thus enhancing efficacy of cross- pollination. Some tree
species synchronize their flowering after downpours,
an adaptation that may increase pollination efficiency
by concentrating the number of pollinators. These
patterns reflect the reality that most species of tropical
trees rely on animals to cross- pollinate them, rather
than wind- blown pollen dispersal, which is common
in many temperate forest tree species (oaks and pines,
for example). Dry season pollination also enables more
seedlings to survive, because they sprout at the onset of
rainy season, when there is adequate moisture available
to ensure their initial growth.
Patterns of plant reproduction vary, even within
a single area. A study by N. C. Garwood of 185
plant species on Barro Colorado Island in Panama
determined that most seedlings emerged within the
first two months of the eight- month rainy season.
That seems logical, given the plants’ need for water
to grow. Forty- two percent of the plant species
studied underwent seed dispersal during dry season
and germination at the onset of rainy season. Forty
percent of the species experienced seed dispersal
at the beginning of rainy season, with germination
occurring later in rainy season. So although time of
seed dispersal varied (likely due to how the seeds were
dispersed), germination always occurred at some point
in rainy season. Approximately 18% of the species
produced seeds that were dispersed during one rainy
season, dormant during the next dry season, and
germinated at the onset of the second rainy season. The
species most sensitive to the onset of rainy season were
“pioneer” tree species, lianas, canopy species, and both
wind- and animal- dispersed species. Understory and
shade- tolerant species were less sensitive.The Increasing Prevalence of ENSO
in the TropicsBeginning in the mid- 20th century, short- term but
major climatic shifts began happening with increasing
frequency in the tropics. The shifts are due to ENSO,
El Niño/Southern Oscillation. (Note that the original
term for this climatic event was simply El Niño, but
with increasing understanding of what is actually
happening, the term now in use is ENSO.) ENSO,
though becoming more frequent, remains difficult to
fully explain with regard to exact cause. El Niño occurs
approximately every two to seven years when a high-pressure weather system that is normally stable over
the eastern Pacific Ocean breaks down, destroying
the pattern of the westward- blowing trade winds. The
trade winds weaken and sometimes reverse from their
normal westward direction. Warm water from the
western Pacific flows eastward, causing an influx of
abnormally warm water toward the western coast of
South America. This water column reaches a depth of
up to about 150 m (approx. 500 ft) and, because it is
warm and thus less dense, flows over and blocks the
colder, more nutrient- rich waters below. This prevents
the upwelling of nutrients into the upper water column,
where they become available to phytoplankton. The
result is that oceanic food chains are severely disrupted.
When that happens, warm waters flow along the
normally cold South American coast. Weather systems
change, resulting in heavy downpours and flooding
in some regions and droughts where there should be
rainfall, effects which range ecologically from mildly
stressful to highly significant.
The El Niño of 1982– 83 was considered at the time
to be the most powerful of the 20th century, and is
estimated to have caused $8.65 billion worth of damage
worldwide. There have been eight major El Niño events
since 1945, including several since 1982, and their
frequency seems to be increasing. A severe El Niño
occurred in 1986– 87, and another occurred in 1994–
95, comparable to the two of the 1980s. Satellite data
indicated that the northern Pacific Ocean was nearly
20 cm (8 in) higher than normal, due to the influx of
warm surface waters. Yet another El Niño occurred in
1997– 98, and its combined global effects are estimated
to have resulted in 2,100 human casualties and property
damage totaling a staggering $33 billion. In 2009– 10
another El Niño altered weather patterns around the
world. The winter of 2015– 16 was characterized by
the most significant ENSO event ever recorded, with
massive amounts of rainfall and flooding in the Pacific
Northwest and a severe dry season in parts of the
tropics such as Panama (plate 2- 2).
The specific causal factors responsible for the
periodicity of El Niños are thus far unknown but it
is clear that the Intertropical Convergence zone, a
complex system of oceanic and air currents, migrates
to a lower latitude, raising sea surface temperatures
and destroying the normal upwelling pattern along
the western coast of South America. The cessation of
an El Niño occurs when the ITC returns northward to
its normal position. Though El Niño has global effects,chapter 2 why it is hot, humid, and rainy in the tropics 35