Food web stability

Question 1

Omnivores complicate food chains. True or false?

Answer 1

True.

Question 2

Which kind of communities are more easily upset, simple or complex?

Answer 2

Simple.

Question 3

Define an attractor in terms of stability.

Answer 3

A ‘cloud of abundance’ or resources that the community clusters around.

Question 4

Is there only ever one stable state for a community?

Answer 4

No, there can be multiple.

Question 5

Stability is possible even with internal fluctuations. True or false?

Answer 5

True.

Question 6

Give an example of extrinsic factors that can cause instability.

Answer 6

Weather

Question 7

Define a pulse perturbation.

Answer 7

A relatively instantaneous alteration of species numbers, after which the system relaxes back into its equilibrium.

Question 8

Define a press perturbation.

Answer 8

A sustained alteration of species density, often leading to the complete elimination of a species. The unperturbed species is the one that is measured until it reaches a new state of equilibrium.

Question 9

Which kind of communities are more easily upset, simple or complex? Why?

Answer 9

Simple: they are more susceptible to internal oscillations and invasions. More diversity creates more stability.

Question 10

Give an example of extrinsic factors that can cause a) pulse and b) press perturbations.

Answer 10

a) Extreme weather event, like a storm

b) Gradual climate change

Question 11

Stability fluctuates with loss/gain of species. True or false?

Answer 11

True.

Question 12

In a system with algae and coral, the algae outgrows the coral. Which species is needed to maintain stability?

Answer 12

The parrot fish, to eat the excess algae.

Question 13

Stability fluctuates with loss/gain of species. True or false?

Answer 13

True: extinction and invasion have huge effects on stability.

Question 14

In a system with algae and coral, the algae outgrows the coral. Which species is needed to maintain stability?

Answer 14

The parrot fish, to eat the excess algae. Without the herbivore the algae and coral cannot coexist.

Question 15

Using Lokta-Volterra to model stability, the equation is as follows:

a(SC)1/2

Answer 15

a = average interaction strength
S = no. of species
C = connectance, or the number of other species a species interacts with

Question 16

Based on the equation a(SC)1/2

Answer 16

If there are many interactions but they are all weak, as they would be in a highly diverse community, stability can be reached.

Question 17

Using Lokta-Volterra competition models it was found that average interaction strength decreased as communities got bigger. What was found in terms of invasion?

Answer 17

Larger communities were more resilient and it was harder for new species to invade.

Question 18

If larger communities benefit from weaker interactions, if this the case with small communities too?

Answer 18

No: small communities gain stability from stronger interactions.

Question 19

In larger communities, low variability of interactions is beneficial. Is this the same with small communities?

Answer 19

No: highly variable interaction strengths can be beneficial.

Question 20

What did Yodzis, 1981 find about a) intraspecific and b) interspecific competition when he assigned the L-V consumer-resource model to 40 independent food webs?

Answer 20

a) Strong intraspecific interference stabilised dynamics

b) Strong interspecific interference destabilised dynamics

Question 21

In a large community, all interactions have equal probability but those that persist are…?

Answer 21

Weaker: low connectance = stability

Question 22

Give an issue of trying to assess stability in a large population.

Answer 22

When there are lots of species data is hard to collect.

Question 23

According to Mcann, 2000 what kind of predators help to stabilise a community, generalist or specialist?

Answer 23

Generalist.

Question 24

According to Mcann, 2000 what effect does removing a species have on mean interaction strength?

Answer 24

It increases interaction strength, meaning less species creates instability.

Question 25

According to Mcann, 2000 an invader can destabilise a community if…?

Answer 25

Interaction variability is increased.

Question 26

Give an example of a simple food web that is unstable.

Answer 26

Killer whales’ normal prey is overfished. They switch to eating sea otters.

Sea otters are being eaten and no longer able to eat sea urchins.

Sea urchins increase and decimate kelp forests.

If there were more species involved, changing one would have had less of an effect on the others.

Question 27

What did Williams et al. in 2002 find about connectance in their food webs?

Answer 27

> 95% of species were within 3 links of each other, thus increasing species diversity decreases connectance.

Question 28

What effect does a loss in biodiversity have on an ecosystem?

Answer 28

It increases connectance between the remaining species and make the ecosystem more vulnerable to invasion/fluctuations.

Question 29

The productivity of a whole system may remain constant even if the species diversity changes. How?

Answer 29

Different species have different favourable conditions. Diversity ensure ecosystem survival.

Basically having high species diversity is the same as having lots of alleles - the system is more able to cope with change as it has more resources.

Question 30

The productivity of a whole system may remain constant even if the species diversity changes. How?

Answer 30

Different species have different favourable conditions. Diversity ensure ecosystem survival.

N.B. Basically having high species diversity is the same as having lots of alleles - the system is more able to cope with change as it has more resources.

Question 31

What are the problems with modelling food webs? List 3.

Answer 31

1. They are incomplete
2. There are life-history stage effects that are unaccounted for
3. They are not representative of complex trophic interactions

Question 32

What did Yodzis, 1981 find about a) intraspecific and b) interspecific competition when he assigned the L-V consumer-resource model to 40 independent food webs?

Answer 32

a) Strong intraspecific interference stabilised dynamics
b) Strong interspecific interference destabilised dynamics

Basically weak interactions between species were beneficial.

Question 33

What effect does a loss in biodiversity have on an ecosystem?

Answer 33

It increases connectance between the remaining species and makes the ecosystem more vulnerable to invasion/fluctuations.

Question 34

The productivity of a whole system may remain constant even if the species diversity changes. How?

Answer 34

Different species have different favourable conditions. Diversity ensures ecosystem survival.

N.B. Basically having high species diversity is the same as having lots of alleles - the system is more able to cope with change as it has more resources.