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Body Changes in Migratory Song Birds

As the global climate undergoes significant, wide-ranging changes from human activities, the effect on the biosphere, or collection of ecosystems and the organisms that inhabit them, is being examined to observe ecological responses. A recent study from the University of Michigan School for Environment and Sustainability (SEAS) used an interesting data set of migratory song birds to illustrate one such set of long term changes.

‘Smashing Good Data’

The specimens used for the investigation was a from a collection of birds from 1978 to the present that died from hitting urban structures in the city of Chicago. The aerial roadkill, scooped up by the Field Museum in the windy city, provide a surprisingly complete record of several migratory song bird species and their morphological changes in the last 4 decades. Specifically, the data of interest lay with the body size and the wing length of each bird and how these size measures have changed with corresponding temperatures.

The migration ended here for the studies’ sample subjects

Drawers Full of Birds

The study’s lead researchers, including UM’s Benjamin Winger, Benjamin Weeks, and co-authors at the Field Museum, painstakingly measured the tarsus (lower leg / foot bone), bill length, body mass, and wing length of the more than 70,000 specimens to look for trends across the 4 decades of data.

U-M biologist and bird expert Ben Winger: definitely a bird guy

Previous research has suggested that rapid warming periods lead to decreased body size in birds and these findings seemed to confirm this with surprising accuracy. After measuring more than 70,000 bird samples, researchers found that the trend of bird body size has decreased significantly while the length of bird wings from tip to tip has increased.

Measures in all 52 species of birds showed a decrease in body size (with 49 statistically significant decreases) while 40 of the bird species showed a statistically significant increase in wing size. Measured tarsus length decreased 2.4% while wing length increased by a mean length of 1.3%. Both physiological changes the researchers are attributing to a warming climate over the past 4 decades.

David Willard of the Field Museum, pulling out all the stops on this bird collection organ

Heating Up: A Case for Efficiency

The UM researchers discuss the implications of their findings as evidence of how a warming climate affects individual species. Unlike ice core, coral reef, or tree ring samples, this is a study showing direct impact of a warming planet on animal morphology or physiology as opposed to measuring actual warming trends or atmospheric greenhouse gas concentration.

The trend towards a smaller body size of these birds suggests the relationship of warmer summer breeding grounds (around a 1 degree C increase), to a reduced body size (a small, but significant amount ~10 g in mass). Flight migration is an extremely energy intensive effort and, prevailing theory contends, having a smaller body size yields a higher surface area to volume ration for the organism allowing for greater heat exchange with the outside environment. The smaller body size correlates with increased temperatures while the larger wing size, according to the researchers, suggests an offset for the loss in body mass to account for efficient flight.


Does the impressive data showing changing body size suggest that the birds were / are actively adapting or showing ‘developmental plasticity’ as rapid adaptation to climate change? That proposed question is being tested by the researchers in concurrent studies, but an alternative idea that holds more weight may be that the warming climate is a selective pressure driving “microevolution”. Simply put, warmer temperatures may be altering bird morphology at the genetic level by selecting individuals that are smaller in mass and size. In examining this study as a marker for how animals adapt to climate change, further research will be driven by examining changes through either rapid adaptation (plasticity) or evolution (selection).

“To date, there is no direct evidence that body size decreases in birds and mammals are an adaptive response to climate change.”

Teplitsky, C., & Millien, V. (2014)
Warblers: among the many species at risk

Plastic or not, what is a disheartening connection is a previous large-scale study on North American song birds showing a risk of extinction from warming temperatures for up to 67% of all species. This grand species loss may indicate that if some species are indeed adapting, many, unfortunately are not.

Sources and Further Reads:


Fire Regime and Climate Change Impacts

Fire is an integral part of many ecosystems and is even a requirement for certain organisms’ life cycles. Anthropogenic climate change has increased the severity and frequency of fire with consequences for natural systems and human populations. This has been magnified by high profile fires in the Amazon, Australia, and California. More severe fire has been attributed to warmer temperatures and more fuel from fire suppression by land management. But how exactly does this happen? More importantly, how has increased fire affected wildlife populations and what will impacts look like under future climate scenarios? We’ll examine these questions from recent literature researching the impacts of changing fire regimes on ecosystems and the integrated services they provide to humans.

