Catherine E. de Rivera

Date of Award


Document Type


Degree Name

Doctor of Philosophy (Ph.D.) in Environmental Sciences and Resources


Environmental Sciences and Resources

Physical Description

1 online resource (xv, 187 pages)




Wildlife need connected habitats to move across the landscape to meet foraging needs, reproduce and establish new territories. Increasingly, habitat areas are lost due to conversion to alternative uses such as agriculture or urban development and being carved into pieces by roads and other transportation infrastructure. Roads are considered a major contributor to habitat fragmentation particularly as they are long, linear features prevalent throughout the landscape. The potential barriers species encounter and interact with on the landscape, such as roads or fences, could be permeable to some species and a near complete barrier to others. This creates a challenge when conservation professionals work on methods to plan for enhancing and preserving connectivity across the landscape.

While roads can present weak to complete barriers to wildlife, depending on the animal and traffic volume, mitigations such as under-crossings and green bridges on highways at least partially increase the permeability of the landscape to some of these species. The few studies evaluating the effectiveness of these structures for at least three years typically focused on a single species. Here, we monitored the crossing structure under Boeckman Road, in Wilsonville Oregon, for wildlife activity across summer seasons for ten years, since construction of the road and subsequent opening to traffic. This long-term multi-species dataset, which includes monitoring when the road was closed to traffic has provided a unique opportunity. Wildlife activity was collected using sand track pads monitored during summer seasons from 2009 to 2018. Wildlife activity showed a significant community level response from year to year and species-specific responses to year, vegetation change, disturbance, detection area, and previous experimental additions of artificial light. Black-tailed deer showed a significant negative association with disturbance, i.e. the presence of traffic and construction activity. Average annual detections of coyote, bullfrog, cottontail rabbit, and eastern gray squirrel demonstrate a dramatic but not significant response to the road closure period. In addition, it appears that the transition between species with preferences for lower canopy cover, and those preferring greater canopy cover, co-occurs with the road closure period, particularly in 2013. This is also the year that invasive plant management activities (mowing and spraying) stopped. Long-term studies such as this one can help researchers and managers design monitoring programs to best account for variable responses over time by documenting changes in use and working to identify covariates and interactive effects that may be driving those changes. Managers working on projects where vegetation disturbance or restoration is being conducted next to crossing structures may decide to delay monitoring until vegetation communities and/or habituation responses have had time to stabilize, avoiding erroneous conclusions about structure use.

Roads create barriers to animal movement through collisions and habitat fragmentation. Investigators have attempted to use traffic volume, the number of vehicles passing a point on a road segment, to predict effects to wildlife populations approximately linearly and along taxonomic lines; however, taxonomic groupings cannot provide sound predictions because closely related species often respond differently. We assess the role of wildlife behavioral responses to traffic volume as a tool to predict barrier effects from vehicle-caused mortality and avoidance, to provide an early warning system that recognizes traffic volume as a trigger for mitigation, and to better interpret roadkill data. We propose four categories of behavioral response based on the perceived danger to traffic: Nonresponders, Pausers, Speeders, and Avoiders. Nonresponders attempt to cross highways regardless of traffic volume. Pausers stop in the face of danger so have a low probability of successful crossing when traffic volume increases. Hence, highway barrier effects are primarily due to mortality for Nonresponders and Pausers at high traffic volumes. Speeders run away from danger but are unable to do so successfully as traffic volume increases. At moderate to high volume, Speeders are repelled by traffic danger. Avoiders face lower mortality than other categories because they begin to avoid traffic at relatively low traffic volumes. Hence, avoidance causes barrier effects more than mortality for Speeders and Avoiders even at relatively moderate traffic volumes. By considering a species' risk-avoidance response to traffic, managers can make more appropriate and timely decisions to mitigate effects before populations decline or become locally extinct.

Barriers to animal movement can isolate populations, impacting their genetic diversity, susceptibility to disease, and access to resources. Barriers to movement may be caused by artificial light, but few studies have experimentally investigated the effects of artificial light on movement for a suite of terrestrial vertebrates. Therefore, we studied the effect of ecological light pollution on animal usage of a bridge under-road passage structure. On a weekly basis, sections of the structure were subjected to different light treatments including no light added, followed by a Reference period when lights were off in all the structure sections. Sand track data revealed use by 23 mammals, birds, reptiles and amphibians, nine of which had N30 tracks for species-level analysis. Columbia black-tailed deer (Odocoileus hemionus columbianus) traversed under unlit bridge sections much less when neighboring sections were lit compared to when none were, suggesting avoidance due to any nearby presence of artificial light. Similarly, deer mouse (Peromyscus maniculatus) and opossum (Didelphis virginiana) track paths were less frequent in the lit sections than the ambient. Crossing was correlated with temporal or spatial factors but not light for three of the other species. These findings suggest that artificial light may be reducing habitat connectivity for some species though not providing a strong barrier for others. Such information is needed to inform mitigation of habitat fragmentation in the face of expanding urbanization.

The effectiveness of a landscape for habitat connectivity is relative to individual species, particularly those of differing taxa and mobility types. Given limited resources, conservation planning efforts are restricted to a subset of representatives, or surrogate species. The goal of a surrogate species approach is to use a few species to best represent the needs of the larger community. It has been common practice in connectivity planning and mapping to utilize habitat generalists or umbrella species as surrogates. Recent research suggests that these species typically function as poor representatives and may not encompass the connectivity needs of a diversity of species. Rather, surrogate species selected based on diverse habitat needs are thought to better represent actual connectivity needs on the landscape for an array of species. We propose a process to guide the selection of species for connectivity planning and assessment purposes and provide an example application of the process. Steps include: 1) clarification and articulation of project goals; 2) acquisition of data to prepare for analysis; 3) hierarchical cluster analysis to group species based on habitat associations; 4) refinement of species clusters; 5) feedback from species experts and/or literature to support selections; and 6) final species selections. Several of these steps permit stakeholder participation and local expert input into the selection process, enhancing engagement in and broadening awareness of the project. This process, which uses project goals to select surrogate species in an inclusive and tractable way, can be applied to connectivity mapping across a range of geographic scales.

Through the work conducted herein we provide contributions to the understanding of how elements of the built environment impact wildlife communities ability to move across the landscape. Additionally, we provide new tools to support resource managers in barrier mitigation and connectivity planning. Habitat fragmentation effects are a complex set of issues that require resources and collaboration to reach meaningful solutions. The work presented here can also support decision-making, communication, and collaborative efforts that will ultimately result in on-the-ground impacts to reduce fragmentation effects and mitigate existing barriers effectively to promote the long-term viability of wildlife and the systems they depend on.

Persistent Identifier

Available for download on Thursday, March 19, 2020