The Amazon’s Extreme Seasonal Rhythm
The Amazon Rainforest
undergoes tremendous yearly fluctuations throughout the seasons. The rainy
season is when rivers overflow, quickly engulfing forest floors, turning large
areas of land into extensive flooded landscapes. The dry season is when water recedes,
creating exposed mud flats, narrowed channels, and dry, cracked dirt. Over
time, this pattern alters all habitats, creating a need for adaptation by all
species of both large and small size.
Life in the Wet Season: When the Forest Turns to Water
Between December and
May, due to the high amounts of rain, the Amazon Basin becomes a large
connected body of water, full of flooded forests (Várzeas) that can be hundreds
of miles long. This forces animals that usually live on the ground into tree
branches where they can access food high up in the trees. Another group of
mammals, including monkeys and sloths, depend on being in the trees to survive.
During this wet season,
fish are at their peak growth rates, with many types (such as tambaqui and
pirarucu) moving upstream to find food as the rains cause flooding to create
additional sources of food (fruits and seeds) for them. As a result, the
rainforest becomes an extensive nursery for juvenile fish, which have many
options for food and shelter. Even apex predator species like jaguars must
change their hunting behavior by swimming through flooded areas to hunt for
caimans and other unsuspecting fish.
Aquatic Adaptations: Thriving in a Water-World
Some animals, such as
the pirarucu, take advantage of seasonal flooding by using air-breathing
capabilities to traverse low-oxygen areas that few of its competitors could
survive in. River dolphins utilize their sonar skills to find fish in submerged
trees as they explore deep into flooded forests. Manatees, though quite slow
compared to the speed of floodwaters, would travel with the rising waters in
search of new aquatic plant growth.
The seasonal flooding
is not simply an obstacle for aquatic species; it is a significant source of
life for aquatic species that create new habitat for them to occupy and also
broadens the number of available habitats available to them.
Transitioning from the Rainy Season to the Dry Season: Once again, a new landscape; new problems
The coming of the dry
season means the end of the Amazon as it was. With the rainwater leaving the
system, the vast waterways that were once surrounded by dense forests slowly
disappear, leaving only fragmented bodies of water like pools and sandbars.
Many of those animals will have to adapt to a very different environment
because their relationships as predator/prey have changed due to diminishing
food supply. Only the most adaptable species of animals will be left standing.
Life During the Dry Season: When Water Disappears
As water gets less
available to fish during the dry season, fish living in temporary lakes and
rivers will struggle to survive in the dry months due to lack of oxygen in the
few remaining pools of water. The lungfish survives by burrowing into mud at
the bottom of lakes and entering into a state known as "aestivation."
The pirarucu can survive in shallow areas, where fish would usually die from
lack of water, by taking in air directly through its gills.
Reptiles such as
caimans also begin to defend themselves more aggressively as their habitat
shrinks and water becomes increasingly scarce. Their natural habitat is often
reduced to few remaining large lakes and lagoons. Due to their migratory
nature, it is common for female caimans to nest during this time of year, as
there is plenty of land exposed by the continual decrease of water levels.
Primates, herbivores, and birds have all developed ways to deal with limited food supply
The dry months of the
year are a time for low food supply for fruit- and leaf-eating animals. For
example, capybaras, tapirs, and deer migrate toward the last sources of water.
Also, when the quantity of fruit decreases, monkeys must change their diet to
include the tougher seeds and bark that are commonly available. Squirrel
monkeys congregate into large mixed flocks so that they can forage more
efficiently by taking advantage of the group's ability to collectively search
for remaining food.
The same changes occur
during the seasonal transitions in regards to the availability of food for many
species of birds. Most birds breed during the rainy season when insects are
plentiful. However, as the dry season approaches, many insect-eating birds must
adapt their diets or migrate to a wetter habitat. Alternatively, wading birds
benefit from high concentrations of fish during the dry season, making hunting
much easier.
Amphibians and Insects Are the Two Most Successful Animals Surviving in Microhabitats
Amphibian populations
thrive during their peak reproductive periods in the breeding pools created by
seasonal rainfall. Frog species produce extremely loud vocalizations during
their reproductive cycles and the noise produced in a rainforest likely fills
the forest with sound at certain times of each year. As the rainforest dries
out (after rain), a large number of amphibian species dig burrows, crawl under
leaf litter or find an area under the shaded crevice of a tree to escape or
otherwise retard water loss (dehydration). Some amphibians remain active even
after rainfall because they forage for food within small humidified microhabitats.
Insects react rapidly
to the changes in seasonal weather patterns. During rainy seasons, there is an
explosion of insect populations and they become a highly important source of
food for insectivores. When the dry season ends and the number of insects drops
due to lower humidity and temperature, many insectivores also move to moist
areas or other localised moist habitats to obtain food. Leafcutter ants live
(forage) year round and base their foraging activities and route decisions on
the distribution of vegetation.
Floods Provide Bounty, Drought Requires Strategy.
The seasonal extremes
of the Amazon play an extremely important role in the ecosystem - floods
produce tremendous growth for plants and animals as well as new places to mate
and a place for them to feed; and in drought years, create the conditions under
which animals and plants may adapt themselves to survival, by changing their
migration routes, their feeding habits, and/or their behaviours. The annual
flooding and drying cycle creates balance, movement and diversity within the
amazon jungle.
Threat of Climate Change to a Delicate Balance of Seasons
The wildlife of the
Amazon relies on predictable cycles of wet and dry seasons for their survival -
however, the impacts of Climate Change are now disrupting the rainfall pattern
of the Andes mountain range by extending droughts in certain areas and
producing heavier-than-normal rainfalls in other areas than have been
historically recorded. Since some species depend on timing to survive, these
changes could prove catastrophic for their continued existence (e.g. Fish
spawning during rising water levels, Frog reproduction occurring following the
initial rainfall of the year).
Should these cycles of
seasonal weather patterns be significantly compromised, the resultant
disruption could destroy the balance on which all living organisms rely.
Therefore, understanding how the Wildlife in the Amazon is surviving under the
present natural climate cycles demonstrates a need to preserve and protect the Amazon
from further disruption of the balance that has existed in this region for
thousands of years.
Resilient Spirit of The Amazon
While their environment
continues to be challenged by human activity, the Wildlife of the Amazon
exhibits remarkable resilience. Each group of Organisms (i.e. Fish, Mammals,
Birds, Reptiles, Amphibians, Insects) has evolved to successfully navigate the
persistent and drastic fluctuations in environmental conditions found in the
Amazon's rainforest; therefore, the Evolutionary History of these organisms
(and all the Amazon's wildlife) serves to collectively demonstrate that
Adaptation to change will continue to flourish through resiliency - also, this
remarkable adaptability serves to highlight the delicate beauty of the world’s largest
rainforest.
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