Graduate Thesis Or Dissertation
 

Effects of plant community characteristics on insect abundance : implications for sage-grouse brood-rearing habitat

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8623j288m

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  • A causative factor in declining greater sage-grouse (Centrocercus urophasianus) populations is reduced annual recruitment due to poor habitat quality. Sage-grouse population decline is concurrent with a decline in the extent and quality of the sagebrush (Artemisia sp.) biome. However, current research has shown a positive relationship between sage-grouse brood and chick survival and the abundance of Lepidoptera larva (caterpillars of moths and butterflies). This two-year (2007-2008) study focused on linking the abundance of litter and ground dwelling insects with plant community characteristics in sagebrush steppe ecosystems, in anticipation of improving sage-grouse brood-rearing habitat management. Focus was placed on insects that have been found in the diet of sage-grouse chicks and included ants, grasshoppers, darkling beetles, and scarab beetles, with a special emphasis on caterpillars. Four sites were chosen in central Oregon for this research. Two sites were dominated by A. tridentata ssp. vaseyana (mountain big sagebrush) and were managed under two different seasons of cattle grazing, spring and winter. The remaining two sites were split between a Chrysothamnus viscidiflorus (yellow rabbitbrush) dominated upland and an Ericameria nauseosa (rubber rabbitbrush) dominated meadow. Line-point intercept, plant height, and basal gap intercept were employed to measure plant community structure and composition. Insect abundance was measured two ways: 1) pitfall traps for ground crawling insects, and 2) black light traps to capture adult moths. Identification of Lepidoptera species by caterpillars is difficult; therefore, documenting Lepidoptera species within an area by conducting an inventory of the adults was necessary. Results show the meadow site had more rabbitbrush, shrub, and vegetative cover, as well as taller shrubs and smaller basal gaps than the upland rabbitbrush site. Although the impact of grazing management was not quantified in this study, spring grazed sites exhibited more sagebrush and shrub cover, taller grasses and shrubs, and larger basal gaps when compared to the winter grazed sites. Within the A. tridentata ssp. vaseyana sites, forb cover, total vegetative cover, grass heights, and species richness were different between years. The meadow site provided the highest abundance of caterpillars compared to all other sites. Both rabbitbrush sites provided more caterpillars throughout May and June than the A. tridentata ssp. vaseyana communities, corresponding to early sage-grouse brood-rearing. The winter grazed A. tridentata ssp. vaseyana site and the upland rabbitbrush site exhibited the highest abundance of grasshoppers. Correlations of vegetation characteristics with insect abundance highlighted several relationships: 1) caterpillars were negatively associated with percent basal gap, mean basal gap size, and sagebrush cover, 2) caterpillars were positively associated with perennial grass cover, rabbitbrush cover, shrub height, and total vegetative cover, and 3) darkling beetles were positively associated with annual forb and annual grass cover. Moth abundance and species richness were highest during July, August, and September, with relatively few moths being caught in May or June. Overall, 222 moth species were present at the study location. Rabbitbrush and sagebrush sites had 145 species in common, with the rabbitbrush sites having 194 species overall and A. tridentata ssp. vaseyana sites having 173 species overall. Moth abundance was negatively correlated with perennial grass cover, basal cover, rabbitbrush cover, shrub and grass height, and total vegetative cover. Additionally, moth abundance was positively correlated with basal gap percent and size, as well as sagebrush cover. In conclusion, the upland rabbitbrush site exhibited the highest abundance of moths, whereas the meadow site presented the most diverse and unique number of moth species. The results of this study suggest rabbitbrush communities may be an important and intricate component within the sagebrush-steppe landscape, contributing to the quality of sage-grouse brood-rearing habitat.
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