Impacts Of Anthropogenic Disturbances And Drought On Breeding Bird Abundance And Diversity In The Rolling Plains Ecoregion Of Texas
Hubbard, Natalie Ellen
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Bird species, especially grassland species have undergone widespread decline in arid grasslands due to anthropogenic disturbance such as different land use including cattle grazing and agriculture and addition of physical structures such as buildings, roads and fences which can negatively impact both animals and native vegetation. The objective of this research was to determine the impacts of anthropogenic disturbance (i.e., presence of oil pump jacks and cattle grazing) and drought on breeding bird community dynamics in the Rolling Plains Ecoregion of Texas. The study involved four sites encompassing different land uses including a natural-non-cattle grazed, cattle-grazed, oil field pump jacks and mixed oil field pump jacks/cattle-grazing sites. Bird abundance, species richness and diversity (H') were compared across these sites from 2008-2012 excluding the drought year of 2011. Bird counts were undertaken on 10 sub-sites within each of the sites during the bird breeding season and the data used to determine sub-site species richness and diversity. Percent vegetation cover (i.e. woody shrubs or woody shrubs plus grasses) was determined for each sub-site by analysis of Google Earth© images available for the pre-drought year of 2008 and the post-drought year of 2012. Examination of the effects of climate involved analysis of mean June daily high temperature and January-June total precipitation data for the city of Post, Texas, relative to that of North American Breeding Bird Survey (BBS) data for the City of Post, TX, Route from 1969 to 2011 in the vicinity of the four study sites. Data on sub-site bird abundance, species richness and species diversity index (H') were used to make three response variables (i.e., response 1: data for all years and sites combined, response 2: pre-drought mean data across all sites for 2008, 2009, and 2010 minus that for 2012, and response 3: data for all sites in 2009 minus that for all sites in 2012). These three responses were analyzed separately for bird abundance, species richness and species diversity using a Multiple Factor ANCOVA with repeated measures for the fixed effects of oil pump jacks and cattle grazing on sub-sites with percent sub-site woody vegetation cover as a covariate. Sub-site differences were further examined for the effects drought, cattle grazing and presence of oil pump jacks using a multiple factor ANOVA with sites and year (2009 versus 2012) as factors. Least Squares Linear Regression Analyses were utilized to determine possible relationships with percent cover (woody shrubs and grasses) and the dependent variables of mean sub-site bird abundance, species richness and species diversity index (H') using combined pre- and post-drought data 2009 and 2012. Climate effects were examined using Least Squares Linear Regression Analyses using Post, TX, BBS route data for bird abundance, species richness and species diversity index (H') versus June average high daily temperatures and January-June total precipitation values as independent variables. Regression analyses were also conducted on these data using the prior year temperature and precipitation to examine the latent effects of drought on the avian community. Although post-drought vegetation cover was reduced by 20% across all sub-sites in 2012 after the 2011 drought, it did not impact (p >0.05) 2012 bird abundance, species richness, and species diversity values across the study sites. Nor was BBS data for these three variables significantly correlated (p >0.05) with either mean June daily high temperature or January-June total precipitation. The only significant correlation (p <0.05) recorded for BBS data was that of species richness versus January-June precipitation in the prior year. My results also did not reveal a significant impact (p >0.05) of cattle grazing relative to the natural grassland site, but presence of oil pumps jacks, significantly (p <0.05) increased bird abundance and, to a lesser extent, species richness relative to the cattle-grazed and natural grassland sites. The results, in general, appeared to agree with those of previous studies examining similar impacts on grassland avian communities in which moderate disturbance associated with the presence of oil pump jacks increased bird abundance but did not greatly impact species richness or diversity. The lack of impact of drought on grassland bird abundance, species richness and diversity revealed in this study is similar to that reported in other studies. However, negative drought impacts have been recorded for some bird communities especially when examined over a broad geographical scale. Overall, my results indicated that the Rolling Plains Ecoregion grassland bird communities studied appeared relatively resistant to the impacts of both anthropomorphic disturbance and drought.