Mary O'Connor


Relevant Thesis-Based Degree Programs

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Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Nutrient effects on metabolic rate in a changing world : a multiscale approach (2023)

As climate change warms the planet, ecosystems require increasing amounts of resources to meet increasing metabolic demand. Understanding whether resource supply will meet this increasing demand is central to predicting and mitigating the ecological effects of warming. Photosynthesis drives the global biological carbon pump and is, as such, a major focus of climate modelling research. In my dissertation, I completed three novel research projects that explore how nutrient supply affects photosynthetic rates in a wide range of ecological systems. First, I tested the hypothesis that there may be broad and predictable effects of warming on resource supply by focusing on a major global source of bioavailable nitrogen: nitrogen fixation. I used theory and data synthesis to understand how rates of nitrogen fixation change with warming across diverse ecological systems. I found that, on average, nitrogen fixation is more temperature-sensitive than photosynthesis, but temperature sensitivity varies across systems. Next, I conducted a lab experiment to test whether this broad-scale pattern could be observed in a simple, two-species phytoplankton system. I also tested the hypothesis that, because of its high temperature-sensitivity, nitrogen fixation can buffer photosynthesis against nitrogen limitation with warming by providing sufficient nitrogen to meet photosynthetic demand. I found that nitrogen fixation did support photosynthesis and growth in nitrogen-limited conditions, but this effect was weak and photosynthetic productivity remained nitrogen limited in the presence of nitrogen fixation. Finally, I conducted a field experiment to understand the role of nutrient limitation in the recovery of eelgrass ecosystems in Eeyou Istchee, Quebec, in order to inform decision-making for local communities. I found that light availability, rather than nutrient availability, is likely limiting eelgrass production in this system. Altogether, I learned when and how nutrient supply influenced photosynthetic productivity in a few specific systems and generated new predictions for how nitrogen supply via nitrogen fixation may influence photosynthesis with warming.

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Scaling from species' traits to the maintenance of biodiversity in a changing world (2023)

Patterns of biodiversity and the processes generating them vary over scales of time, space, and ecological organization. This scale dependency is one of the foremost obstacles to understanding how biodiversity responds to global change. The problem of scale has inspired and shaped many theoretical frameworks in ecology, but despite many attempts to unify ecological understanding across scales, this goal remains elusive. It is becoming increasingly clear that a more unified view of ecology will require us to draw from multiple ecological theories. Attributes of organisms that determine their fitness, known as “species’ traits”, are a common feature of many scaling theories and may help to unify our understanding of ecology across scales. In this thesis, I drew from The Metabolic Theory of Ecology, Modern Coexistence Theory, and Metacommunity Theory to understand how processes that determine patterns of abundance and diversity relate to measurable species’ traits. I combined theory with experiments in which I manipulated temperature, a key factor in climate change, and distance between habitat patches, a key element of land use change and fragmentation. I conducted laboratory experiments in a pond zooplankton system to test how species’ traits determining thermal performance relate to how populations and communities respond to temperature. Contrary to theoretical predictions, I showed that temperature sensitivities increase with scale from individual-level metabolism to resource consumption to population growth. My results also reveal that an essential element that determines how strongly species compete, stabilizing niche differences, change with temperature altering competitive outcomes, and the maintenance of biodiversity. This is the first test of the temperature dependence of niche differences. Lastly, I conducted field experiments in an eelgrass community to test whether the movement of invertebrates at the regional scale affected their diversity at the local scale. I showed that invertebrate species richness decreases with increasing eelgrass patch isolation, but this was not explained by species’ traits. My thesis has revealed scale dependence in key processes that maintain biodiversity. These results can help us better predict how biodiversity will change under climate change and habitat loss.

