The double life of a squirrel:seed disperser and predator

12.05.2012Jakob Nalley, Liz Schultheis and Tomomi Suwa

Today’s Outline

• Background on dispersal and predation• Field Experiment• Project Squirrel (citizen science project)

Seeds

Why is there so much diversity in seed shape and size?

Seeds evolved many strategies to interact with other species

Seeds encounter …• Dispersers• Predators

DispersalOrganism moves away

from parent or current population

Stickiness

Bright color & fruits

Mass Production

Air & Wind

Water

DispersalWhy is it important to disperse?

Seed Predators

Post-dispersal seed predators: larger, mobile and generalist. e.g. rodents, birds, ants

Pre-dispersal seed predators: small, sedentary, specialist feederse.g. insect larvae

Seed’s dilemma: Attractive for dispersers

Defensive against predatorsvs.

Squirrel as disperser and predator

Advantages of caching (from seed perspective):• Disperse seeds• Predators do not eat all the seeds they cache

(some are lost)• Death rates are lower for buried seeds than

seeds exposed on the surface.

Caching – a food storing behavior of animals (hoarding)e.g. birds, rodents, ants,

Chocolate-chip cookie study

Steven Lima, Thomas Valone, and Thomas Caraco. 1985. Foraging-efficiency -- predation-risk trade-off in the grey squirrel. Animal Behaviour 33:155-165.

Squirrels are more likely to carrycookies over shorter distances

Chocolate-chip cookie studySquirrels are more likely to carry

larger cookies

Size of Cookie

If you give a squirrel a cookiesunflower seed

Today’s Experiment

ForestOpen Lawn

Experimental Design

Open Lawn

ForestWhat are the differences between these habitats from the perspective of a squirrel?

Do you think a squirrel will behave more like a predator or a disperser depending on the habitat?

HypothesisWhat are the differences between these habitats from the perspective of a squirrel?

Do you think a squirrel will behave more like a predator or a disperser depending on the habitat?

I predict that squirrels will behave more like a ___________ in the forest, compared to the open field. This is because _______________________.

predator/disperser

give a reason you might predict this

Fox Squirrel Gray Squirrel Chipmunkblack

morph

Data to collect:Data on seed consumption:• Number of seed coats remaining (divide by 2 to

get a number of seeds consumed at the tray)• Number of full seeds remaining• Number of seeds without coats remaining• Difference between number of seeds placed into

tray (100, or 50g) and number remaining

Data on squirrel visitation:• Number of footprints in the sand• Squirrel observations when collecting trays• Squirrel behaviors in each habitat type

Open Lawn

Forest

Experimental Data

Open Lawn Forest

observations?

Data to collect:

Data Analysis and Interpretation

http://www.youtube.com/watch?v=Dxhn4W9NKnY

Citizen Scientists Unite!

Extra slides

PredatorsOrganism that kills and feeds on its prey

Pathogen• is a microorganism that causes

disease in its hoste.g. virus, bacteria, fungi,

• Pathogen do not necessarily kill the host animal/plant

Need more pictures here

II. Scientific MethodSteven Lima, Thomas Valone, and Thomas Caraco. 1985. Foraging-

efficiency -- predation-risk trade-off in the grey squirrel. Animal Behaviour 33:155-165.

Predict that tendency to carry a food item should decrease with distance of food from cover (predation risk) and increase with item size (food reward). Both risk and reward should influence behavior

Experiments were conducted in Highland Park in Rochester, New York. The reward was chocolate-chip cookies, cut to weigh 1, 2, or 3 g. More “natural” foods were buried rather than eaten and cookies may be a “natural” food for a park squirrel anyway. Food was placed at different distances from trees.

II. Scientific Method

Steven Lima, Thomas Valone, and Thomas Caraco. 1985.

II. Scientific MethodSteven Lima, Thomas Valone, and Thomas Caraco. 1985. Foraging-

efficiency -- predation-risk trade-off in the grey squirrel. Animal Behaviour 33:155-165.

The authors conclude that the results support their hypothesis. Simple models that only incorporate foraging rate or only exposure to predators are insufficient, as both are important.