Pyrophytes and Fire Cycles

Many species of trees and plants require fire to complete their life cycles and reproduce. Famous examples of such trees include the Giant Sequoia of Yosemite fame, the lodgepole and jack pines of the Boreal forests, and sand plain gerardia and wood lily of Eastern habitats.

Famous Trees: California State Parks. Photo by Brian Baer

The trees have serotinous cones protected by thick waxy coatings, that require the heat of a fire to release seeds. Fires are part of natural ecosystem cycles that pave the way for new growth, increase biodiversity (pyrodiversity), and naturally manage the health of the forest. Fires open up space and quickly regenerate soil nutrients for new trees (whose seeds have just been released by fire) and plants to germinate. They also clear old growths dominated by few species (or invasives) to open up meadows and grassland spaces, and animal deaths from fire become food for scavengers or detritus for further nutrient cycling. Fire has been a natural event in ecosystems for a long time and organisms are well-adapted to it.

So, what’s the problem?

Problems, really if we want to answer this truthfully. To quote the musical “Hamilton”, you should “look arou-nd, look arou-nd…”

“There’s a lot more fire, Eliza….”

The number, severity, and frequency of fires has increased significantly in the last half century in the Western U.S. and this has impacted human civilizations and wildlife populations. While huge individual fires that have burned towns to ash, caused plumes of smoke to drift across the United States, and turned urban skies eery shades of orange get the most media attention, it is the patterns of these events that matter more and give us context.

Interestingly, fire regime change from climate impacts globally is difficult to assess and shows less of a clear pattern (though more updated research is needed). This is due to the dynamics of climate change such as increased atmospheric moisture from warmer temperatures and less fuel that can reduce fires in some areas (tropics and deserts) while increasing fire rates elsewhere (arid grasslands and upper latitude climes.)

Why have fires increased in the Western U.S.?

This has been a subject of much study because there are many factors involved in pinpointing the causes of increased fire. For people that do not want to put the climate change stamp on the fire increase, the reasons are to do with forested land management, increased proximity of human settlements to forests, and natural climate variations.

Climate change stamp, seen from space

Humans have indeed decreased the fuel clearing fires (fire suppression) that naturally occur and so lots of detritus and organic matter (read: wood) has built up in forests while the fires that do occur, are closer to human settlements that have encroached onto previously uninhabited land. So is this the reason fires have increased?

In short, no, according to a 2016 UW Madison study among others (1,2,3,4)

While more fuel from fire suppression certainly is part of the equation, that wood has to burn first. What increases the likelihood of that wood burning is how dry it is (fuel aridity). What has been found is that increasing drought, which has been part of a long-term “mega drought” in the Western U.S., and rising temperatures caused by anthropogenic climate change (ACC) have increased the aridity of the air and decreased atmospheric vapor pressure across Western US forests. While natural climate variability does account for some of the observed drying out, ACC was found to be responsible for the majority (~55%) between 1979 – 2015. The study’s authors also found that ACC contributed 4.2 million more hectares of forest burned, nearly twice as much as would have otherwise occurred between 1984 – 2015, and double the average number of days of the fire season between 1979 – 2015.

In addition, a great deal of the fuel from fire suppression has also come from dead plant matter as a result of drought and wood-boring beetle outbreaks (themselves wrought by changing climate conditions, see image below). Drought, likely exacerbated by ACC, has caused the die-off of many forest plants and contributed to the increase in fire fuel. Fire suppression in the 20th century certainly provided more fuel for the increased fire regime and has modulated the fire data, but the effects of ACC to dry out and add to that fuel has led to much of the increases in fire we see in the present.

Forest Insects & Wildland Fire, Ann M. Lynch, Ph.D., US Forest Service & University of Arizona, Laboratory of Tree-Ring Research

How have increases in fires affected wildlife?

