so Mary and her husband, fellow UCR biology professor Nickolas Waser, offered to tag along to help him collect data. “The Granite Mountains site is a beautiful transect,” Price explains. “You have a steady 3,000- to 4,000-foot gradient, along an uplifted piedmont with only minor changes in the soil substrate, so the vegetation changes gradually with the elevation.”
Because the days were short and cold, the fieldwork was extremely hectic. The trio had to set and clear their traps, take soil samples, and conduct vegetation transects at five different elevations in as short a time as possible. The wind was howling. “We were running like crazy the whole day just to get everything processed before dark,” Price recalls.
When they were finally done, the exhausted researchers began looking for a campsite. At their first site, the wind was blowing so hard they couldn’t keep the stove lit. Noticing some large rocks in the distance, they decided to seek a more sheltered spot. They repacked their gear and headed out along a dirt road in the general direction of the rocks. The road led them to the hoped-for sheltered spot, but with an additional surprise: an abandoned house.
“We were entranced,” says Price. “Not only did we have shelter from the wind, but also it was a beautiful site. Our first thought was, ‘Can we buy this place?’” Out of the reach of wind and weather, at last, the three chatted over a hot dinner about how great it would be to have a field station in the East Mojave.
When Price returned to campus the next week, she mentioned their discovery to Mayhew and was surprised to learn that he knew about the place. He’d taken classes there many times, and in fact the Natural Reserve System was negotiating to buy the property as part of a much larger reserve they were putting together. Purchase of the land was completed in 1985; today the now-restored house is part of the reserve’s Norris Camp. Price’s dream of having a base of operations with electricity, running water, data connections, and all the other amenities required for long-term fieldwork is a reality.
The Company One Keeps
For Price, field stations are important not only for the facilities they provide, but also for the people they attract. When researchers spend extended time in a single area, they begin to notice the subtle factors that are crucial in understanding how a natural system functions. Price calls this developing one’s “natural history intuition.” This process is accelerated at a field station because one is surrounded by other trained researchers who are also observing the system closely, though perhaps from different perspectives. Price has found that a colleague’s casual observation can sometimes cast her own research focus in a new light.
“Field stations allow information about a place to accumulate,” Price adds, “so researchers can build on a base of knowledge and don’t have to start from square one. In a sense, knowledge begets knowledge.”
Soon after the University had established the Granite Mountains Reserve — now designated the Sweeney Granite Mountains Desert Research Center — Price and Waser began spending as much time as possible there. “Nick and I were trying to do behavioral observations,” Price explains. “So during vacations or sabbaticals, we’d head out there to do the fieldwork. We colonized the little rock house that Philippe and Cindy [Philippe Cohen and Cindy Stead, who managed the reserve from 1986 to 1993] had used as a storage shed. We more or less mouse-proofed the place so people could stay.”
Over the years, Price’s long stays at the research station have given her deeper understanding of the natural systems of that reserve, as well as some surprising, new insights into the lifestyles of the animals that make this desert their home.
Competition versus Coexistence
Much of Price’s research focuses on the heteromyidae, a family of small to medium-sized rodents that includes such diverse species as kangaroo rats and pocket mice. Price had become interested in the coexistence of multiple species of heteromyids while doing her graduate work at the University of Arizona. Because all such species share the same food source — seeds — logic would seem to dictate that one species, or perhaps one body type, should prove superior and out-compete the other species. Instead, diverse heteromyids coexist. When researchers put traps out in the desert, they often capture four or five different heteromyids, as well as an assortment of other small rodents and ground squirrels.
To uncover the secrets of this coexistence, Price decided to study two very different heteromyids: the medium-sized, bipedal kangaroo rats and the tiny, quadrupedal pocket mice. Her hypothesis was that coexistence between the two species was based on “resource partitioning,” a strategy employed by many other animals. Resource partitioning is the behavior of coexisting species using different parts of a single, shared resource. It leads to coexistence if each participating species is specially adapted to use a specific microhabitat for locating food and avoiding predators. In this case, Price reasoned, the larger, quicker kangaroo rats might be better at avoiding predators in the open space, while the pocket mice might be more efficient at finding seeds in the richer, coarser soils found underneath shrubs.
For resource partitioning to lead to coexistence, however, each species must be better than the others at using some part of the partitioned resource. This difference in performance, called a “trade-off,” is what prevents one species from usurping all of the available resources.
