• Reduction in nutrition causing starvation
• Reduction in competitive ability to protect territory
• Increased vulnerability to predation
• Increased vulnerability to secondary infection that kills host

Reduction in Host Fecundity due to parasites is caused by:

• Reduction in female condition creates weaker offspring
• Reduction in female competitive ability
• Reduction in # of offspring
• Parents unable to care for offspring
• parasites cause castration in males and sterilization in females

Anderson and May model equations

dH/dt=??

Number of hosts=(natural birth rate of host-natural death rate of hosts)*#hosts-(host mortality by parasite+parasites affecting host fecundity)*#parasites

dH/dt=(a-b)H-(α+δ)P

Anderson and May model equation

dP/dt=?

number if parasites in parasitic stage=amount of parasites that get into host (both infective and free-living parasites) aka transmission stage-(natural mortality of host and parasite and mortality of hosts caused by parasites)*#parasites-level of aggregation

dP/dt=βWH-(μ+b+α)P-αP^2(k+1)/kH

Anderson and May model equation

dW/dt=?

P*/H*=(a-b)/α

-when birth rate of host is larger than death rate caused by parasite

-coexistence can occur through alternation of host and parasite population growh (cyclic)

When net infection> host growth rate

β-(μ+b+α)> (a-b)*(k+1/k)

• if α too high (parasites kill host)
• if α too low (parasites have a low effect on host)
• low k- aggregate (very few hosts can infect)

parasite induced reduction in fecundity(corrected for aggregation)> parasite induced mortality

δk>α

• Aggregated distribution
• Non-linear increase in parasite induced mortality
• Density dependent constraints on parasite population
• Parasite induced host mortality is higher at high parasite densities

Regular distribution k-->∞

parasite induced reduction in host fecunity,δ>α

Time delays in life cycle

Multiplication within host

• If aggregation is very high nomatter the population of host- unregulated host population.
• Between mild and high aggregation get a damped cycle.
• If parasites not too virulent, w/ a random aggregation and decreasing population of hosts=cycles.

• Highly virulent doesnt matter the # of hosts b/c parasites will kill hosts (unregulated).(Hosts die b/c too many parasites)
• If virulence is lower than the host population, it can be regulated. (Hosts dont die b/c virulence is low).

• If you put a constrain on growth of parasites, increase virulence = host death. (unregulated)
• Need to have a lot of hosts and medium to little (mild) virulence for regulation.

• Tendency for parasites to maintain host population abundance at some constant density or density trajectory.
• Requires Net infection >Host growth rate
• Regulation results in host population (and parasite population) to change in abundance during the course of the infection.