Laura Hoyt
12-9-12
The Effects of Increased Drought on the Diversity and Distribution of Organisms
Climate change is the long-term shift in weather patterns, in a specific region or globally
(Perlmutter and Rothstein 2011). These changes could be in factors such as precipitation,
temperature, cloud cover, and many others. There is some debate over the cause of current
climate change, with many believing that anthropogenic activities have the greatest influence and
others natural causes. One of these human actions is the release of greenhouse gasses into the
atmosphere, leading to an increase in temperature called global warming. The basic mechanism
of global warming is that greenhouse gasses absorb solar radiation and help to trap heat near
surface of the Earth, creating a warming effect. Over the course of many years, these two forces,
climate change and global warming, can lead to dramatic changes in precipitation across the
globe. This could result in more intense precipitation in some areas, and in others drought. The
causes of drought are uncertain, but are thought to be related to a change in atmospheric
circulation where storm tracks move farther from the equator as the Earth warms (Schmidt and
Wolfe 2009). Still, the effects of drought can be far reaching and are predicted to have an
enormous impact on organisms in drought affected regions. As drought becomes more common
in these areas, the biodiversity of both plants and animals is expected to decline, while their
distributions could change dramatically.
Biodiversity is an important measure of the health of an ecosystem, generally a higher
level of diversity leads to a healthier ecosystem. This is because life on earth is extremely
ordered and structured, with every organism found in specific locations playing particular,
interconnecting roles (Schmidt and Wolfe 2009). In addition, biodiversity provides an ecosystem
with more genetic diversity throughout its species, allowing it to better adapt to change. As a
result, a shift in just one species can have a drastic effect on its ecosystem, often making it more
vulnerable and more susceptible to changes (Schmidt and Wolfe 2009).
In the future, plant diversity is expected to decrease in areas modified by climate change
induced drought. A study on the potential impact of global warming on Mediterranean shrub
land found that it might, “change inter-specific competitive relationships,” in favor of more
drought resistant species (Prieto et al. 2009). They also found that after a disturbance, like a
wildfire, the transformation from shrub to pine tree dominated vegetation slowed, which could
lead to a decline in the abundance of pine species, as drought induced fires become more
common. In a similar study, where areas of mesic grassland in the Pyrenees were transplanted to
lower elevations to simulate drought and warming, researchers found a strong shift in plant
diversity (Sebastia et al. 2008). Specifically they noticed that many dominant and rare grass
species declined significantly in the new conditions, while other previously rare species began to
thrive. This indicates that in the coming decades drought and climate change could cause a
decline in the abundance of both dominant and rare species, while aiding a few uncommon plant
species. As a result an ecosystem composed of a wide variety of plants could be replaced by one
with selection of well adapted species, which would mean a huge loss of plant diversity. And in
South Africa, an area expected to be hit hard by drought in the coming years, researchers also
noted a decline in biodiversity due to an increase in drought resistant plant species under
simulated drought (Hoffman et al. 2009). All of these studies were done in distinct ecosystems,
yet they all had similar findings, in the future drought resistant plants could begin to outcompete
other species, leading to a loss of diversity.
Similarly, an increase in drought is also predicted to decrease animal diversity. In a study
measuring the vulnerability of fifteen animal species to climate change in Coronado national
forest in Arizona, scientists observed that vulnerability was highest in higher-elevation
woodlands and lower in desert scrub (Davison et al. 2012). Comparable to plants, these results
indicate that future drought might allow desert animal species to outcompete other species,
particularly those adapted to higher elevations. This would result in a net loss of species from an
ecosystem, reducing its diversity. Another study on the effects of drought on freshwater
vertebrates in 350 rivers across the United States reported similar declines in animal diversity in
response to drought (Spooner et al. 2011). During the experiment, researchers reduced the flow
to sections of these rivers to mimic predicted water levels in 2070. They then observed how large
an impact drought conditions would have in the diversity of fish species in each of the rivers.
