Food webs Flashcards
Why are food webs important?
Help us to understand how ecological communities are structured, and how changes to the abundance of one species can propagate. E.g. changes in a top predator (cat) can influence the number of flowers (through mice and bees).
Which interactions do food webs ignore?
Mutualisms or interference as they’re not trophic interactions.
What is a linkage web?
Shows which species eat other species. Binary representation so can be represented in a matrix.
What is an energy flow web?
Shows the quantity of energy flowing between species. Can be represented in a weighted matrix.
What is a functional strength web?
Shows the most important interactions structuring the community. Requires experiments.
Keystone species.
What is a keystone species?
A species that has a disproportionately large effect on its environment relative to its abundance
If these species go extinct, then effects will be widespread
Connectance of food webs?
S - number of species in web
L - number of links
C - fraction of possible links in the web which actually occur
C = L/(S(S-1)/2)
Why should we study food webs?
- summarise the complexity of community interactions. More realistic than models of a few interacting species
- help us understand community structure. Which species occur in a community and why some are rare and some are common
- help us understand community dynamics. What happens if we interfere with ecological communities
Main conclusions from modelling food webs?
Food chains should be short as longer chains aren’t stable.
Complexity may reduce stability which contradicts conventional wisdom.
Species feeding on more than one trophic level (omnivores) should be rare, but where they are stable, should be v stable.
Why are food chains short?
Productivity - energy attenuation hypothesis
Energy is lost at each trophic level (heat, respiration, etc.). 2nd law of thermodynamics
Eventually there is not enough energy for a further trophic level.
Communities with more efficient energy transfer should have longer food chains, which sometimes happens for endotherms vs ectotherms.
Communities with higher productivity should have longer food chains. At low productivity they do, but there is a limit of about 4 levels.
Experimentally manipulate resources. Little evidence that food chain length is increased by increased resources, except at v low productivity.
Instability of long chains - trophodynamics explanation
Chance variations in population sizes are amplified up the food chain leading to unpredictable dynamics for the top predator. Food chains are indeed shorter in unpredictable environments.
Experimentally manipulate food chain length. Long food chains show more variable dynamics.
Does complexity reduce stability?
Elton - conventional wisdom that complexity promotes stability. Expect species feeding on many prey species to be buffered against chance fluctuations in prey abundance.
May - more species could lead to decreased stability. More connections lead to decreased stability.
Randomly assembled webs with different numbers of species (S), connectance (C) and interaction strengths (β). Webs were stable (populations returned to equilibrium after a small disturbance) if β(SC)^1/2<1. So complexity (more species and/or more connections) appears to decrease stability
How does extinction vary with web size?
Probability increases with web size.
Importance of weak interactions?
In models, weak to intermediate strength links are important in promoting community persistence and stability.
Weak links dampen interactions between consumers and resources, keeping population densities further from 0 and hence decreasing the chance of a population extinction. A rocky shore field experiment provides support for the models.
Importance of indirect interactions?
Most food web models consider direct (trophic) interactions, e.g. A eats B
Indirect effects occur e.g. when B influences the population size of C via the action of A
In this example, B and C share a predator
Changes in the abundance of B may influence the abundance of C, by increasing the abundance of A
This particular example is called ʻApparent Competitonʼ
Example of apparent competition?
Red squirrel decline due to non-native grey squirrels transmitting squirrel pox virus.
