Our work since our last progress report has focused on continuing to sample our long-term projects and to continue several new studies initiated with our latest grant. The new animal work involved expanding the work on prairie dogs begun in 1997. Belowground work centers on understanding the effects of grazing on microbial communities.

Spatial distribution of weeds: Weeds, especially exotic plants, represent one of the most serious challenges to the current structure and function of ecosystems. A key first step in understanding the problem in a particular area is to investigate which species are present, where they are located and how abundant they are. We initiated a study in 1998 to begin answering these questions for the Central Plains Experimental Range and western section of the Pawnee national Grassland (1024 km2). Our initial approach is to use the roads and stream courses to do the assessment. This approach assumes that the roads and the riparian areas along stream act as conduits for the movement of species as well as loci for invasion of the adjacent grassland. The products from this work will be maps of the distribution and abundance of exotic plant species (total and by species) for roadsides, riparian areas and upland grassland. Our expectation is that this information will lead to experimental work on mechanisms controlling invasion of grassland by exotic species. The work involves two graduate students, two REU’s and one additional undergraduate funded by a NSF research program on campus.

The effects of increasing temperatures on shortgrass steppe vegetation: There is now general agreement that global warming is apparently primarily a result of increases in daily minimum temperatures (TMIN) rather than increases in daily maximum temperatures (TMAX). We examined long-term weather data from the SGS LTER and determined that this pattern has held true for the period 1964-1993. Evaluation of vegetation data indicates that total ANPP and density of several species, including Opuntia polyacantha, were most closely correlated with precipitation. However, ANPP of the dominant grass species, Boutella gracilis, and several other important species, was most closely and negatively, correlated with spring TMIN. Also of interest was the finding that density of plants in the " exotic forb" functional group increase significantly with an increase in TMIN. Thes results suggest that this ecosystem may be sensitive to increases in minimum temperatures similar to those that are being observed globally (Alward et al. in prep).

We constructed four automated shelters to cover plots from sundown to dawn from spring until fall (except during periods of precipitation) to reduce radiative heat losses and warm plots primarily during nighttime hours. Compared with adjacent plots without shelters, covered plots had 1.5 to 2.0 C higher temperatures from 10cm above the ground surface to at least 30 depth in the soil. The higher temperatures resulted in more rapid drying of the soil in covered plots. Among the major findings related to increased nighttime temperatures, we have observed (a) increased tillering rates and advanced phenology, but decreased inflorescence production, in B. gracilis; (b) seasonal shifts in insect herbivory damage Pascopyrum smithii, a common C3 grass, such that there was less damage early in the growing season but more damage later; and increased cladode production but decreased flower and frui production in the abundant and widespread cactus, O. polyacantha (Alward and Detling, in prep).

Animal Monitoring Programs: Since 1994, we have estimated population sizes of nocturnal small mammals, rabbits, and terrestrial carnivores on the SGS-LTER (Stapp 1997, a,b,c,d and in press). These monitoring programs continued in 1997-1998, including live-trapping studies in may and roadside counts of rabbits and canids quarterly. We continued monthly warm-season surveys of terrestrial macroarthropods. Captures of major insect tax are counted in 90 pairs of pitfall traps placed along a 1km topographic gradient in new long-term monitoring studies. In 1998, we began arthropod pitfall trapping on trapping webs established for monitoring abundance of small mammals. Pitfall traps are run for 4-5 consecutive days on thee occasions during summer months to track changes in abundance of arthropods, which are important food items for rodents in shortgrass steppe, as a possible determinant of trends in rodent numbers.

Beginning this year, we modified our roadside census route to include areas of the Pawnee national grasslands (PNG), taking advantage of the 1996 SGS-LTER site expansion. The new route includes more upland prairie habitats used by white-tailed jackrabbits (Lepus townsendii) and swift foxes (Vulpes velox), while continuing to allow us to monitor rabbit and canid populations on portions of the Central Plains Experimental Range.

