Lecture 7 - Predation

Question 1

What is competitive exclusion?

Answer 1

It can be stated as follows:

if two competing species coexist in a stable environment, then they do so as a result of niche differentiation, i.e. differentiation of their realized niches.

If, however, there is no such differentiation, or if it is precluded by the habitat, then one competing species will eliminate or exclude the other. Thus exclusion occurs when the realized niche of the superior competitor completely fills those parts of the inferior competitor’s fundamental niche that are provided by the habitat.

Question 2

What is a realised niche?

Answer 2

Realised niche is the actual space that an organism inhabits and the resources it can access as a result of limiting pressures from other species (e.g. superior competitors).

Question 3

For a realised niche what do we assume?

Answer 3

There is competitive exclusion
Resources are limiting
The community is saturated
The system is at equilibrium

Question 4

If there is a large niche width what does this suggest?

Answer 4

That there are fewer species in the population

Question 5

What is ‘niche overlap’?

Answer 5

When two populations are occupying the same niche, more overlap means there are more resources shared. The overlap is the competition coefficient.

Question 6

What are plants and predators regulated by?

Answer 6

Competition

Question 7

What are herbivores regulated by?

Answer 7

Predation

Question 8

What is species packing?

Answer 8

The number of species that can be packed along a niche axis. We assume competitive exclusion is present, resources are limiting, the community is saturated and all populations are at K.

Question 9

What is saturation with regards to niche curves?

Answer 9

The niches are spread along the axis to reduce similarity.

Question 10

What is ‘w’ with regards to niche curves?

Answer 10

‘w’ is the niche width, it is the distance to the point of inflexion, where 68% of the usage occurs.

Question 11

What is ‘d’ with regards to niche curves?

Answer 11

‘d’ is the distance between niche peaks. A lower d value means more overlap and sharing of resources.

Question 12

What is the critical point for niche curves?

Answer 12

When d/w = 1 there is substantial overlap and the species peak coincides with the point of inflexion. Half the resources are shared and there is therefore more competitive exclusion.

Question 13

In a realistic model what do we assume?

Answer 13

In realistic models food resources are seldom constant. The environment varies resulting in the carrying capacity, K, to vary.
The model is subject to stochasticity, K varies around the mean and species are distributed normally.
The prediction is that there will be broader niches resulting in more generalists.

Question 14

Explain the water boatman study regarding coexistence.

Answer 14

There is a pool of water on Mount Pellegrino that has two different species of water boatman, there is a large size difference. The differences in resource use results in competitive exclusion being avoided

Question 15

What number is the scaling ratio, the ‘magic number’?

Answer 15

1. 3
   eg. Fruit pigeons of the East Indies feed on various fruit sizes, each size class of fruit is eaten by a different group of pigeons. The weight differences between the groups are about 2 (1.3^3)

Question 16

What experiment in coastal lakes in Canada showed rapidly diverging species?

Answer 16

In coastal lakes in Canada a rapidly diverging species complex can be observed.
If two species are in the lake they exist in two forms:

'Benthic' = large, deep bodied, wide mouth, coarse filter feeder
'Limnetic' = small, thin, narrow mouth, fine filter

However if only one species is present in the lake it is a intermediate generalist with variable morphology.

This experiment was recreated in an artificial pond. It began with just generalised fish who showed no change in growth rate or morphology when they were the only species in the pond. However, if a limnetic looking fish was introduced, the generalised fish greatly reduced its growth rate in order to become more limentic looking. Therefore growth rate is related to fitness.

Question 17

Describe 2D niche packing graphs.

Answer 17

The major axis are two resources, food and habitat. ‘d’ is the distance between the centre spots of the niche circles. The overlaps are shared with more than one species as each species has 6 neighbouring species.

Question 18

Describe the desert lizard study.

Answer 18

Observed desert lizards in North America, the Kalahari and West Australia.
Their food type, habitat use and the number of spp. in the community was noted.
It was found the limiting factor was water, it correlated with the number of species present. This was caused by the variability in rain fall.
The maximum tolerable overlap could be predicted.
The results matched the prediction. A greater niche overlap resulted in fewer lizard species present.
This matches the niche theory for competition for resources that increased overlap results in competitive exclusion and the extinction of certain species so only generalists remain occupying multiple niches.

Question 19

What is the paradigm of coexistence?

