Isotopic Tracing of Resource Movement by Animal Consumers

Lee, P-J., S. A. Macko and M. A. Bowers

University of Virginia

TWS2004, Calgary, Canada

Background: Pulse resource in space and time

0

1

2

3

4

1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000acor

n pr

oduc

tion

inde

x

Data: Virginia Department of Wildlife and Inland Fisheries

-- acorn mast in Quercus spp.

Background: Isotopic tracing of resource movement

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

13Cstd (‰): PDB

15Nstd (‰): Atmospheric N2

Stapp and Polis (2003)

Marine resources penetrate

inland areas through predator

spillover effects and subsidize

Peromyscus maniculatus

populations

Methods: Mast simulation with C4 millet seeds

-3

0

3

6

9

12

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

13C

15N

millet seeds

background plants

Timing: September-October 2003

Intensity: 50 pounds of seed / 0.25 hectare / 10days

Total amount: 200 pounds of seed / 0.25 hectare

Methods : Mice sampling and isotopic analysis

capture-recapture

trapping grids in forests and fields

Isotopic analysis

taking blood and hair samples

Methods : Quantifying resource taken by mice

Mixing equation:

Δ xE (‰) = fb x (xEb) + (fm) x (xEm)

where (fb + fm) = 1

(MacAvoy et al. 2003)

Isotopic turnover in mouse (Mus musculus) blood

13C 15N

Half-life 17 20

Equilibrium 70 56

Methods : applying mouse turnover to P. leucopus

R2 = 0.92

10

15

20

25

30

0 3 6 9 12 15ln(body mass)

ha

lf-lif

e o

f 13

C in

blo

od

(d

ays

).

Black bear(Ursus americanus)

Mink(Mustela vison)

Mouse

Body mass of P. leucopus is similar to that of Mouse = 20grams

Result: Fraction in adult diets derived from millet seeds

Forest

0

0.1

0.2

0.3

0.4

0.5

0.6

0 25 50 75 100 125

Days

Frac

tion

of m

illet s

eeds

in m

ouse

di

et (l

east

squ

are

trans

form

ed)

control

mast

Field

0

0.1

0.2

0.3

0.4

0.5

0.6

0 25 50 75 100 125

Days

Frac

tion

of m

illet s

eeds

in m

ouse

die

t (lea

st s

quar

e tra

nsfo

rmed

)

control

mast

Result: seed mast penetration into neighboring areas

--no immigration from control grids to mast grids detected:

mice commute to gain access to resources

mast resources

could penetrate

~140 meters into

neighboring areas

in forests

y = -0.0034x + 0.4782

R2 = 0.7743

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 50 100 150Distance from masting patch (m)

Fra

ctio

n of

mill

et s

eeds

in

mou

se d

iets

Forest

Field

Result : Mice population density and reproduction output

Forest

0

5

10

15

20

25

30

35

10 50 105 305

days

Mic

e de

nsity

(pe

r grid

)

controlmast

Field

0

5

10

15

20

25

30

35

10 50 105

Days

Mic

e de

nsity

(pe

r gr

id)

controlmast

-- Equal number of juveniles per adult female between mast and control grids

-- Juvenile mice incorporate equal amount of millet-derived nutrients

towards hair tissue synthesis: 32%

Conclusions

--Resource transport efficiency via consumer movements

patchy resources could penetrate hundreds of meters

--Mode of resource movement across neighboring food webs

mice commute to mast grids at a scale of hundreds of meters

--Reproductive output is equal between control and mast grids

local patchy resources could promote regional population dynamics

Acknowledgement

Stephen McAvoy

Clay Morris

Kimberly Spalding

Blandy staff

Funding:

Blandy Experimental Farm

Virginia Museum of Natural History

The Wildlife Society student travel grant

Questions?