Rapid Evolution and Adaptation to Climate Change: Salmon
by: Young Writers Posted on: December 04, 2012
Photo: Kiva Stevens, Nooksack River
Editor’s Note: Are species beginning to evolve in response to climate change?
Kiva Stevens is a Junior at Western Washington University and Huxley College of the Environment in Bellingham Washington. She is pursing the major Environmental Science with a focus on fresh water ecology. Last spring she interned with a local nonprofit organization, Nooksack Salmon Enhancement Association (NSEA). She loves nothing more than spending time on rivers and whitewater raft guiding!
Predicting the future has never been an easy task. That’s because it is always moving, but in some cases, this is for the best. If worrying about the future, climate change in particular, makes your stomach drop, you’re not alone. Some estimates place future climate-related species loss at a staggering 43% in high-diversity parts of the globe (Malcolm et al. 2006). However, an emerging field from within evolutionary biology – the study of rapid evolution – offers hope that this dim tale may not be the end of the story, because evolution itself is the story of a future constantly in flux.
Over the last decade, ecologists developed what are known as climate envelope models to try to grasp the potential scale of extinctions that might result from climate change. In these models, populations have at most three options based on their biology: stay put and hope for the best, move to a place where their needs can be met, or go extinct. Yet from the very beginning, biologists acknowledged a fourth potential option (but one on which little data was available): adapt or evolve.
While adaptation can be linked to behavior and not necessarily evolution, rapid evolution is the ability of a species or population to quickly adapt genetically to changes in its environment before falling extinct. Examples of species showing evolving traits in response to climate change include populations of the wood frog (Rana sylvatica), insects like the fruit fly (Drosophila melanogaster), and trees including the engelmann spruce (Picea engelmanii) and the ponderosa pine (Pinus ponderosa).
Can Columbia River sockeye do the same? The Columbia River sockeye salmon (Oncorhynchus nerka) are fighting to survive changes in their home waters. Two serious threats are warmer water temperatures and alterations in the timing and volume of water flows. Compared to historical records, today’s Columbia River flows are earlier and greater in the springtime and lower in the summer.
Fish can’t breathe when water gets too hot; for salmon, temperatures above 68ºF hinder freshwater migration, spawning, and rearing. Average temperatures in the Columbia River rose 2.5 degrees F over the past 40-70 years (Ekwurzel). Water temperature controls the timing of salmon reproduction and development, and warmer water causes eggs to hatch earlier. Premature fry are too small to compete against predators, and are out of sync with the lifecycles of their prey. Warmer waters also cause migration and breeding patterns to change: temperatures over 70 degrees F can prevent migration. Additionally, with climate change comes earlier peak stream flows in spring (up to 30 days earlier) that flush young salmon from rivers to estuaries (where rivers and the ocean meet) before they are physically prepared. Decreased flows due to climate change in the summer create shallower and thus warmer water in the summer and fall.
Regardless of these challenges, sockeye salmon in the Columbia River are showing rapid adaptation to these river changes by altering their migration patterns. A National Oceanic and Atmospheric Administration (NOAA) study found that sockeye salmon are now migrating upstream 10 days earlier on average than in the 1940s, in direct correlation with rising water temperatures and flow changes. So, are the salmon evolving new tolerances, or avoiding harmful conditions through non-genetic changes like behavior, in a phenomenon known as phenotypic plasticity?
The distinction is important because these two different responses can lead to very different fates in a population over time. To understand the distinction, lead researcher Dr. Crozier developed a model to help infer evolutionary change. Using their model and data from the past 60 years they concluded that earlier migration was primarily the result of natural selection in response to temperature, plus behavioral changes in response to altered river flow. “Evidence of an evolutionary response in Columbia River sockeye salmon is good news, because it appeared to reduce their exposure to potentially lethal river temperatures in recent years” said Dr. Crozier.
Despite positive signs, most data shows a clear relationship between the Columbia River’s increase in water temperature and the decreasing survival rate of sockeye salmon; salmon populations in the Columbia River system are down by 90 percent (Ekwurzel). Rapid evolution and population declines are not mutually exclusive. Species may even show quick evolution responses to a changing environment, only to see their rate of evolution decay over time due to a loss of genetic variation from population decline.
Other factors limiting a population’s capacity to evolve must be considered. For one, the climate may just change too fast before populations can adapt. Slow-reproducing, long-lived species like trees or large mammals are the hardest hit. Past changes in climate demonstrate that thousands of years may elapse before these species begin to respond to a new climate system (Skelly). While extinction projections that exclude evolution may overshoot, there is no arguing the risk is urgent and real.
If the story of adapting salmon tells us anything, it is that we should do all that we can to enhance the chance of biological adaptation through our conservation, planning, and management plans. Crozier states, “this study gives managers insight into the multiple processes that help salmon persist…and augments our toolbox for predicting how other species might respond to similar changes.” Adaptation awareness should not diminish the urgency with which we work to conserve genetic diversity and manage natural resources in the face of climate change; rather it can enhance our efforts and our reasons for hope that by working with these processes we can support the diversity of life around us.
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