April 19, 2009
In the cerrado—a tropical region of grasslands and dry forest—Emmons watched as the Brazilian guinea pig Cavia aperea went locally extinct, and three other rodent species declined significantly: the South American giant rat Kunsia tomentosa, the Paraguayan Bolo mouse Necromys lenguarum, and the Huanchaca mouse Juscelinomys huanchacae. Surveying two different areas in the cerrado, Emmons found a nearly 96 percent decline in small mammals, from a high point of 232 individuals surveyed to a low of ten. In 2004 guinea pigs vanished entirely from the cerrado and have not returned.
Brazilian guinea pig with eartag just before the disappeared from study site. Photo by: Louise Emmons.
Another change in the ecosystem, however, caught Emmon's eye. “When the study began in 1998, we were soaked to the waist from dew on the grass, shoes squelching, each morning when checking savanna traps,” she writes in a paper published in Biotropica. “In later years the morning grass was usually dry.”
The sudden disappearance of nighttime dew coinciding with the rodent declines caused Emmons to wonder if the small mammals had depended either directly or indirectly on the dew. “We do not really know yet whether the small animals and plants need or use dewfall for survival during hot, dry weather, but it is a strong possibility,” she says, describing her theory as an 'anecdotal hypothesis' which requires further research.
Heavy dew on morning grass. Photo by: Louise Emmons.
Farmers and ranchers burn the Brazilian Amazon annually to make more room for agriculture and ranching. According to satellite data, recent years have seen some of the worst fires in the Amazon. Data collected by a Bolivian airport showed a direct correlation between weather changes in the cerrado and the seasonal burning in both the Brazilian Amazon and the Santa Cruz department of Bolivia.
The South American rat is endemic to the cerrado. It suffered significant declines during the survey. Photo by: Louise Emmons.
According to Emmons, an in-depth study of the connection between dew and small mammals is needed to determine the validity of her theory. “We really do not know much at all about the role that dew might play in the lives of tiny plants and animals in dry habitats,” she admits. “Two of the mice that declined on our study area eat mainly insects, the other two eat plants. Possibly dew could be important for the insects and plants, and not directly for the mice, but we don’t know. The plants might need dew, then the insects might need to eat those plants, then the mice need to eat those insects, then the maned wolves need to eat those mice . . .You see how it goes. Or the mice might need to drink dew directly during hot, dry weeks – we saw a fox drinking dew by lapping it up from grass, but we have no information about the needs of the little creatures. We need to study the whole cycle of dew formation, smoke, and temperature, and to get down on our hands and knees to study the chains of relationships hidden in the grass.”
A relationship that is currently being studied by one of Emmons' students is how the disappearance of rodents has already affected one of the area's largest predators, the long-legged maned wolf. The Brazilian guinea pig is one of the major food sources for the maned wolf in the cerrado. Before their disappearance guinea pigs appeared in 80 percent of maned wolf scats, however after the decline the number of wolf scats including guinea pig dropped to zero. Increasingly, maned wolves, who are omnivorous, are being forced into vegetarianism: in 2007 over half of the wolf's scats had no meat while four years earlier over 80 percent contained meat.
The maned wolf lost prey due to the mysterious disappearance of small mammals in the cerrado. Smoke from seasonal burning can be seen behind the wolf. Photo by: Louise Emmons.
If Emmons' hypothesis turns out to be correct, it would mean a new environmental factor for conservationists to take into account. In addition, the theory may help explain other mysterious mammal declines.
“Australian dry habitats have shown numerous declines and extinctions of small mammals while wetter habitats have not,” Emmons points out. “Although introduced predators (cats, foxes) have likely caused many of the declines, some researchers do not think this can explain those in the Northern Territory, where it has also been shown that patterns of fire have changed, and visibility has decreased because of smoke. I think it is worth taking a second look at what is happening in northern Australia and in other regions with strong dry seasons.”
Fires (red points) and smoke on a single day over all of Bolivia (24 August, 2004) study area marked as Los Fierros, photo from NASA - Modis, courtesy of Louise Emmons.
Pointing to high levels of endemic species, both of plants and animals, Emmons says that the cerrado is millions of years old and includes many undiscovered treasures: “for example, during my work in a tiny area of Noel Kempff Mercado National Park, we have discovered and described four rodent species new to science, and I am sure that we have not yet completely inventoried the mammal fauna.”
CITATION: "Long-Term Variation in Small Mammal Abundance in Forest and Savanna of Bolivian Cerrado". Louise H. Emmons. Biotropica, March 9th, 2009.
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