While it is true that natural ecosystems are well-adapted to fire cycles, conditions wrought by ACC are negatively impacting wildlife directly and indirectly. The obvious impact of more frequent fires is that some wildlife populations are lost directly from the fire itself (which has also claimed many human lives). Some of these populations were already vulnerable due to habitat loss and other ACC caused stressors such as invasive or pest species impact. The well known bark beetle outbreaks that have killed large stands of forest have an interesting relationship with fire as some of the wood-boring insects can survive fires and subsequently attack fire damaged trees.

Beetle species, such as Mountain Pine Bark beetle have devastated certain trees in the Western U.S. have left many forests more vulnerable following a fire.

In order for wildlife to survive a fire and be part of natural cycles, they must be able have unburnt refuges to thrive in while the burnt habitat recovers (which takes time). With habitat loss and higher intensity of fires which burn more completely, refuges are smaller with fewer available resources, handicapping organism capacity of surviving with a fire cycle. Another worrying impact of fire on wildlife is the loss of non-pest insects and the ecosystem services they provide for humans as well as nature, mainly in the form of pollination.

Increases in fire frequency, severity, and longevity may have long lasting impacts on natural ecosystems. Forest trees may see their ranges shift (due both to climatic factors and fire), fire-dependent species may thrive in some areas, but ultimately this will depend on fire frequency. If fires are too frequent to permit growth, than slow growing pyrophytes such as sequoias will not have sufficient time to grow past the juvenile stage. This scenario could lead to current forested areas being left treeless. This rapid change in primary production source has implications for the entire ecosystem, which could lead to rapid die-offs of communities or migrations of species that are dependent on forests.

Future Scenarios:

The future outlook of fire disruption is highly biome-specific. Global models have shown that certain regions of the world expected to receive higher precipitation levels or that expect to have less vegetation (due to drought / desertification) may be less fire prone. Other regions, such as boreal forests, tundra, and mountain grasslands may be at much higher risk especially as the intensity and frequency of drought from ACC and continued levels of arid fuel are present.

Fire increases under different modeled scenarios, Moritz, et al, 2012

The increase in fire season length and intensity along with the impacts that follow will continue in these areas as the climate continues to warm and as long as there is fuel to burn. The obvious long-term solution is part of the challenge of climate change in general which is to reduce planet-warming carbon emissions. Other short-term solutions may involve mitigating the impact of these more frequent fires. These short term solutions could include adjusting forest management to increase prescribed burns, using integrated pest management to control some bark beetle outbreaks, limiting human settlements to reduce direct impacts from fire, and to target fire support to critically endangered populations during fire seasons (such as using fire repellents around stands of endangered tree populations).

These short term solutions will be for nothing however if the longer term of goal of reducing carbon emissions to prevent further warming and its consequences are ignored or not taken seriously. We already know the impact of climate change and the projected outlook for the future will be looked at through smoky, orange colored (not rose colored) glasses.

Sources and Further Reads:

Abatzoglou, John T., and A. Park Williams. “Impact of Anthropogenic Climate Change on Wildfire across Western US Forests.” Proceedings of the National Academy of Sciences, vol. 113, no. 42, 2016, pp. 11770–11775., doi:10.1073/pnas.1607171113.

Akaike, H., et al. “Forest Ecosystems, Disturbance, and Climatic Change in Washington State, USA.” Climatic Change, Springer Netherlands, 1 Jan. 1974,

Aponte, Cristina, et al. “Forest Fires and Climate Change: Causes, Consequences and Management Options.” CSIRO PUBLISHING, CSIRO PUBLISHING, 4 Aug. 2016,

Aponte, Cristina, et al. “Forest Fires and Climate Change: Causes, Consequences and Management Options.” CSIRO PUBLISHING, CSIRO PUBLISHING, 4 Aug. 2016,

Canada, Natural Resources. “Government of Canada.” Natural Resources Canada, / Gouvernement Du Canada, 7 July 2020,

Flannigan, Mike D., et al. “Implications of Changing Climate for Global Wildland Fire.” CSIRO PUBLISHING, CSIRO PUBLISHING, 10 Aug. 2009,

