Shrub Dominance Associations
Both direct and indirect gradient analyses were used to visualize associations between bird species, elements of shrub dominance, shrub species, and bioclimate zone. Results from both the direct and indirect gradient analysis of the elements of shrub dominance and bird species were included. Only the direct gradient analysis of the shrub species and bird species were included.
Direct Gradient Analysis
The direct gradient analysis revealed a number of bird species significantly associated with elements of shrub dominance. Based on their location relative to the shrub height vector, Dusky Flycatcher (DUFL), Dark-eyed Junco (DEJU), and Swainson's Thrush (SWTH) are all significantly positively correlated with shrub height, and a number of other species show weaker correlations towards shrub height . White-crowned Sparrow (WCSP) and Wilson's Warbler (WIWA) appear to be more strongly associated with shrub density than shrub height, and have a more negative correlation to shrub stem count (Figure 15, Table 2).
Fewer bird species were associated with shrub stem count, but Horned Lark (HOLA) and American Pipit (AMPI) showed strong positive correlations with stem count and strong negative correlations with shrub density (Figure 15, Table 2).
Fewer bird species were associated with shrub stem count, but Horned Lark (HOLA) and American Pipit (AMPI) showed strong positive correlations with stem count and strong negative correlations with shrub density (Figure 15, Table 2).
Figure 15: Direct gradient analysis of bird species abundance and elements of shrub dominance. Green and orange points and ellipses represent sampling sites in the subalpine and alpine habitats respectively. Shrub dominance elements (density, average height, and average stem count) were ordinated using an NMDS and are represented by red vectors. Bird species are identified by a unique four-letter code. Bird species vectors have been removed to make the plots easier to interpret. Bold bird species are significantly associated with shrub variables (p <0.05). Highlighted bird species are highly significantly associated with shrub variables (p <0.01).
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Table 2: Table depicting the R squared and p-values for the direct gradient analysis depicted in Figure 14. Bold bird species are significantly associated with shrub variables (p <0.05). Highlighted bird species are highly significantly associated with shrub variables (p <0.01). Also pictured are the six highlighted bird species.
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Indirect Gradient Analysis
The indirect gradient analysis involved ordinating the bird species using an NMDS, and then plotting the shrub dominance variables as vectors. The majority of the relationships between specific bird species and the elements of shrub dominance are the same as seen in the direct gradient analysis above. The indirect gradient analysis also confirms that there is a difference in bird species composition between the subalpine and alpine because the subalpine and alpine are still distinct groups when bird species are used in the initial ordination (Figure 16).
There are some minor differences between the direct gradient analysis and the indirect gradient analysis, namely regarding shrub variable vector length. As above, average shrub height is strongly associated with the subalpine habitat and shows positive and negative associations with similar bird species as above. However, the shrub density and stem count vectors are shorter in the indirect gradient analysis, indicating that they are not highly correlated or significantly associated with any bird species (Figure 16, Table 3).
There are some minor differences between the direct gradient analysis and the indirect gradient analysis, namely regarding shrub variable vector length. As above, average shrub height is strongly associated with the subalpine habitat and shows positive and negative associations with similar bird species as above. However, the shrub density and stem count vectors are shorter in the indirect gradient analysis, indicating that they are not highly correlated or significantly associated with any bird species (Figure 16, Table 3).
Figure 16: Indirect gradient analysis of bird species abundance and elements of shrub dominance. Green and orange points and ellipses represent sampling sites in the subalpine and alpine habitats respectively. Bird species were ordinated using an NMDS and are identified by a unique four-letter code. Bird species vectors have been removed to make the plots easier to interpret. Shrub dominance elements (density, average height, and average stem count) are represented by red vectors. Bolded and highlighted shrub dominance elements are significantly associated with bird species (p <0.01).
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Table 3: Table depicting the R squared and p-values for the indirect gradient analysis depicted in Figure 14. Bold and highlighted shrub variables are significantly associated with bird species (p <0.01).
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Shrub Species Associations
A direct gradient analysis was used to visualize associations between bird species, shrub species, and bioclimate zone. Two shrub species were strongly associated with the subalpine habitat: (1) Dwarf Birch (Betu_gla) and Willow species (Sali_spp), and there were a number of significant associations between bird species and shrub species (Figure 17, Table 4). However, the bird species that were most significantly associated with shrub species appear to be more positively correlated with Dwarf Birch than other shrub species (Figure 17). Finally, although there is no significant or even strong correlation between bird species and Heather spp (Phyl spp), it is worth noting that Heather spp is ordinated towards the alpine habitat and the birds associated with it. These birds also show strong negative correlations to both shrub height and density based on their ordination (Figure 16).
