The Effect of Global Warming on Animals
Over the last 100 years, the average global surface temperature has increased
approximately 0.8oC. Even with this amount of warming, which seems small
compared to what might occur in the relatively near future (2 o to 6oC or more
increase by 2100), wild animals are already exhibiting discernible changes. This
is because all living things are affected by temperature in one way or another.
Several types of changes have already been seen in the wild, including: 1) shifts
in ranges boundaries (e.g., moving north in the Northern Hemisphere) and/or
shifts in the density of individuals from one portion of their range to another (e.g.,
the center of the abundance pattern moving up in elevation), 2), shifting in the
timing (i.e., phenology) of various events primarily occurring in spring and/or
autumn, 3) changes in the genetics, behavior, morphometrics (e.g., body size or
egg size), or other biological parameter, and 4) extirpation or extinction, the latter
of which is the final irreversible change. Given what is known about physiological
requirements of species, these changes are consistent with those expected with
increasing ambient temperatures.
Changes in ranges and shifting densities
The temperature occurring at any given location can greatly influence whether a
particular animal is present or absent in the habitat around that location. As the
globe warms we find that species are extending their ranges poleward and up in
elevation. The reason for these shifts is that habitats previously avoided by
various species have now warmed sufficiently to allow colonization. For
example, 35 of 56 (65%) non-migratory butterflies in Europe have been found to
be expanding their ranges northward this century by 35-240 km, while only 3% (2
out of 56) are shifting southward.
These types of changes in species ranges are of concern for at least three
reasons. First, the synergistic effect between rapid global warming and habitat
modification is probably the most important problem facing species today. This
will become an even larger problem in the future because temperatures will
continue to rise quickly and as the human population grows we will continue to
modify more landscape. The movements (dispersal) of species forced by rising
temperatures are frequently slowed and often blocked by human modifications of
the landscape. Dispersing individuals must not only find suitable habitat through
which to travel, but appropriate habitat in which to colonize. This is relatively
easy for highly mobile species like butterflies, birds and bats, but certainly
scorpions, salamanders, and shrews an the like will have trouble navigating
across highways and through farm fields or cities. When temperatures were
increasing in prehistoric times, the rate the temperature was rising was
significantly lower and the habitat species were moving through was not modified
in the manner it is today. Second, species near the poleward side of continents
(e. g., South Africa fynbos) will have no habitats into which they can disperse.
Additionally, the same will be true for species occurring in high mountain
habitats. Once they reach the top of the mountain there is nowhere else for them
to go—unless humans intervene and translocate them to some remaining
suitable habitat. Third, in addition to not having habitat into which they can
disperse, species living at the tops of mountains and poleward side of continents
will have the added stress of species from lower latitudes and altitudes invading
their habitats and thereby creating new competitive situations.
All three of these reasons can significantly increase the danger of extinction. For
example, pikas are currently living in montane habitats in western North America
where the ambient temperature is quite close to the maximum this small mammal
can endure. Moving up in elevation to cooler regions is not possible because
the type of habitat needed by pikas is not available. Another example is a
subspecies of a checkerspot butterfly in Baja California. It will probably go
extinct in the near future because it too has a low tolerance to hot temperatures
and cannot shift in to cooler regions because Tijuana and San Diego are blocking
its way. Fourth, species movements will be independent of shifts of other
species. The reason for this is that the physiological constraints of temperature,
food availability, and many other factors are unique for each species. This
independence will become more and more evident the higher the temperature
goes. The independent movement will likely cause the disruption of species
interactions. As an example, if, on the one hand, the range of a predator shifts
and the range of its prey does not, then this could be a benefit if the prey is an
endangered species. Without the predator, the abundance of the endangered
prey could increase. If, on the other hand, the prey is a pest on one of our food
crops, then the increase in its population due to the movement of its predator
could certainly become detrimental.
Changes in timing
The timing of many different biological events is strongly influenced by warming
temperature. Such events for animals include the timing of: hatching or birth,
arrival after spring or fall migration, arousal from hibernation and sexual
receptivity. Using the timing of spring events as an example, coauthors and I
found three different patterns. First, we found that species are acting
progressively earlier. Over the last 30 years, around 700 species (plants and
animals together) from locations around the globe were found to be changing the
timing of a spring event by around 5 days earlier per decade. Second, species
are acting later, but this is quite unusual, being that only 6 out of the same 700
species (<1%) showed this type of change. Third, species are exhibiting no
change in the timing of their spring events. The number of species in this group
is very difficult to determine. Scientist and journal editor rarely report on species
not changing, unless this information is by chance reported along with
information addressing change in another species. For migrating birds, I believe
that the unlike the birds showing change, the unchanging species are probably
cueing on day length rather then climate as a migration trigger. More study is
needed, however, on the topic of migration triggers.
Rapid phenological changes of species are of concern because for over tens of
thousands of years or more, animals have been adjusting to the timing of those
species around it. For example, as the globe warms farmers may have to
change the timing of their planting and might even change the type of crop
grown. Either of these changes could provide an insect with a food resource that
was previously limited, thereby allowing the population size to grow. If the insect
feeds on the nectar from the flowers of the crop, then the farmer could
experience a benefit owing to the plants being pollinated. If, however, the insect
feeds on the tissue of the crop plant, then the increasing size of the insect
population could be seen as a detriment that must be countered in some manner
(e.g., pesticide). In wild communities, changes in timing could mean that a food
source of a species is not available at the time it is needed. This in turn could
cause the species stress, either in time and energy looking for food or in
competitive interactions over the little food available. Such stress may lead to
lower fecundity rates, which, if not rectified could lead to extinction.
