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SYEFEST Activity: Giving Up Density
Institute of Ecosystem Studies
Millbrook, New York
Contributor: Alan Berkowitz
Starter Questions:
- Which food stations are utilized most?
- Which animals are likely to visit the food stations?
- How does giving up density change in different habitats?
- What environmental, physical, or biological factors may encourage animals to feed at a site?
Overview: Many animals are active all year round and have to adapt to their environment throughout the changing seasons. Every action the animal makes has both benefits and costs. For example, when animals forage they may be exposed to predation or inclement weather. These are risks, or costs, associated with foraging at a particular site. The benefit of a food patch is primarily determined by the nutrition value and density of food in an area. A high density of food may encourage an animal to take greater risks whilst feeding. However, as an animal feeds, there is a critical point at which the density of food available no longer out ways the cost of feeding at that site. This is called the giving up density. When the giving up density of a food patch is reached, the animal will leave the site and forage somewhere else. Winter is a time when resources are particularly limited so behavioral decisions are very critical. To survive, an animal has to adapt its behavior so that the balance of its actions result in a net benefit.
This study will allow you to ask questions about how giving up density changes in different habitats. Further investigation may indicate some of the physical and environmental factors that affect giving up density at each site and so allow an unique insight into the decision making processes of foraging animals.
Materials:
- Aluminum baking trays
- Striped sunflower seeds
- Sand
- Sieves to separate sand and sunflower seeds
- Pans to sieve sand into
- Flagging
- Data sheets and clipboards
Procedure:
- Prepare food stations
- Half fill aluminum baking trays with dry sand.
- Count sunflower seeds and place them in each baking tray. The number of seeds depends on how long trays will be left outside, the habitats in which the trays will be placed, and the number of foraging animals in the area. Start with 100 seeds per tray left outside for 2 nights. Run some trials before starting the main project to see if this is a good number.
- Mix the seeds into the sand to make your food station.
- Choose site locations
- Choose sites where food stations can be located in 2 different habitats (plots). For instance, in brush and in the open.
- Locate 2 or more food stations in each habitat. Having a number of food stations at each plot will help the results show the real effect different habitats have on giving up density as opposed to results being influenced by chance or random factors.
- Mark your plots with flagging or tape so they can be found easily. In this study, at each site plot 1 is in a covered area and plot 2 is in the open.
- Running the experiment
- Place the food stations at each site for two or more days. This gives animals time to find the food and get used to the new objects in their environment.
- Collecting data
- After two or more days, collect each food station and count the number of whole seeds that remain. Don't count empty shells, since some animals are just messy eaters and leave these behind after their meal.
- Calculate the number of seeds eaten:
Number of seeds eaten = 100 - number seeds remaining
- Enter the information from each site in the table on this page.
- Extending the activity
- There are many ways to investigate the differences in giving up density in different habitats. Perhaps temperature or light intensity could be measured.
- Animal antifreeze is a suggested follow-up activity.
Click here to view a Sample Worksheet for your "Giving Up Density" activity. Please post any questions or comments in our Giving Up Density forum. We'd love to hear from you!
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