Their results show a large increase in the number of fish species extirpations under drought
conditions, highlighting the susceptibility of freshwater species to drying conditions (Figure 1).
Underground invertebrates could also feel the harmful effects of drought. In a thirteen year
experiment where patches of ground were put under false drought conditions, scientists found a
35-65% loss of enchytraeid species in the plots (Holmstrup et al 2012). They also reported that
species loss was highest in enchytraeids that had weaker migratory potentials and in the driest
patches.
Figure 1. The effects of drought on fish species extirpations (Spooner et al. 2011).
As global warming and climate change progress, many believe that global precipitation
patterns will be severely altered. It is thought that there will be an increased variation in
precipitation everywhere, with more extreme fluctuations between precipitation and drought. In
addition, climate scientists Shugart and Woodward state that in the future there are expected to
be, “increases in precipitation at the warming high latitudes but significant reductions in rainfall
at the mid-latitudes” (Shugart and Woodward 2011). This along with a predicted increase in
precipitation in equatorial regions means that current dry areas will probably become even drier,
while wet areas become wetter.
The distribution of plants across the globe is hypothesized to change with shifts in the
precipitation pattern. So, as the mid-latitudes become drier, desert plants will expand their range
and force plants adapted to more rain further towards the poles. In addition, some studies have
shown that harsher droughts could aid the spread of invasive and exotic plants. In a study that
used satellite images to examine the impact of drier weather on a region of California, scientists
reported that after periods of drought damaged native vegetation, exotic plants could more
efficiently invade (Elmore et al. 2003). Another study that used models to predict how drought
and climate change will alter plant distributions in Western Australia stated that, “between 5%
and 25% of species were projected to suffer range losses of 100% by 2080… with the greatest
losses of species projected to occur in a transition zone between wet coastal areas and interior
arid regions which is projected to become more arid in the future,” (Fitzpatrick et al. 2008). The
projected loss of these species implies that, in the future, drier weather could cause the ranges of
many species to collapse instead of move. In areas that are expected to have drier conditions in
the future, the range of forests is predicted to change as well. A study that used model to depict
plant distributions in fifty-five countries on five continents under different climate scenarios,
noted that the Ukraine showed a 39-47% decrease in forest area (Dixon et al. 1999). In addition,
Venezuela had a 40-50 x 10^6 increase in tropical dry and extremely dry forests, replacing
wetter jungles (Figure 2). Both of these alterations in forest range were correlated with a rise in
drought.
Figure 2. Current and predicted plant distributions in Venezuela due to climate change.
Comparable to plants, the distribution of animal species is also expected to change
drastically in response to drought. Desert species will probably also expand their ranges as the
mid-latitudes dry. Yet, many believe that arid species are already at the limit of their drought and
heat tolerance as they live in their current environments, and that as deserts become even drier
they could lose some of their habitats (Barrows 2011). Still, in many areas they will gain territory
as drying environments become unsuitable for current animal species. An experiment modeling
the ranges of twelve vertebrates in North Queensland Australia reported that the distribution
areas decreased by 50% on average (Meynecke 2004). This result is somewhat unique, since
Australia is an island, which will prevent its animal species from moving with new precipitation
patterns (Figure 3). However, it does illustrate how drastically the ranges of some species could
be reduced due to drought in other regions where the movement of animals is restricted.
Figure 3. The current and predicted ranges of H. albispecularis from several climate models (Meynecke 2004).
Many recent studies indicate that climate change will radically alter life on Earth, and
that drought will play a significant role. Throughout the mid-latitudes it could decrease the
biodiversity of both plants and animals, weakening ecosystems and making them more
susceptible to invasive species. It could also shift the ranges of many organisms, benefiting some
species and collapsing the ranges of many others. The effect that this will have on humans is
uncertain. However, it is clear is that as the environment adapts to more frequent and severe
droughts, we will be forced to change as well.
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