Friday, August 8, 2008

PS 85-144: Effects of large-mammal grazing and arthropod abundance on ecosystem processes

Karen A. Haubensak1, Randall D. Jackson2, Claudio Gratton2, Jack Donaldson2, and Susan Crane Lubetkin3. (1) Indiana University, (2) University of Wisconsin-Madison, (3) University of Washington


Large-mammal grazers can have significant impacts on ecosystem processes like decomposition and NPP; above- and belowground arthropods can influence plant species composition and productivity and thus influence ecosystem processes as well.  Interactions between large mammal grazing effects and arthropods, and the effects of those interactions on ecosystem processes, are less well understood.  We conducted two separate studies to assess the relative importance of grazing, plant species composition and arthropod abundance on litter decomposition in pasture systems of the upper midwest.  In the first study, we collected large intact pieces of turf from grazed farmlands which had been continuously grazed, intensively/rotationally grazed, with an ungrazed control; we then measured arthropod and ecosystem processes in them for over one year in greenhouse mesocosms.  We hypothesized that potential effects of predators and detritivores on litter decomposition would be mediated by grazing history, whereby their impacts would be more detectable (greater) in continuously grazed areas with lower productivity and dominance by species that produce more recalcitrant litter.  We used a separate field-based study to assess the structure and function of arthropod food webs in grazed and ungrazed plots by labeling litter material with 15N, and tracking its signal through all arthropod guilds over 10 months.  

We found that while decomposition of a common substrate did not differ across grazing treatments, decomposition of native litter material did.  This suggests that the soil microbial community may not differ among grazing treatments but that plant species composition differences caused by grazing may contribute to variation in litter decomposition rates.  On the other hand, arthropod abundances did not vary significantly by grazing treatment in the mesocosm or field study, leading us to speculate that any effect of arthropods on decomposition is likely subsumed by the primary importance of litter quality or quantity in these systems.