Image © 2009 Jupiterimages Corporation.Living things are intimately connected to their physical surroundings. Even small changes in the
temperature of the air, the moisture in the soil, or the salinity of the water can have significant
effects. Each species is affected by such changes individually, but those individual impacts can
quickly reverberate through the intricate web of life that makes up an ecosystem.
In particular, two important types of ecological impacts of climate change have been
observed across the United States: shifts in species' ranges (the locations in which
they can survive and reproduce), and shifts in phenology (the timing of biological
activities that take place seasonally). Examples of these types of impacts have been
observed in many species, in many regions, and over long periods of time.
As Earth warms, many species are shifting their ranges to areas with more tolerable
climate conditions, in terms of temperature, precipitation, and other factors. About
40 percent of wild plants and animals that have been studied over decades are relocating
to stay within their tolerable climate ranges. Some organisms—those that cannot
move fast enough or those whose ranges are actually shrinking—are being left with no place to go. For example, as arctic sea ice shrinks,
so too shrink the habitats of animals that call this ice
home, such as polar bears and seals. As these habitats
contract toward the North and South poles, the animals
that depend on them will reach the end of the Earth as
they know it.
Plant hardiness zone maps, used by gardeners to determine which areas are suitable
for certain plants. Warmer colors indicate warmer zones. A new map was created in
2006 to reflect changes in climate since the 1990 map was created.
2006 map courtesy of the National Arbor Day Foundation.
Climate change is also driving changes in the timing of
seasonal biological activities. Many biological events,
especially those in the spring and fall, are based on
seasonal cues. Studies have found that the seasonal
behaviors of many species now happen 15–20 days earlier
than several decades ago. Migrant birds are arriving
earlier, butterflies are emerging sooner, and plants are
budding and blooming earlier.
If all of the species in an ecosystem shifted their seasonal
behavior in exactly the same way, these shifts might not create
problems. But when a species depends upon another for survival
and only one changes its timing, these shifts can disrupt important
ecological interactions, such as that between predators and their prey.
For example, a small black-and-white bird called the European pied
flycatcher has not changed the time it arrives on its breeding grounds even
though the caterpillars it feeds its young are emerging earlier. Missing the peak
of food availability means fewer chicks are surviving, in turn causing the flycatcher's
population to decline.
In addition to shifting ranges and seasonal behaviors, other ecological impacts of climate
Change—some of which will appear in the examples described in this booklet—include changes
in growth rates, in the relative abundance of species, in processes like water and nutrient
cycling, and in the risk of disturbance from fire, insects, and invasive species.
Winners and Losers?
The ecological impacts of climate change are not inherently
"bad" or "good." The concept that a change is beneficial or
detrimental has meaning mainly from the human perspective.
For an ecosystem, responses to climate change are simply
shifts away from the system's prior state.
Other Human Activities Compound the Effects
of Climate Change
Plants and animals are simultaneously coping with climate change and many other human-created
stresses. Rivers—many of which are polluted by fertilizers or other chemicals—are
dammed to provide water for crops or for people. Roads, cities, and farms break up habitats
and migration routes, and human activities carry nonnative species into new ecosystems. Many
of the species and ecosystems described in this booklet are being affected by these other human
influences in addition to those related to climate change.
Ecosystems are generally resilient to some changes. For example, they can often cope with a
drought or an unusually hot summer in ways that alter some aspects of the ecosystem but do
not cause it to change in a fundamental way. When such changes remain within the limits of
an ecosystem's resilience, the ecosystem may not appear to be affected. There is often a threshold
point, however, that results in dramatic transformations. Such threshold points are like the
moment when water overtops a levee. As long as the water level is even slightly below the top,
functioning is normal. But once it rises above the levee, there is a flood—permanently transitioning
the ecosystem into a new state. The many ways humans have altered the planet could
act as compounding factors that make it harder, or even impossible, for already stressed species
to adjust to climate change.
Biodiversity and the Permanence of
Extinction
Artist's depiction of the mastodon,
driven to
extinction by human hunting
compounded by a
reduction in habitat due
to climate change.
Ecological processes—even those that seem to represent
the activities of a single species—depend on interactions
among an interconnected web of vital and unique species.
Honey, for example, is produced in a beehive, but the bees
depend on pollen and nectar from the plants they pollinate.
These plants, in turn, depend on the bees that pollinate
them, the worms that aerate the
soil, the microbes that release
nutrients, and many other
organisms. This diverse array
of creatures is key to the functioning
of the entire system.
Among all the possible impacts
of climate change on ecosystems,
the most permanent is extinction.
Once a species is lost, it
cannot be recovered. And since
no species lives in isolation, its
entire ecosystem can be affected.
The number of extinctions
caused by climate change so far
may be small, but if a level of
warming occurs in the range of
3.6 to 5.4°F—somewhere in the
low-to-mid projected range—it is
estimated that about 20 to 30
percent of studied species could risk extinction in the
next one hundred years. Given that there are approximately
1.7 million identified species on the globe, this ratio
would suggest that some 300,000 to 600,000 species could be
committed to extinction—primarily as a result of human
activities.
This web page is based on the National Academies' educational booklet
Ecological Impacts of Climate Change.
