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The Crab Connection: Crabs in Trouble Equals Cranes in Trouble!
The Importance of Fresh Water for People and Wildlife

How do cranes eat such crabs? See Blue and Crabby.
Photo: Chesapeake Bay Program
Overview
Whooping 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 the 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
Aerial view over Aransas National Wildlife Refuge, an estuary where blue crabs (and the cranes that eat them) should flourish.
Whooping cranes 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. These 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, they 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.


Although whooping cranes eat many other foods, especially acorns, clams, and other kinds of crabs, blue crabs are by far their most important winter food.

Winter

Blue Crab Supply for Cranes

Number of Cranes that Died that Winter

1993-94

poor

7

1994-95

adequate

0-1

1995-96

adequate

0-1

1996-97

adequate

0-1

1997-98

adequate

0-1

1998-99

adequate

0-1

1999-2000

adequate

0-1

2000-01

poor

6


Ups and Downs: Natural Populations
The 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.
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 water in 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
Whooping Crane numbers steadily rose for many years. Then what happened? Do we have the time and the resolve to save cranes?
 
Most populations 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 Texas:

Human beings are the smartest animals on the planet. Can we figure out a way of helping the people of Texas get enough water while still conserving enough water in the Guadalupe and San Antonio Rivers? Is it possible for humans to 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."


Try this! Activities
  • Discuss Or Write: 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 to give people living near a wildlife refuge stricter rules than other people have? Do we have a responsibility to protect wildlife such as the Whooping Crane?
  • Act! Be a water conservationist. Learn about our water resources and how you can help conserve them in our Water Conservation Lesson.

National Science Education Standards

  • Science investigations involve asking and answering a question and comparing that to what scientists already know about the world.
  • Organisms 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.
  • Humans depend on their natural and constructed environments. Humans change environments in ways that can be either beneficial or detrimental for themselves and other organisms.
  • Environments are the spaces, conditions, and factors that affect an individual's and a population's ability to survive and their quality of life.
  • Changes in environments can be natural or influenced by humans. Some changes are good, some are bad, and some are neither.

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