Portrait of a Dungeness crab (Cancer magister). Credit: Kevin Cole
Earth’s increasing temperature is affecting ecosystems from the depths of the sea to the tippy top of Mt. Everest. In two unrelated studies published this week, researchers in Michigan and Toronto have now linked climate change to crashing crab populations and smaller, long-winged birds.
Scientists at the University of Michigan analyzed data from 129 bird species: 52 migratory species breeding in North America and 77 South American resident species in a study published in PNAS. The 86,131 specimens were actually collected as part of two unrelated studies in Chicago and the Amazon. The large and complementary datasets provided a unique opportunity for Michigan researchers to test whether body size and generation length shaped the birds' responses to rapid environmental change.
Data on the migrating birds in Chicago showed the birds were getting smaller and longer-winged over time. The non-migrative birds in the Amazon displayed the same widespread declines in body size with concurrent increases in wing length.
Then, when the University of Michigan researchers combined the data, they found an even more striking pattern: in both studies, smaller bird species declined proportionately faster in body size and increased proportionately faster in wing length.
Both the Chicago and Amazonian studies attributed the reductions in species body size to increasing temperatures over the past 40 years, suggesting that body size may be an important determinant of species’ responses to climate change. Even so, exactly why smaller-bodied species are changing faster remains an open question, according to the Michigan researchers.
It could be that smaller-bodied birds are adapting more quickly to evolutionary pressures. But the available data did not allow the Michigan team to test whether the observed size shifts represent rapid evolutionary changes in response to natural selection.
"If natural selection plays a role in the patterns we observed, our results suggest that smaller bird species might be evolving faster because they experience stronger selection, are more responsive to selection, or both," said co-senior author Brian Weeks, an evolutionary ecologist at the U-M School for Environment and Sustainability. "Either way, body size appears to be a primary mediator of birds' responses to contemporary climate change."
Crabs with no sense of smell
Meanwhile, scientists at the University of Toronto Scarborough have discovered that climate change is causing a marine crab to lose its sense of smell—which could partially explain why their populations are thinning.
The research was done on Dungeness crabs, an economically important species found along the Pacific coast, stretching from California to Alaska. They are one of the most popular crabs to eat and their fishery was valued at more than $250 million in 2019.
Like most crabs, Dungeness crabs have poor vision, so their sense of smell is crucial for finding food, mates, suitable habitats and avoiding predators. They sniff through a process known as flicking, where they flick their antennules (small antenna) through the water to detect odors. Tiny neurons responsible for smell are located inside these antennules, which send electrical signals to the brain.
In their study, published in Global Change Biology, the researchers tested the electrical activity in the crabs’ sensory neurons to determine their responsiveness to odors. According to the results, when exposed to ocean acidification, the crabs did less flicking. Not only were the crabs’ sensory neurons 50% less responsive to odors, but the neurons also physically shrank by as much as 25%.
“Crabs increase their flicking rate when they detect an odor they are interested in, but in crabs that were exposed to ocean acidification, the odor had to be 10 times more concentrated before we saw an increase in flicking,” said co-author Cosima Porteus, assistant professor in the Department of Biological sciences.
The Toronto researchers say reduced food detection could have implications for other economically important species, such as Alaskan king and snow crabs, because their sense of smell functions the same way.
“If crabs are having trouble finding food, it stands to reason females won’t have as much energy to produce eggs,” said Porteus.