Crab Connection: Crabs in Trouble Equals Cranes in Trouble!
Importance of Fresh Water for People and Wildlife
eat such crabs? See Blue and
Cranes are in trouble. Tom Stehn, Whooping Crane Coordinator for the U.S,
writes from Aransas
National Wildlife Refuge: "The reduction of freshwater inflows
is a huge threat to the Whooping crane that could lead to extinction."
What's going on? What is causing freshwater inflows to be reduced, and
what does it have to do with cranes? We'll explore
number one threat to Whooping crane survival. We'll also learn about some
simple things we can do to help protect cranes, and help protect the rivers
wherever we live.
Background Information: Balanced Diet
Aransas National Wildlife Refuge, an estuary where blue
crabs (and the cranes that eat them) should flourish.
eat a wide variety of foods, from frogs, snakes, and an occasional fish
to seeds, fruits, and tubers from plants. Even though they eat many
things, every winter they need to eat mostly one special thing: blue
crabs. When there are enough blue crabs, cranes may eat 80 every
day! In the best winters, 80-90% of the Whooping crane diet is blue
crabs have so much protein and other nutrients that they get crane bodies
in top condition. When cranes have to switch to other foods in winter,
sometimes get less energy than their bodies need for their daily activities.
Without enough blue crabs, some cranes die. The cranes that
do survive are less healthy than
they should be; they're often in too poor condition to reproduce after
spring migration to the nesting grounds.
Now look at this table. It shows the results of an
8-year study where scientists compared Whooping crane winter deaths
to the quantity
of blue crabs available.
whooping cranes eat many other foods, especially acorns, clams,
and other kinds of crabs, blue crabs are by far their most important
Crab Supply for Cranes
of Cranes that Died that Winter
Ups and Downs: Natural Populations
San Antonio River feeds into the Guadalupe River outside of this map.
The now-larger Guadalupe River feeds into the San Antonio Bay at the green
arrow. The imput of freshwater into the bay and surrounding marshes keeps
the salinity at a good level for blue crabs.
live in tidal estuaries where the land meets the
sea. The water in estuaries comes partly from rivers emptying into the
ocean and bays.
So estuary water is brackish—salty, but not nearly as salty
as in the ocean.
The rivers that run into Aransas National Wildlife Reserve are the San
Antonio and the Guadalupe Rivers. They provide the main source of fresh
the estuary, which keeps the salinity (saltiness) low for enough for blue
crab survival. These rivers also carry nutrients and sediments that
the crabs need to stay alive. Some years the river levels are high, other
years low. These natural cycles affect the crab population.
Natural cycles of the ocean, too little or too much rainfall on land, and
all kinds of other things can make the blue crab population fall or rise
in one place or another. Drops in blue crab numbers in one place wouldn't
matter if there were thousands of Whooping cranes spending the winter in
many different places. But Whooping cranes belong
to a badly endangered species. The entire surviving wild-breeding population
spends the winter in Aransas National Wildlife Refuge. If anything bad
happens to the blue crabs at Aransas, it hurts EVERY wild whooping crane!
If virtually no Whooping cranes breed in a year, it hurts their population,
but isn't a disaster. If they can't breed two years in a row, it becomes
very serious. Three years in a row, and the population could fall so dangerously
that it might never recover. And if this is happening when cranes are dying
in Aransas because of poor feeding conditions, it can be disastrous.
Ups and Ups: Human Population Growth Impacts Crabs and Cranes
Crane numbers steadily rose for many years. Then what happened? Do we
have the time and the resolve to save cranes?
go up and down, but the population of one species just goes up and up.
That species is us!
We humans have mastered so many wonderful methods for providing ourselves
and our children with food, shelter, and protection from natural disasters
that our population is steadily increasing. The human population in Texas
alone is expected to double in the
next 50 years. What does this mean for Whooping cranes and blue crabs?
Think about all the uses people have for clean water, which we get from
rivers, lakes, reservoirs, and underground wells. We all need fresh water!
But the more people there are in Texas, the more water will be taken out
of the San Antonio and Guadalupe Rivers before they reach the estuary. If
we don't figure out a solution, Aransas will become more salty. Blue crabs
will become much more scarce. And the Aransas Whooping Crane population
may become extinct.
Many concerned Texans, including Tom Stehn, are working on this problem.
Tom writes, "Human consumption of river water in Texas is a growing
resource issue as the State's population continues to expand. This is
a very worrisome
trend since Texas water law reserves water for people but has few provisions
for wildlife." Should our use of fresh water be regulated to ensure
that there will be fresh water enough for wildlife as well as humans?
Read more about water rights and the debate raging in
are the smartest animals on the planet. Can we figure out a way of
the people of Texas get enough water while still conserving enough water
in the Guadalupe and San Antonio Rivers? Is it possible for humans
thrive along with blue crabs and the Whooping cranes that need them?
Dr. Seuss wrote in The Lorax, "Unless someone like YOU cares a whole
awful lot, nothing is going to get better. It's not."
What do you think are appropriate activities on a refuge and in the
area outside of a refuge but close enough to affect it? Is it fair
people living near a wildlife refuge stricter rules than other people
have? Do we have a responsibility to protect wildlife such as
Be a water conservationist. Learn about our water resources and how
you can help conserve them in our Water
Science Education Standards
investigations involve asking and answering a question and comparing
that to what scientists already know about the world.
have basic needs. Organisms can survive only in environments in which
their needs can be met.
- An organism's
behavior patterns are related to the nature of that organism's environment.
When the environment changes, some plants and animals survive and reproduce,
and others die or move to new locations.
depend on their natural and constructed environments. Humans change
environments in ways that can be either beneficial or detrimental for
themselves and other organisms.
are the spaces, conditions, and factors that affect an individual's
and a population's ability to survive and their quality of life.
in environments can be natural or influenced by humans. Some changes
are good, some are bad, and some are neither.