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Effects of human activities, environment, and time on eelgrass-associated epifaunal communities (2022)

Eelgrass (Zostera marina) is a coastal marine angiosperm found in the Northern Hemisphere and forms expansive meadows in sheltered estuaries and bays, providing habitat for a high diversity of organisms, including algae, invertebrates, microbes, fish, birds, and mammals. It also provides invaluable ecosystem services for humans, including coastal storm protection, carbon sequestration, and nutrient cycling. However, eelgrass meadows are threatened worldwide by numerous anthropogenic activities, including shoreline modification, overfishing, land-based nutrient loading, and climate change. In British Columbia, Canada, eelgrass ecosystems are vulnerable to human activities such as habitat destruction, fishing, and pollution, and warming waters, which can cumulatively lead to negative impacts on eelgrass and associated fauna. To understand how environmental variation and BC-specific human activities affect eelgrass ecosystems temporally and spatially, I used a three-year, monthly observational time series of two trophic groups, statistical modeling, and an experiment to study how eelgrass communities and species interactions vary at the micro and macro scales. I show that eelgrass and epiphytic algae biomass change seasonally, and the timing of their peak biomasses varies interannually and is driven by nutrient availability and mesograzer abundances. I also show that human activities have a negative effect on eelgrass biomass, epiphytic algae, and epifaunal invertebrate abundances and species richness. Lastly, I show the first experimental evidence that eelgrass leaves host a core microbiota, eelgrass leaf-associated microbial communities are primarily driven by their surrounding environment, yet there may be host influence because microbial communities are resistant to environmental change within short periods of time. This research provides new insight to how environmental variability and human activities shape eelgrass-associated epifauna communities. This information can be used to predict how eelgrass ecosystems may change over time, which can help inform management decisions for restoration and conservation practices. Implementing more regulations for coastal anthropogenic activities may help ensure that eelgrass, and the organisms and humans that depend on it, will be sustained well into the future.

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Spatial dimensions and dynamics of bushmeat hunting: predicting defaunation and managing for sustainability (2022)

Humanity continues to depend on wild meat for subsistence and livelihoods, particularly in the Global South. Here, harvest sustainability is often undermined by ineffective wildlife management, exacerbated by limited data and research capacity. A key challenge to decision makers, then, is how to manage harvest systems in the face of data scarcity. The goal of this dissertation is to provide data-limited decision support tools for wild meat management, with particular focus on (1) predicting effort distribution across large scales, and (2) assessing spatial management strategies’ ability to robustly meet conservation and harvest goals. In my first chapter, I developed a novel hunter movement model based on landscape resistance and circuit theoretic algorithms. These algorithms build on commonly used accessibility metrics by incorporating diffusive, multi-path movement from settlements into surrounding landscapes. To facilitate broad application, I built the landscape-scale map from freely available geospatial datasets and open source software. Validating this approach with camera trap observations of hunters from Malaysian Borneo, I found that the new accessibility metric had far more explanatory power than any other accessibility-based or environmental predictor. To assess other drivers of spatial foraging patterns, in my second chapter I extended my scope across the tropics. Compiling images of foragers taken from cameras in 10 protected areas (PAs), I explored which types of variables—accessibility, wildlife value, or environmental features—best informed forager presence on the landscape. While the results of this chapter should be interpreted cautiously, given limited observations, I found no variable could consistently explain observed forager presence across PAs except for elevation. In my final chapter, I leveraged simulations and decision analysis to assess traditional spatial management. I found that large, fixed reserves could robustly maximize both harvest and conservation goals, especially compared to strategies with small or dynamic alternatives. However, outcomes were strongly influenced by the intensity of hunting effort, hunters’ adherence to spatial rules, and mammal dispersal patterns. While the models and simulations I apply here do not capture the full diversity of hunter behaviour and game responses, they can provide guidance and insights where local data collection is infeasible.

