Degree Name

Master of Science (MS)

Semester of Degree Completion

2000

Thesis Director

Eric K. Bollinger

Abstract

Leafy spurge (Euphorbia esula) is an exotic invasive weed in the northern Great Plains. We examined the effects of leafy spurge infestation on densities and nest success of breeding birds in grasslands on the Sheyenne National Grassland (SNG), ND. We categorized spurge-infested grasslands into three levels of infestation, based on the area covered by spurge patches: (a) low (0-20%), (b) medium (20-60%) and, (c) high (> 60%). We surveyed 60 100-m radius circular plots (20 in each category), and searched for nests in three 16-ha plots (one in each category). There were no statistically significant differences in mean species richness or mean species diversity among the three types of survey points. Of the eight most abundant grassland birds, only Upland Sandpiper (Bartramia longicauda) densities were significantly different among spurge categories, with highest mean density (13.5 ± 4.1 birds/100 ha) occurring on medium-spurge points. However, none of these species occurred in highest densities on high-spurge points. Le Conte's Sparrows (Ammodramus leconteii) and Savannah Sparrows (Passerculus sandwichensis) were significantly negatively correlated with spurge infestation (-0.23 and -0.24, respectively). Spurge infestation was not correlated with grazing intensity (number of stems/m2: rs = -0.01; % cover: rs= -0.03). Le Conte's Sparrows were negatively correlated (-0.34) with grazing intensity; whereas Savannah Sparrows were positively correlated with this factor (0.28). The low-spurge plot contained the most nests (n = 24), but nests on the high-spurge plot (n = 11) experienced the highest nest success (0.745, χ2 = 13.2, df = 2, P < 0.01). There were no significant differences between successful and unsuccessful nests or between nests and nearby paired sites with respect to number of spurge stems/m2 or percent cover of spurge. However there were significant differences for other measured vegetational features. Thus, based on these data, most birds appeared to show little response to leafy spurge per se. Birds may choose microhabitats based more on characteristics of vegetation structure (e.g. ground cover, vegetation height, vertical density, litter depth) than on particular plant species. Circumstantial evidence suggests that spurge may even provide benefits for certain species through foraging opportunities and nest protection. Assuming spurge can alter vegetation structure to the detriment of grassland birds, infestation may not be high enough over much of the SNG to show a strong negative effect on bird community parameters. Other factors may obscure relationships between bird densities and spurge infestation including strong avian preferences for other vegetation characteristics, cattle grazing intensity, and habitat productivity.

We surveyed the breeding bird communities of the five major habitat types (grassland, sedge meadow, wetland, savanna, and woodland) of the SNG. Grasslands contained the greatest number of total species (47), but species richness/point and species diversity/point were not significantly different among habitat types (F = 1.33, P = 0.29; F = 0.65, P 0.63, respectively). The most abundant species on grassland survey points was the Western Meadowlark (47.5 birds/100 ha). The Red-winged Blackbird was the most abundant species on sedge meadow and wetland survey points (88.5 and 382.2 birds/100 ha, respectively). Ground foragers were the most abundant guild on savanna and woodland points (236.6 and 229.3 birds/100 ha, respectively). The complex interspersion among habitat patches, combined with the relatively broad range of habitat preferences and flexibility displayed by many bird species probably lead to the observed patterns of species overlap among communities.

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