Abstract
Individual variation in disturbance vulnerability (i.e. the likelihood that disturbance negatively affects an individual's fitness) can affect how disturbance impacts animal populations, as even at low disturbance levels some individuals could be severely affected and die. Individual variation in vulnerability can arise due to different responses to disturbance. We propose a new hypothesis that even when individuals respond similarly to disturbance, time‐limited individuals are more at risk that their condition deteriorates since they have limited ability to extend their foraging time to compensate for disturbance. We investigate this ‘time‐limitation hypothesis' both empirically and mathematically by studying how individuals that differ in their average foraging time (presumably due to differences in foraging efficiency) are affected by disturbance. We used tracking data of 22 wintering Eurasian oystercatchers Haematopus ostralegus to compare time budgets between disturbed and undisturbed tidal periods. In three tidal periods with severe disturbance by transport airplanes, more than a third of the variation in additional flight time and foraging time loss was explained by individual differences. Inefficient individuals that foraged longer in undisturbed tidal periods experienced higher costs in disturbed tidal periods, since they lost more foraging time. We next used an analytical energy balance model to study how time‐limited individuals differed in their maximum disturbance thresholds. Both our theoretical model and empirical study suggest that inefficient individuals in a time‐limited environment may be unable to increase their foraging time to compensate for the effects of disturbance. Consequently, the magnitude of individual variation in time budgets strongly determines what proportion of the population is at risk that their condition deteriorates due to disturbance. Our hypothesis implies that, when assessing disturbance effects on a population, it is not only important to consider individual variation in disturbance responses, but also variation in time budgets that determine the consequences of disturbance.
Highlights
The increasing number of human activities in nature is of concern for policy makers, since the resulting disturbance can heavily affect wildlife populations (Pirotta et al 2018)
We propose a second pathway via which individual variation can affect how vulnerable individuals are to disturbance: differences in their ability to compensate for disturbance, for example by increasing foraging time (Urfi et al 1996, Blanc et al 2006, Houston et al 2012), may cause individual variation in disturbance vulnerability, even when disturbance responses are similar
Mathematical modelling of disturbance costs as a function of foraging time Using our analytical model parameterized with field and literature data from our model species, we explored disturbance thresholds, energetic costs and potential population consequences of disturbance for varying amounts of individual variation in time budgets
Summary
The increasing number of human activities in nature is of concern for policy makers, since the resulting disturbance can heavily affect wildlife populations (Pirotta et al 2018). Human disturbances can elicit flight responses in wildlife (Stankowich 2008), which are often energetically costly and may have additional costs such as a loss of foraging time (Houston et al 2012) or reduced intake rate (Rutten et al 2010). Individual variation in disturbance vulnerability (i.e. the likelihood that disturbance will reduce an individual’s fitness) can alter the effects of disturbance on wildlife populations. In populations where individuals vary in their disturbance vulnerability, the most vulnerable individuals are expected to suffer higher energetic losses when disturbance occurs and their condition may be affected even under low levels of disturbance (RodríguezPrieto et al 2010, Griffin et al 2017). Individual consistency in disturbance responses (often measured as the consistency in flight initiation distances) is observed, but the extent varies strongly among species (Thiel et al 2007, Ellenberg et al 2009, Carrete and Tella 2013, DeVault et al 2017). Individual variation in disturbance responses can be caused by personality (Ellenberg et al 2009) and sex (Thiel et al 2007, Ellenberg et al 2009), but can arise due to habituation (Runyan and Blumstein 2004, Ellenberg et al 2009, Rodríguez-Prieto et al 2010)
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