Goss10, Michael, et al. “IOPscience.” Environmental Research Letters, IOP Publishing, 20 Aug. 2020,

“The Long-Lasting Impact of Wildfires on Wildlife, WWF Explains.” PBS, Public Broadcasting Service,

Moritz, Max A., et al. “Climate Change and Disruptions to Global Fire Activity.” The Ecological Society of America, John Wiley & Sons, Ltd, 12 June 2012,

“What’s the Deal With a Lack of Water in the West?” What’s The Deal With…, 26 Apr. 2015,

“A Rare Species of Tree Was Saved from Australia’s Wildfires. And Something Else Happened.” Bulletin of the Atomic Scientists, 27 Aug. 2020,

Forest Response to Climate Impacts in the Anthropocene

Since trees, like corals in the ocean, form the physical structure of many terrestrial biomes, their response to climate change will be vital in understanding effects on ecosystems. Along with their more conventional ecological role as primary producers and physical habitats, trees provide global services such as Carbon, Nitrogen, and water storage. These factors which connect the biotic realm to the global climate system, show how trees and forests both affect and are impacted by changes to climate. As the global climate continues to change with increasing temperature, atmospheric carbon levels, and altered precipitation, how trees adapt or are affected will have both a local and global feedback. This short set of current research looks into how some forests have responded and may change in response to climate change.

How Trees Respond to Higher CO2 Concentration

As anthropogenic activities have increased atmospheric carbon levels, this might seem like a boon to trees, who as any secondary school student will tell you, gain their chemical energy through photosynthesis of carbon dioxide from the air and water from soil. That same secondary school student will also tell you that trees produce water as a waste product from respiration; so trees that photosynthesize more are also at risk of water loss through leaf pores known as stomata that transpire into the air (evapotranspiration).

6CO2 + 6H2O –> C6H12O6 + 6O2 !!!!!!

The ratio of carbon intake for photosynthesis over conductance through stomata is known as intrinsic water use efficiency, iWUE*. This metric can be used to examine tree response to higher CO2 levels and consequent increases in temperature.

Schematic examining Carbon & water through leaf stomata (stomata and guard cells shown in dark green)

A joint-research team from China, Australia, the UK, and Ireland found that evergreen trees increased iWUE more rapidly than deciduous trees in response to rising CO2 levels across a 25 year period. This response suggests a competitive advantage for evergreens in future drier climates. The authors conclude that past climate warming, such as in the Eocene (55 mya) may have led to evergreen expansion at that time period.

Eastern White Pine: An evergreen with a higher level of iWUE

Challenges to applying this simplified tree response system to understanding future forest growth systems include other climatic variables such as warmer temperatures and resulting increased water access.

*this response can be seen at the cellular level as higher CO2 levels and warmer temperatures can influence guard cells to contract the opening of stomata in leaves / needles.

While Some Decline, Others Thrive

While many forests around the world have declined in productivity and biomass due to global changes, some forested regions have grown. In the Tibetan Plateau, a joint Chinese – USA research team found that trees have grown steadily there since the 1900s and are at their highest growth levels since the 18th century. So why the growth?

Study location on the Tibetan Plateau involving individuals, mixed, and forest level community

Scientists examining the trees used Carbon, Oxygen, and Nitrogen isotopic analysis from dendrochronology (historical tree ring data) to infer that higher carbon levels in the past had led to increased water & nutrient availability. The scientists postulate that the warmer temperatures from increased atmospheric CO2 has led to higher precipitation levels and melting of permafrost. The melted permafrost has not only allowed access to soil moisture but also nutrients trapped within the permafrost layer.

Permafrost melting in Svalbard, Norway.

So, trees could respond to higher CO2 levels by increasing water efficiency or by thriving due to new water resources made available by a warmer climate. But these responses are limited to certain climates and tree types. Higher CO2 and temperature levels can also lead to lower precipitation rates, increasing severity of drought and tree mortality.

Drought Resistance & Ecosystem stability

As studies are showing, anthropogenic climate warming is likely behind the “mega-drought” in the Western U.S. – the long term severe drought from 2000 – present. How trees respond to continuous drought stress is important for conservation, fire management, and understanding ecosystem stability as droughts become longer, more frequent, and more intense.