Changes in Genetics, Behavior, Morphometrics, etc.
Most studies concerning how rapid global warming has influenced animals focus
on range and phenological shifts. The number of studies, however, are
increasing that show other factors, such as genetics and behavior, are also
changing along with rapidly warming temperatures. For example, Bradshaw and
others found day-length cue that initiates dormancy in a mosquito in eastern
North America is genetically controlled. With global warming the habitats where
this mosquito is found are staying warmer longer in the fall. Based on
temperature, dormancy can occur later in the season, when the day length is
shorter. As a result the genetic control of the day-length cue has changed to a
shorter day length. Normally a very large number of generations is required to
observe genetic changes. Consequently, enough time has not yet past for us to
observe genetic changes in most wild species. Indeed, the concern is that the
genetic changes needed for the survival of a species may not happen quickly
enough to “keep up” with the rapidity at which the planet is warming. Without a
change in gene frequency and not being able to disperse into hospitable habitat,
a species could be heading toward extinction.
An example of a behavioral change associated with global warming is the
foraging habits of polar bears (Ursus maritimus). As the globe has warmed,
these bears are increasingly foraging in garbage dumps rather than on seals.
This change in behavior arose in response to the bears being less able to
capture seal, their regular prey, because to hunt seals requires bears to be
standing on sea ice. With global warming the ice is thinning and melting earlier
in the spring, before the bears have put on enough depot fat to get them through
the winter. By necessity bears now forage, unfortunately, much more at dumps.
The type of food and the quantity are not sufficient to sustain the previous
number of bears. Hence, numerous bears have been found dead or lacking the
nutrients needed for reproduction. The population size of these bears has
dropped. Additionally, other animals that depend on the polar bear as a keystone
species (e.g., arctic fox (Alopex lagopus) and ivory gull (Pagophila eburnean))
may also be in significant trouble as the bears catch fewer and fewer seals,
leaving fewer and fewer carcasses on the ice for these other scavengers.
Extirpation and Extinction
The size of a species’ range, the density of individuals within their range, and
how rare or common a species’ preferred habitat is within its range are all
important factors influencing a species risk of extinction. All of these factors can
be directly affected by global warming. Species with small ranges, low densities
and few preferred habitats have a much higher probability of extinction.
Extirpation (a population in a given location disappears) of populations on the
hotter side of its ranges reduces the overall range size; a step in the direction of
extinction. Additionally, extirpation decreases the density of individuals, which is
again a step in the direction of extinction. For species able to extend their
ranges—appropriate habitat is available and they are not at the poleward edge of
continents or at the tops of mountains—rapid global warming will most likely only
cause extirpation of populations in the warmest portions of their ranges. At the
same time the populations dispersing and colonizing in high latitudes and
altitudes could easily counter this decrease in population size.
Because temperatures are increasing rapidly around the globe, basically all of
the species in the world are going to be affected. With “only” about 0.8 oC
increase in the average global temperature thus far, numerous species are
already speeding toward extinction. These “facultatively extinct” species are
those that cannot move to a different location as the temperature increases. This
includes species occurring: at the poleward edge of continents, on tops of
mountains, or on islands, whether they are surrounded by water or inhospitable
land (e.g., desert or city). For example, the Mallee Emu-wren (Stipiturus malle) is
quite sedentary (rarely moving farther than 5 or 6 km) and has a small
fragmented range that is threatened by fires. It cannot move until its habitat
moves, which will likely be much slower than the speed at which the emu-wren
will need, given the rate of temperature increase. Unless humans intervene and
translocate individuals to suitable habitat farther south, this bird will most likely go
extinct within the next 25-50 years. Unfortunately only about 2000 km2 of
suitable native habitat is available today, and creating suitable habitat farther
south, which will require 2-3 year old spinifex grass (Triodia irritans). After the
birds are moved to a new habitat, then we would need to ensure that both the
habitat and emu-wren survive, which will necessitate prohibiting a fire cycle that
is less than 10 to 15 years. Such interventions by humans would be difficult and
likely not practical for this species or for other species in the same
circumstances. We do not have money, land, personnel, or political will with
long-term commitment to accomplish such an enormous task. This is why many
scientists are predicting that we are standing at the brink of a mass extinction
that will be caused by one very careless species. Projections are that between
15% and 40% of all species alive today will go extinct due to human
enhancement of atmospheric green house gases. Given that there are
somewhere between 5 million to 50 million species (only around 1.75 million are
described), that means somewhere between 750,000 and 2 million or 7.5 million
to 20 million species could go extinct primarily due to our use of fossil fuels and
the dumping of their combustion products into the atmosphere as if it were an
unpriced sewer.
Further Reading
Bradshaw, W. E., S. Fujiyama and C. M. Holzapfel. 2000. Adaptation to the
thermal climate of North America by the pitcher-plant mosquito, Wyeomyia
smithii. Ecology 81: 1262-1272.
Derocher, AE, N. J. Lunn and Stirling, I. 2004 Polar bears in a warming climate.
Integrative and Comparative Biology.44:163-176.
Grayson, D. K. 2000. Mammalian responses to Middle Holocene climatic
change in the Great Basin of the western United States. Journal of Biogeography
27: 181-192.
Parmesan C., and G. Yohe. 2003. A globally coherent fingerprint of climate
change impacts across natural systems. Nature 421: 37-42.
Reynolds, A. 2002. Warnings from the Bush. Climate Action Network Australia.
http://www.cana.net.au/bush
Root, T. L., J. T. Price, K. R. Hall, S. H. Schneider, C. Rosenzweig, J. A. Pounds.
2003. Fingerprints of global warming on wild animals and plants. Nature 421:
57-60.
Terry L. Root