The path of most resilience

Human activities are changing ecosystems around the world. Some species take these environmental changes in stride and are capable of surviving, or even thriving, in an altered landscape. However, other species are more sensitive, and decline or disappear. This variation in resilience to environmental change is a puzzle for scientists. Why there is such variation among species?

There are two possible paths to resilience in the face of environmental change. The first is for individuals to acclimatize, and alter their physiology so that they are better able to handle the new environment. The other path to resilience is through adaptation of the population, where only individuals that are suited to the altered environment survive and reproduce. So how do hardy species manage? Do they acclimatize or adapt?

One of the biggest current threats to marine ecosystem is ocean acidification. With increasing human consumption of fossil fuels, atmospheric CO₂ is rising. Atmospheric CO₂ then dissolves into ocean waters and increases the acidity. An international team of researchers led by Piero Calosi from Plymouth University in the UK collaborated to investigate how species develop resilience to high CO₂. To conduct this research, the scientists took advantage of natural shallow CO₂ vents. While most species cannot survive if the CO₂ levels are too high, these natural ocean vents create localized areas of high CO₂, and are surrounded by a collection of species that thrive in the acidic conditions.

The researchers identified species of polychaete marine worms that live near natural CO₂ vents and are tolerant of elevated CO₂. The scientists also identified species of polychaetes that are sensitive to CO₂, and are never found in high CO₂ areas. The team then conducted two experiments. First, to determine how the tolerant and sensitive species differ in their responses to high CO₂, Calosi and colleagues collected both tolerant and sensitive polychaete species in low CO₂ areas. They transplanted these individuals near natural CO₂ vents, where CO₂ is high. Second, to determine whether tolerant species are acclimatizing or adapting to high CO₂, the researchers took tolerant species from natural vents and transplanted them to low CO₂ areas. In both experiments, the researchers measured metabolic rates of the polychaetes in their natural environment and in the transplanted environment. Metabolic rate is related to many crucial physiological processes, and gives insight into the scope of an organism for growth and reproduction.

The researchers found that tolerant species easily handled transplantation from low CO₂ to high CO₂ areas. However, the sensitive species responded dramatically. When transplanted near CO₂ vents, two of the sensitive species drastically lowered their metabolic rates, while individuals from the third sensitive species drastically increased their metabolic rates. The results were even more interesting when tolerant species were transplanted from natural CO₂ vents to more favourable low CO₂ areas. In one species, individuals adjusted their metabolic rates only slightly. In the second tolerant species, individuals increased their metabolic rates considerably when transplanted to low CO₂ areas, suggesting that there has been adaptation to high CO₂ in this species. Genetic analysis confirmed that this species could be divided into distinct groups originating from different high and low CO₂ regions.

These results have implications for both understanding extinctions and predicting species resilience in the face of future environmental change. Short-term acclimatization is a viable strategy, until long-term adaptation of the population can occur. There are indeed two paths to resilience, and on an evolutionary time scale, organisms need to exploit both of these paths if they are to survive and thrive in changing environments.