Patterns and Drivers in Counts of Migratory Raptors at Hawk Mountain Sanctuary, USA
Understanding the mechanisms driving biodiversity patterns amidst an era of global environmental change is the core of modern ecological research. The magnitude of biodiversity losses associated with anthropogenic activities has prompted resource managers and ecologists alike to identify strategies...
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ScholarWorks
2020
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| Online Access: | https://scholarworks.boisestate.edu/td/1710 https://scholarworks.boisestate.edu/cgi/viewcontent.cgi?article=2841&context=td |
| Summary: | Understanding the mechanisms driving biodiversity patterns amidst an era of global environmental change is the core of modern ecological research. The magnitude of biodiversity losses associated with anthropogenic activities has prompted resource managers and ecologists alike to identify strategies to address conservation issues. Broadly, two types of approaches are employed to answer ecological research questions: 1) single-species and 2) ecosystem-based approach. Single-species approaches are often useful to elucidate mechanisms driving population trajectories of individual species. On the other hand, ecosystem-based approaches can help in identifying general patterns that may be useful for multi-species management. Here, I used both approaches in assessing broad-scale patterns and mechanisms driving count trends of migrating raptors recorded at Hawk Mountain Sanctuary (HMS), Pennsylvania. In the first chapter, I used a hierarchical breakpoint model to identify the assemblage-wide and species-specific timing of the shifts in count trends. Then I evaluated if changes in trend directionality of counts were linked to species’ traits (body size, population size, migratory behavior, tolerance to human presence, DDT (dichlorodiphenyltrichlorethane) susceptibility, habitat or dietary specialization). I found that an assemblage-wide shift in counts occurred around 1974, and this timing was common among 14 of the 16 species in the assemblage. Moreover, I found that habitat specialization appeared to explain the synchronous positive and negative count trends of multiple species. Other traits that I evaluated were not consistently associated with either types of trends. The temporal shift in trends in 1974 emphasized the relative importance of DDT, an organochlorine known to have adversely influenced several wildlife species and was banned in the US around the 1970s, in driving population dynamics of raptor species. However, because the counts of species susceptible to DDT were highly variable after 1974, this may suggest that a suite of additional factors, acting together, affected the recovery of species from DDT-associated declines. Additionally, the potential role of habitat specialization in count trends may suggest important linkages between habitat use and demography. In the second chapter, I used a generalized linear mixed-effects model to assess the relationships between changes in the count totals and total proportional cover of major land-use types in nine states located in the northeastern US (Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont). The hierarchical modelling approach that I used allowed me to identify average and species-specific responses to the proportional cover of forested and urban area. These land-use variables were not associated with overall raptor counts. However, species-specific responses were variable and significant. I found that counts of Northern Goshawk, American Kestrel, Rough-legged Hawk, Sharp-shinned Hawk, and Red-tailed Hawk were positively associated with forest cover. On the other hand, Turkey and Black Vultures, Bald Eagle, and Peregrine Falcon were positively associated with urban cover. Moreover, Red-shouldered Hawk, Broad-winged Hawk, and Northern Harrier were not significantly associated with forest cover but were negatively associated with urban cover. Merlin and Cooper’s Hawk exhibited similar non-significant associations to forest but positive associations with urban cover. Finally, Golden Eagle and Osprey were not significantly associated with either land-use variables. These results provided insights on the potential influence of land-use changes on the demography of migrating raptors. Thus, these findings may be useful in improving our predictions of the population trajectories of these species in future landscape scenarios. These results illustrate the utility of evaluating species-level and assemblage-wide patterns in long-term count data. In this case, it allowed me to identify general patterns in counts of migrating raptors and gain detailed insights on the responses of individual species to land-use changes. In doing so, I was able to better understand the potential drivers of their ecological dynamics. By integrating information from these two approaches, we can expect to obtain a better understanding of natural systems and consequently, increase the probability of successful conservation outcomes. |
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