Thermal Envelopes & Climate Change

Stream temperatures profoundly affect the distribution of fish species throughout the Pacific Northwest. The reason is their physiology.

Most fish species, unlike us mammals, are ectothermic, meaning they can’t stuff their faces with food to rev-up their metabolisms and generate body heat. Nor, should they find themselves outside their preferred temperature comfort zones, do ectotherms possess other mammalian physiological adaptations, such as sweating or shivering.

Instead, ectotherms maintain their body temperatures by staying within their evolutionarily determined thermal niches. All this means that stream temperatures can serve as predictors of where and what kinds of fish species (or “fishes” to scientists and managers)—not to mention other aquatic organisms, including amphibians—may occupy a stretch of stream now and into the future as climate change heats things up.

Developing a better understanding of the relationship between stream temperatures and the temperature-dependent aquatic animals that occupy them is the topic of a new paper published in Ecological Applications.

In their study, authors Dan Isaak and colleagues mined databases of surveys collected since the mid-1980s. From this, the researchers identified the presence of 14 species of fish and amphibians (12 fish and two frog species) in streams throughout the Northern Rocky Mountains, a range that includes most of Idaho, parts of western Montana, and parts of eastern Oregon and Washington. Examining the range of stream temperatures that these 14 species occupied, the researchers then took a further step: they estimated the probability that any given species will inhabit a given location found under a given temperature.

Consider the rainbow trout (Onchronchus mykiss). The fish is found throughout the Northern Rockies in a wide range of temperatures. However, based on Isaak and colleagues’ investigation, the rainbow trout is most likely to be found in streams measuring 17 degrees Celsius (63 degrees Fahrenheit). Move to streams above or below 17 degrees Celsius, and rainbows may still occur, but the odds become less and less the further the temperature is from that optimum 17 degrees. (Math nerds refer to this as a quadratic relationship.)

This work helps define what’s called a thermal envelope for each species. Think of a thermal envelope as a kind of range that defines how much cold or warmth each species can tolerate outside of its optimal temperature. For example, the cutthroat trout (O. clarkii) is most likely to occur at 9 degrees Celsius (48 degrees Fahrenheit), but may still be found in streams with temperatures more than 6 degrees Celsius (10 degrees Fahrenheit) above or below that optimum.

Understanding a species’ thermal envelope—or cold and warm edge transitions, as Isaak and colleagues call them—are important for a major reason: this research and others like it will be essential to help keep streams in the Western US filled with fish and frogs as both the region’s air and stream temperatures warm under climate change.

Here’s the problem.

As a result of our warming climate, many species are migrating, chasing those thermal envelopes they have evolved to tolerate. The current study, as well as other work by Isaak and colleagues, strongly suggests that this migration is already happening and will continue to happen in the Mountain West as certain species move to cooler habitats upstream, seeking refuge as downstream habitats continue to warm under climate change.

For the adaptation efforts of managers, the consequence of this migration to cold-water refuges is clear: in order to aid migration, downstream and upstream reaches will need to be connected and navigable. In practice this means removing barriers—including impassable road culverts or adding passage at dams or water diversions—so that species can move with the changing temperatures across the landscape, or what the authors call the region’s changing thermalscape.

This type of work may be complemented with restoration of riparian areas to increase stream shading, particularly in areas with historical land use changes to buffer warming. However, the study’s authors note, these tactics may not be as useful for species with streams already pushing the edges of their thermal envelopes, such as bull trout (Salvelinus confluentus).

Bull trout are highly acclimated to cold water. In fact, the species has an upper temperature edge of just around 6 degrees Celsius (43 degrees Fahrenheit). This constrains the animal to all but the coldest waters in the Northern Rockies. In other words, the bull trout are already at the limit of their upstream habitat. For this reason, Isaak and colleagues suggest that adaptation efforts should focus on maintaining or conserving these areas as cold-water refugia.

The current Ecological Applications paper builds on a previous work by Isaak and colleagues to map multiple aquatic species’ thermal envelopes against the actual streams of the Western US. Called the NorWeST Stream Temp project, this effort includes a database filled with highly detailed historical as well as probable future temperature distributions for stream networks throughout the region.

By combining NorWeST with their more recent work covering cold and warm edge species transitions, Isaak and his fellow researchers can now improve estimates of how aquatic species are currently distributed under today’s climate as well as track how that distribution is likely to change under future climate change scenarios.

This work was been done through the USDA Forest Service Rocky Mountain Research Station, where Isaak is a research fish biologist.

Isaak recently wrote a story about his work tracking stream temperatures and species migrations for Northwest Climate Magazine. Northwest Climate Magazine is a joint effort led by CIRC, the Northwest Climate Science Center, and North Pacific Landscape Conservation Cooperative.

John Stevenson is CIRC’s Regional Extension Climate Specialist, a position jointly-funded by CIRC and Oregon Sea Grant. Follow him on Twitter @CIRC_Extension

Study: Ecological Applications

Citation: Isaak, Daniel J., Seth J. Wenger, and Michael K. Young. “Big biology meets microclimatology: defining thermal niches of ectotherms at landscape scales for conservation planning.” Ecological Applications (2017).

Photo Caption: John Day River, home to the endangered bull trout. (Photo credit: U.S. Department of Agriculture, some rights reserved.)

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