To determine the trade-offs between kangaroo rats and pocket mice, Price brought her subjects into her lab and exposed them to different feeding and predator-avoidance conditions. But she couldn’t find any trade-offs. When she tested their abilities to avoid predators in open spaces versus under shrubs, all species were found to be much safer under shrubs. When she tested their abilities to harvest seeds in different types of soils, the kangaroo rats were always faster at harvesting seeds. So why didn’t the larger, behaviorally dominant kangaroo rats simply drive out the pocket mice? Why did they live mostly in the riskiest open spaces?
Price agonized over this problem, racking her brain to consider every factor. She studied soil samples with the idea that each species might prefer certain soil textures with different amounts of organic materials. She took soil seed samples, thinking that maybe the secret was in the clumping of seeds, or their numbers, or their size, or their spatial distribution. But nothing seemed to reveal the required trade-offs.
Price’s aha! moment came while she was reviewing her data. As part of her field experiments, she had decided, for the first time, to calibrate her measurement of the “seed bank” (seeds in the soil) by recording “seed rain” (seeds falling on the surface). What she found amazed her. Even during “seed pulses,” when the plants were producing and dropping many more seeds than the animals could eat, very few of the falling seeds were finding their way into the seed bank.
Price began taking a closer look at seed rain. In an article co-written by Price and UCR undergraduate Jamie Joyner and published in Ecology in 1987, the investigators summarized the results of a 19-month study conducted in the Granite Cove section of the Sweeney Granite Mountains Reserve. In that paper, they conclusively established that input from the seed rain did not accumulate in the soil. Even though the seed rain averaged 262 seeds per square meter each day during the study period, the seed bank actually declined by a daily average of 114 seeds per square meter.
So what was happening to all those seeds? As Price recalls: “I suddenly realized that all of that seed was being sequestered, snatched up by granivores and stored. We were all wrong about the resources these animals were using. They were not using the seed bank; they were using the seed rain. Changing that perspective opened up a lot of possibilities we hadn’t thought about before. It was an accidental observation, but now that we understood that seed production in a given year limits the population, we knew this explained the major population fluxes we’d seen between wet, seed-intensive years and dry, seed-scarce years.”
Following this breakthrough, Price looked at the desert ecology differently. She now began to observe who was gathering the newly fallen seeds and tried to determine what they were doing with them. Even when she put out trays filled with seeds, no matter how rich, all of the seeds disappeared overnight. The animals were removing many more seeds than they could eat.
This discovery led her to uncover a time-honored, universal mechanism for coexistence — theft. As she noted in a recent presentation: “Suddenly we realized there are two phases to the competition: a scramble to harvest newly produced seeds, followed by a much longer period during which the animals use previously cached seeds. While the kangaroo rats are more efficient in the first phase, collecting and caching seeds, the pocket mice are experts at locating and stealing the seeds once they’re cached by the larger animals.”
The Future
Price retired from the University in order to spend more time publishing papers based on the mountains of data she has accumulated over the years. “It’s a much better use of time,” she says with a laugh, “than sitting in meetings.” Though she will no longer be at the University, she won’t stop conducting field research. In fact, she seems to be widening her horizons for future work. A recent trip to Australia’s “Red Center” amazed her because the deserts were so different than what she had become accustomed to in the United States. And not long ago, she completed an intensive Spanish-language course in preparation for doing more fieldwork in Mexico.
She’s also working on a book proposal that focuses on the role of “place-based” research in developing a general understanding of larger ecological processes. “It’s interesting that ecologists haven’t really started to talk about how they gain understanding and how they know it’s useful or accurate,” she explains. “Ecologists tend to do focused work on one system for many years. So, how can you make much progress, given that there are billions of places out there and relatively few ecologists? How do you gain a general understanding that’s useful beyond that one tiny place on earth where you happened to study? There’s a paradox there, so we’re asking ecologists to talk about it.”
Despite these larger philosophical thoughts, Price has no regrets about spending much of her career conducting place-based research. “I do this because I like it,” she muses. “There’s something I find very personally satisfying about thinking about how nature is put together, and trying to figure out a strategy that will force nature to tell me something. That I find fascinating. My career choice has given me the freedom to do things I like to do.” — JB
Editor’s note: Mary V. Price is Professor Emerita of Biology at University of California, Riverside, an adjunct professor in the School of Natural Resources at the University of Arizona, Tucson, and affiliated with the Rocky Mountain Biological Laboratory, Crested Butte, CO. She can be contacted by email at: <mary.price@ucr.edu>. |
