A spearman correlation analysis was run to determine statistical significance, seen in figure 3. There is no significant correlation between abundance of individuals per island and time since fire, distance from mainland, volume of basal area, or volume of coarse woody debris. The strongest relationship exhibited is between abundance of birds and time since fire.
Figure 3. Spearman correlation plots and reported statistics for avian abundance (number of individuals present) as a function of a series of response variables.
Sample of code used to generate R and p-values, as well as figure 3.
The analysis of this data was subject to a series of limiting factors, primarily my individual (in)capability of using R. The currently generated figures and statistical reports have been derived from a temporarily modified dataset, and as such will shift once the full avian dataset is compiled and delivered.
How does avian species richness vary as habitat parameters shift?
Avian species richness in boreal ecosystems is linked to forest structure recovery post-disturbance, particularly the aspects that provide foraging opportunities and cover [4]. Among the critical habitat parameters that species seek out in early successional stages is downed wood, as it provides refuge for myriad size classes of taxa, and foraging substrate for many more. As well, basal area post-disturbance can be comprised primarily of snags, dependent on the severity of the fire [5]. The relationships between time since disturbance and forest recovery are explored in figure 4 below.
Figure 4. Avian species richness as a function of changing habitat parameters.
As we observe recovery post-fire in boreal ecosystems, we can observe an influx of a wide base of generalist avian species who use downed wood as refuge, habitat, and foraging substrate [4]. As the volume declines, so too does presence of species in an assemblage [5]. CWD and basal area of core aspects of forest structure, and often influence habitat quality and suitability, and therefore avian diversity.
How does time since fire influence habitat suitability and diversity?
Figure 5. Volume of basal area and volume of coarse woody debris as functions of time since fire. Each bullet represents the corresponding data point from an individual island site.
Effects of wildfires on habitat characteristics is well-documented, and the relationship of suitability and succession can be further observed in figure 5. The relationship between time since fire and CWD is apparent; fires create a large volume of downed logs within an island, and as time goes on, decay reduces the presence.
Comparatively in figure 4, as a habitat recovers from a burn, so too does volume of basal area. Growth and colonization of trees across an island increases, and basal area shifts from snag dominance to large living trees [4,5]. How this forest structure shifts ties directly in with avian assemblage structure, and those physical components have a direct influence.
How does forest structure and island biogeography influence diversity?
Structure characteristics are directly linked to successional stage and maturity of a forest, thereby attracting avian colonizers through a range of temporal and spatial factors [4]. The relationship between time since fire and number of species within the taxon is visualized in figure 7. There is a slightly weak negative relationship between assemblage diversity as time elapses since fire. While abundance declines, we can infer that with mature forests come established territories and family units, as well as the attracting of a tighter assemblage of birds that occupy specialized niches [10]. Avian assemblages working solely as a function of time since fire does not reflect the complexity of avian behaviour, and the decline in richness could be accounted for by an extreme variety of factors [6,7].
Figure 8 shows a strong positive relationship between number of species detected and island area. While it does not account for fine-scale topography, it does display a relatively simple relationship. The greater an island sites size was, the greater the diversity of an assemblage. With more habitat available, more territories can be allocated to a wide range of species, both generalist and specialist [5,7]. Size of an island and number of species detected has a fairly positive relationship, and the larger an island is, the more motivated a bird may be to expend energy and travel to it.
Does distance to the mainland have any affect on diversity and dispersal rates?
As birds have evolved with the ability to fly, it can be reasonably assumed that distance from mainland does not particularly impede their ability to disperse from island to island. In figure 12, we see a slightly negative relationship between number of species detected and distance to mainland. However this cannot be expressly relied upon, as the figure also displays a large number of sites clustered near the mainland with fewer samples as distance increases.
Figure 6. Number of avian species detected as a function of island sites' distance from the mainland. Each point represents the corresponding data point from an individual island site.
While we can infer that a site's distance from the mainland has a negative affect on assemblage diversity, we cannot make any conclusive statements regarding efficacy based on the sample size; there are many variables that could account for why less species are detected upon islands further from the mainland.