Broderick, et al. used remote sensing to analyze drought resistance in California forests by examining canopy water content (CWC) or the total amount of water held in the tree canopy. By looking at changes to CWC over time using satellite and ground-based data, tree vulnerability to drought can be examined. A tree’s drought resistance, identified by the authors, would be measured as the amount of drought withstood by the tree prior to CWC levels falling below a long term level. The authors found that California’s forest showed heterogeneous drought resistance with certain forest types, such as the Redwoods, showing greater resistance than other forests such as the lower elevations of the Sierra Nevada ranges.

Differential Drought Resistance

Whether the Redwoods took advantage of additional water sources, improved iWUE like their evergreen cousins from the 1st mentioned studies, or another strategy is unclear. The authors caution though that drought resistance does not confer drought resilience, or the capacity to recover fully from the abiotic stress. If drought induced stress does lead to extensive tree mortality, this may have largescale impacts on whole ecosystems and could induce succession of other tree species able to colonize these stressed regions.

Climate Induced Range Expansion

Defined boundary between hardwood forest and coniferous evergreen forest – elevation or climate can define where this boundary is located

In southern Canada near the Great Lakes region, Goldblum & Rigg from the University of Wisconsin-Whitewater and N. Illinois University found that 3 primary tree species residing along the boundary between boreal and temperate forests will likely see different ranges under a warming climate by 2080. They examined tree core records (dendrochronology) of sugar maple, white spruce, and balsam fur and pollen accumulation to examine growth responses to temperature and precipitation. These data when applied to future modeled warming scenarios, showed a change in the location of the three tree populations. Under all future warming & precipitation scenarios, the sugar maple would show a northward growth trend while the balsam fir would likely show a significant decline.

The Forested Zones may need to be updated

These changes indicate that the temperate dwelling maple may thrive at more northerly latitudes in the warmer future and that the organismal composition of future forest biomes may be altered. But forests around the world are not homogeneous and their responses to climate change will vary as many different factors play into their growth.

Many, Many Variables to Consider

Foster, et al., examined boreal forests in Alaska and the many interactions between trees and the soil, fire events, and climate to make predictions about warmer climates using a highly complex model. Their model showed an overall decrease in biomass in the Tanana Valley River Basin in Alaska with significant declines in spruce forests (the dominant tree species) but smaller gains in deciduous trees such as Quaking Aspen.

Many factors play into the success of trees in the Alaskan boreal forest under increases in temperature and atmospheric carbon

Their model and the assumptions of tree response touch on many of the earlier studies mentioned above including thresholds to consider for trees under climatic stress and the complex interplay of many variables. While White Spruce is a drought-tolerant species and can apply strategies such as iWUE, there is a limit to this tactic. Under long-term moisture stress and increased temperature as shown in the researchers model, spruce forests declined after initial growth from higher CO2 and photosynthetic capacity.

Many other variables added to the model showed differentiation between individual tree site, soil type, water availability due to moss cover and organic soil layer depth, rate of drought and fire, and other factors. The researchers stress that despite the improvement of their model of including many variables to determine future tree growth under warming climate scenarios, their model is missing some key information such as bark beetle infestation which has had significant impact on forests in southern latitudes.


While the small sample of research above is by no means a sufficient review of the literature, it does show the complexity and challenge of predicting forest response to climate change in specific locales. While this post examines some specific responses to climate change, other literature provides a more comprehensive outlook of forests under warming scenarios.

As forests attempt to adjust to increased atmospheric carbon and warmer climates, researchers are able to see tree responses and ecological impacts in real time. This work can help to predict what future forests may look like, ecosystem impacts to the communities that depend on forests for shelter or fodder, and global budgets of carbon and water storage that impact and feedback into the climate system.

What we have seen so far of rapid warming and forest response is not promising despite the many arrows that trees have in their quiver. This somber reminder is needed in order to accelerate research and change global goals for carbon emissions.

