L11 + L12 Multi-species dynamics: Predation and herbivory

What are examples of couples oscillations?

Predator -> prey

Herbivore -> plant

Parasite -> host

What is the Lotka-Volterra model, that relates to predation and herbivory?

Captures how efficient a predator is at catching prey, and conversion of prey biomass into predator biomass

What are zero isoclines?

no change in population size

What are the 2 lotka-Volterra graphs?

What happens when you overlay the graphs?

Coupled oscillations, with predator lagging behind prey

1st shows prey and predator

2nd shows against time

What happens when there is a disturbance in the lotka-volterra model?

Oscillations are unstable

What is a functional response?

Describes how consumption rate of predator reacts to change in prey numbers

What are the 3 types of functional response?

- Type I

- Type II

- Type III

What is a type 1 functional response?

Consumption rate increases linearly with prey abundance (until plateau is reached)

Type I functional response is assumed in LV equations (constant a = attack rate)

What is a type 2 functional response?

Consumption rate increases with prey abundance, but in a decelerating way

Caused by predator spending more time on handling prey than finding prey at high densities

What is a type 3 functional response?

- Like type II at higher prey densities, but lower than expected at low prey densities

- Could be caused by 'switching'

- Leads to s-shaped ('sigmoidal') curve

- Refugium for prey at low densities

Will prey numbers vary across time and space?

yes

Where do predators aggregate?

where prey numbers are high

Refugia where prey numbers are low

How do we get stabilisation of oscillations?

through a negative feedback mechanism

What are features of herbivory - grazing?

Plant usually not killed

Large variation from strict monophagy (feeding on one species) to full polyphagy (feeding on lots)

What is the critical difference between predation and herbivory?

- plant not normally killed

- animal is

What do monophagous herbivores normally consist of?

- Small insects (exception is koala)

What do polyphagous herbivores usually consist of?

Large mammals (exception is locusts)

What is the strict monophagy of herbivory?

Plant abundance determined by herbivore traits; herbivore abundance by plant traits

Because herbivore is food-limited, the faster the plant grows, the faster the herbivore eats it

What happens if there is full polyphagy in herbivory?

- Herbivore is polyphagous, so abundance does not depend on focus plant -> constant rate of herbivory

- Plant abundance determined by plant and herbivore traits; the faster it grows, the higher its biomass

- Herbivore is not limited by focal plant

What is a trophic cascade?

when top consumers reduce the biomass of the next lowest trophic level and this reduction cascades down food chain

What control is it when predator pop increases, prey decreases and plant increases?

top-down control

When will bottom up control occur?

If dynamics driven by competition amongst plants

Plants decrease, prey decrease predator decrease

Do top-down and bottom-up control play a role on the species?

Both processes likely to play a role, with relative importance depending on the system

How can we try to understand what happens in natural systems?

construct food webs

Simplest food web:

"who eats whom"

Is modelling a food web impossible?

yes as it is so complicated

What do people try to do instead of modelling a food web?

Characterise food webs, with aim of understanding stability (how resistant to change, how fast to revert back after change)

What is connectance?

fraction of all possible species pairs that are connected in the food web

Is there a relationship between species richness and conectance?

Yes but complex and not always significant

What is a keystone species?

Species whose impact is proportionally large relative to its abundance

Has a large beta ij with lote of species

Conceptually important, but difficult to measure objectively

Important for conservation

What are the 2 big problems with food webs?

Problem 1: doesnt necessarily taxonomic detail

- Not all 'groups' included

- Not all 'groups' equally detailed taxonomically

Problem 2: food web interactions basically 'yes' or 'no'

(i.e. βij not estimated)

What do quantitative food webs show?

Not only "who eats whom", but also "who eats how much of whom", "who is eaten how often by "whom"

What are indirect effects of complex food webs with many species?

- Intra-guild predation

- Apparent competition

What is intra-guild predation?

Predator eats prey and predator of prey

Top predator has direct and indirect effect on middle trophic level

Raptor & Reptile Centre, Ringwood Top predator has direct and indirect effect on lower trophic level

What is apparent competition?

Blackbird population increases

Owl population increases

Frog population decreases

Decrease of frog population as a result of increase in blackbird population looks like competition

But blackbirds and frogs do not share resource