Abstract
Defining demographic and ecological threshold of population persistence can assist in informing conservation management. We undertook such analyses for the Indo-Pacific humpback dolphin (Sousa chinensis) in the Pearl River Delta (PRD) region, southeast China. We use adult survival estimates for assessments of population status and annual rate of change. Our estimates indicate that, given a stationary population structure and minimal risk scenario, ~2000 individuals (minimum viable population in carrying capacity, MVPk) can maintain the population persistence across 40 generations. However, under the current population trend (~2.5% decline/annum), the population is fast approaching its viability threshold and may soon face effects of demographic stochasticity. The population demographic trajectory and the minimum area of critical habitat (MACH) that could prevent stochastic extinction are both highly sensitive to fluctuations in adult survival. For a hypothetical stationary population, MACH should approximate 3000-km2. However, this estimate increases four-fold with a 5% increase of adult mortality and exceeds the size of PRD when calculated for the current population status. On the other hand, cumulatively all current MPAs within PRD fail to secure the minimum habitat requirement to accommodate sufficiently viable population size. Our findings indicate that the PRD population is deemed to become extinct unless effective conservation measures can rapidly reverse the current population trend.
Highlights
Under Criterion A of the IUCN Red List Categories and Criteria Version 3.1 1, the population status, either NT (Near Threatened), VU (Vulnerable), EN (Endangered) or CR (Critically Endangered), is classified by the percentage of decline within three generations[1] which comes from relevant trend analyses[6] and has been applied across variety of species[7,8,9,10,11,12]
The threshold values of the rate of population change representing decline of 30%, 50% and 80% within three generations, which corresponds to the status change from NT to VU, VU to EN, and EN to CR under the IUCN Criterion A3b1 were −0.0058, −0.0113 and −0.0263, respectively (Table 1)
Applying this equation to the recent r estimate (r =−0.0249)[8], the mean survival rate (Sa) of the PRD humpback dolphins was estimated at 0.930, which is close to the threshold value that corresponds to the status classification as CR (Fig. 2)
Summary
Under Criterion A of the IUCN Red List Categories and Criteria Version 3.1 1, the population status, either NT (Near Threatened), VU (Vulnerable), EN (Endangered) or CR (Critically Endangered), is classified by the percentage of decline within three generations[1] which comes from relevant trend analyses[6] and has been applied across variety of species[7,8,9,10,11,12]. Indo-Pacific humpback dolphins (Sousa chinensis), locally in China and Taiwan known as Chinese White Dolphins, inhabits shallow coastal waters of the eastern Indian Ocean and western Pacific[27,28] Their preferred inshore habitats are often in close proximity to areas of intense human activities, which exposes them to various anthropogenic impacts such as coastal fisheries, urban and industrial developments, pollution and disposal of hazardous materials, alteration of coastlines and various degrees of habitat degradation[29,30,31,32]; all of which have been suggested to lead to population decline[4,5,29,33,34,35]. This classification, has been challenged[5] and, following a recent taxonomic revision of the genus Sousa[28], there are numerous indications that all currently recognised species of humpback dolphins may have long been under far greater threat throughout their range than it was previously recognized[5,37,38,39]
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