Measuring human disturbance using terrestrial invertebrates in the shrub–steppe of eastern Washington (USA)

Abstract

Abstract Hanford Nuclear Reservation in eastern Washington is an ideal place to study biological responses to diverse human activities because minimally disturbed areas of native shrub–steppe exist in close proximity to areas that have been substantially altered. This range of conditions provides an opportunity to test which attributes of terrestrial invertebrate assemblages change systematically along a gradient of human influence and to select from these a set of biological metrics that can be used for site assessment. We sampled invertebrates at 25 sites at or near the Hanford Reservation: some sites showed past or present influence from agriculture, waste disposal, urbanization, or construction; others had a history of minimal human disturbance. We evaluated 57 attributes of terrestrial invertebrate assemblages to determine if they varied systematically with the intensity of human disturbance. Attributes were measured as taxa richness or percentage relative abundance of key taxonomic, trophic, or ecological groups. Ten attributes were consistently associated with disturbance in 3 years of independent sampling, and another 12 attributes were significant in 2 out of 3 years. Those selected as metrics were total number of invertebrate families (1) number of Diptera families (2) taxa richness of Acarina (3) predators (4) detritivores (5) ground-dwellers (6) and percentage relative abundance of detritivores (7); all declined with increasing disturbance. The percentage relative abundance of Collembola (8) and taxa richness (9) and relative abundance (10) of polyphagous Carabidae increased with disturbance. Two metrics (7 and 10) were excluded from further consideration because they were redundant with other metrics (6 and 9). We transformed the remaining eight metrics to a standard scale and added them to yield a multimetric index—a single value summarizing the biological condition of each site. Undisturbed sites had the highest index values; sites with physical disturbance related to construction or waste disposal ranked next; and agricultural sites had the lowest values, indicating the most severe changes in their resident biota. Index values were lower for more frequently disturbed sites, although the time since last disturbance produced no differences in index values. Repeat sampling at five sites indicated that index values varied little across years and that year-to-year differences were smaller at undisturbed than disturbed sites. This first effort to develop a terrestrial monitoring approach modeled after the aquatic index of biological integrity (IBI) shows promise, but validation of the metrics at other places will be needed before a terrestrial index of biological integrity (T-IBI) can reliably guide management, restoration, or policy decisions.