Dr Eoin O’Gorman from the Department of Life Sciences explores how species might cope with rising global temperatures.
Our planet is getting warmer. That is now an undeniable fact, even by the most conservative predictions for the next century. Climate change has begun to take centre-stage in international politics, Nobel prize decision-making, and even Oscar acceptance speeches.
Warmer temperatures will have numerous consequences for us humans, but also for the ecological systems that we rely on for food and resources. More energy is expended in a warmer environment, due to higher metabolic rates, and more needs to be consumed as a result.
Increasing appetites put a strain on predators in nature, who may struggle to get enough energy to survive in a warmer climate. And the bigger the predator, the greater its energy demands. Our best predictions about the fate of different species under warming suggest that the biggest predators at the top of the food web (e.g. sharks in the ocean, fish in our rivers, or lions on land) will be the first to go extinct.
Geothermal streams: simulating climate change
In order to verify these predictions (and answer a variety of other questions related to climate change), I have been working with a large team of researchers in Iceland over the past few years. We have spent many years sampling the communities found in freshwater streams (including fish, snails, insect larvae, and microscopic algae) in the Hengill geothermal valley, near Reykjavík. All the streams in this valley are connected to the same main river and so, in theory, could foster the same communities. But different geothermal heating of the bedrock and the groundwater that flows over it means that some streams can be very cold, while others are a little warmer, and some are quite a bit warmer.
We noticed that the communities change quite a lot as stream temperature increases, with certain cold-adapted species unable to survive in the warmer streams and new warm-adapted species coming in to take their place.
The trout that bucked the trend
However, we also noticed some surprising patterns in our naturally-heated stream system. There is only one fish found in the system – brown trout – and these top predators (who eat the snails and insect larvae) become bigger and more abundant as stream temperature increases. This goes against all the predictions about the fate of large consumers in warmer environments. So what is causing these surprising effects in our system?
We studied the diet of the trout over several years, by catching all the fish in each stream and stomach flushing them. This involved passing a plastic tube through the mouth of the fish and into the stomach. We then gently forced water through the tube with a syringe until the trout regurgitated its stomach contents into a small sample bag. We then preserved this in ethanol and identified the contents under a microscope in the laboratory.
We compared the diet of the fish to the food that was available to them in each stream and discovered that they became more selective in their feeding habits as temperature increased. In the colder streams, the fish were happy to eat whatever came their way, particularly the bulky snails and low quality material in the drift – their equivalent of sitting on the couch and filling up on junk food. In the warmer streams however, they actively sought out higher quality resources, such as juicy blackfly larvae and predatory insects – similar to dining in fine restaurants. The second option might cost more because of the additional energy expended in searching, but it is a great way of getting more energy from your food without needing to eat as much.
This showed us that trout can adapt their behaviour to deal with the constraints of living in a warmer environment. We need to take these adaptive responses into account when we are building our predictive models for the future if we want to make the best guesses about how our natural resources will change.
Our goal now is to test whether these adaptive responses are somehow unique to our stream system in Iceland, or whether they are a general feature of warmer environments. Over the next few years, we will look at naturally heated streams in Greenland, Alaska, Russia, and Svalbard . We will also carry out more controlled warming experiments in artificial ponds and stream channels closer to home.
We hope that this work will make a valuable contribution to our ability to deal with the unavoidable issue of a warming world.