drivers of population-level effects of parasite virulence life-history meets meta-analysis maggie j....
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Drivers of population-level effects of parasite virulence
Life-history meets meta-analysis
Maggie J. Watson Charles Sturt University, Australia
A central goal of population ecology is to identify factors controlling population dynamics.
Population Size• Intrinsic Death Rate• Parasites• Predators• Disease
Population Size• Intrinsic Birth Rate• Food Supply
Parasites—the focus of MANY experimental studies
Do parasites affect individuals?
Parasites—the focus of MANY experimental studies
Do parasites regulate populations?
Population regulation by parasites has been identified in Red Grouse Lagopus lagopus scoticus
Hudson et al. 1998
Svalbard Reindeer Rangifer tarandus platyrhnchus
Albon et al. 2002
and Soay Sheep Ovis aries
Gulland 1992
...but, they are all managed populations
Thus, the question still remains—are parasites significant drivers of population-level effects and what host life-history traits drive
observed parasite virulence?
• Identify a common measure of EFFECT SIZE resulting in a weighted average
• Estimate, with great power, the TRUE EFFECT SIZE
A meta-analysis of 38 experimental studies (31 avian, 6 mammalian, 1 fish) of the costs of parasites to population-level measures of natural, free-ranging hosts revealed an overall effect size,
d =0.49
% breeding succ., # young produced, survival, clutch size, hatching succ.
d =0.49
This is HUGE!
But, a very small pool…
• 1 Perciformes• 2 Galliformes• 1 Columbiformes• 1 Ciconiiformes• 1 Pelicaniformes• 2 Charadriiformes• 1 Apodiformes• 23 Passeriformes• 2 Lagomorpha• 3 Rodentia• 1 Artiodactyla
But, a very small pool…
• 1 Perciformes• 2 Galliformes• 1 Columbiformes• 1 Ciconiiformes• 1 Pelicaniformes• 2 Charadriiformes• 1 Apodiformes• 23 Passeriformes• 2 Lagomorpha• 3 Rodentia• 1 Artiodactyla
Not
eve
ryth
ing
is a
pas
serin
e…
So where to now?
What host traits influence parasite
virulence?
Some predictions…
Expect greater effects in:
• Cavity nesters• Colonial species• Tropical species• Species with high
mortality rates
Møller et al. 2009 A meta-analysis of parasite virulence in nestling birds. Biol. Rev. 84: 567-588
Nidelet et al. 2009. Effects of shortened host life span on the evolution of parasite life history and virulence in a microbial host-parasite system. BMC Evolutionary Biology 9:65.
…cavity nesters?
-2
-1
0
1
2
3
4
Effec
t Siz
e
Open Nest Cavity Nest
Z*=-0.34p-value=0.97
…coloniality?
-2
-1
0
1
2
3
4
Effec
t siz
e
Not Colonial Highly Colonial
Z*=0.36p-value=0.72
…latitude?
0 15 30 45 60 75 90
-2
-1
0
1
2
3
4
Effec
t Siz
e (d
)
Z*=-1.30p-value=0.19
0 5 10 15 20 25 30 35 40 45 50-2
-1
0
1
2
3
4
Lifespan (years)
Effec
t siz
e
…host lifespan (maximum)?
Z*=-1.44p-value=0.15
0 5 10 15 20 25 30-2
-1
0
1
2
3
4
Lifespan (Years)
Effec
t Siz
e…host lifespan (average)?
Z*=-1.93p-value=0.05
0 5 10 15 20 25 30
-2
-1
0
1
2
3
4
Lifespan (Years)
Effec
t Siz
e of
Par
asite
s
…which species?
Some results…
Found greater effects in:
• Cavity nesters • Colonial species • Tropical species • Species with lower
average lifespans
Conclusions
• Yes, parasites do have population level effects, but are they biologically relevant to all species?
• What life-history attributes determine virulence?
• Future areas of inquiry—lifespan of host organism, ideally study (truly) long-lived organisms and species other than tits and swallows and martins!
An aside about data reporting…
• Means• SD or SE• Sample sizes• …and make sure
there are comparable categories to other studies
Now for a parasite removal experiment!
…and lots of pretty pictures!
Host species• Crested Tern (Thalasseus
bergii cristata , Stephens 1826)
• Colonial• Abundant (IUCN Least
Concern)• Ubiquitous (Africa to
Australia)• Naturally heavily
parasitised • And VERY long lived…
Parasite Species• Lice
– Austromenopon atrofulvum
– Quadraceps sellatus – Saemundssonia
laticaudata – …and maybe more!
• Ticks– Ornithodoros capensis– Ixodes kohlsi
Methods• Morphometric changes
– Bill + Head– Mass– Sex specific growth
• Haematological changes– Haematocrit
• Immunological changes– Total WBC counts– Heterophil/Eosinophil Ratio
• Survival• Metabolic (?) changes
– Ptilochronology– Fluctuating Asymmetry
Medication experiment:Sample size: 388; 178 recapturedIvomec vs. Control
Pthilochronology: growth bars
1 2 3
ResultsMedication reduced
parasite numbers (p=0.00051)
High vs. Low Parasites Weight gain p=0.23 Bill + Head p=0.51
Males vs. Females Weight gain p=0.35 Bill + Head p=0.39
Haematocrit NS
Results: Immunological (Total WBC Counts)
0 50 100 1500
5
10
15
20
25
30
R² = 0.378605881014183
Total WBC Count
Num
ber o
f Par
asite
s
Results: Immunological (H/E Ratio)
0 5 10 150
5
10
15
20
25
R² = 0.197254487275589
Heterophil:Eosinophil Ratio
Num
ber o
f Par
asite
s
2008-9 poor food supplies 2009-10 good food supplies
Results: Ptilochronology
0 5 10 15 20 250
2
4
6
R² = 0.02770795174516010 5 10 15 200
2
4
6
R² = 0.43324472117248
Number of Parasites (Rank Order)
Gro
wth
bar
s (m
m)
Results: Fluctuating Asymmetry
low parasites high parasites0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
Asym
met
ry B
etw
een
Tail
Feat
hers
(mm
)
P=0.03
N=24
N=26
What do the non-significant results mean?
• Morphometric changes obfuscated by food supply
• Haematocrit and age• Ptilochronology and
food supply• Ptilochronology and
“quality”• Long-lived species and
survival
What do the significant results mean?
• The benefits of parasites• Immunological “costs”• Metabolic “costs” and
effects on long-term “quality
Conclusions • In highly evolved natural systems, parasites may not affect hosts except during extreme stochastic events.
• Long-lived species are not as affected by their parasites as short-lived species.
Acknowledgements
Trusty fieldworkers: Dave, Douglas, Jack and Charlie Watson, Andrew, and the other Maggie
The notoriously unphotographable advisor: Shane Raidal
Funded by ILWS CSU Scholarship and Holsworth Foundation Grants