Abstract
Partial migration is a common movement phenomenon in ungulates, wherein part of the population remains resident while another portion of the population transitions to spatially or ecologically distinct seasonal ranges. Although widely documented, the causes of variation in movement strategies and their potential demographic consequences are not well understood. Here, we used GPS telemetry data and individual-based photographic surveys to describe evidence for the partial migration of giraffe (Giraffa camelopardalis) in the tropical savanna habitat of Murchison Falls National Park, Uganda. Seasonal movements in giraffe have been described but have not been systematically investigated within the framework of partial migration. We characterized movement behaviors of eight female GPS tracked giraffe across one full year using a model-driven approach of net-squared displacement metrics. To further evaluate these space use patterns at the population-level, we used closed robust design multi-state capture recapture models derived from individually based photographic surveys collected seasonally over three years. We also characterized environmental conditions associated with seasonal space use by conducting ground-based vegetation surveys and analyzing remotely sensed phenology data. Our results from both individually based telemetry models and population-level multi-state models suggest intra-population variation in seasonal space use strategies with three dominant movement classes: 1) Residents in deciduous savanna characterized by Acacia sieberiana, Acacia senegal, Harrisonia abyssinica and Crateva adansonii in the far western end of the park. 2) Residents in the broadleaf savannas characterized by Pseudocedrela kotschyi, Stereospermum kunthianum, Termalia spp. and Combretum spp.in the central sector of the park 3) Male-biased migrants that transitioned seasonally between the acacia savanna in the wet seasons and the broadleaf savanna in the dry seasons. Our results offer insights into how giraffe navigate spatiotemporally dynamic environments at both individual and population levels, providing ecological mechanisms for the emergent population dynamics of these large-bodied topical browsers.
Highlights
Understanding the ecological interactions that influence an organism’s movement decisions and the subsequent fitness consequences of movement remains a major theme in ecology
The average annual rainfall for Murchison Falls National Park (MFNP) ranges from ∼1,100–1,500 mm and is bimodally distributed with the short rains occurring from midMarch to June and August to December, and with the long dry season occurring from late-December to mid-March (Fuda et al, 2016).The current natural distribution of giraffe is limited to the northern portion of the park
We restricted our study to the western half of this area, comprised largely of the Delta and Wankwar sectors since small rivers in the central area of the park limit the potential for giraffe movement across this east/west gradient and our own mark-recapture data suggest that the majority of giraffe occur in these two sectors with little interchange with giraffe further east (Figure 1)
Summary
Understanding the ecological interactions that influence an organism’s movement decisions and the subsequent fitness consequences of movement remains a major theme in ecology. Identified in a diverse suite of taxa (Chapman et al, 2011; Ohms et al, 2019) and observed on a wide range of spatial scales, from several kilometers (Mysterud, 1999; Gaidet and Lecomte, 2013) to hemispheres (Shaffer et al, 2006), partial migration, because of the inherent variation of movement strategies with a population, provides a useful process to evaluate the causes of intraspecific variation in movement behaviors and the fitness consequences of different space-use strategies (Chapman et al, 2011) These variations in movement may have a genetic basis in some systems (Berthold and Helbig, 1992; Bensch et al, 2011; Hess et al, 2016) researchers are increasingly identifying scenarios in which these alternative movement strategies are conditional on the state of an individual and may be plastic over the life of an individual (Sutherland, 1998; Found and St. Clair, 2017). Studies examining conditional migration have suggested that these movement behaviors may be contingent upon asymmetries in sex or social dominance, or the ability of individuals to assess resource conditions and respond to social cues (Chapman et al, 2011)
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