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.