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The connectivity of coastal marine populations: uncovering patterns of dispersal and the implications for management (2022)

Dispersal is the ecological process of organismal movement away from the location of birth.Emerging from the process of dispersal is the concept of connectivity, in which animalmovement links spatially distinct populations, thus generating a landscape-scale pattern fromprocess. Quantifying connectivity is a central challenge in marine ecology, with implications forour understanding of population dynamics and the effective management of biodiversity. Yet,there remains a limited understanding of connectivity for many marine ecosystems in terms ofthe dispersal patterns that emerge from the influence of physical seascape features and biologicaltraits. This knowledge gap presents an opportunity to assess the connectivity of understudiedcoastal ecosystems at novel spatial and temporal scales. In this thesis, I model the potentialdispersal and connectivity of coastal marine species on the Pacific coast of Canada. In Chapter 2,I identify and evaluate connectivity patterns of the invertebrate community associated withseagrass (Zostera marina) habitat. I uncover the spatial and temporal scale of networks of habitatconnected by dispersal, and I identify the contribution of individual seagrass habitat patches tomaintaining overall seagrass network connectivity. In Chapter 3, I assess the connectivity of theexisting Marine Protected Area (MPA) network for a variety of nearshore invertebrate species,and I find that the majority of MPAs are connected and support persistent populations. InChapter 4, I explore how connectivity can cause cascading effects of human activities thatimpact seagrass meadows by altering invertebrate dispersal thereby indirectly affecting othermeadows in the seascape. I find that robust dispersal connections can support the persistence ofmost populations despite the negative effects from human activities. In sum, my thesis generatespredictions on the spatial scale of dispersal, and it highlights the ecological insights gained fromanalyzing the emergent connectivity as a network. Together, the methodological framework andresults from this thesis provide essential ecological knowledge to support our understanding ofbiodiversity and aid in the management and conservation of marine ecosystems.

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Local-scale assessment of regional conservation plans: strategies for the conservation of animals and plants in a tropical rainforest and surrounding mixed-farmland (2021)

There is an urgent need to conserve biodiversity in human-modified landscapes throughout the tropics. Animal conservation has traditionally focused on single species, but it remains unclear whether these strategies will also protect other taxa that co-occur within the ecosystem. These uncertainties can also affect plant conservation if management interventions change plant-animal interactions. I identified steps to mitigate the effects of hunting, forest product extraction, and farming on rainforest animals and plants in Suaka Margasatwa Buton Utara (SMBU) and the surrounding mixed-farmland on Buton Island, Southeast Sulawesi, Indonesia. First, I used Bayesian Network inference to assess whether protecting anoa (Bubalus spp.) habitat might also benefit other animals by modelling species co-occurrences in relation to habitat and human activities. Next, I completed a pantropical Network Meta-Analysis (NMA) to identify where controlling the foraging of granivorous mammals might reduce mortality of management-sowed seeds in human-modified forests. Finally, I used the NMA to guide an experimental assessment of how seed predation might affect the regeneration of nine plant species in the reserve and mixed-farmland. Buton macaques (Macaca ochreata) did not co-occur with anoa and were the only species to avoid human-dominated areas. The government might consider concentrating patrols in easy-to-access areas to increase the distribution of macaques throughout SMBU and the mixed-farmland. The NMA identified that granivore control could help reduce seed mortality, but which seeds to protect depended on the type of human activity that modified the forest. At SMBU, granivore control was not required in the mixed-farmland because seed predation was very low for 78% of the studied plants. Low seed losses in the mixed-farmland suggested that forest regeneration might be enhanced by increasing macaque distribution and natural seed rain throughout those areas. My approach could be used to design projects that conserve animals and plants in human-modified ecosystems. Local co-occurrence analyses can identify species that remain vulnerable to humans under conservation projects focused on other species. In data deficient situations, wide-scale evidence synthesis using NMA can help guide decision making, such as when to use granivore control, in human-modified ecosystems.

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The Thermal Ecology of Populations and Ecosystem Services in a Changing World (2018)

A central goal of ecology is to understand what drives the abundance, distribution and diversity of life on Earth. For centuries, biologists have been addressing these questions from a variety of perspectives, and yet we still lack a coherent and mechanistic understanding of what drives these patterns. One process that is shared by all of life on Earth is metabolism. The development and testing of metabolic scaling theory (MST), which formalizes relationships between body size, temperature and metabolism, has revealed remarkable generality in the way that organisms respond to the environment. In spite of extensive documentation of cross-species metabolic patterns, we still lack evidence for how metabolic constraints propagate from the fine to the broad organizational scales. The large gap in our understanding at the level of populations presents a critical challenge for MST. I have combined theory, experiments and data synthesis to test the metabolic underpinnings of biodiversity and its implications for human well-being. Results showed that the temperature-dependence of population dynamics can be predicted from the temperature-dependence of individual metabolism, thus lending strong support for the role of energetic constraints in governing population growth and abundance. Further, I showed that variation in ecologically important traits such as body size has important implications for the nutritional value of aquatic species assemblages, thus linking the processes that structure ecosystems with the benefits they provide. This approach has revealed that understanding what generates and maintains aquatic biodiversity has direct and immediate consequences for human well-being.

