April 1, 2010
Demonstrating synergy between functional groups: Burrowing mammals and megaherbivores
The black-tailed prairie dogs of the Chihuahuan grasslands have been under enormous pressures due to human activity, mostly from poor land management and overgrazing from cattle in the region. I blogged about a paper back in January from Gerardo Ceballos, Ana Davidson and their colleagues describing the level of colony disruption in the Janos region of Mexico and how the absence of the black-tailed prairie dog, this keystone species, was affecting the populations of other organisms and allowing the encroachment of scrubland to progress more rapidly.
Davidson et al. published another study a few weeks ago in Ecology further exploring the relationships between black-tailed prairie dogs and their much maligned neighbors, Bos taurus, cattle. Prairie dogs have been generally regarded as a danger to cattle by ranchers and removed through poisoning or other means. Overgrazing can lead to desertification, further threatening these animals. But that's a relatively new trend in a long and complex history of interaction between prairie dogs and megaherbivores like cattle.
Bison used to roam the Janos grasslands living side by side with burrowing mammals like the banner-tailed kangaroo rat and the black-tailed prairie dog, but across North American grasslands, free-roaming herds of bison have been replaced by cattle herds. There has been a recent push to reintroduce bison to Chihuahua, to reinstate their ecological status, but Davidson et al. are taking a more realistic approach in this study given the wide range of cattle distribution and the importance of these animals to local economies. A level of functional equivalence has been demonstrated between bison and cattle, and the authors seek to pin down the specifics of how cattle and prairie dogs affect their neighbors and the environment both in cohabitation and in isolation. This study addresses a much larger issue of demonstrating the synergy of organisms in different functional groups having a combined effect on the ecosystem.
The authors established and studied four types of plots to tease out individual effects from synergistic: Prairie dogs and cattle present (P+C+), prairie dogs present, no cattle (P+C-), no prairie dogs, cattle present (P-C+) and neither present (P-C-). Important environmental variables were analyzed for each of these plots, waypoints to illustrate discrepancies: "vegetation (plant height, cover, and biomass), animal activity (fecal counts of rabbits, and soil disturbance by prairie dogs, kangaroo rats, and gophers), mounds (of prairie dogs, kangaroo rats, and harvester ants) grasshopper abundance and prairie dog abundance."
First and foremost, prairie dog abundance doubled in the presence of cattle due to grazing and their combined efforts had a dramatic effect on the plant cover on the P+C+ plots, reducing vegetation height significantly. There was also two to three fold decline in the grasshopper population between the P-C- plots and the P+C+ plots.
The study demonstrated some interesting individual effects as well. In the absence of prairie dogs (P-C+), banner-tailed kangaroo rats activity increased. According to the authors it has been speculated in the past that these animals, who are also considered ecosystem engineers and a keystone species, actually compete with the black-tailed prairie dog. The results seem to favor that notion as well as demonstrate at least a short term boon for the kangaroo rats - another burrowing mammal - in the presence of a megaherbivore, particularly one that has been "responsible" for perpetrating the overgrazing leading to desertification that threatens the kangaroo rat.
The important concept here is combined effects; the individual effects on the environment and other organisms by prairie dogs and cattle were mostly not significant, but when they were combined and allowed to synergize, they apply broad controls to the ecosystem. From a land management standpoint, this natural inclination can be employed to help maintain biodiversity and protect threatened species in the Chihuahuan grasslands without pushing ranchers out.
Davidson, A., Ponce, E., Lightfoot, D., Fredrickson, E., Brown, J., Cruzado, J., Brantley, S., Sierra, R., List, R., Toledo, D., & Ceballos, G. (2010). RAPID RESPONSE OF A GRASSLAND ECOSYSTEM TO AN EXPERIMENTAL MANIPULATION OF A KEYSTONE RODENT AND DOMESTIC LIVESTOCK Ecology DOI: 10.1890/09-1277