Though human-caused climate change is making organisms extinct, we know worryingly little about how it’s happening. That’s what John Wiens and PhD students from Stony Brook University in New York State found after looking at existing research into the subject. Among 136 studies into our changing climate’s effects on life around the world, the 11-strong team found that just seven mentioned a direct – or proximate – cause of extinction. “Understanding the proximate causes of extinction from climate change should be an urgent priority for future research,” they write in a paper published on Wednesday. “For example, it is hard to imagine truly effective strategies for species conservation that ignore these proximate causes.”
Currently, the International Union for Conservation of Nature lists 20 species as extinct worldwide or extinct in the wild potentially because of climate change. And while John’s team found there typically isn’t strong evidence for these links, there is evidence that climate has pushed many other organisms into local extinction. But while climate is the ultimate, driving, force behind their downfall, there can be direct, or proximate, causes of extinction that are less obvious. That means looking beyond overheating and rainfall changes, to include issues like disease and food availability. Change in population of other species is another common proximate cause. That could include population increases of predators or competitors that are harmful to an organism, or declines in beneficial species like prey, pollinators, and hosts for parasites.
John became interested in such causes through his personal interest in biodiversity and climate change’s threat to it. Building on that, he led a class last year for PhD students at his university on whether and how species can adapt to such changes. “As we read papers related to this topic, the students and I realized that it was not really clear what species would need to adapt to in order to survive in a changed climate,” he told me. “Initially, we assumed that they would need to evolve to not overheat. But, as we read and thought more about the topic, we realized that the story was much more complicated.”
Species links wither
That realisation inspired students Abigail Cahill and Matthew Aiello-Lammens to search through about 4,000 papers, before finding 136 relevant ones. They and the other students then divided up and went through the papers to figure out which ones had identified proximate causes of extinction or population declines from climate change. As well as the seven that found proximate causes of climate change-driven local extinctions, they found another seven studies with proximate causes of population decreases. Another four found proximate causes of extinctions related to natural climate patterns.
Among those papers, they found good reason to look more closely at how plants and animals’ numbers decrease. “There has been a lot of attention on species overheating due to climate change,” John said. “We show that there are many different factors that can cause local extinctions, not just overheating. Changes in interactions between species seem to be the most common general cause of local extinctions and declines due to climate change. These extinctions and declines are already happening, after an average change in temperature of less than 1°C. Our results raise the possibility that species and populations might be threatened by changed species interactions long before it gets hot enough for overheating to become the major threat.”
Answering tough questions
Papers looking at proximate causes are rare, John explained, because people aren’t thinking about climate-related extinction in this way. “Instead, papers have focused on documenting shifts in geographic ranges and timing, and in predicting how ranges will shift using modelling approaches,” he said. “These types of studies have been and will continue to be incredibly important. But, at some point, we will also have to know the specific mechanisms and proximate causes that make climate change dangerous.”
Another hurdle comes partly because it’s difficult to get detailed enough evidence on what caused populations to shrink. But John’s optimistic that more scientists will face this challenge. “It is hard to show that populations have declined or gone extinct due to recent climate change, especially when you consider all the other natural and human-related factors that might cause populations to decline as well,” he said. “Once you have shown this, it can be very difficult to isolate a specific proximate cause, as there are so many. But I think that people are beginning to focus more on mechanisms. We hope that our study will also bring more attention to this issue.”
Cahill, A., Aiello-Lammens, M., Fisher-Reid, M., Hua, X., Karanewsky, C., Yeong Ryu, H., Sbeglia, G., Spagnolo, F., Waldron, J., Warsi, O., & Wiens, J. (2012). How does climate change cause extinction? Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2012.1890