Figure 17: Direct gradient analysis of bird species abundance and shrub species. Green and orange points and ellipses represent sampling sites in the subalpine and alpine habitats respectively. Shrub species were ordinated using an NMDS and identified by the first four letter of genus and first three letters of species and are represented by red vectors. Shrub species that did not contribute to ordination were removed. Bird species are identified by the identified by a unique four-letter code. Bird species vectors were removed to make the figure easier to interpret.
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Table 4: Table depicting the R squared and p-values for the direct gradient analysis depicted in Figure 11. Bold bird species are significantly associated with shrub species (p <0.05), and highlighted bird species are highly significantly associated with shrub species (p<0.01).
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Discussion
Subalpine Birds
These data visualizations identified many bird species positively associated with subalpine habitat and shrubs dominance. Birds such as Swainson's Thrush that live and breed in treed habitats (Mack and Yong, 2020), and Dusky Flycatcher and Dark-eyed Junco (Nolan et al., 2020; Pereyra and Sedgwick 2020) that are more generalist were significantly positively correlated with shrub height (Figure 15). If shrubification through height expansion occurs in mountainous habitats, these birds may benefit from increased nesting grounds and preferred habitat. However, if shrubification occurs through cover or density expansion, possibly due to environmental factors like wind and soil quality, these birds may not benefit.
Bird species positively associated with shrub density in the indirect gradient analysis include Wilson's Warbler, Lincoln Sparrow, and American Tree Sparrow (Figure 15; Figure 16) and these three species were also significantly associated with dwarf birch (Figure 17). This is somewhat expected, as dwarf birch is a denser, lower-growing shrub when compared to a species like willow that has more potential to grow tall (Pojar and MacKinnon, 2013). Interestingly, the White-crowned Sparrow was significantly positively associated with shrub density (Figure 15), but did not appear to pull more significantly more towards dwarf birch than willow species (Figure 17). This could be because White-crowned Sparrows could be considered more generalist, often showing great variety in their preferred breeding habitat depending on location (Chilton et al., 2020). These birds that are significantly associated with shrub density and dwarf birch may benefit is shrubs on mountains increase in density, a phenomenon which could be more common in harsher mountain top environments where shrub height is limited by environmental factors such as weather.
Bird species positively associated with shrub density in the indirect gradient analysis include Wilson's Warbler, Lincoln Sparrow, and American Tree Sparrow (Figure 15; Figure 16) and these three species were also significantly associated with dwarf birch (Figure 17). This is somewhat expected, as dwarf birch is a denser, lower-growing shrub when compared to a species like willow that has more potential to grow tall (Pojar and MacKinnon, 2013). Interestingly, the White-crowned Sparrow was significantly positively associated with shrub density (Figure 15), but did not appear to pull more significantly more towards dwarf birch than willow species (Figure 17). This could be because White-crowned Sparrows could be considered more generalist, often showing great variety in their preferred breeding habitat depending on location (Chilton et al., 2020). These birds that are significantly associated with shrub density and dwarf birch may benefit is shrubs on mountains increase in density, a phenomenon which could be more common in harsher mountain top environments where shrub height is limited by environmental factors such as weather.
Figure 18: Examples of bird species associated with subalpine habitats. Clockwise from top left: Swainson's Thrush, Lincoln Sparrow, Wilson's Warbler, Dark-eyed Junco, Dusky Flycatcher, and White-crowned Sparrow (images from Cornell Lab).
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Figure 19: Shrub species strongly associated with the subalpine and the corresponding bird species. From left to right: Dwarf Birch and Willow spp.
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Alpine Birds
These data visualizations also identified bird species highly associated with alpine habitats (Figure 15; Figure 16; Figure 17). The species one would expect to find in alpine habitats during the breeding season include Horned Larks, American Pipits, Ptarmigan species, Surfbirds, and potentially Gray-crowned Rosy Finch, and the majority of these birds were associated with the alpine habitats and negatively associated with tall and dense shrub species (Figure 15; Figure 16: Figure 17).
