Graduate Thesis Or Dissertation
 

Are Bird Habitat Associations Consistent Across Space and Time?

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  • The concept of the fundamental niche is frequently used in ecology to define the set of environmental conditions needed by a species to survive and reproduce (Hutchinson 1957). In contrast, the realized niche constitutes the locations where a species actually occurred, which is a function of both the environmental (abiotic) conditions and biotic interactions (e.g., predation, competition, mutualisms; Soberon and Peterson 2005). Up to now, the realized niche of most species has been mostly examined during breeding seasons and researchers have long assumed that niches should remain constant across species’ ranges and across the avian annual cycle. If this assumption is valid, then the conservation of each species across their full annual cycle is made easier because each species is associated with a single niche. However, if species have more than one realized niche, then we risk misleading conservation efforts by protecting the wrong habitat in some regions or seasons. In this dissertation, I aim to identify whether most species exhibit a single realized niche, or rather, that species have multiple niches that change depending on age and sex (Ch.2), geographic location (Ch. 3) or the stage of the annual cycle (Ch.4). More specifically, in chapter 2, I assembled a large dataset representing the capture (and release) of Rufous Hummingbirds (Selasphorus rufus) and calculated migration routes and timing across age and sex categories. The data showed that adult males, adult females and juveniles tend to migrate using different migration corridors. A greater number of young birds migrated south through California in comparison to the adults which migrated largely through the Rocky Mountain states. Moreover, migration timing also varied across age and sex categories with adult males departing first and young of both sexes departing last – on average one month after the adult males. Overall, the results of this chapter suggest that Rufous Hummingbirds likely encounter and use slightly different sets of environmental conditions during their fall migration across demographic groups. The goal of chapter 3 was to determine if species exhibit consistent habitat relationships across their breeding range. In other words, are niches sufficiently predictable that predictive models can be readily transferred from one location to another? This property is termed “stationarity”. I used Breeding Bird Survey data from across North America, land cover and climate data from remote sensing, and a model transferability methodology to predict avian abundance across space for 131 species. I also assessed whether species’ traits were correlated with levels of stationarity in distributions. Lastly, I tested whether prediction accuracies between modeled regions decreased with 1) geographical distances, 2) level of extrapolation, and 3) were affected by a ‘core-boundary’ effect. The results suggest that, for most species, habitat relationships change across species’ breeding ranges. Species with large distributions, with distributions in regions with less topographic relief, and species with shorter life spans were more likely to have non-stationary distributions. Moreover, results show that predicting across long geographical distances or to novel environments decreases prediction accuracies. Overall, these results suggest that caution should be used when assuming stationarity in models, because the habitat used by birds across regions sometimes differed in measurable ways within a species’ range. These results are important for conservation planning because many conservation efforts such as forecasts of biological invasions, prioritization of land protection, and translocation of endangered species relies on accurately predicting abundance across space (Guisan et al. 2013). Lastly, in chapter 4, I tested whether 83 migratory bird species are consistent in their niches throughout the annual cycle. Here, I used year-round data from the citizen science program, eBird, along with land cover and weather data from remote-sensing sources to calculate niches for each species in four different seasons in each year (2005 – 2020). I compared niches across years (within season) and across seasons (within years) and calculated the percent niche overlap and the drivers of niche dynamics between pairs of niches. As expected, niche overlap was higher across years than across seasons. Consistent with the niche plasticity hypothesis, the niches of migratory birds were largely stable across species’ annual life cycles, but with some small and significant differences among them. These results suggest that migrants have the most similar niches between spring and fall migrations, and most different niches between breeding and over-wintering grounds. Moreover, migratory birds seem to have the most constrained niches during the breeding season. These results suggest that while most migratory birds may rely on common sets of environmental conditions across seasons, the seasonal niches also contracted and expanded significantly throughout the year. Taken all together, the results of these three chapters suggest that each bird species is associated with differing sets of environmental conditions among demographic groups, and across space and time. As such, species are better represented by multiple realized niches, which overlap to different degrees within their fundamental niche. These results highlight the need to identify each species’ collection of realized niches in order to successfully conserve their populations and protect their habitat throughout their full life cycle. Doing so is likely essential to reversing the negative trends observed in our bird populations (Rosenberg et al. 2019).
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  • This research was supported by a Natural Sciences and Engineering Research Council (NSERC) Postgraduate Scholarship, the Oregon State University, College of Forestry, Richardson Family Graduate Fellowship, and the Oregon State University, Graduate School, Diversity Advancement Pipeline Fellowship.
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  • Pending Publication
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  • 2020-12-15 to 2022-01-15

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