Host population ecology and infectious disease Flashcards
Biological population
A group of individuals of one species that live and interbreed in same place at same time
Population Ecology
The study of the processes that affect the distribution and abundance of animal and plant populations
Factors that influence population size
- Births
- Deaths
- Immigration
- Emigration
Assumptions for exponential population growth models
- Change in population size depends on birth and death rates (ignores immigration and emigration)
- Assume unlimited resources
- Birth rates and death rates are constant
- Assume population growth is approximately continuous
Equation for population growth
dN/dt= (b-d)N
b-d = r
b=births
d= deaths
r= instantaneous rate of increase
N= population size
dN/dt= population growth= change in population size over a small interval of time
Units of r
Individuals per individuals per unit of time
Instantaneous rate of increase (r) fates
The value of r determines the fate of the population
r > 0 population grows exponentially (b > d)
r = 0 population remains constant in size (b = d)
r < 0 population declines to extinction (b < d)
What can you compare r in a host population to in a pathogen?
Similar to R0 of pathogens
Ring-necked pheasants on Protection Island
- Example of exponential growth
- In 1937, ring-necked pheasants introduced to Protection Island where the population is closed to immigration and emigration
- Island was essentially predator-free
- In 1937, N = 8 adult birds …. In 1943, N= 2000 adult birds
What is unrealistic about the assumption that the birth and death rates are constant?
There will be limited resources so there will be increased deaths and decreased birth rates
What influences birth and death rates?
- Depend on N (Population size/density). Deaths increase with N, and births decrease with N
- The maximum population size that can be supported by the environment and resources is K. At K, b=d and r = 0 (no change in population size)
Carrying Capacity (K)
The maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water and other necessities available in the environment
Logistic population growth
Population growth when taking into account carrying capacity (ensuring the K is not exceeded)
Logistic growth (dN/dt)= rN (1-(N/K))
What does (1-N/K) represent?
Represents the unused portion of the carrying capacity
Ex. If population uncrowded (7% of K), then equation would be dN/dt = rN (0.93)
Logistic population growth curve
- N vs. time will follow an S-shaped curve
- When N is small, population grows exponentially at rate close to r
- Population growth is at its fastest when at half of carrying capacity (K)
- Growth decreases as population nears K
- If population starts above K, then growth is negative and N will decline towards K
Northern Fur Seal population
- In St. Paul Alaska
- In late 1800s, hunting drove fur seals almost to extinction
- In 1911, hunting banned, fur seal populations grew
- Today, population is at K = ~10,000 seals
- Displays density-dependent population growth
What ecological factors will affect b, d, and r, in a density-dependent manner?
Effects of these factors will increase as N increases
- Disease
- Competition for resources
- Predation
Density-independent ecological factors
Factors that influence population size regardless of population density
- Natural disasters (drought, etc.)
Density-dependent population growth
Growth of population will depend on the size of the population (density-dependent)
- Populations shrink when they are too large (N > K) and grow when they are too small (N < K)
- Important for biologists and conservationists managing fisheries and hunted animal populations
Views of Parasites and pathogens role on populations
- Epidemics causing mass mortality were viewed as natural disasters where ecosystem was out of balance
- Pathogens and parasites were not seen as density-dependent regulators of host populations
- Ecologists thought that if diseased animal was eaten by a predator, its death was due to predation and not infectious disease
Rabies
- Widely distributed across the world
- 55,000 human deaths per year
- 95% of deaths in Africa and asia
- Most human deaths from dogs
- 30-60% are children less than 15yrs old
- Vaccination of dogs and wildlife can eliminate rabies
Rabies epizootic in foxes in Europe
- Foxes were widespread in Europe
- Epizoonotic (1948-1988)- spread from Poland to central and western europe. Moved 20-60km per year
- Dog rabies was non-existent due to vaccination
- Control measures for foxes included shooting, trapping, poisoning, gassing
Why use rabies to study whether pathogens can exert density-dependent control on their host population?
Have to use a pathogen that has high virulence
Rabies has latent period
- Period from becoming infected to the infectious period
- Period where you are exposed but not yet infected
SEI model of Rabies
- Includes births, susceptibles, pre-infectious/exposed, infectious, density-dependent mortality
- Assume that only susceptibles are well enough to give birth
- No recovered!
Pathogen invasion in SEI model of Rabies
Rabies is density-dependent and therefore depends on K
- R0 increases with transmission rate, carrying capacity, and latency rate
- R0 decreases with virulence and birth rate
There is a threshold carrying capacity (KT) that is required for rabies to invade the fox population. If fox population drops below KT then rabies will go extinct
Threshold carrying capacity (KT)
The carrying capacity that is required for a pathogen to be sustained within a population
Why does a pathogen go extinct before the population does?
- Because pathogen requires KT to be sustained in population. When it drops below this, the pathogen will die out and the smaller population will be able to rebuild.
K> KT for pathogen to survive in population
Is rabies more likely in high- quality or low-quality habitat for foxes?
High quality, high population
Graph displaying rabies cycling within the population
- Rabies outbreaks every 3-4 yrs
- Graph does not show whether rabies makes the fox population cycle as well, because there is no denominator (total population). Difficult to know population numbers because lack of funding or interest.
Prevalence of rabies cycles
- Rabies cycle within the population
- Study by foxes shot by hunters and number of dead foxes found.
- Fox population had 3 waves of rabies. Fox population decreased following rabies outbreaks
- Spike in 1967/1968 rabies cases. The following year, there was a population decrease.
If rabies goes extinct, how does it come back?
Only becomes extinct locally. When K rises above KT again, rabies has the chance to come back
Spatiotemporal distribution of rabies
- Shown in NE Switzerland 1967-1972
- Rabies within one area will become extinct when K drops below KT. That area will be free of Rabies until K rises above KT again. At this time, rabies will be in other areas and the same cycle will happen, spreading throughout an area and can cycle back to the original area.
Steps of rabies cycling in population
- Rabies invades population if carrying capacity (K) of the fox population is greater than the threshold carrying capacity for rabies (KT)
- As rabies builds up in the population, rabies-infected foxes die, and the fox population decreases
- Once the fox population drops below KT, rabies goes extinct. The rabies-free fox population now increases towards K
- Once fox population size surpasses KT, rabies can invade population again
How to get rid of rabies in foxes
Oral rabies vaccination distributed in baits in Europe to the main reservoir hosts
- Culling foxes did not work
Saw an average number of rabies decline. If they stop vaccination, rabies likely to come back into population.
What did rabies vaccinations prove about population size and rabies pathogen?
- Rabies vaccination decreases rabies prevalence in a population
- A decrease in rabies saw an increase in population size but most likely minor. Other factors probably played a role in their overall population size growth
Evidence for pathogen regulation of population
- Rabies decreases population size below K
- Rabies causes cycles in population
