Bio‐energetic modeling of medium‐sized cetaceans shows high sensitivity to disturbance in seasons of low resource supply

Abstract

Understanding the full scope of human impact on wildlife populations requires a framework to assess the population‐level repercussions of nonlethal disturbance. The Population Consequences of Disturbance (PCoD) framework provides such an approach, by linking the effects of disturbance on the behavior and physiology of individuals to their population‐level consequences. Bio‐energetic models have been used as implementations of PCoD, as these integrate the behavioral and physiological state of an individual with the state of the environment, to mediate between disturbance and biological significant changes in vital rates (survival, growth, and reproduction). To assess which levels of disturbance lead to adverse effects on population growth rate requires a bio‐energetic model that covers the complete life cycle of the organism under study. In a density‐independent setting, the expected lifetime reproductive output of a single female can then be used to predict the level of disturbance that leads to population decline. Here, we present such a model for a medium‐sized cetacean, the long‐finned pilot whale ( Globicephala melas ). Disturbance is modeled as a yearly recurrent period of no resource feeding for the pilot whale female and her calf. Short periods of disturbance lead to the pre‐weaned death of the first one or more calves of the young female. Higher disturbance levels also affect survival of calves produced later in the life of the female, in addition to degrading female survival. The level of disturbance that leads to a negative population growth rate strongly depends on the available resources in the environment. This has important repercussion for the timing of disturbance if resource availability fluctuates seasonally. The model predicts that pilot whales can tolerate on average three times longer periods of disturbance in seasons of high resource availability, compared to disturbance happening when resources are low. Although our model is specifically parameterized for pilot whales, it provides useful insights into the general consequences of nonlethal disturbance. If appropriate data on life history and energetics are available, it can be used to provide management advice for specific species or populations.

Methods for Monitoring for the Population Consequences of Disturbance in Marine Mammals: A Review

“…This conclusion is supported by analyses of the potential effects of body condition on vital rates using bioenergetics models. For example, Hin et al (2019) used a dynamic energy budget (DEB) model of the North Atlantic long-finned pilot whale to investigate the potential effects of disturbance on lifetime reproductive success. They found that disturbance could lead to a large reduction in the proportion of calves that survived to weaning.…”

“…Body condition also varies markedly seasonally and during different life history stages. For example, the DEB model developed by Hin et al (2019) indicates that body condition (as estimated by the proportion of body mass available as an energy reserve) of undisturbed lactating pilot whales and their calves may fall to potentially lifethreatening levels during the first year of lactation. It is therefore important, whenever possible, to identify the life history stage of each individual when estimating its body condition.…”

A decision framework to identify populations that are most vulnerable to the population level effects of disturbance

“…Figure 3 shows the pooled distribution of the experts' predictions of the maximum number of days of disturbance that can be tolerated before it has any effect on calf survival for harbor seals, harbor porpoise, and bottlenose dolphins. If data on this tolerance is lacking it may be possible to obtain suitable values using expert elicitation or bio-energetic modeling (e.g., Hin, Harwood, & de Roos, 2019;McHuron, Costa, Schwarz, & Mangel, 2016). Clearly, this threshold will not be appropriate for all taxa: species with low body weights and high basic metabolic rates (such as rodents and songbirds) may only be able to tolerate short periods of disturbance, whereas large poikilotherms may be able tolerate much longer periods.…”

“…Clearly, this threshold will not be appropriate for all taxa: species with low body weights and high basic metabolic rates (such as rodents and songbirds) may only be able to tolerate short periods of disturbance, whereas large poikilotherms may be able tolerate much longer periods. If data on this tolerance is lacking it may be possible to obtain suitable values using expert elicitation or bio-energetic modeling (e.g., Hin, Harwood, & de Roos, 2019;McHuron, Costa, Schwarz, & Mangel, 2016).…”

“…These species rely almost entirely on stored energy reserves accumulated immediately before and during pregnancy to cover the costs of lactation (McHuron et al, 2016;Villegas-Amtmann et al, 2015). For species that rely on an increase in energy intake to cover these costs (income breeders), "lactating" is the life stage that is potentially most sensitive to disturbance (McHuron et al, 2016), particularly in situations where food resources are limited (Hin et al, 2019;Pirotta, Schwarz, Costa, Robinson, & New, 2019).…”