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 ???