drivers of population-level effects of parasite virulence life-history meets meta-analysis maggie j....
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![Page 1: Drivers of population-level effects of parasite virulence Life-history meets meta-analysis Maggie J. Watson Charles Sturt University, Australia](https://reader030.vdocument.in/reader030/viewer/2022032804/56649e4f5503460f94b46026/html5/thumbnails/1.jpg)
Drivers of population-level effects of parasite virulence
Life-history meets meta-analysis
Maggie J. Watson Charles Sturt University, Australia
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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
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Parasites—the focus of MANY experimental studies
Do parasites affect individuals?
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Parasites—the focus of MANY experimental studies
Do parasites regulate populations?
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Population regulation by parasites has been identified in Red Grouse Lagopus lagopus scoticus
Hudson et al. 1998
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Svalbard Reindeer Rangifer tarandus platyrhnchus
Albon et al. 2002
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and Soay Sheep Ovis aries
Gulland 1992
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...but, they are all managed populations
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Thus, the question still remains—are parasites significant drivers of population-level effects and what host life-history traits drive
observed parasite virulence?
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• Identify a common measure of EFFECT SIZE resulting in a weighted average
• Estimate, with great power, the TRUE EFFECT SIZE
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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.
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d =0.49
This is HUGE!
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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
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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
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Not
eve
ryth
ing
is a
pas
serin
e…
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So where to now?
What host traits influence parasite
virulence?
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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.
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…cavity nesters?
-2
-1
0
1
2
3
4
Effec
t Siz
e
Open Nest Cavity Nest
Z*=-0.34p-value=0.97
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…coloniality?
-2
-1
0
1
2
3
4
Effec
t siz
e
Not Colonial Highly Colonial
Z*=0.36p-value=0.72
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…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
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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
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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
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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?
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Some results…
Found greater effects in:
• Cavity nesters • Colonial species • Tropical species • Species with lower
average lifespans
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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!
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An aside about data reporting…
• Means• SD or SE• Sample sizes• …and make sure
there are comparable categories to other studies
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Now for a parasite removal experiment!
…and lots of pretty pictures!
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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…
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Parasite Species• Lice
– Austromenopon atrofulvum
– Quadraceps sellatus – Saemundssonia
laticaudata – …and maybe more!
• Ticks– Ornithodoros capensis– Ixodes kohlsi
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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
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Pthilochronology: growth bars
1 2 3
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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
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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
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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
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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)
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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
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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
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What do the significant results mean?
• The benefits of parasites• Immunological “costs”• Metabolic “costs” and
effects on long-term “quality
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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.
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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