Pei-Jen LeeUniversity of Virginia

Portland, 2004

Omnivory Driven by Pulse Resources

in White-footed Mouse Peromyscus leucopus

The Question: What drives omnivory in a trophic generalist

species?

Dose pulse resources drive the behavioral switching between generalist and specialist strategy?

Coll and Guershon (2002):

nutrition balance / toxics dilution obligate omnivores

food supplementation / competition avoidance opportunistic omnivores

Ostfeld and Keesing (2000)

Background: Optimal Foraging Theory (Fretwell, 1972)

Original Resource DistributionSpecialized

Consumption

Resource Distribution after Specialized Consumption

Generalized Consumption

Resource Quality Gradientoptimal

Ab

un

dan

ce /

Co

nsu

mp

tio

n

The Study System: white-footed mouse in oak masting forests

(Whitaker 1966; Wolff 1985)

0

1

2

3

4

5

summer fall winter yearInse

ct :

Pla

nt ra

tio in

die

ts

Eatsern Deciduous Forest Agricultural Field

Tools and Assumptions I: Stable isotopes and carbon isotopic mixing

xE (‰) = [(xE/ yE)sample / (xE/ yE)std – 1 ]*1000

13Cstd (‰): 15Nstd (‰):

Pee Dee Belemnite Atmospheric N2

-- Mixing equation:

Δ 13Cmouse (‰) = fc4 x (13Cc4 – 13Cc3)

-- Carbon isotope mixing effect: i.e. C3 versus C4 plants

Assumptions: constant fractionation, equilibrium with food etc.

Tools and Assumptions II : Trophic level based on nitrogen isotopes

Let 15Ndiet-mouse = 2.7 ‰

15Ndiet - intermediate consumer = 3 ‰

15Nplant = a ‰

15Nmouse = x ‰

Consumer trophic level: x-a

Deviation from typical omnivory: |x-(a+3.9)|

mouse

Intermediate Consumers

Producers

P 1-PHerbivore Insectivore Omnivore

P=0,1-P=1 P=1,1-P=0 P=1-P=0.5

a+2.4 a+3+2.4 a+3.9

-- Nitrogen isotopic trophic effect:

Experiment in 2003: simulation of seed mast event across two habitat

types

-3

0

3

6

9

-35-30-25-20-15-10

δ13C %o

δ15N %o

Result I:Fraction of diet derived from millet seeds

0

0.1

0.2

0.3

0.4

1 week 7 weekFrac

tiona

of m

illet s

eeds

in d

iets

control

food addition

Food addition effect: p=.01

Time effect: p<.0001

Food by Time: p=.01

Result II:Intra-population divergence in trophic levels

0

10

20

30

40

50

Before 1 weeks 7 weeks 15 weeksCV

of n

itrog

en is

otop

ic ra

tios

control food addition

Start End p= .02

Result III:Individual became specialized at millet seeds

Pearson r = .7, p=.05

0.00.20.40.60.81.01.21.41.6

0.0 0.1 0.2 0.3 0.4

Fraction of millet seeds in diets

Dev

iatio

n fro

m o

mni

vory

.

Pearson r = .7, p=.02

0

1

2

3

4

5

0.0 0.1 0.2 0.3 0.4

Fraction of millet seeds in diets

Trop

hic le

vel

Summary

Pulse resources could drive the switching between generalist and specialist

strategy in trophic generalist consumers.

Generalist consumers could trace the changes in food abundance efficiently

at population level.

Intra-population trophic divergence (CV of nitrogen isotopes) is greater in poor

environment and smaller in rich environment.

Deviation from omnivory and trophic level of individual consumers are directly

related to the fraction of pulse resource in their diets.

Acknowledgement

Roulston, T’ai

Wilbur, Henry

Swap, Bob

Funding:

Blandy Experimental Farm / Virginia Museum of Natural

History

Carr, Dave / Morris, Clay

Drummond, Brie / Sheu, Yu-Cheng

ALL Blandy Farmers!

0

5

10

15

20

25

before 1 week 7 weeks 15 weeks

po

pu

lati

on

de

ns

ity

control

mast