CLIMATE CHANGE COULD HAVE MANY EFFECTS
climate change is being tracked and ecological changes have been confirmed
by ecologists in many ways. Researchers have documented premature flowering
in some plants, earlier nesting by wood ducks, and some reptiles emerging
from hibernation in the winter. I have written about the following phenomenon
before. It is yet another way in which gradually rising temperatures
could affect the world's organisms over a few decades.
of environmental temperatures on animals throughout the world is incontestable.
One way is by altering sex ratios in some species. For example, in some
turtles and alligators one sex is produced at high incubation temperatures
and the other at low temperatures. Jim Spotila of Drexel University
has even suggested that the influence of temperature on sex determination
may have caused the extinction of some dinosaurs. At the end of the
Mesozoic era, major temperature changes occurred on a global scale.
If incubation temperatures determined sex in some of these ancient reptiles,
when average temperatures throughout the world rose or dropped several
degrees over a short period of geologic time, some species may have
begun producing young of only one sex. If so, whether male or female,
the final result would be no more mating. And no more offspring.
coastal fish, the Atlantic silverside, also becomes male or female depending
on temperature. Although genetics has an influence, the sex of these
fish is determined to some degree by the temperature of the water during
the larval period. Along the South Carolina coastline, silversides born
during the cool temperatures of spring are predominately female. Those
born during summer are mostly male. Because silversides are born during
both spring and summer, over the course of a year approximately equal
numbers of males and females are produced. If ocean temperatures continue
to rise, the silversides in South Carolina would presumably shift toward
being all males. Not a good strategy for any species. But one study
with silversides suggests that the species can adjust its sex ratio
in response to rising temperatures.
studying Atlantic silversides in Nova Scotia found that at that latitude,
temperature has no effect on sex determination. Instead, sex is determined
genetically. In between South Carolina and Nova Scotia, along the New
York coast, cold water still results in a shift toward more females,
but not as dramatically as further south. This suggests that an intermediate
state exists between genetic and environmental influences.
sex ratio is characteristic of most animal populations, with one female
being born on average for every male, although temperature clearly can
affect the sex ratio in silversides. In laboratory experiments, researchers
raised thousands of Atlantic silversides at constant high or low temperatures,
simulating those temperatures most likely to produce males or females
in South Carolina and New York. After each generation, the scientists
determined the number of each sex. Some of the experimental populations
started with many more of one sex than the other. But, sure enough,
after several generations each experimental population had reached a
balanced sex ratio of the same number of males as females, regardless
of the water temperature.
if left at a constant temperature, silversides from South Carolina and
New York should have an excess of one sex or the other. Yet the sex
ratio gradually became balanced during the experiments. The explanation
is a complex relationship between genetics and environmental conditions,
and how the adjustment is made is not completely understood. The findings
with Atlantic silversides emphasize the subtlety of the response of
organisms to environmental change and also confirm the complex and sensitive
balance in which the earth's ecosystems rest.
because Atlantic silversides can adjust their sex ratio with changing
temperatures should not make us complacent about what might happen if
we experience the global warming trend predicted by many scientists.
Will all life on earth be able to adjust to global warming the way silversides
can, or will some go the way of the dinosaurs?
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