The effect of colony size The effect of colony size on energy acquisition on energy acquisition

rates in rates in PogonomyrmexPogonomyrmex

Melanie E. MosesSevilleta LTER Symposium

January 14, 2004

Introduction

• Most organisms show a nonlinear relationship between body size and energy acquisition rates

• Jun et al (2003) predict a nonlinear relationship between ant colony size and energy acquisition

• Predictions were tested with field observations at the Sevilleta LTER and in Portal, AZ.

• A revised model is proposed and will be tested with future field experiments

Summer 2003 Field Sites

Pogonomyrmex species at the Sevilleta and Portal, AZ

Species # Workers # Foragers

P. desertorum 400 160

P. maricopa 2000 400

P. rugosus 10,000 1000+

P. barbatus 12,000 2500

Colony growth is sigmoidal, and oscillates with seasons

Species are relatively similar in ecology, behavior and ant size

Foraging in Pogonomyrmex

• Single load, central place foragers• Generalist seed diet + termites

• High foraging efficiency (Espent/Eacquired < 1%)

• Time minimizers• Temp, humidity, rainfall greatly impact foraging• Foraging time = travel time + search time

Tf = Tt + Ts Ts >> Tf

Allometric Foraging ModelJun et al 2003

• Derived from West et al allometric models, considers how colony size and use of space (territory area) affect foraging times

• Assumes that when diet, environment, food resources, etc. are equal, larger colonies require a larger territory in which to forage

• Predicts that the average time to acquire each seed is a function of the number of foragers (F) in the colony:

Tt ~ F1/3

• Diminishing returns/self shading– The rate of energy acquisition per forager declines ~ F-1/3

– Total energy acquired by the colony ~ F2/3

Field Methods

• Follow marked foragers of each spp to determine– Travel time (Tt) – Search time (Ts)– Total foraging time (Tf)– Distance to seeds (d)– Territory area (A)

• Estimate # of foragers– Flow of ants * Tf

Foragers in bigger colonies travel furtherbut exponent is < model prediction of 1/3

Foraging Distance vs Forager Population

d = 2.1F0.20

R2 = 0.931.00

10.00

100 1000 10000

Forager Population (#)

Fo

rag

ing

Dis

tan

ce (

m)

P. barbatus travel significantly further (10m) than P. desertorum and P. maricopa (6m)

Travel time is proportional to travel distance

Outbound Time vs Outbound distance

time = (0.41) distance

R2 = 0.49

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0 5 10 15 20 25 30

Outbound distance (m)

Out

bou

nd tr

avel

tim

e (m

)

barbatus californicus desertorum rugosus

BUT Tf is constant across colony sizes

Total Foraging Time vs Travel distance

0.00

5.0010.00

15.00

20.0025.00

30.00

35.00

40.0045.00

50.00

0 5 10 15 20 25 30

Travel distance (m)

Fo

rag

ing

tim

e (

m)

barbatus californicus desertorum rugosus

1.00

10.00

100.00

100 1000 10000

F (number of foragers)

To

tal

Fo

rag

ing

tim

e (m

inu

tes)

Total foraging time vs F

Summary of field data

Larger colonies had • larger territories• longer foraging distances• longer travel times

• but total foraging times were not significantly different across colony sizes

Why are large colony foraging times shorter than model predictions?

Model• Assumes search time is negligible

• Assumes foragers fill space at constant density

• Assumes seed density is constant across foraging areas

Data• Search time is > 60% of foraging time

• Forager density in large Pogo colonies is twice that in small colonies

• Seed density was not measured

A revised model of ant foraging

Use the approach of the Jun model:

ants are “space filling” and rate minimizing

Three changes to the model:

• Consider Ts as a component of foraging time

• Allow density of foragers (F ) to vary

Increased F decreases travel time (Tt)

increases search time (Ts)

• Allow large colonies to exploit high density seed patches

Model Predictions

• Density of foragers: F ~ F1/3 (s

4/3)

• Territory Area: AT ~ F2/3(s-4/3)

• Travel time: Tt ~ F1/3(s-2/3)

• Search time: Ts ~ F1/3 (s-2/3) + (1/s)

Foraging time (Tf) is equal if P. barbatus forage in areas with twice the seed density (s) of P. desertorum

Next steps

• Validate model with simulations• Test model with additional field data • Experimental seed manipulations

– Does forager density increase when seed density increases?

– Are larger colonies better at exploiting dense seed patches?

Conclusions

• Large Pogonomyrmex colonies have larger territories and longer foraging travel times than small colonies

• Large colonies may compensate for increased travel times by– Finding richer seed patches– Increasing forager density

• Seed manipulation experiments are being designed to test these predictions

Thanks!

Bruce Milne & labJim Brown & labRic CharnovHilary Lease

“The Ant Course” & Southwest Research Station Sevilleta LTEREPA STARNSF BiocomplexityNM GRD NM AGEP

QUESTIONS ???