Future changes in precipitation patterns as the climate warms may affect biodiversity and ecosystem processes.  While climate models predict variable precipitation patterns in temperate areas of the planet, effects of long-term alteration in moisture on biota is still uncertain.  To address this, we examined the effects of precipitation augmentation or removal from an intact hardwood forest on important ground fauna as part of the Throughfall Displacement Experiment (TDE) at Oak Ridge National Laboratory.  Over a 13-year period approximately 33% of natural rainfall was either removed (dry treatment) or added (wet treatment) to plots 80 X 80 m in size.  A third plot received normal levels of rainfall and served as a control.  Pitfall traps were used to quantify beetles in the Family Carabidae and litter cores used to quantify litter mass, moisture and chemistry in 10 8 X 8 M plots per treatment multiple times within a single season.  The long term TDE data archive on soil moisture was used to relate this parameter to faunal responses.  These observations evaluated if precipitation alteration would change physical factors of the forest floor important for carabid beetles and if changes in beetle community structure would be observed between treatments.  We focused our analysis on collections made mid-summer, the time of year when beetles were most abundant and separation between wet and dry treatments most pronounced.   

Results/Conclusions

Carabids were significantly lower in both abundance and tribe-level richness in the dry plots, while evenness significantly increased with water limitation.  This result is largely explained by a shift in abundance within three tribes, Harpalini, Pterostichini and Callistini, in plots with water added.  For Harpalini, 7X more beetles were collected in wet than dry plots, supporting previous work that found this beetle group constrained by moisture deficits.  Our finding that instantaneous litter moisture did not vary between treatments suggests no clear direct effect of litter moisture on the beetle community.  Instead, beetle community responses are most likely dictated by a moisture regime reflected in the underlying mineral soils.  The best predictors for tribe abundance and richness were litter mass (significantly higher in dry plots) and soil moisture (lowest in dry plots).  These data suggest that long-term alteration of precipitation in temperate forests will affect key physical parameters important for ground dwelling beetles, leading to shifts in richness and evenness in taxa most adapted for change.