Pueblo Chemical Depot post-grazing habitat monitoring

By Max Canestorp, PCD Natural Resources Manager
and Renée Rondeau, CNHP Ecologist

For many years Pueblo Chemical Depot hosted a grazing program on the eastern and southwestern portions of the installation. The livestock owner the depot worked with lived in another state, but was allowed to graze his cattle on PCD property on an outlease basis. However, there was little oversight of the program, and far more cows were allowed on PCD than what the grazing prescriptions called for. Then, in the mid-1990s, ecological surveys were conducted on the depot and it was determined that the leased areas were significantly overgrazed, to the point that vegetative communities had changed. Consequently, in 1998, the grazing program was suspended and the cattle were removed from depot properties. During Team Pueblo meetings a decision was made by cooperators (PCD, the Local Reuse Authority, and local citizens) to initiate a post-grazing habitat monitoring study that would determine how the vegetative communities might recover with the cessation of grazing. There have been many such studies in several different vegetative types in western states, but the depot project was enhanced by including grasshopper and small mammal populations in the study to determine grazing effects on those biological communities as well.

The post-grazing study was set up with plots established in both grazed and ungrazed areas within the four primary natural vegetative communities found on the installation: shortgrass prairie, greasewood scrub, sandsage, and riparian woodland. Data on the plants, grasshoppers, and small mammals were collected from the plots for five years. Then, in accordance with the project's design, after a five-year hiatus the plots were visited again for a long-term assessment of the vegetative communities.

In the initial 5-year portion of the study, it was found that 36 common grasshopper species were found in equal numbers in both grazed and ungrazed vegetative communities. However, 16 uncommon species were absent or in low numbers at ungrazed plots while common in grazed sites. A total of 52 grasshopper species were recorded during this portion of the study!

The small mammal group found that although there were certain species' preferences for vegetative types (e.g., deer mice preferred greasewood habitats and kangaroo rats preferred sandsage habitats), there were no apparent differences in small mammal populations between grazed and ungrazed vegetative communities. This could be because the timeline for small mammal response to grazing is short on these systems, or simply because no differences existed.

Regarding vegetation, we found that after 12 years without grazing there is more grass and less bare ground in 2010 than in 1998; however, some species are slow to respond, e.g., needle-and-thread grass is just now starting to show signs of coming back into the areas where it was previously grazed out. It was also found that if blue grama grass, the quintessential shortgrass prairie species, has been significantly reduced through overgrazing or other harmful influences, it may be very difficult to recover or restore. Incidental findings include the significant increase of shrubs after the 2002 drought, indicating that an increase in droughts could possibly change the grassland into more of a shrubland, impacting the animal community. Other fun facts: do you know that cow fecal pats are still present on the prairie after 12 years of no grazing?

How does the post-grazing habitat monitoring study benefit PCD and the shortgrass prairie ecosystem? Determining the effects of grazing, and especially overgrazing, can help in interpreting past influencing factors in shortgrass prairie systems. It can help us learn how to restore overgrazed lands. And finally, it can help us determine grazing prescriptions by monitoring vegetative communities. Pueblo Chemical Depot may someday resume a grazing program. However, greater attention will be given to monitoring livestock numbers and grazing impacts on vegetative communities.