Optimal foraging theory

Transcription

1 Optimal foraging theory Christopher Furlong/Getty Images John Ascher Where is Moriarty going? Purpose of trip: buy ice cream for home Moriarty wants to get home as fast as possible Moriarty knows layout of local streets 2012 Google How to build an optimality model 1. Choose the decision you want to predict 2. Make assumptions about: Fitness currency Constraints on behavior Where is Moriarty going? Purpose of trip: buy ice cream for home Moriarty wants to get home as fast as possible Moriarty knows layout of local streets 2012 Google How to build an optimality model 1. Choose the decision you want to predict 2. Make assumptions about: Fitness currency Constraints on behavior The habitat 3. Predict animal s behavior assuming it maximizes fitness currency within constraints 4. Test your prediction Some classical optimal foraging models: Optimal diet: What types of food to include in diet? Marginal value theorem: How long should one forage in a patch? 1

2 handling and # of Optimal diet food item at a How to determine optimal diet # Rank foods based on handling 1: 2: 3: Always eat best food Eat next-best too if best food scarce Eat third-best too if best 2 foods scarce Etc. Testing the optimal diet model Results of conveyor belt studies: When high quality items scarce, animals generalize When high quality items abundant, animals usually reject low quality items, regardless of their abundance But: low quality items never completely dropped from diet e.g., Krebs et al Optimal diet handling and # of energy intake food item at a Aquatic Terrestrial Energy per gram 3.8 kcal 3.0 kcal If both food types are abundant, what does our model predict moose should eat? Time needed to eat 1 gram 0.05 min 0.06 min 2

3 Optimal diet handling and # of energy intake food item at a Rumen capacity is limited and aquatic are very bulky Aquatic Terrestrial Energy per gram 3.8 kcal 3.0 kcal Bulkiness (% rumen capacity/g) 0.07% 0.01% Moose need sodium in their diet Terrestrial have almost no sodium but aquatic are a good source Aquatic consumed (g) A more realistic model Rumen capacity Minimum energy requirement A B Terrestrial consumed (g) C D Belovsky 1978 Minimum sodium requirement Marginal value theorem takes 3

4 Marginal value theorem takes Forager knows: Quality of current patch Average patch quality and travel Energy in patch Rate of = Time in patch + Energy in patch Rate of = Time in patch + Which gives the highest rate of? Slope = Marginal rate of = in next micro-moment Marginal Value Theorem: Animal should leave patch when its marginal rate of = the overall average rate of it can achieve in its habitat 4

5 What happens if you change travel? Bumblebees visiting Delphinium nelsonii Hodges 1985 Paul Slichter John Ascher Flowers visited Observations: Bees leave when nectar/flower declines to a threshold (Hodges 1985) MVT correctly predicts #s of flowers visited/plant (Pleasants 1989) plant density -> # flowers visited/plant (Cibula and Zimmerman 1984) The Hummingbird Game A web exercise on optimal foraging in the form of a game: Optional Exercise! For class participation credit, play the hummingbird game: Design a set of rules used to leave patch Collect data on how the different rules perform Report results and interpretation in 1-2 typed pages Hints for full credit: Be systematic in how rules vary. Use multiple replicates per rule. Patch use in bluegill sunfish # of prey eaten Crowley et al (min) OK Dept. of Wildlife Conservation Fish stay in patches much longer than predicted! Many other animals also stay longer than predicted (Nonacs 2001) Why??? Marginal value theorem takes Forager knows: Quality of current patch Average patch quality and travel 5