Our world is currently in the midst of a mass extinction, with species declining and disappearing at alarming rates. However, not all are faring so poorly. While many populations and species are disappearing, others are maintaining their numbers, or even becoming more abundant, and a critical question is why: why are some species and populations more vulnerable to decline and extinction than others?
Many researchers have identified that body size predicts vulnerability to population decline and species extinction. Populations and species with larger individuals are more likely to decline and go extinct than those with smaller individuals. However, this relationship between body size and population decline and extinction is probably because larger animals tend to have other life-history traits – like lower rates of reproduction, slower growth rates and delayed sexual maturity – which make them less able to recover when the mortality rate increases. There is a great deal of variation in life-history traits, even within animals of a given body size. Thus, although body size may provide a reliable rule-of-thumb for predicting vulnerability to decline and extinction, it is only a rough measure.
Maria José Juan-Jordá, from the Universidade da Coruña, Spain, and Simon Fraser University, Canada, with collaborators from both universities, as well as the European Commission Joint Research Center, Italy, decided to tackle the challenge of identifying whether other life-history traits, such as growth rate, can better predict vulnerability to decline and extinction than body size alone. The researchers focused on the Scombrid fishes, which are the tunas, mackerels and bonitos. Because tunas and their relatives are important commercial food fishes, most species have faced high mortality as a result of fishing pressure at some point. Life-history data are also available for most fishes in this family, so the researchers were able to select species with known life-history traits and only included species that have declined when faced with fishing pressure. The team was then able to control for fishing pressure, and use general and logistic linear models to determine whether traits related to body size or traits related to the speed of life (i.e. growth rate, age-at-maturity, longevity) better predicted the extent of the species’ decline, and the probability of overfishing.
Analysing the results of their calculations, Juan-Jordá and colleagues found that the models that included both growth rate and fishing pressure best predicted the extent of a species’ decline and whether a species was considered overfished; models that included only fishing pressure were less reliable. The models that included body size and fishing pressure were the least able to accurately predict the extent of species decline and overfishing. Thus, the researchers demonstrated that life-history traits related to the speed of life, such as growth rate, are better at predicting whether a species will decline and become overfished than body size.
The researchers also identified that tunas, mackerels and bonitos from higher latitudes have declined more dramatically than fish from lower latitudes. For fish, water temperature is tightly linked to growth rate, with fish growing and maturing more slowly in cold water. Thus, this study both identifies a global pattern in fisheries population declines and offers an underlying explanation. Across broad global scales, the slower-growing fish at higher latitudes decline faster when faced with fishing pressure.