Geographic range size, water temperature and extrinsic threats predict the extinction risk in global cetaceans

Despite that cetaceans provide significant ecological contributions to the health and stability of aquatic ecosystems, they are highly endangered with nearly one-third of species assessed as threatened with extinction. Nevertheless, to date, few studies have explicitly examined the patterns and processes of extinction risk and threats for this taxon, and even less between the two subclades (Mysticeti and Odontoceti). To fill this gap, we compiled a dataset of six intrinsic traits (active region, geographic range size, body weight, diving depth, school size and reproductive cycle), six environmental factors relating to sea surface temperature and chlorophyll concentration, and two human-related threat indices that are commonly recognized for cetaceans. We then employed phylogenetic generalized least square (PGLS) models and model selection to identify the key predictors of extinction risk in all cetaceans, as well as in the two subclades. We found that geographic range size, sea surface temperature and human threat index were the most important predictors of extinction risk in all cetaceans and in odontocetes. Interestingly, maximum body weight was positively associated with the extinction risk in mysticetes, but negatively related to that for odontocetes. By linking seven major threat types to extinction risk, we further revealed that fisheries bycatch was the most common threat, yet the impacts of certain threats could be overestimated when considering all species rather than just threatened ones. Overall, we suggest that conservation efforts should focus on small-ranged cetaceans and species living in warmer waters or under strong anthropogenic pressures. Moreover, further studies should consider the extinction risk of species when superimposing risk maps and quantifying risk severity. Finally, we emphasize that mysticetes and odontocetes should be conserved with different strategies, because their extinction risk patterns and major threat types are considerably different. For instance, large-bodied mysticetes and small-ranged odontocetes require special conservation priority.

尽管鲸目动物（Cetacea）对水生生态系统的健康与稳定具有重要的生态贡献，但它们正面临极高的灭绝风险：目前已有近三分之一的评估物种被列为受威胁物种。然而迄今为止，鲜有研究明确探讨该类群的灭绝风险模式、过程及其面临的威胁，且针对两个亚支——须鲸亚目（Mysticeti）与齿鲸亚目（Odontoceti）之间的对比研究更为匮乏。为填补这一研究空白，本研究整合了一套数据集，包含6项内在性状（活动区域、地理分布范围、体重、潜水深度、集群规模与繁殖周期）、6项与海表温度及叶绿素浓度相关的环境因子，以及2项鲸类公认的人为威胁指数。随后，本研究采用系统发育广义最小二乘（PGLS）模型与模型筛选方法，分别识别全鲸类以及两个亚支的灭绝风险关键预测因子。研究结果显示，地理分布范围、海表温度与人为威胁指数是全鲸类及齿鲸类灭绝风险的最重要预测因子。值得注意的是，须鲸亚目类群的最大体重与灭绝风险呈正相关，而齿鲸亚目类群的最大体重则与灭绝风险呈负相关。通过将7种主要威胁类型与灭绝风险进行关联分析，本研究进一步发现，渔业兼捕是最为普遍的威胁类型，但在纳入所有鲸类物种而非仅受威胁物种进行分析时，部分威胁的影响可能被高估。综上，本研究建议保护工作应重点关注分布范围狭小的鲸类、栖息于暖水环境或承受较强人为压力的鲸类物种。此外，后续研究在叠加风险图谱与量化风险严重程度时，应充分考虑物种的灭绝风险特征。最后，本研究强调须鲸亚目与齿鲸亚目应采取差异化的保护策略，因为二者的灭绝风险模式与主要威胁类型存在显著差异。例如，大体型须鲸与分布范围狭小的齿鲸需要获得优先保护。