Biosolids application increases grasshopper abundance in the short term in a northern Canadian grassland

Abstract

Degraded grasslands are common worldwide, often due to overgrazing by livestock; such degradation often reduces plant growth and water quality, while increasing soil erosion, wildfires, and invasive species. Recent restoration efforts have used organic amendments to increase soil nutrients, improve water retention, and increase forage production. Biosolids, the stabilised and pathogen-treated remains from wastewater treatment plants, have strong impacts on soil nutrients and plant growth, but there is very little known about impacts on higher trophic levels. We worked on northern grasslands in British Columbia, Canada, to test whether biosolids applications changed grasshopper abundances, body sizes, or species richness. We used hoop transects to measure density and timed net samples to determine richness and evenness. There were significantly higher (~3.8×) grasshopper densities at sites where biosolids were applied 1–2 years before sampling compared with control sites or sites where biosolids were applied in the year of sampling. Tibia lengths of grasshoppers varied with treatment, species, and sex, but there was no clear signature of biosolids leading to bigger body sizes. There were no significant differences in species richness or equitability in relation to the year of the biosolids application. Collectively, our results show that biosolids have large impacts on grasshopper densities, but no clear impact on community structure or body size. Because grasshoppers can be dominant insect herbivores and are critical prey for many birds and mammals, our results suggest biosolids could be an important tool in the context of site restoration or efforts to improve populations of insectivorous vertebrates.

Niche partitioning of avian predators in northern grasslands amended by biosolids

“…During 2014-2016, biosolids were spread by pull-spreaders on 11.1 km 2 (24.6%) of the study area, at a rate of 57,000 kg/ha (unpublished data, SYLVIS Environmental Ltd.). We documented ~3.8 fold increases in grasshopper densities 1-2 years postapplication of biosolids compared with untreated sites (Gaudreault et al, 2019). There was a distinct and large peak in vole numbers in 2017, detected via incidental observations of voles, runways, and grass clippings during our fieldwork every summer from 2015 to 2020.…”

“…This reliance on Orthoptera echoes work on American Crows further north in BC, at the Prince George airport; crows there strongly preferred recently mown areas, where grasshoppers were more visible than in longer grass (Kennedy & Otter, 2015). We also observed this selection for habitat over prey abundance, because grasshopper densities were higher on areas with biosolids than areas without (Gaudreault et al, 2019), but crows preferred to forage on pastures without biosolids. This result likely indicates a preference for the sparser grass growth itself.…”

“…Orthoptera were more abundant on sites with biosolids applications (Gaudreault et al, 2019). Abbreviations: AFO, absolute frequency of occurrence (% of pellets containing each prey type); RFO, relative frequency of occurrence (% of a given prey type out of all prey recorded).…”

“…We counted birds by the number of beaks in a pellet; if no beaks were present, we assumed feathers indicated one bird was consumed. Grasshoppers were identified from mandibles (Buers et al, 2019); samples of each species of grasshopper present on the ranch were available as a reference collection (Gaudreault et al, 2019). Grasshoppers were counted by the number of mandibles present, with two mandibles for the same species representing one grasshopper eaten; 6 of the 243 pellets contained grasshopper legs but not mandibles, and we were unable to count individuals or resolve species for these pellets.…”

“…Further, pastures amended with biosolids supported more grasshoppers (Gaudreault et al, 2019) and songbirds (unpublished data).…”