BIO 2600 unit 13- Exploitative interactions: Predation, herbivory, parasitism and disease

Exploitation

Interaction that enhances fitness of one individual while reducing fitness of the exploited individual

Predators

kill and consume other organisms

Parasites

Live on host tissue and reduce host fitness, but do not generally kill the host

Parasitoid

Parasite insect larva that consumes the host

Pathogen

A parasitic organism that causes disease

Predators, parasites, and pathogens influence:

The distribution, abundance, and structure of prey populations

Exploitative interactions have potential to influence

Prey and host populations

Mange mites (Pathogenic parasite) kill dogs and foxes (predator) in turn increasing

The number of mountain hares (prey) by 2-4x

Overpopulation theory

High growth leads to disease, stress, and starvation.

Would predator/prey cycle work the same if there were more predator and prey species?

Yes, but the patterns become more complex and usually more stable because predators switch prey and predators compete with each other

Predation can account for ______to ______ of hare mortality during peak densities

60 - 90%

Indirect effects of predation

With more predators, the increased amount of chasing may stress the female hares, which then reduces their reproductive output. This predation stress can be inherited (epigenetic effects) and can contribute to low hare populations for 2-4 years

Krebs et al. found increased hare numbers with increased food availability:

Reduced predation and a combination of the two - Conclude that hare population cycle is result of interaction among 3 trophic levels

Lotka-Volterra assumes

Host population grows exponentially, and population size is limited by parasites, pathogens, and predators

Lotka-Volterra assumes parasite/predator growth rate is determined by

Rate of conversion of food into offspring minus mortality rate of parasite/predator population

cpNhNp =

Conversion rate of hosts into parasite/predator offspring

Increased predation =

More predators -> higher exploitation rate

Host reproduction immediately translated into

Destruction by predator

Larger predator population eventually reduces host population, in turn

Reducing predator population

Modelers say:

"All models are wrong, but some are useful"

Unrealistic assumptions of Predator-Prey model

-Prediction of eternal oscillations along narrow path.

-Neither population is subject to carrying capacities.

-Changes in one population result in immediate response by other (the eating a prey results immediately in offspring)

Most lab experiments producing L-V oscillations lead to extinction of one population within a relatively short period. This made us think of what we missed - and one possibility was:

Refugia

Refugia

Places where species can survive during bad environmental conditions

Refuges can take a variety of forms

-Flight for a bird, large size, etc.

-Space.

• Spatial refuges include burrows, trees, etc.

• Invasive Opuntia stricta cactus has small isolated populations as spatial refuges.

• Herbivorous insects don't find them

Prey consumed/predator x predators/area =

Prey consumed/area

Living in a large group provides a

Refuge

Predator satiation defense

Prey can reduce individual probability of being eaten by living in dense populations

- Beyond a threshold, increases in prey density do not lead toincreases in predator density or feeding rates

The Ecology of Fear

The presence of predators can alter the behavior of prey to avoid high-risk locations

To compensate for L-V unrealistic assumptions (like conversion of prey to predator offspring is immediate), we can use

A ratio-dependent model in which not only number of prey, but also the number predators competing for it, matter.

L-V assumptions (like conversion of prey to predator offspring is immediate, and size of predator population has no effect on individual predator feeding rate - 'prey dependent') are only valid in

Very low predator densities, or for micro-predators with fast life cycles

The prey-dependent functional response model assumes

The predator's consumption rate (g) depends only on prey abundance (N)g

In the prey-dependent model, what is g a function of?

g = g(N)

What model is an example of a prey-dependent functional response?

Holling's model

What are the two key factors included in Holling's model?

Searching efficiency (a) and handling time (h).

Formula for Holling's functional response?

g(N) = aN/1 + ahN

What does the ratio-dependent model assume?

Predator consumption rate depends on the ratio of prey to predators (N/P)

In the ratio-dependent model, what replaces the parameter "a"?

α (alpha), the rate at which prey become available instead of searching efficacy

In the ratio-dependent model, what replaces N (prey abundance)?

The ratio N/P (prey per predator)

Formula for the ratio-dependent functional response?

g(N/P) = α (N/P)/1 + αh (N/P)

What does g represent in both models?

The per capita rate of prey consumption, meaning - per one predator, how many prey does it eat in a given time?

Arditi and Ginzburg suggest that ratio dependent model is

Most appropriate for large-scales studies of large organisms with slower life cycles.

______________ interactions weave populations into a web of relationships that defy easy generalization

Exploitative

Some parasites _______ hosts behaviour

Alter

Photoaxis

Orientation of an organism to light

What happens to infected amphipods' behavior?

They show positive phototaxis (move toward light).

Why does positive phototaxis benefit the parasite?

It brings amphipods near surface predators that act as the next host.

What is the main idea of complex exploitative interactions in ecosystems?

Many species interact through predation, parasitism, and competition, creating a large web of connections where changes to one species can affect many others