Declining home range area predicts reduced late-life survival in two wild ungulate populations.

Hannah Froy,Michael J Crawley,Daniel H Nussey,Sean Morris,Alison Morris,Luca Börger,Jill G Pilkington,Tim H Clutton‐Brock,Charlotte E Regan,Josephine M Pemberton

doi:10.1111/ele.12965

Abstract

Demographic senescence is increasingly recognised as an important force shaping the dynamics of wild vertebrate populations. However, our understanding of the processes that underpin these declines in survival and fertility in old age remains limited. Evidence for age-related changes in foraging behaviour and habitat use is emerging from wild vertebrate studies, but the extent to which these are driven by within-individual changes, and the consequences for fitness, remain unclear. Using longitudinal census observations collected over four decades from two long-term individual-based studies of unmanaged ungulates, we demonstrate consistent within-individual declines in home range area with age in adult females. In both systems, we found that within-individual decreases in home range area were associated with increased risk of mortality the following year. Our results provide the first evidence from the wild that age-related changes in space use are predictive of adult mortality.

Highlights

Demographic senescence, the age-related decline in survival probability and fertility, is widely observed in nature and increasingly acknowledged as an important force shaping the ecological and evolutionary dynamics of wild vertebrate populations (Nussey et al 2013; Bouwhuis & Vedder 2017; Lema^ıtre & Gaillard 2017)
This selective disappearance was responsible for the increase in home range area observed at the oldest ages in the raw data means for deer (Fig. 1a; Fig. S5); there was no evidence for a quadratic association with age (v21 = 2.41, P = 0.121)
The slope of the age-related increase in home range quality predicted by the model was shallower than seen in the raw data (Fig. 1c), since the model accounted for the negative association between home range quality and home range area, which increased with age (Table 1c)

Summary

Introduction

Demographic senescence, the age-related decline in survival probability and fertility, is widely observed in nature and increasingly acknowledged as an important force shaping the ecological and evolutionary dynamics of wild vertebrate populations (Nussey et al 2013; Bouwhuis & Vedder 2017; Lema^ıtre & Gaillard 2017). Our understanding of the processes responsible for senescent declines in natural populations remains limited (Nussey et al 2013) Ecological traits such as foraging behaviour, habitat selection and space use represent potentially important proximate drivers of demographic variation because of their association with resource acquisition (Stephens et al 2007; Gaillard et al 2010). Evidence directly linking such age-related changes in behavioural traits to reduced adult survival or fecundity are currently lacking in wild systems

Methods

Results

Conclusion