Sources and Further Reads:

Brodrick, P. G., et al. “Forest Drought Resistance at Large Geographic Scales.” AGU Journals, John Wiley & Sons, Ltd, 1 Mar. 2019,

Foster, Adrianna C., et al. “Importance of Tree- and Species-Level Interactions with Wildfire, Climate, and Soils in Interior Alaska: Implications for Forest Change under a Warming Climate.” Ecological Modelling, Elsevier, 3 Aug. 2019,

Silva, Lucas C. R., et al. “Tree Growth Acceleration and Expansion of Alpine Forests: The Synergistic Effect of Atmospheric and Edaphic Change.” Science Advances, American Association for the Advancement of Science, 1 Aug. 2016,, Wuu Kuang, et al. “Rising CO2 Drives Divergence in Water Use Efficiency of Evergreen and Deciduous Plants.” Science Advances, American Association for the Advancement of Science, 1 Dec. 2019,

Apple Trees & Phenological Mismatch

Plant / pollinator mismatch is a growing area of study as scientists examine effects of ongoing anthropogenic climate change in natural ecosystems. Plant phenological mismatch, the missed timing of flowering plants and their pollinators, also has huge implications for agricultural systems and the economics of food systems. Understanding warming’s effects on these systems, therefore, are of great interest to a rapidly expanding human population and the required agricultural yields for feeding the global population. In this vein, researchers in Hungary tested Apple trees (Malus x domestica) at various stages of phenological stage to observe effects on numbers of herbivores, pollinators, and other insects. They found substantial evidence for effects of mismatch but their study is limited and further research is needed for direct impacts to be examined for both natural and agricultural systems.

Phenology is the study of seasonal observable change within organisms and includes features such as changing fur color in mammals, feather production in birds, and migration patterns in many organisms. For deciduous plants, phenological features include foliage color and drop, and the timing of budding, bursting, flowering, fruiting, and leafing. These features have become important markers of a changing climate globally as historic phenology records can be used as further evidence of temperature change.

The timing of phenological features has important implications across ecosystems as flowering plants (angiosperms) form the base of many ecosystems. Flowering plants depend of course on coevolved pollinators for reproduction and growth of populations. A mismatch in timing with pollinators (ie. migrating or emergence) could have drastic effects from the base of the food web upwards including on the numerous organisms that inhabit and form communities on trees.

A Study in Hungary: An Apple Delay…

Researchers in Hungary manipulated the phenology of apple trees to examine the effects on pollinators and the communities of critters that inhabit the trees. The scientists kept one group of apple trees in cold storage (delayed) to delay bud burst and flowering of the apple trees before planting and another group of apple trees in climate controlled greenhouses (advanced) to facilitate the early onset of budding and flowering prior to planting. Both these experimental groups were compared to trees planted outdoors and experiencing seasonal climate.

keeps Pollinators Less Varied

When each of the tree groups were sampled for arthropod abundance (insects, spiders, etc…), they found that the manipulated trees (delayed or advanced) had less biodiversity than the control trees. Overall numbers for each of the groups varied, but the study showed that specialized interactions, such as herbivores or insectivores with limited diets were more susceptible to the altered timing of the trees.

Noble Chafer at work amongst the apple blossoms

One interesting effect not anticipated was that in all trees, pollinator abundance (ie. bees) was similar despite the different timing of flowering. The researchers suggest that a significant diversity of wild bees hedged the erratic timing of phenological mismatch by being able to respond and ensure fruit set. This, however, was noted in turn that the overall pollinator diversity was lower on the manipulated trees compared to the control.

Ok. Wait, What Does This Mean?

The climate induced changes to phenology of both wild plants and agriculturally important crops has important implications that begin to be examined by this study. Mismatch of the timing of flowering for the apple trees in this study, while not showing impacts on fruiting, did show effects on the diversity of pollinators. In many areas around the world, pollinators are already showing signs of mismatch with plants. If commercially important crops flower without enough pollinators, this obviously could lead to lower yields, higher food prices, and global food shortages.

