Evolving Alaskan habitats: how global warming may disrupt the prey-predator balance

Figure courtesy of wikipedia.org

Figure courtesy of wikipedia.org

The polar bear scrambling onto a small piece of floating ice is the emotional icon of global warming.  We know that dramatic loss of sea ice is threatening their habitat and will likely reduce their population and change their distribution.  But what about other species in the northern climates and Arctic?  Is global warming dangerous for all animals or will some see a survival advantage?

A new study in Climatic Change provides some answers to these questions by predicting how habitats will evolve with global warming over the next century and connecting this data to various species.  As temperatures increase, certain types of vegetation and land cover, known as ecotypes, will expand while others diminish, helping and hurting species based on their natural habitat.  Focusing on about 1500 square kilometers in northwest Alaska, the study used historic data over the past 30 years regarding ecotype changes to predict how they will evolve until the end of the century.  With these trends in hand, the authors then constructed three models:

1) Conservative: just project the same rates of change historically into the future to identify how habitats will change.

2) Temperature-sensitive: in addition to including the historic trends, the authors connected these trends to changes in air temperature to take into the effect of global warming.  Using these correlations, they correct the model to account for future warming – this will lead to more drastic predicted changes in climate over the next century, but also may be the most realistic.

3) Biophysical: The most complex of the models, including feedbacks and lags to the ecotype changes due to environmental factors (read here for more details)

With predictions about ecotypes in hand, the authors then connected this data to species use of each habitat to determine which will be most affected, positively or negatively.  In particular, the authors plotted habitat change between 2010 and 2000 for each major species with habitat in at least ten percent of the whole 1500 square kilometer area, doing it separately for birds and mammals.

First up are the bird species!

Figure courtesy of [1]

Figure courtesy of [1]

The species names across the bottom may be a bit small to read, but the major trend shows greatly increased habitat for raptors and grouse like hawks (lefthand side of plot).  The predicted change varies between 30-80 percent depending on the model used (green is temperature, red is conservative, diamond is biophysical), showing how significantly temperature plays a role in the pace of habitat change.  The righthand side of the plot shows 10-20 percent declines in habitat for loons, shorebirds, and gulls.

This division between rising and declining habitat for various bird species is mainly due to the drastic increase in tree ecotypes with warming temperatures.  Hawks and grouse enjoy the black spruce forests of Alaska like the one shown below:

Figure courtesy of [1]

Figure courtesy of [1]

These forests, along with white spruce and larger shrubs, are all predicted to expand over the next century, much to the advantage of top bird predators.  In contrast, shoreline habitats, barrens, and meadows will all decline, leading to reduced regions for the other bird species.  The loss of these meadow-type habitats also impact the mammals, shown here:

Figure courtesy of [1]

Figure courtesy of [1]

Here, even less mammals show an increase in habitat, due to most species being small to medium-sized rodents that spend most of their time out of the forested regions.  In particular the arctic fox (fourth from right), could lose up to 50% of its habitat based on the temperature-dependent model.

Other notable species such as shrews and muskrats will also lose 10-20 percent of their habitat.  It’s important to note that even if predatory birds gain more forest habitat, they will have a difficult time surviving if their typical prey can’t find a place to live!

These data paint a different picture than the message propagated throughout the media – not better or worse, but more complex.  Not all species are necessarily hurt by global warming; some may find more room available for breeding grounds and hunting.  But such sharply changing habitat will affect the food web in a complex way due to predator-prey relationships that could swing out of balance.

Although the study offers some intriguing insights, the results feel more exploratory than anything else.  Percent changes are an important way to look at the magnitude of effects, but there was little statistical analysis done with that data, other than fitting some patterns of habitat changes that don’t seem very convincing (which I don’t show here).   Some more systematic statistical analysis may help further these findings in the future.

Obviously this is a narrow study geographically, so generalizations to other Arctic and northern climates should be made with caution.  The results provide some intriguing results that should motivate others to continue this line of research. Highlighting the effect of habitat change on interactions between trophic levels is especially insightful – birds like the red-tailed hawk may see a survival advantage due to increases in tree ecotypes, but they may also be crippled by the sharp decline in habitats conducive for their prey, like the shrew.  By elucidating these relationships, this study shines a light on the complexities of food networks and how global warming will truly disrupt these delicate ecosystems as they are now.

References

1)

Marcot, B., Jorgenson, M., Lawler, J., Handel, C., & DeGange, A. (2015). Projected changes in wildlife habitats in Arctic natural areas of northwest Alaska Climatic Change DOI: 10.1007/s10584-015-1354-x

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