Answer 19

This considers a set of species existing at the same trophic level without interbreeding but they are not forced to extinction by competitive exclusion - they coexist.

The reason they can coexist is the ‘invasibility criterion’, that is they can increase in number when one species is rare and the other is at its usual abundance.

Question 20

How does competition act between trophic levels?

Answer 20

Predation acts between carnivores, herbivores and seed predators.

Question 21

What is the HSS hypothesis?

Answer 21

Hairston proposed that consumers play a much more important role in structuring plant dominated ecosystems. The World is green because higher trophic levels control herbivore abundance. Grazers are not limited due to the great amount of green material on Earth, instead they are limited due to the predators that keep their populations and the negative impact of plants in check. The world is kept green by a tri trophic interaction, where predators control grazers that would otherwise overgraze and eliminate vegetation.

Question 22

Describe White’s inadequate environment theory.

Answer 22

White’s theory predicts that bottom up effects, that is food shortages result in trophic cascades and predation is of minimal importance.

Question 23

What is Sih’s review and what conclusions were made?

Answer 23

Looked at prey numbers, relative abundance, diversity and size structure in 139 experimental manipulation studies.

The conclusions made were that predation is very important, the pattern does not support White or HSS, but does support Menge, that all trophic levels affect population size but herbivores to the greatest degree.

Question 24

Give an example of how predation shaped prey traits

Answer 24

Sparrowhawks prey on small great tits but not blue tits and generate strong selection on body size as a result.

Question 25

Describe predator prey networks.

Answer 25

Two prey species that do not compete but share a predator. Prey 1 increases and Prey 2 decreases when predator numbers increase, this appears to be competition but it is mediated through a predator.

Question 26

Describe competition for enemy free space.

Answer 26

Two prey species that do not compete but do share a predator.
One prey may develop an anti predator mechanism that results in prey 2 being eaten more, this is competition for space that is free of natural enemies and occurs in evolutionary time.

Question 27

What is a keystone predator?

Answer 27

Keystone predators can control the diversity of an entire system.

A classic keystone species is a small predator that prevents a particular herbivorous species from eliminating dominant plant species. Since the prey numbers are low the keystone predator numbers can be even lower and still be effective. Without the keystone predator the herbivorous prey numbers would explode and potentially wipe out the dominant plants and alter the entire ecosystem.

Question 28

If populations are at equilibrium and controlled by bottom up forces, what is this termed as?

Answer 28

Competitive exclusion

Question 29

If populations are at equilibrium and controlled by top down forces, what is this termed as?

Answer 29

Predation

Question 30

If populations are not at equilibrium does competitive dominance occur?

Answer 30

No, competitive dominance does not occur as this is prevented by predation, resulting in coexistence. This is an interaction between competition and predation.

Question 31

How do predators affect competition?

Answer 31

Predators reduce the effect of competition by allowing inferior predators to exist. Predation has a greater effect on survival than competition.

Question 32

Is predation important at all trophic levels?

Answer 32

Predation is important at lower trophic levels. Enemy free space, trophic cascades and keystone species are important.

Question 33

Are predator-prey cycles common?

Answer 33

They are common in nature but rare in lab studies.
An example is the interaction between a snow hare and the lynx.
Hares cycle every 10 years and feed on shrubs and tree roots. Their densities are unpredictable, their main source of mortality is predation.
Lynx peaks 1-2 years after the hare
When lynx are present they stress the female hard which results in a maternal effect that is transmitted through 2 generations.
The survival of the hare is linked directly with predation.
The lynx cycle is driven by food supply and movement, it involves the delayed effects of reproduction.

Question 34

What is the Nicholson-Bailey model?

Answer 34

Describes the random searching by parasitoids (an insect whose larvae live as parasites which then kill their hosts)

The model assumes that parasites search for hosts at random, and that both parasites and hosts are assumed to be distributed in a non-contiguous (“clumped”) fashion in the environment.

aP = nP/N :
(the average number of attacks per host)(parasitoids) = [(number of hosts per lifetime)(parasitoids)]/(hosts)

The model is unstable
It moves from its norm, then fluctuates which leads to a crash and extinction

Question 35

How can the ‘a’ term be fixed in the Nicholson Bailey model

Answer 35

Decrease numbers at high density, allow increases at low density, prey density will increase resulting in ‘a’ (the av. number of attacks per host to increase)