You are where you eat.
Submitted by editor on 18 May 2015.
Get the paper!Central place foragers make consistent returns to a central place between foraging bouts, generally a den or a nest. Territorial animals often aggressively defend their territories and as such regularly monitor the boundaries of their home range. Home range development for both of these groups has been modelled using mechanistic models based on rules for movement or resource accumulation strategies. Mechanisms underlying the development of home ranges for foragers that move without a central place and maintain multimodal home ranges remain less well understood. Published in Oikos in 2009, van Moorter et al.[i] proposed a mechanistic home range model for these generic foragers, simulating movements and subsequent home ranges based upon optimal foraging theory and a two-part memory system. Although a promising model for free-ranging ungulates and other forager species, van Moorter et al.’s model has never been evaluated empirically.
Enter the new study by Dana Paige Seidel and Mark Boyce out of the University of Alberta: “Varied tastes: home-range implications of foraging patch selection.” Seidel and Boyce explore the foundational assumptions of van Moorter et al.’s model in a study of two populations of elk (Cervus elaphus) in southwestern Alberta.
Adult cow elk fitted with a GPS radiocollar. Photo by Mark S. Boyce.
van Moorter et al.’s model asserts that foragers are attracted to and remember distinct foraging patches and that these patches have a higher value or utility than the surrounding matrix. To test these assumptions, Seidel followed 12 radiocollared elk and identified foraging and nonforaging use sites, “patches” and “non-patches” respectively. Field sampling for forage biomass and species composition along with remotely sensed covariates at each site were used in a matched-pairs design to compare patches used for foraging and alternative use sites outside these patches but within individuals’ home ranges. Although differences in habitat selection were found across herds, sexes, and seasons, in general, elk selected patches with higher forage biomass and forest cover, at higher slopes, and at sites with lower vehicular traffic. To investigate the role of foraging patches in the structure of home ranges, the unique distribution of foraging patches from each elk was compared to their utilization distribution built from all GPS relocations of the elk over the summer season (example shown below Fig 1). Most maps revealed multimodal use patterns with several hot spots distributed within the boundaries of the home range. Importantly, the density of identified patches was strongly correlated with the underlying patterns of use over the season. This correlation between density of patches and density of overall use suggests that the location of successful foraging sites may be a critical factor determining the ultimate size and structure of elk home ranges, just as conceptualized by the original model.
Fig. 1 Patches overlaid on utilization distribution of elk E159.
“Despite individual variation, this visualization of patches across existing home ranges is an important presentation of support for the use patterns and movements
predicted by Van Moorter et al. (2009).”
This study offers early empirical support for van Moorter et al.’s model of underlying mechanisms to forager home range development. Additionally, the results illuminate the complexity of variables influencing patch value and selection by elk including the influence of human disturbance, sex, habitat availability, and season. Further investigation of the mechanisms underlying animal space use and movement patterns may prove useful for predicting range shifts of foraging species in rapidly changing environments.
The authors, through Dana Paige Seidel
[i] Van Moorter, B. et al. 2009. Memory keeps you at home: a mechanistic model for home range emergence. - Oikos 118: 641-652.
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