Dissertation
Urban intensity impacts on small mammal distributions, population ecology, and community interactions
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2024
DOI: 10.25820/etd.007340
Abstract
Over half of the world’s human population is urban and this proportion is only expected to increase, simultaneously increasing the extent and ecological impact of urban areas. Native species that inhabit urban or peri-urban areas are directly impacted by processes such as habitat fragmentation, non-native species introductions, and human activity, which affect important wildlife community interactions such as competition and trophic interactions, yet we still do not fully understand these changes or the mechanisms that drive them. A critical knowledge gap exists in our understanding of how species’ responses to urban environments impact their interactions with other species, and how these changes in interactions may affect urban ecosystem processes such as predation and competitive exclusion. Additionally, despite being important prey items for larger vertebrates and seed predators themselves, an additional critical knowledge gap remains regarding our understanding of the effects of urbanization on mammalian prey species, which in turn limits our understanding of the processes that structure urban mammal communities. These gaps prevent us from understanding a critical determinant of urban species occurrence (i.e., the extent to which interspecific interactions determine occurrence) and prevent us from managing cities to sustain populations of mammalian prey, which themselves sustain charismatic fauna and are critical to ecosystem functioning.
My dissertation research consists of three studies that help to fill these gaps by analyzing the distribution of key mammalian prey species, namely small rodents, shrews, and tree squirrels, in urban environments. Study one assessed how the potential for competition among tree squirrel species affected their distribution in nine cities across the US. Utilizing a multi-city network of wildlife cameras, I assessed relationships between local land cover attributes (e.g., impervious cover, vegetation) and squirrel occurrence and co-occurrence using a multi-species occupancy model. Overall, I found that species occurrence and co-occurrence was highly variable among, but less variable within, cities. Tree squirrels appear to partition urban habitat in similar ways to the habitat partitioning seen in non-urban systems, indicating species interactions still play important roles in structuring urban wildlife communities, and indicating that managing urban habitat to provide variation in urban tree canopy cover promotes occupancy by a diverse tree squirrel community.
In study two, I analyzed the effects of urbanization on native small mammal communities and species-specific responses to urbanization in the Iowa City, Iowa metropolitan area. To do so, I live-trapped small mammals along the urbanization gradient and modeled population persistence and recruitment and their relationships with local land cover using a hierarchical Bayesian community abundance model. Overall, I found species richness increased as proportional human-modified land cover (i.e., impervious surfaces and turfgrass) increased. This trend was not simply a gain of species, but rather, a change from low-diversity, generalist communities to species-rich, prairie-specialist communities as forest habitat was replaced by tall herbaceous vegetation communities along riparian corridors and in bioswales near housing developments. Thus, my research shows that community-level responses to urbanization do not always capture important species-specific responses, and that urban landscapes that are managed for prairie-like vegetation appear to attract and retain prairie-specialist communities.
Finally, study three investigated the relative influence of habitat characteristics and predation on the abundance of deer mice (Peromyscus spp.) in the Iowa City metropolitan area. I used a model that integrated wildlife camera detections of common mouse predators with mouse abundance data collected using live-trapping to identify factors that control mouse population dynamics. Deer mice were significantly less likely to persist in urban habitats where domestic cats (Felis catus) are also present. However, the strength of this relationship decreased as natural vegetation cover increased; deer mice had similar persistence rates at sites with and without cats when natural vegetation cover was at its greatest. Red foxes (Vulpes vulpes) did not appear to strongly affect deer mouse populations, potentially indicating a loss of an important predator-prey interaction in urban environments. Urban deer mouse populations appear to occur in “sink” patches that rely on immigration from the surrounding environment to avoid extinction. The continued presence of these species in urban ecosystems may be tenuous unless urban landscapes provide adequate connectivity to ensure dispersal success. Additionally, managing urban habitats to provide vegetation heterogeneity (e.g., trees, shrubs, tall vegetation) may provide important refuges for urban prey to escape predation by domestic cats.
This research not only deepens our understanding of small mammal ecology in urban ecosystems, but also identifies important effects of changes in species distributions on species co-occurrence and potential interactions (e.g., competition, predator-prey interactions) and adds to our knowledge of urban community ecology, broadening understanding of understudied taxa (i.e., small mammals) that play key roles in terrestrial trophic webs. Understanding the urban biotic community, how and why it varies across urban landscapes, and how urban habitat composition and structure can change the potential for interspecific interactions, can help us design and manage urban environments to maintain biodiversity and ecological functioning, and is becoming increasingly critical to achieving both conservation and human well-being objectives.
Details
- Title: Subtitle
- Urban intensity impacts on small mammal distributions, population ecology, and community interactions
- Creators
- Rachel N. Larson
- Contributors
- Heather Sander (Advisor)Matthew Dannenberg (Committee Member)Marc Linderman (Committee Member)Andrew Forbes (Committee Member)Mason Fidino (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Geography
- Date degree season
- Spring 2024
- DOI
- 10.25820/etd.007340
- Publisher
- University of Iowa
- Number of pages
- xiv, 126 pages
- Copyright
- Copyright 2024 Rachel N. Larson
- Language
- English
- Date submitted
- 04/16/2024
- Description illustrations
- illustrations (some color), color maps
- Description bibliographic
- Includes bibliographical references (page 94-104).
- Public Abstract (ETD)
For the first time in history, more people live in cities than in the surrounding rural landscape. Growing cities often come with negative consequences for local plants and animals, however, many also survive and even thrive within cities. Designing sustainable cities, that support both people and nature, requires understanding of how and why species use urban landscapes. My research seeks to improve our understanding of why wildlife species occur where they do within a city, how these patterns differ among cities, and how city living changes how wildlife species interact with each other. I used a combined dataset consisting of images from motion-triggered cameras and live-trapped animals to investigate the roles of potential competition and predation risk on populations of urban prey. My research shows there are clear winners and losers in the struggle for limited habitat within cities. Many habitats within cites can only support one species of squirrel or one kind of small mammal community. Even adaptable species such as deer mice cannot escape negative consequences of urban living such as increased predation risk. However, my research also shows that providing habitats with different types of vegetation (e.g., parks with trees, shrubs, and wildflowers) can protect prey species and provide habitat diversity to support diverse wildlife communities. By better understanding the behaviors of our wildlife neighbors, cities can be designed more sustainability, to support the diversity of life that calls cities home and to provide city-dwelling people with opportunities to experience and enjoy urban nature and its benefits.
- Academic Unit
- Geographical and Sustainability Sciences
- Record Identifier
- 9984647257002771
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