Abstract
AbstractVariation in adult sex ratio (ASR) affects population demography and dynamics of large mammals. The mechanisms behind this variation are largely unclear, but may be partly related to climatic drivers and density dependence operating differently on the adult male and female segments of the population. Here, we examine such drivers of annual changes in ASR in the predator‐free wild Svalbard reindeer (Rangifer tarandus platyrhynchus), a high Arctic subspecies whose population dynamics are shaped by climate. Using up to 35 year long time‐series of population count data from two populations, we disentangle drivers of fluctuations in ASR by first analyzing how climate and density dependence affect sex‐specific adult population growth rates through effects on mortality. There were a positive population size trend and an overall female bias in ASR throughout the study period. Increased winter precipitation, a proxy for the harshness of winter feeding conditions, was found to significantly reduce adult population growth rates through reduced survival in males, but not in females. However, increased population size tended to cause a stronger immediate decline in female than in male adult population growth rates. As a consequence, the female bias in ASR increased with harsher winter conditions and declined with higher population size. As expected from the increased frequency of rainy and icy winters due to climate warming, a recent trend toward increased female bias in ASR was evident. This demonstrates that climatic drivers of both short‐term fluctuations and long‐term trends in demography need to be accounted for in the management and population dynamic predictions of Arctic ungulates.
Highlights
Understanding how adult sex ratio (ASR) is influenced by population size and environmental variability, such as climate change, is of relevance for the conservation, management, and harvest quota setting of vulnerable populations, and in particular small populations that are sensitive to demographic stochasticity (Lee et al 2011)
By separating long-term population monitoring data into sex-specific components, we have shown that winter harshness and direct density dependence influence the adult male and female segments of Svalbard reindeer populations differently, causing changes in ASR
The female bias in Svalbard reindeer ASR increased with winter severity, but tended to decline with population size (Fig. 4)
Summary
Alterations in the adult sex ratio (ASR) are likely to influence population characteristics such as mean parturition date (Mysterud et al 2002, Holand et al 2003), female fecundity (Solberg et al 2002), offspring sex ratio (Sæther et al 2004), sexual size dimorphism (Garel et al 2006), timing of the rut, and mean age of males holding harems (Clutton-Brock et al 1997).❖ www.esajournals.orgFebruary 2017 ❖ Volume 8(2) ❖ Article e01699 PEETERS ET AL.Variation in ASR may influence population growth rate (Milner et al 2007, Lee et al 2011, Schmidt and Gorn 2013). The observed skew in ASR results mainly from a higher average mortality in the rarer sex Both juvenile and adult mortality rates are typically higher for male than for female ungulates due to higher energy requirements and, higher susceptibility to food shortage (Clutton-Brock et al 1985, Toigo and Gaillard 2003). Survival of the two sexes can be high under good environmental conditions (Cobben et al 2009), it is typically more variable in males than in females (Toigo and Gaillard 2003) Extrinsic factors such as predation (Berger and Gompper 1999) can induce sex-biased mortality rates
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