As it relates to natural systems, the earlier onset of budding and flowering can have impacts on those producers on the rest of the ecosystem. For trees reliant on the arrival of migratory birds for dispersal for example, mismatch could mean heavy impacts on producer population growth and all the organisms that depend on them. Studies using historical data on phenology have shown that mismatch in this area is already occurring.

Bohemian waxwing enjoying a berry and unwittingly being part of a coevolved dispersal strategy


Many of the studies examined on phenological mismatch require further research. Looking back into historical records, such as Henry David Thoreau’s writings on nature or farmer’s almanacs are helpful to set a baseline from which to compare, but they are sparse and obviously don’t offer a global picture. Also of interest is if plants and pollinators will change on smaller scales through plastic responses (ie. behavioral changes to migration times or budding) to reduce mismatch. If this is occurring, not only would it be an interesting area of research, it could reduce a huge potential detriment to global food systems.

Sources and Further Reads:

  • Kőrösi, Á, Markó, V., Kovács-Hostyánszki, A., Somay, L., Varga, Á, Elek, Z., . . . Báldi, A. (2018). Climate-induced phenological shift of apple trees has diverse effects on pollinators, herbivores and natural enemies. PeerJ, 6. doi:10.7717/peerj.5269

Dynamics in Caribou Migration Patterns

Researchers at the University of Maryland have found an important dynamic underlying changes to Caribou (American reindeer, Rangifer tarandus) migration and population sizes. In recent decades, many herd populations in North America have experienced significant decline. Based on a large scale study that included data found from 80% of all migrating caribou over 20 years, researchers found that the amount of food resources available (ie, lichen) was not a driving factor in changing migration habits and that migration patterns showed significant changes only when it came to seasonal weather conditions and likely effects thereafter. This article discusses those underlying changes to an arctic keystone species as an example within the context of global climate change.

On dasher!, on dancer!, on prancer!,…… on dasher 25!, on dancer 25!…

Movin’ On Up: From Boreal Forest to Arctic Tundra

Caribou constitute the largest migrating mammal population in the world1. Their migrations surpass even the famous blue wildebeest migration in the savannas of Africa in terms of land distance covered as they migrate from wintering grounds in the boreal forest to summer calving grounds in the Canadian tundra to reproduce. Like many arctic animals, caribou have experienced significant decline in population as the climate has warmed. The lack of understanding of the specific connection between a warming climate and declining population have driven this research.

1. Kcutlip. “Who Goes Farthest? The World’s Longest Wild Animal Terrestrial Migrations And Movements.” College of Computer, Mathematical, and Natural Sciences, 25 Oct. 2019,
Surprisingly, an annual global migration of 9 caribou on December 24 – 25 was considered an outlier for this study and thus was not considered for the data set.

Availability of food resources can be a climactic and biotic factor in driving migration patterns. This was one suspected cause of these changes and subsequent declines of caribou from the current literature. As temperatures in the arctic have warmed significantly, forage is available at earlier times, plants can develop chemical defenses earlier, inedible shrubs can replace lichen in the tundra, and insects can harass large herbivores for longer periods of time that can lead to loss of foraging time2. These known factors led to hypotheses surrounding forage as an expected important dynamic in caribou decline.

2 Pearson, R. G., S. J. Phillips, M. M. Loranty, P. S. A. Beck, T. Damoulas, S. J. Knight, and S. J. Goetz. 2013. Shifts in Arctic vegetation and associated feedbacks under climate change. Nature Climate Change 3:673–677.
Caribou herds and migration locales

Calm and Warm Lead to Poor Maternal Health

Because of the large and varied data generated from collared animals from multiple herds and attempting to impress individual organism statistics to a local population, the researchers developed a complex model to test factors on altered migrations. The model tested factors such as: regional climactic patterns including ocean driven climate cycles (ie. North Atlantic Oscillation), seasonal snow coverage, and other weather data (ie. wind speed and temperature). The model weighed these variables compared to when in the spring the caribou left their wintering grounds (mainly in the boreal forest or taiga) to migrate to calving areas in the northern tundra of Canada and Alaska.