Prairie Dog Studies: Prairie dogs (Cynomys ludovicianus) influence their surroundings in many ways. These herbivorous mammals change local plant community structure by cropping vegetation, and they modify the soil structure by burrowing and building mounds. These changes in vegetation and edaphic factors may influence insects such as beetles and grasshoppers that live in prairie dog towns. We have completed one year of research aimed at understanding the relationship between short-horned grasshoppers (Orthoptera: Acrididae), darkling beetles (Coleoptera: Tenebrionidae) and prairie dogs. Preliminary results indicate that there are fewer grasshoppers on prairie dog towns than on the surrounding prairie, suggesting that prairie dogs may alter prairie habitats in a way that may be unfavorable for grasshoppers. Tenebrionid beetles were sampled by pitfall traps once every 3 weeks from July through mid-September 1997. During 1998, traps will be set four times between late May and mid-September. This work is being conducted by a graduate student (Junell) under the direction of Bea Van Horne. An REU student is also working with P. Stapp to assess the influence of prairie dogs on avian abundances this summer.

We have initiated new work on the genetics of prairie dog populations (Roach, under direction of Van Horne and Antolin). Prairie dog colonies in shortgrass steppe exist in spatially isolated subpopulations that are connected by dispersal, traits typical of metapopulations. The dynamics of these subpopulations are also determined by local colony extinction, which results from plague, agricultural control efforts and recreational shooting. Difficulties in quanitifying dispersal behavior have complicated efforts to document the degree of connectedness between isolated colonies. However, patterns of genetic relatedness among populations can provide an estimate of the degree of linkage between subpopulations and e used to generate hypothese about the types of behavioural processes responsible for these patterns. In this study, we plan: 1) to establish levels of genetic heterogeneity within and among black-tailed prairie dog populations and 2) to evaluate proposed models of recolonization based on the degree of genetic differentiation among recolonized and established colonies. We sampled 13 prairie dog colonies on the SGS site from May-December 1997: 6 at Central Plains Experimental Range (CPER) and 7 at Pawnee national Grasslands (PNG). A tissue sample was collected from each individual for genetic analysis. We are currently measuring genetic variability within and among populations of prairie dogs using microsatellite loci markers. Cluster analysis of data from the microsatellite markers will reveal patters of relatedness among populations and will be used to evaluate the role of dispersal in maintaining genetic diversity of the metapopulation. Results from this study may be used to describe patterns of prairie dog metapopulation dynamics. Dtermining the effects of extinction and recolonization events on genetic diversity can provide predictions about the stability of metapopulations and provide possible guidelines for maintaining rare and endangered species in highly fragmented habitats.

Last, we are studying the above prairie dog towns to distinguish between effects associated with soil churning and movement and thos associated with consumption of vegetation, and to determine the magnitude of each effect. In doing so, we are characterizing changes in plant species composition, cover, and biomass in areas inhabited by black-tailed prairie dogs and in nearby uncolonized areas (Detling and Milchunas), and examining the effects of prairie dogs on plant nitrogen content and potentially mineralizable nitrogen in soil (Detling and Burke). A graduate student and REU are involved in the latter work on mineralizable nitrogen. We anticipate expanding the scope of this project during 1999 with the addition of at least one new graduate student working with Dr. Detling. Measurements of plant biomass, species composition and cover, and N content will continues as before to determine to what extent modifications induced by prairie dogs change over time as intensive grazing continues.

Belowground Processes

Impacts of Grazing on Soil Biota and Processes: This project involves manipulating cattle grazing in historically non-cattle grazed and cattle grazed pastures and tracking the changes in above and belowground communities. We moved fences in 1991 to expose previously ungrazed exclosures and to create new exclosures and to create new new exclosures on historically grazed pastures. Yearly sampling has shown a marked shift in the microbial community. Grazed pasture that was taken out of grazing now possesses a community structure that is similar to the ungrazed pasture, and ungrazed pasture that was allowed to be grazed is similar in structure to the historically grazed pastures. The microbial communities began to diverge from one another two years following the manipulations. The complete shift has taken six years to materialize, and was only evident after the last growing season. We are in the middle of processing this year’s samples from these sites (Moore in prep).