Life-history traits inform on population trends when assessing the conservation status of a declining tiger shark population
Published on 20. August 2018
Life-history traits inform on population trends when assessing the conservation status of a declining tiger shark population
Christopher Brown, George Roff
ABSTRACT:
The assessment of the conservation status of wide ranging species depends on estimates of the magnitude of their population trends. The accuracy of global trend estimates will depend on how many locations within a species range are sampled and where in the range population size is sampled. We ask how the spatial extent of sampling within a species range interacts with non-linear patterns in long-term trends to affect estimates of decline in the tiger sharks (Galeocerdo cuvier) population on the east coast of Australia. We use Bayesian random effects models to estimate long-term population trends. We compare estimates of trends where we use all data spanning 11.5 degrees of latitude to estimates that used spatial subsets of the data. We also introduce a method to that uses prior information on species life-history to inform on the expected level of population variation. As more regions were included in the model the trend estimates converged towards an overall decline of 65% over three generations. Trends estimated from data only from northern regions underestimated the regional decline, trends estimated from data only from southern regions overestimated the regional decline. When a subset of regions were modelled, rather than the full data-set, the estimated trend depended on the choice of the Bayesian prior for population variation. The prior informed by life-history traits performed well, a prior incorrectly informed by life-history traits of a much slower growing shark species performed poorly. Synthesis and applications: The rate of decline in tiger sharks is consistent with a listing East Coast Australia tiger sharks as endangered under local legislation. Monitoring programs that aim to estimate population trends should attempt to cover the extremes and mid-points of a population’s range. Life-history information can be used to inform priors for population variation and may give more accurate estimates of trends that can be justified in debates about red listing of threatened species, particularly when sampling is limited.
bioRxiv, Preprint, DOI 10.1101/395509