Estimation of population size and trends for highly mobile species with dynamic spatial distributions

Abstract

Aim: To develop a more ecologically realistic approach for estimating the population size of cetaceans and other highly mobile species with dynamic spatial distributions.Location: California Current Ecosystem, USA. Methods:Conventional spatial density models assume a constant relationship between densities and habitat covariates over some time period, typically a survey season. The estimated population size must change whenever total habitat availability changes. For highly mobile long-lived species, however, density-habitat relationships likely adjust more rapidly than population size. We developed an integrated populationredistribution model based on a more ecologically plausible alternative hypothesis: (1) population size is effectively constant over each survey season; (2) if habitat availability changes, then the population redistributes itself following an ideal free distribution process. Thus, the estimated relationship between densities and habitat covariates adjusts rather than population size. We constructed Bayesian hierarchical models corresponding to the conventional and alternative hypotheses and applied them to distance sampling data for Dall's porpoise (Phocoenoides dalli), a highly mobile cetacean with distribution patterns closely tied to cool sea-surface temperatures.Results: The Dall's porpoise data provided strong support for the hypothesis based on an ideal free redistribution process. Our results indicate that the population size of Dall's porpoise within the survey region was relatively stable over each summer/fall survey season, but the distribution expanded and contracted with the extent of suitable habitat. Over multiple survey seasons, the model partitioned variation in observed densities among three sources: variation in population size, the density-habitat relationship and measurement error, leading to lower and more ecologically plausible estimates of interannual variation in population size. Main conclusions:We conclude that the integrated population-redistribution model (IPRM) presented here represents an ecologically plausible model for use in future assessments of the population size and dynamics of cetaceans and other highly mobile long-lived species with variable spatial distributions.

Predicting cetacean abundance and distribution in a changing climate

“…Although Model 1 included a linear SST function (see Figure in Appendix in Supporting Information), which should have captured the increase in the number of striped dolphins in 2014, the overall predictions were too low. This could indicate that the magnitude of the species–environment response for striped dolphins changed during 2014, similar to the pattern identified by Boyd et al () for Dall's porpoise when the amount of suitable habitat within the study area changed substantially. Our study area represents the northern portion of the range of striped dolphin (Mangels & Gerrodette, ; Perrin, Scott, Walker, & Cass, ), and the unusually warm water temperatures in 2014 may have increased the available striped dolphin habitat markedly, changing the nature of the species–environment relationship within our study area.…”

“…This could indicate that the magnitude of the species-environment response for striped dolphins changed during 2014, similar to the pattern identified by Boyd et al (2018) for Dall's porpoise when the amount of suitable habitat within the study area changed substantially. Our study area represents the northern portion of the range of striped dolphin (Mangels & Gerrodette, 1994;Perrin, Scott, Walker, & Cass, 1985), (Table 5).…”

“…The final prediction grids thus account for the varying oceanographic conditions during the 1991–2009 summer/fall SWFSC cetacean surveys and provide a “multiyear average” of predicted daily cetacean species densities. Given that the greatest source of uncertainty in these models is the interannual variability in population density due to movement of animals within or outside of the study area (Barlow et al, ; Boyd et al, ; Ferguson et al, ), this source of uncertainty was used to produce lognormal 90% confidence intervals for the spatial density estimates (Forney et al, ). We clipped the prediction grid to the boundaries of the approximate 1,141,800‐km 2 study area to ensure that predictions were not extrapolated outside the region used for model development, particularly since longitude and latitude were included as potential predictors in one of the three alternative models.…”

“…New periods of climatic and oceanic extremes could result in unexpected interactions, state changes in species-environment relationships, or changes in the intensity of the response (e.g., Boyd et al, 2018), and these could foil predictions in novel situations (Myers, 1998). For example, during the 1982-83 El Niño, the coastal stock of bottlenose dolphins (Tursiops truncatus) expanded its range northward into central California and has remained there ever since Wells et al, 1990).…”

Abundance estimates of six species of Delphinidae cetaceans off the Pacific coast of Japan between 1985 and 2015

“…Boyd et al . () recently developed Integrated Population‐Distribution Model that estimates time‐series animal abundance by combining a population process and distribution process models within the same model. Although the present study provides valuable baseline for conservation and management of delphinids, there is clearly need for more extended approaches.…”