Interactions between populations I: Predator-prey and host-parasite interactions Flashcards
Describe the broad classification of interactions
antagonism, competition and mutualism
Describe competition
negative effect on both species
Describe antagonism
- one species gaining nutrition from another across trophic levels
- involves a negative effect on only one
- e.g. predation, parasitism and parasitoidy
Describe mutualism
- confers a positive effect on both species
Describe predation
- prey is killed by the predator
- confers a benefit to the predator and a cost to the prey
- many times in a predator’s lifetime, but only once in a prey’s
Describe parasitism
- host is harmed by a parasite which lives on (an ectoparasite) or in (an endoparasite)
- confers a benefit to the parasite, and a cost to the host
- cost to a parasite host is less than the cost of a predator’s prey; only part of the host is consumed
- rarely a fatal antagonism
- at a species level, only one, or very few, hosts are consumed during a parasite’s life history
Describe parasitoidy
- (likely arthropod) host is killed by a parasitoid which lives on (ectoparasitoid) or in (endoparasitoid) it
- parasitoid will consume only one host in its lifetime.
Describe herbivory
- primary producer has tissue removed by a herbivorous consume
- temporary parasitism, where only part of the tissue is consumed, and therefore the cost lesser
Describe a primary producer
the resource individual
Population size is determined by the
relative rates of natality and mortality.
The equation for population change over time can be written thus:
Nt+1 = Nt + Births – Deaths
A population is said to have reached its carrying capacity (K), when …
natality is equal to mortality
The factors influencing natality and mortality are:
- intra- and inter-specific competition for resources
- natural enemies
natural enemies aka
antagonists
Predator-prey dyanmics can be analysed using the
inter-trophic Lotka-Volterra predator-prey model
Describe the Lotka-Volterra predator-prey model
- prey undergo exponential increase
- but their removal rate from the population is dependent upon the product of predator-prey encounters (P x N) and on the attack rate of the predator (a).
Give the Lotka-Volterra predator-prey model equations
dN/dt = rN – aPN
dP/dt = faPN – qP
Describe predator mortality rate under the Lotka-Volterra predator-prey model
- increases exponentially under starvation pressure
- q
Describe zero isoclines
solving for dN/dt = 0 in the Lotka-Volterra predator-prey model
Describe a prey zero isocline
when the combinations of predator and prey lead to an unchaning prey population (r/a)
Describe a predator zero isocline
when combinations of predator and prey lead to an unchanging predator population (q/fa).
How do we explore the dynamics of both predator and prey species acting in concert?
Superimposing dP/dt on top of dN/dt graphically under the Lotka-Volterra predator-prey model
What does the Lotka-Volterra predator-prey model reveal?
population cycles
Population cycles
- aka coupled oscillations
- display linkage between the rises and falls of predator and prey abundances
- intimate linkages are rare in nature: regular cycles are the exception rather than the rule
Describe the assumptions of the Lotka-Volterra model
- fragmenting the populations in space: ignoring other interactions between them
- Type I response between prey density and number of prey consumed by a predator
Type I
linear
Type III
- sigmoidal
- density-independent
Describe the functional response between prey intake rate of the consumers and prey density (how attack rates scale with prey density)
non-linear
Describe what determies whether predation is density-dependent or density-intendent
- the shape of the predator response curve
- the range of prey densities
Describe what happens when prey population levels are very high (>D)
- predation is likely to become inversely density-dependent
- destabilisng upon the prey population
- role in prey density limitation becomes weaker and weaker
Average prey population
labelled D
Describe what happens when prey population levels are low (<D)
- predation will become density-dependent
- stabilising upon the prey population, since prey are hard to find, and predators are dispersed relative to their population
Describe density-dependent population limitation through host parasitism
- cliff swallow population Hirundo pyrrhonota
- nest colonially
- number of ectoparasites per nestling increases with the size of the colony
Describe the Janzen-Connell hypothesis
‘pest pressure’ from specialised herbivores, or pathogens, cause density-dependent survival of seeds or seedlings, promoting diversity.
