The Roles of Habitat Area, Fragmentation and Vegetation Diversity in Bird and Snake Habitat Quality in Agricultural Landscapes in Iowa, USA

Submitted by barogers on Fri, 12/09/2022 - 10:54

Publication Type:



Wildlife Ecology, Volume Doctor of Philosophy (2022)


<p>Widespread loss of native grasslands has led to major population declines of grassland wildlife worldwide, including in North America. Continent-wide, 71.2% of the historical extent of the North American tallgrass prairie has been converted to cropland, contributing to the decline of 74% of grassland bird species and the threat of extinction for 12% of Palearctic reptile species. In this dissertation my objectives were to investigate what environmental variables drive grassland nesting passerine nest success in extensively cropped landscapes, determine the best supported framework for explaining brown-headed cowbird (Molothrus ater) parasitism on grassland nesting birds, and to survey snake and lizard biodiversity in these landscapes.</p>

<p><br />
I provide an in-depth examination of the factors that drive grassland-nesting passerine nest density and survival in landscapes dominated by agricultural production. Red-winged blackbirds (Agelaius phoeniceus) and Dickcissels (Spiza americana) were the only species for which we found enough nests to allow models to converge with a useful number of predictor variables, so we also modeled grassland nesting passerines as a guild which also included vesper sparrows (Pooecetes gramineus), common yellowthroats (Geothlypis trichas), meadowlarks (Sturnella spp.), and four other species. Nest density and survival for Red-winged blackbirds, Dickcissels, and grassland nesting passerines as a guild were best predicted by both habitat amount and configuration, often interacting such that configuration effects reversed direction depending on the amount of habitat on the landscape. Vegetation diversity and density consistently positively predicted nest density and survival, demonstrating that local habitat quality had similar effects as landscape composition. Landscape woody land cover amount had a negative effect on both nest density and survival.</p>

<p><br />
I also compared three existing and two new hypotheses on how brown-headed cowbirds located nests to parasitize, including the edge effect hypothesis, the perch proximity hypothesis, the nest exposure hypothesis, the Habitat Amount Hypothesis (HAH), and the vegetation diversity hypothesis. I found that the covariates on nest parasitism occurrence that best fit the<br />
data included habitat amount and distance to crop edge. This lent support to the edge effect hypothesis but not the others.</p>

<p>Finally, I investigated how habitat amount, habitat configuration, and vegetation diversity affected terrestrial reptile occupancy, presence, and biodiversity in low habitat, highly fragmented agricultural landscapes. Snake and lizard encounter rates were very low. Although I encountered terrestrial reptiles infrequently in these fragmented agricultural landscapes, the majority of encounters were with Iowa Species of Greatest Conservation Need. The most important variables in a global model for detection-corrected occupancy were patch isolation, patch area, and patch count interacting with nearby habitat amount. I also found that snake presence was positively predicted by vegetation diversity, patch size, and grassland habitat amount and negatively predicted by edge density.</p>

<p>Through these studies I demonstrated that habitat configuration remains an important predictor of demographic parameters after accounting for habitat amount on the landscape. Additionally, I provided evidence that vegetation diversity had positive effects on presence and/or reproductive success of multiple taxa of wildlife in a landscape with a dearth of high<br />
diversity vegetation. Finally, I demonstrated that grass contour strips and terraces may be population sinks or ecological traps for grassland breeding passerines. In contrast, prairie contour strips offered similar reproductive success to larger patches of restored prairie in the landscapes studied, but larger tracts of habitat are necessary to attract areas sensititive bird species, reptiles and lizards.</p>