The scientists model showed inconsistent results from snow cover and vegetation when compared to migration times; an unexpected result meaning that earlier loss of snow cover or earlier emergence of vegetation could not account for earlier migration times. What did show significant effect was the weather in the previous summer on the migration times in the following spring meaning that something occurring during the warmer months on the tundra was causing delayed or earlier departure times from wintering grounds for migration.

Based on the study, researchers identified that maternal (cow) health was a primary factor in a changing arrival at calving grounds. The likely culprit for delayed arrivals and poor maternal health? Pesky insects – namely mosquitos, black flies, and bot flies.

The significant effects proposed by the authors shown in dotted red lines

The researchers model results showed how warmer weather during the summer combined with calm wind conditions resulted in a delayed migration time for caribou populations the following spring. The warmer temperatures and decreased wind speed would enhance insect harassment on mother caribou. A decreased ability to forage and build healthy tissue during the summer due to the insect attacks would likely delay migration starts the following spring because of decreased energy reserves and poor health of calves. Reinforcing this analysis, where weather data showed cooler summer temperatures and higher average wind speed, the following spring migration tended to be earlier and not delayed – a likely function of decreased insect harassment and improved maternal health.

Limited Say

What can we say with this study regarding changing migrations? While this study certainly constitutes a large-scale look at effects on caribou migration, the model the researchers used leaves many possibilities still frozen in the permafrost, so to speak. While their research was inconclusive when it came to forage availability, there were many unexplored tracts to this point. For example, snow cover was examined only from satellite data and leaves snow quality from the review which means that perhaps tramped down or iced over snow from warmer than usual weather events would be a contributing factor to altered migration times*. In addition, the specific rationale for earlier migration is still unconfirmed and should be duly tested. Perhaps this would involve a measure of caribou maternal body mass as a quantitative data point to connect previous summer weather with spring migration times. Specific tracking of individual cows and their experience surrounding insect harassment and measured health factors could add further support to the researchers findings.

*iced over or tramped down snow is easier to walk long distances on despite the decreased ability to forage underneath

Phenological Tracking and Plasticity

The significance of the changing caribou migration is just one example of how a keystone* species is being affected by a changing climate. Interestingly, the authors note how the migration time across the continent is relatively plastic for caribou; that is, having the capacity to change migration times strategically based on conditions. This capacity for changing migration times may help caribou “surf the snow line” (follow the emerging forage or stable ice for ease of commuting) but its ability to retain healthy herd populations with warming conditions remains to be tested as the arctic continues to rapidly warm and climactic conditions trend towards instability.

The rapid effects of anthropogenic climate change are driving alarming trends in what is known as phenological mismatch3 most prominently seen in arctic biomes. Warmer and earlier spring arrivals are causing producers (plants) to flower at earlier times often before the arrival of pollinators which can lead to population decrease as plants would lack their coevolution partner in reproduction and pollinators would miss an important food source. The effects on producers may in turn have consequences for larger herbivores such as the caribou.

*Caribou are keystone species in that many species in multiple trophic levels in the arctic ecosystem depend on them either as a food source, source of dispersal.
3 “Phenological Mismatch with Abiotic Conditions Implications for Flowering in Arctic Plants.” Ecology, U.S. National Library of Medicine, Mar. 2015,

Sources and Further Reads:

Gurarie, Eliezer, et al. “Tactical Departures and Strategic Arrivals: Divergent Effects of Climate and Weather on Caribou Spring Migrations.” The Ecological Society of America, John Wiley & Sons, Ltd, 12 Dec. 2019,

Kcutlip. “Caribou Migration Linked to Climate Cycles and Insect Pests.” College of Computer, Mathematical, and Natural Sciences, 12 Dec. 2019,

Saalfeld, Sarah T, et al. “Phenological Mismatch in Arctic-Breeding Shorebirds: Impact of Snowmelt and Unpredictable Weather Conditions on Food Availability and Chick Growth.” Ecology and Evolution, John Wiley and Sons Inc., 16 May 2019,, Helen C, et al. “Phenological Mismatch with Abiotic Conditions Implications for Flowering in Arctic Plants.” Ecology, U.S. National Library of Medicine, Mar. 2015,