Case study 3 - parasitism in shags

the impact of seasonal breeding on parasite burden throughout life in european shags

Parasitims

• negatively impacts individual hosts in terms of health and fitness levels

• might scale up to impact populations

There is variation in response to infection

• can be lethal

• sub-lethal => impacts in reproduction, survival or life history

• can be acute

• can be chronic

What are some host factors that can impact parasitism

• behaviour

• social environment

• Physiology (immune system)

these all cause diff manifestations of parasitism in individuals and may vary over time aka different behaviours between years

what is an example of impacts of seasonality on parasitism

differences in exposure to hosts over time => prevelence of diff parasites may vary across seasons => in nigeria, lassa fever peaks between december and feb because vectors for disease move into humans to breed and hibernate

How might this variation of parasitism across seasons impact individuals

• could be related to the observed decline in breeding success as the season progresses

• this may be due to increased exposure to parasites

BUT

could also be due to resource availability or aging of individuals

what could variations in seasonal breeding cause aka differences between individuals breeding at diff times or being born at diff times

• intrinsic : age of breeders, breeding experience

• or differences in: exposure to parasites, resources available for immunity, age-related immunity differences

why does seasonal breeding cause variation

• if individuals are born at diff times they may be exposed to diff conditions which may impact their parasite burden

How do we measure parasiite burden

By measuring parasite abundance

• this refers to the number of parasites per individuals in a population

• includes both infected and non-infected individuals and ranges from 0 upwards

• common way of measuring macro-parasites such as ticks

• can also measure this in prevelance like number of hosts infected at a time => used to measure microparasites like viruses

what does this graph show

it shows the effect of seasonal breeding driven early life effects on parasitism=> breeding later on makes individual more susceptible to parasites=> experiencing differences in early life may impact individual and parasite burden well into adulthood

Why is understanding seasonality’s impact on parasitism important?

• it can impact parasitism and so can early life

• this variability also impacts fitness

• understanding these patterns can help us understand season declines in fitness that we see across breeding seasons in wild populations

How can we study these effects to allow us to predict the impact of phenological shift in breeding date on parasite burden

• by using a long term study population that varies in time of breeding and experiences variability in parasite burden

What study system is used

Isle of May European shag system, isle of may is small 500 m by 1km

What makes shags a good study system

(1) Chicks incubate for 60 days

(2) Individuals are trackable with rings

(3) They have variations in timing of breeding with peak breeding times varying across years and different hatching times between years

(4) Variation in winter life with ½ migrating and ½ residential

(5) Variation in parasite burden

What parasitises Shags

• nematode worms through fish and through chicks being fed infected prey

• contraaecum rudolphii

What are the effects of these parasites

effects can be sub-lethal

• Impacts on reproductive success

• this is only seen in females, females with more parasites have lower success

• differences between sexes in how they respond => males have parasites

• removing parasites from parents increased survival in chicks early but not late in the season

When measuring parasite load in females compared to their reproductive success, what was observed?

• The later females breed, the higher their parasite load

• this implies that as the breeding season progresses, the immune investment may decrease or parasite exposure may increase

• This shows a direct seasonal effect on parasite load in breeding females

When measuring chicks for parasite load and comparing this to their hatching date, what trend was observed?

• the later chicks were hatched, the higher their parasite load at 30 days

• this shows that later born chicks have a weaker immune system

• seasonal breeding effects early-life parasite exposure

When measuring parasite load in adult males compared to the date they were hatched, what trend is seen

In males that were hatched later parasite load was higher

• this is in direct contrast to what is seen in females and in chicks

• it suggests that early-life timing may have an lasting, long-term impact on males

• maybe due to compensatory development, immune priming or selective survival

What main conclusions can be drawn from the results of the Shag Isle of may study?

(1)  Seasonal breeding affects parasite burden—but differently across life stages and sexes - the trend in males is opposite to the trend seen in females, possibly due to compensatory development, immune priming or selective survival. females and males are likely effected differently by seasonal effects between life stages

(2) Sex differences matter - adult females are more effected by breeding season, adult males are more effected by when they were hatched - this could be due to different immune function, behaviour or life history strategies

(3) Immune investment pattern mirrors parasite burden patterns - Later-hatched chicks and later-breeding adults both show lower immune investment.

• This supports the idea that immune function is a key mediator of the observed parasitism patterns.

 

What is important to keep in mind about the conclusions drawn from the shag study on the isle of may?

• the results show correlation, not causation

• they’re good observations, off which more studies can be done

• results can inform scientists about why animals that breed later have lower fitness which can help inform conservation attempts as well as climate-driven shifts in breeding seasons