Food web stability Flashcards
Omnivores complicate food chains. True or false?
True.
Which kind of communities are more easily upset, simple or complex?
Simple.
Define an attractor in terms of stability.
A ‘cloud of abundance’ or resources that the community clusters around.
Is there only ever one stable state for a community?
No, there can be multiple.
Stability is possible even with internal fluctuations. True or false?
True.
Give an example of extrinsic factors that can cause instability.
Weather
Define a pulse perturbation.
A relatively instantaneous alteration of species numbers, after which the system relaxes back into its equilibrium.
Define a press perturbation.
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.
Which kind of communities are more easily upset, simple or complex? Why?
Simple: they are more susceptible to internal oscillations and invasions. More diversity creates more stability.
Give an example of extrinsic factors that can cause a) pulse and b) press perturbations.
a) Extreme weather event, like a storm
b) Gradual climate change
Stability fluctuates with loss/gain of species. True or false?
True.
In a system with algae and coral, the algae outgrows the coral. Which species is needed to maintain stability?
The parrot fish, to eat the excess algae.
Stability fluctuates with loss/gain of species. True or false?
True: extinction and invasion have huge effects on stability.
In a system with algae and coral, the algae outgrows the coral. Which species is needed to maintain stability?
The parrot fish, to eat the excess algae. Without the herbivore the algae and coral cannot coexist.
Using Lokta-Volterra to model stability, the equation is as follows:
a(SC)1/2
a = average interaction strength S = no. of species C = connectance, or the number of other species a species interacts with
Based on the equation a(SC)1/2
If there are many interactions but they are all weak, as they would be in a highly diverse community, stability can be reached.
Using Lokta-Volterra competition models it was found that average interaction strength decreased as communities got bigger. What was found in terms of invasion?
Larger communities were more resilient and it was harder for new species to invade.
If larger communities benefit from weaker interactions, if this the case with small communities too?
No: small communities gain stability from stronger interactions.
In larger communities, low variability of interactions is beneficial. Is this the same with small communities?
No: highly variable interaction strengths can be beneficial.
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?
a) Strong intraspecific interference stabilised dynamics
b) Strong interspecific interference destabilised dynamics
In a large community, all interactions have equal probability but those that persist are…?
Weaker: low connectance = stability
Give an issue of trying to assess stability in a large population.
When there are lots of species data is hard to collect.
According to Mcann, 2000 what kind of predators help to stabilise a community, generalist or specialist?
Generalist.
According to Mcann, 2000 what effect does removing a species have on mean interaction strength?
It increases interaction strength, meaning less species creates instability.
According to Mcann, 2000 an invader can destabilise a community if…?
Interaction variability is increased.
Give an example of a simple food web that is unstable.
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.
What did Williams et al. in 2002 find about connectance in their food webs?
> 95% of species were within 3 links of each other, thus increasing species diversity decreases connectance.
What effect does a loss in biodiversity have on an ecosystem?
It increases connectance between the remaining species and make the ecosystem more vulnerable to invasion/fluctuations.
The productivity of a whole system may remain constant even if the species diversity changes. How?
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.
The productivity of a whole system may remain constant even if the species diversity changes. How?
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.
What are the problems with modelling food webs? List 3.
- They are incomplete
- There are life-history stage effects that are unaccounted for
- They are not representative of complex trophic interactions
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?
a) Strong intraspecific interference stabilised dynamics
b) Strong interspecific interference destabilised dynamics
Basically weak interactions between species were beneficial.
What effect does a loss in biodiversity have on an ecosystem?
It increases connectance between the remaining species and makes the ecosystem more vulnerable to invasion/fluctuations.
The productivity of a whole system may remain constant even if the species diversity changes. How?
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.