The Horned Lark in particular was the most significantly negatively associated with shrub density, shrub height, and both shrub species driving the classification of the subalpine habitat (willow species and dwarf birch), with the American Pipit being a close second (Figure 15; Figure 16; Figure 17). While the Horned Lark did show the strongest negative associations with shrub height and density, it is worth noting that this species is not dependent on alpine tundra for nesting. The Horned Lark nests in barren, open country both at low and high elevations (Beason, 2020), and therefore may be more adaptable to finding nesting sites in grasslands at lower elevations. In contrast, American Pipits breed exclusively in open habitats above tree and shrub-line (Hendricks and Verbeek, 2020), indicating that this species may face extirpation as shrub-lines continue to advance up mountains.
These two bird species were also positive associated with shrub stem count (Figure 15). While this may seem like an unexpected result, it is likely because shorter shrubs in the alpine expand through cover rather than height, and each plant may cover a large area with many small stems. This would increase the stem count, while also causing shrub plants to grow farther apart, and thus decreasing shrub density. While not significant, the vector for heather species (Phyl_spp) is the only shrub species pointing towards the alpine habitat grouping (Figure 17), and indeed this shrub is low growing and matted, with each plant often containing over 50 stems. Future research may consider estimating total stem count in surrounding areas rather than stem count of each plant, and thus would be more likely to yield expected relationships between shrub density and stem count and show different relationships with bird species.
White-tailed Ptarmigan and Surfbirds were also species driving the distinction between subalpine and alpine locations (Figure 16), but were not as negatively associated with shrub height and density as other alpine specialists (Figure 15). This could mean that while these species likely would not benefit from increased shrubification, they may not be as vulnerable as other alpine specialists.
The Horned Lark in particular was the most significantly negatively associated with shrub density, shrub height, and both shrub species driving the classification of the subalpine habitat (willow species and dwarf birch), with the American Pipit being a close second (Figure 15; Figure 16; Figure 17). While the Horned Lark did show the strongest negative associations with shrub height and density, it is worth noting that this species is not dependent on alpine tundra for nesting. The Horned Lark nests in barren, open country both at low and high elevations (Beason, 2020), and therefore may be more adaptable to finding nesting sites in grasslands at lower elevations. In contrast, American Pipits breed exclusively in open habitats above tree and shrub-line (Hendricks and Verbeek, 2020), indicating that this species may face extirpation as shrub-lines continue to advance up mountains.
These two bird species were also positive associated with shrub stem count (Figure 15). While this may seem like an unexpected result, it is likely because shorter shrubs in the alpine expand through cover rather than height, and each plant may cover a large area with many small stems. This would increase the stem count, while also causing shrub plants to grow farther apart, and thus decreasing shrub density. While not significant, the vector for heather species (Phyl_spp) is the only shrub species pointing towards the alpine habitat grouping (Figure 17), and indeed this shrub is low growing and matted, with each plant often containing over 50 stems. Future research may consider estimating total stem count in surrounding areas rather than stem count of each plant, and thus would be more likely to yield expected relationships between shrub density and stem count and show different relationships with bird species.
White-tailed Ptarmigan and Surfbirds were also species driving the distinction between subalpine and alpine locations (Figure 16), but were not as negatively associated with shrub height and density as other alpine specialists (Figure 15). This could mean that while these species likely would not benefit from increased shrubification, they may not be as vulnerable as other alpine specialists.
Conclusion
Results from this research identified birds and shrub species associated with both the subalpine and alpine habitats on mountains. It also identified subalpine birds that may benefit from shrubification through increased shrub height such as Swainson's thrush, dusky-flycatcher, and dark-eyed junco, and those that may benefit from increased shrub density such as Wilson's warbler, Lincoln sparrow, and American tree sparrow. These birds may be able to expand their habitat ranges, find more nesting sites and food resources at higher elevations, and increase their populations.
This research also identified bird species associated with the alpine habitat such as horned lark, American pipit, surfbird, and white-tailed ptarmigan. In addition, horned lark and American pipits were significantly negatively associated with shrub height and density, indicating that they may be forced to re-location or face extirpation if shrubs continue to grow taller and more dense. They may also face compeition from generatlist and shrub specialists moving farther up slope and encroaching on alpine habitat. These species should be of particular conservation concern as shrub dominance continues to increase on mountain habitats.
This research also identified bird species associated with the alpine habitat such as horned lark, American pipit, surfbird, and white-tailed ptarmigan. In addition, horned lark and American pipits were significantly negatively associated with shrub height and density, indicating that they may be forced to re-location or face extirpation if shrubs continue to grow taller and more dense. They may also face compeition from generatlist and shrub specialists moving farther up slope and encroaching on alpine habitat. These species should be of particular conservation concern as shrub dominance continues to increase on mountain habitats.
References
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