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Heat wave impacts across scales in the splash pool copepod, Tigriopus californicus (2017)

Temperature influences biological processes at all levels of biological organization. As such, temperature is a fundamental abiotic variable affecting ectotherm fitness. As human induced climate change persists, the need to understand biological responses to temperature has never been more pressing. One consequence of rapid climate change is an increase in the frequency and intensity of heat waves. Despite substantial interest in the physiological effects of heat stress, it is less clear how individual responses to extreme heat events influence population-level responses such as persistence and population growth. Energy balance is a unifying concept that integrates the effect of temperature across scales. The Energy Limited Tolerance to Stress (ELTOS) framework links physiological models of temperature and oxygen availability to dynamic energy budgets in order to predict the effect of stress on individual reproduction and population growth. Drawing on the ELTOS framework, I tested three critical predictions about individual thermal experience, and how the influence of heat stress scales up from physiological to individual to populations, utilizing the splash pool copepod, Tigriopus californicus. In Chapter 2, I tested the prediction that aerobic energy production declines with increasing heat wave intensity and duration, and that individual reproduction declines similarly. In Chapter 3, I tested the assumption that short-term impacts of heat waves on individual reproduction have persistent effects on longer-term population dynamics. In Chapter 4, I tested for an effect of spatial variation in thermal history on subsequent heat wave survival. Together, these tests provide a comprehensive examination of ELTOS predictions of biological responses to heat waves across scales. My data are consistent with ELTOS predictions that heat wave intensity affects energy balance and subsequent individual reproductive effort. However, I did not find consistent patterns of population dynamics over both short and longer-term time scales. The opposing effect of temperature on different life-history traits that occur over different time-scales likely underlies differences in the effect of short-term heat wave effects on population dynamics over short and longer time periods. Lastly, I found that spatial variation in thermal history, particularly recent heat accumulation, explains reduced survivorship during experimental heat waves.

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An Evaluation of the Effectiveness of Rockfish Conservation Areas in British Colubmia, Canada (2016)

Networks of Marine Protected Areas are implemented to conserve fish populations, yet their effectiveness is rarely comprehensively examined or adaptively managed. In this dissertation, I evaluate a network of Rockfish Conservation Areas (RCAs) implemented to reverse population declines of inshore Pacific rockfishes (Sebastes spp.). First, I used SCUBA surveys to examine patterns of Black Rockfish abundance compared to spatial and temporal variability in recruitment to determine how recruitment influences population density in and around a RCA. Habitat variables such as complexity and rocky substrate predicted adult Black Rockfish abundance while recruitment did not. Next, I surveyed the fish communities of 35 RCAs and adjacent unprotected areas using a Remotely Operated Vehicle (ROV). Habitat features such as percent rocky substrates and depth influenced the density of Quillback (S. maliger), Yelloweye (S. ruberrimus), Greenstriped Rockfishes (S. elongatus), Kelp Greenling (Hexagrammos decagrammus), Lingcod (Ophiodon elongatus) and all inshore rockfishes combined, while reserve status did not. The results give little indication that demersal fish populations have recovered inside the RCA system. I used aerial observations of recreational fishing from surveys before, during and after 77 RCAs were established and found there was no evidence of a change in fishing effort in 83% of the RCAs. Compliance was related to the level of fishing effort around the RCA, the size and perimeter-to-area ratio of RCAs, proximity to fishing lodges and the level of enforcement. Non-compliance in RCAs may be hampering their effectiveness and impeding rockfish recovery. Lastly, I modeled rocky reef habitat using Random Forest Classification to assess habitat in the RCAs. I combined three habitat metrics with data on compliance, RCA size, rockfish bycatch, and connectivity into a single Conservation Score. The Conservation Score is related to the log reserve ratio, a measure of relative abundance, for Quillback Rockfish. RCAs with low Conservation Scores are not likely to be effective and managers should evaluate the reasons for low scores and address reserve shortcomings in an adaptive spatial management framework. Education and enforcement efforts are critical to the recovery of depleted fish stocks. Continued monitoring and evaluation of the RCAs is essential to rockfish conservation.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Estimating blue carbon storage capacity of Canada’s eelgrass beds (2023)

