Dichotomous terms to reference scale-dependent habitat selection
Many studies dichotomize habitat selection into “local” or “landscape” level effects, with little explanation regarding what each represents. Ambiguous use of these terms across studies may confound observations of selection operating across different spatiotemporal resolutions and impede synthesis of scale-dependent habitat selection. I examined the consistency of the use of the terms “local” and “landscape” level habitat selection to evaluate potential implications for conservation science. I reviewed 136 multi-level studies observing local and/or landscape effects. From each study, I identified which of Johnson’s selection orders were observed at the local and landscape scale. I assessed the consistency in the selection order observed at each scale. I found local and landscape scales inconsistent across studies, making comparisons and synthesis difficult. This inconsistency also makes the terms “local” and “landscape” meaningless, which may lead to misidentification of limiting factor(s) most important for conservation efforts. I suggest dichotomous terms no longer be used in reference to levels of selection, but in reference to specific explanatory variables whose characteristic scale(s) best fit the appropriate definition.
Heisler, L.M., R.G. Poulin, and C.M. Somers. 2017. Stop using dichotomous terms to reference levels of scale-dependent habitat selection in ecology. Landscape Ecology: Accepted.
Use of owl pellets for landscape-level assessments of small mammal communities
Small mammal community composition is almost universally estimated from conventional trapping, which is logistically difficult to scale up for landscape-level assessments. Owl pellets may be a more effective alternative for measuring small mammal community composition over large geographic areas due to the relative ease and low cost of field collections. However, owl pellets may introduce sampling biases that differ from those associated with conventional trapping. A thorough comparison to conventional traps is required before owl pellets can be widely adopted as an alternative research tool for small mammal studies. We conducted a literature review of owl diet-prey availability studies to: (1) compare small mammal community composition between owl pellets and trapping when the two methods were used simultaneously, and (2) assess the influence of owl genus and habitat type on community composition estimated by these two methods. We used data from 27 published studies, which allowed for 32 comparisons between owl pellets and trapping conducted simultaneously. These studies included 15 owl species from five common genera from different major habitats. Small mammal community composition estimated via pellets was better represented compared to estimates from conventional trapping. Composition metrics from the two methods were consistent and not affected by owl genera or habitat type. Thus, owls are an effective alternative for landscape-level assessments of small mammal communities.
Heisler, L.M., C.M. Somers, and R.G. Poulin. 2014. Owl pellets: an effective alternative to conventional trapping for broad-scale studies of small mammal communities. Methods in Ecology and Evolution 76: 96-103. DOI: 10.1111/2041-210X.12454
Landscape-level environmental factors affecting rodent species distributions across the Canadian prairies
The Canadian prairies are a heterogeneous region characterized by landscapes varying in climate, soil, and land use that likely affect small mammal species distributions. The expense and time involved in trapping small mammals across such a large study area makes landscape-level studies impossible. Previous studies have used prey remains in owl pellets to estimate small mammal community composition across broad landscapes. There are 2 owl species that are widespread across the Great Plains: burrowing owls and great horned owls. Both are dietary generalists that consume a variety of small mammal species. Their pellets provide an excellent opportunity to examine landscape-level environmental factors influencing small mammal community composition across the Canadian prairies.
My M.Sc. research used the owl pellet catalog at the Royal Saskatchewan Museum to estimate the relative importance of climate, soil, and land use as determinants of small mammal community composition. I found soil texture was the primary landscape feature driving small mammal composition, whereas agricultural cropland significantly altered the composition of these assemblages (Heisler et al. 2013). I also identified population responses to seasonal weather variation among deer mice, meadow voles and sagebrush voles on the Canadian prairies. I found all three species responded to seasonal weather variation. However, deer mice and sagebrush voles showed much weaker responses, indicating factors other than weather variation determined their abundance. In contrast, meadow voles showed a 5-fold increase in abundance following winters of persistent, deep snow cover (>=20cm depth). This indicates irruptions of this species on the Canadian prairies was at least partially weather dependent (Heisler et al. 2014).
Heisler, L.M., C.M. Somers, T.I. Wellicome, and R.G. Poulin. 2014. Rodent populations on the northern Great Plains respond to weather variation at a landscape scale. Journal of
Mammalogy 95: 82-90. DOI: http://dx.doi.org/10.1644/13-MAMM-A-115.1
Heisler, L.M., C.M. Somers, T.I. Wellicome, R.G. Poulin. 2013. Landscape-scale features affecting small mammal assemblages on the northern Great Plains of North America. Journal of Mammalogy 94: 1059-1067. DOI: http://dx.doi.org/10.1644/13-MAMM-A-022.1
My Ph.D. research will continue using the Royal Saskatchewan Museum’s owl pellet catalog to identify the impacts of agriculture on rodent diversity. Conversion of grassland habitat for agricultural land use (i.e., annual cropland, tame forage) resulted in the direct loss of grassland habitat from the region, as well as the increased isolation of remaining grassland patches from each other. Studies of the impact agricultural conversion has had on species distributions has focused on charismatic prairies species, such as the burrowing owl, swift fox, and sage grouse. My current research will focus on the impacts agricultural conversion has had on rodent diversity. Rodents are small bodied mammals with low dispersal capabilities. These characteristics may impede their ability to cope with the high amounts of grassland loss and isolation of remaining grassland patches in some landscapes of the Canadian prairies. However, rodents are also characterized by low space requirements and high reproductive capacity, which may facilitate population persistence within the remaining small grassland patches. My current research will shed much needed insight into how agricultural conversion has affected the distributions of grassland rodents on the Canadian prairies.