Merrill, Evelyn

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  • Data package
    Data from: Study "Ya Ha Tinda elk project, Banff National Park, 2001-2020 (females)"
    (2020-07-24) Hebblewhite, Mark; Merrill, Evelyn; Martin, Hans; Berg, Jodi E.; Bohm, Holger; Eggeman, Scott L.
    (1) Migratory animals are predicted to enhance lifetime fitness by obtaining higher quality forage and/or reducing predation risk compared to non‐migratory conspecifics. Despite evidence for behavioural flexibility in other taxa, previous research on large mammals has often assumed that migratory behaviour is a fixed behavioural trait. (2) Migratory behaviour may be plastic for many species, although few studies have tested for individual‐level flexibility using long‐term monitoring of marked individuals, especially in large mammals such as ungulates. (3) We tested variability in individual migratory behaviour using a 10‐year telemetry data set of 223 adult female elk (Cervus elaphus) in the partially migratory Ya Ha Tinda population in Alberta, Canada. (4) We used net squared displacement (NSD) to classify migratory strategy for each individual elk‐year. Individuals switched between migrant and resident strategies at a mean rate of 15% per year, and migrants were more likely to switch than residents. We then tested how extrinsic (climate, elk/wolf abundance) and intrinsic (age) factors affected the probability of migrating, and, secondly, the decision to switch between migratory strategies. (5) Over 630 individual elk‐years, the probability of an individual elk migrating increased following a severe winter, in years of higher wolf abundance, and with increasing age. (6) At an individual elk level, we observed 148 switching events of 430 possible transitions in elk monitored at least 2 years. We found switching was density‐dependent, where migrants switched to a resident strategy at low elk abundance, but residents switched more to a migrant strategy at high elk abundance. Precipitation during the previous summer had a weak carryover effect, with migrants switching slightly more following wetter summers, whereas residents showed the opposite pattern. Older migrant elk rarely switched, whereas resident elk switched more frequently to migrate at older ages. (7) Our results show migratory behaviour in ungulates is an individually variable trait that can respond to intrinsic, environmental and density‐dependent forces. Different strategies had opposing responses to density‐dependent and intrinsic drivers, providing a stabilizing mechanism for the maintenance of partial migration and demographic fitness in this population.
  • Data package
    Data from: A multi-scale test of the forage maturation hypothesis in a partially migratory ungulate population
    (2016-01-29) Hebblewhite, Mark; Merrill, Evelyn
    The forage maturation hypothesis (FMH) proposes that ungulate migration is driven by selection for high forage quality. Because quality declines with plant maturation, but intake declines at low biomass, ungulates are predicted to select for intermediate forage biomass to maximize energy intake by following phenological gradients during the growing season. We tested the FMH in the Canadian Rocky Mountains by comparing forage availability and selection by both migrant and nonmigratory resident elk (Cervus elaphus) during three growing seasons from 2002–2004. First, we confirmed that the expected trade-off between forage quality and quantity occurred across vegetation communities. Next, we modeled forage biomass and phenology during the growing season by combining ground and remote-sensing approaches. The growing season started 2.2 days earlier every 1 km east of the continental divide, was delayed by 50 days for every 1000-m increase in elevation, and occurred 8 days earlier on south aspects. Migrant and resident selection for forage biomass was then compared across three spatial scales (across the study area, within summer home ranges, and along movement paths) using VHF and GPS telemetry locations from 119 female elk. Migrant home ranges occurred closer to the continental divide in areas of higher topographical diversity, resulting in migrants consistently selecting for intermediate biomass at the two largest scales, but not at the finest scale along movement paths. In contrast, residents selected maximum forage biomass across all spatial scales. To evaluate the consequences of selection, we compared exposure at telemetry locations of migrant and resident elk to expected forage biomass and digestibility. The expected digestibility for migrant elk in summer was 6.5% higher than for residents, which was corroborated with higher fecal nitrogen levels for migrants. The observed differences in digestibility should increase migrant elk body mass, pregnancy rates, and adult and calf survival rates. Whether bottom-up effects of improved forage quality are realized will ultimately depend on trade-offs between forage and predation. Nevertheless, this study provides comprehensive evidence that montane ungulate migration leads to greater access to higher-quality forage relative to nonmigratory congeners, as predicted by the forage maturation hypothesis, resulting primarily from large-scale selection patterns.