Despite the importance of coastal ecosystems for the global carbon (carbon) budgets including obligations to the United Nations (UN) and growing policy frameworks for natural climate solutions, knowledge of coastal carbon storage capacity and the factors driving variability in storage capacity is still limited. Here we provide an estimate of the magnitude and variability of Canadian eelgrass (Zostera marina) sediment carbon stocks. We sampled 22 eelgrass meadows across three coastal regions to quantify and explain variation in organic carbon stocks in eelgrass associated sediments. Eelgrass carbon stocks integrated over 25-cm depth ranged from 55 to 196 Mg carbon ha⁻¹ (mean 18.2 Mg carbon ha⁻¹ ). Organic carbon stocks predicted from the model also ranged from 39.65 to 85.37 Mg carbon ha⁻¹ with an average of 59.02 ± 0.35 Mg carbon ⁻¹. These estimates are consistent with estimates of temperate Z. marina blue carbon values (3.18 ± 0.10 to 265.23 ± 6.67 Mg carbon ha⁻¹ ) (Röhr et al 2018). Overall, from 89 cores collected in this study, I found that variation in organic carbon stocks was explained by substrate cover type, coast, sediment silt content, exposure, and sediment depth. The high spatial carbon storage variability urges caution in extrapolating carbon storage capacity from broader seagrass carbon studies as well as between geographical areas.

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An experimental test of how microbial communities respond to warming in systems connected by dispersal (2021)

Community dynamics and structure are greatly affected by climate change through warming. Temperature directly affects the rates of biochemical reactions that in return affect growth, resource use, organismal abundance, species interactions, and, therefore, communities and biodiversity. In addition, low connectivity can limit dispersal between communities, reducing the potential for demographic rescue effects. Therefore, the effects of temperature on diversity and community structure in patchy landscapes can depend on the degree of connectivity among landscapes. We tested whether the effects of temperature on communities contingent on the degree of connectivity using experimental pond metacommunities, each comprised of four-1000L mesocosms spanning a 4.5°C spatial temperature gradient and connected by one of three dispersal rates. This spatial temperature gradient was maintained, while also allowing the mesocosm temperatures to fluctuate temporally with seasonal weather variation. Bacterial communities in the mesocosms were sampled in the summer to evaluate whether dispersal rate at the metacommunity level affects local and regional community response to seasonal fluctuations in temperature. We predicted that higher levels of dispersal would raise local (alpha) diversity and decrease species turnover among ecosystems (beta diversity) and metacommunity-level (gamma) diversity. However, we found no effect of dispersal on local and regional diversity metrics. We also predicted that dispersal rates would differently affect species compositional differences along the thermal gradient. At low dispersal rates among communities, we observed differences in species composition associated with temperature. At higher dispersal rates, communities were not structured by temperature and composition was similar within a metacommunity, which was not observed in any other dispersal treatment. This emphasizes the homogenizing effect high dispersal has on bacterial community structure. Our findings demonstrate that bacterial diversity metrics do not follow metacommunity predictions about dispersal effects on diversity. However, we found support for the hypothesis of high dispersal homogenizing communities. This suggests there are other processes that influence bacterial community diversity patterns, but dispersal can erode the effect of the environment on bacterial community structure.

