Effects of grass seed agriculture on aquatic invertebrate communities in seasonal wetlands of the southern Willamette Valley, Oregon

Graduation date: 2012An estimated 99% of native wet prairie has been lost in the Willamette Valley, Oregon, but a large number of seasonal wetlands remain on private lands dedicated to grass seed production within the historical wet prairie landscape. Because agriculture is known to alter physical and chemical wetland conditions, I investigated if aquatic invertebrate communities differed among annual grass seed farmed wetlands, perennial grass seed farmed wetlands, and native wet prairie. I sampled 30 southern Willamette Valley wetlands (n = 10 in each land use category) in spring of 2009 and resampled 28 in spring of 2010 that are hydrogeomorphically classified as Flats. Invertebrate community composition in native wet prairie differed from farmed wetlands (A-stats = 0.37, P < 0.001). Specifically, taxa richness (26.2 ± 1.2 in 2009 and 27.4 ± 1.0 in 2010) and diversity (3.0 ± 0.1 in 2009 and 3.1 ± 0.1 in 2010) were higher. From Indicator Species Analysis, taxa representative of wet prairie included a flightless beetle (Apteraliplus parvulus), a cladoceran (Dumontia oregonensis), an isopod (Caecidotea sp.), Calanoida copepods, and Lymnaeidae snails. Invertebrate density (individuals/m²) did not differ among land-use groups, but biomass (mg/m²) in perennial grass wetlands was greater than annual grass wetlands both years. Biomass in perennial grass farmed wetlands was also higher than native wet prairie in 2009 and similar in 2010. Wetland invertebrate composition was strongly associated with reduced vegetation cover and increases in water turbidity and conductivity with farmed sites having lower cover and higher turbidity and conductivity. These findings suggest grass seed farming practices such as soil tillage and field leveling have affects on invertebrate communities by lowering diversity and richness and altering community composition. However, farmed seasonal wetlands still provide habitat for aquatic invertebrates, contribute to the region's biodiversity, and are substantial food resources for wildlife. This grass seed agricultural landscape is an important seasonal wetland habitat and needs to be incorporated into regional conservation plans