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Community structure and availability of edible primary producers to herbivores along a temperature gradient (2015)

Warming can influence the rate of plant-herbivore interactions through direct effects on individual metabolism, resource use, and growth rates, and via indirect effects on the properties of plant resources and behavior of consumers. Through these processes, temperature can affect the structure and function of food webs, though whether these overall responses reflect primarily direct or indirect effects of temperature is unclear. To begin to address this problem, I quantified the effects of temperature and grazing on primary producer traits and relative abundance to understand how temperature directly and indirectly affects an important aspect of food webs: resource availability to herbivores. I hypothesized that warming would decrease the availability of edible resources to consumers through decreased abundance, body size and shifts among dominant functional groups, and that these effects would be strengthened in the presence of consumers. I tested this hypothesis in freshwater algal-grazer communities maintained across an 11°C temperature gradient over 11 weeks. I observed direct, positive effects of temperature on whole-system oxygen fluxes (i.e. through net primary productivity and ecosystem respiration), and direct negative effects on phytoplankton abundance and body size, with higher relative abundance of small phytoplankton. Herbivores drove shifts in phytoplankton size distributions across the temperature gradient through size-selective consumption of large phytoplankton. Warming shifted species composition among algae from plankton-dominated to periphyton-dominated assemblages, consistent with indirect effects of warming on competitive interactions. Taken together, shifts in abundance, body size and functional group dominance over the temperature gradient decreased the availability of preferred plant resources to filter-feeding zooplankton at warmer temperatures, which may alter food web structure and function, especially under increased grazing pressure. I conclude that resource-availability shifts are predictable with warming, and that temperature-dependent community theory can be expanded to include these indirect effects of temperature on species interactions.

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Seagrass epifaunal communities of Barkley Sound: Epifaunal diversity varies across small spatial and temporal scales (2014)

The assembly and persistence of ecological communities is a phenomenon that occurs across large spatial and temporal scales. However, the relative effects of regional versus local processes on community structure are not well understood in marine ecosystems. In order to understand how scale can alter processes that drive variation in community assembly it is necessary to determine patterns of diversity across multiple scales. Here, I used invertebrate epifaunal communities in the foundation species Zostera marina to test 1) whether this marine community exhibits meadow-scale variability through time, and 2) whether we can identify patterns of connectivity and diversity within and among meadows in the same region. I found that seagrass epifaunal communities are variable in terms of their rarefied richness, alpha and beta diversity, and evenness among meadows. In addition, differences in these metrics were detected over the course of a summer season.

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Quantifying the effect of hiking disturbance on American pika (Ochotona princeps) foraging behavior (2013)

An animal's need to balance energy intake with predator avoidance results in trade-offs predicted by optimum foraging theory. These trade-offs may include reducing foraging activity if 1) perception of predation risk is high or 2) abiotic conditions are suboptimal for foraging. Human disturbances such as hikers can influence an animal's perception of predation risk, yet little is known about how hiking affects the foraging behaviour of species in the alpine zone. American pikas (Ochotona princeps) are small, food-hoarding mammals whose foraging ability is restricted by heat. If pikas' ability to forage is further decreased by hiking disturbance it could negatively impact their survival due to reduced food storage. To quantify the effect of hikers on foraging time, I evaluated multiple hypotheses of risk avoidance by simulating disturbance events for 48 pikas in Glacier National Park, BC. I assessed pikas' response to hikers using four indicators of risk behaviour: alert distance (DA), flight initiation distance (DF), exit delay (TE) and delay in return to forage (TR). To test if temperature or distance to trail was more influential to pikas' foraging activity, I conducted behavioural observations on 17 pikas. All simulated disturbance events elicited anti-predator response behaviours in foraging pikas, reducing time available to forage and increasing time spent alert and vigilant. Distance to trail was an important predictor of TE and TR. Pikas near trails (100 m away from trails, which lost an average of 13.2 (SE=1.7, n=16) minutes of foraging time per disturbance event. Temperature, not distance to trail, was the strongest predictor of pikas' foraging activity over a 4-6 hour period. This suggests that human disturbance may be partially mitigated by pikas' behavioural adaptation at less frequented sites. Monitoring pika populations near and away from trails would be well-advised given projected trends in warming climate and potential increases in hiking traffic.

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