Food chains Flashcards
lecture 6
how much energy is converted to another trophic level in food webs?
- energy is limited - only 10% E available to next level
- short food chains
- restraints at the top of the food chains
- stability of food chains is a result of how many links there are in a food chain, more chains are less stable in larger food webs.
Directionality of control?
bottom up food chains
top down food chains
Which links are strong?
little food chains depict the strength of the interactions between species.
two compartments of the oceanic food web?
- benthic macroalgal food web (on the seafloor)
- pelagic food web (sea surface)
How to deal with complexity in food webs?
modularise = study small, important modules from the broader web.
Nature of interactions in food webs?
algea -weak direct effect> limpets -strong direct effect> birds -indirect effects>algae
Indirect and unexpected effects in food webs?
e.g. Cat and birds and rats
cats escaped from boats and dramatically directly impacted the bird populations on islands but indirectly affected the rat populations.
Indirect effect thwarted the conservation programs for the birds.
difference between direct and indirect effects
direct - can be sort of be predicted
indirect - cannot be predicted
Directionality and effect on food chains?
Bottom up forcing (limpets:birds) positive relationship non-linear.
Top down forcing (bird:limpet) negative relationship
effects are non- linear = phase shifts and result in different states in communities.
Real life examples
James Estes
Single food chains - work provides a lot of insights.
Kelp forests (macrocystis) and sea otters
How top predator changes the food chain dynamic:
With otters present dense kelp thriving forests.
Otters removed - sea urchins take over and remove kelp from the system.
This food chains is top down controlled.
Top-down forcing trophic-cascade.
Effects are not as big in California as in Canada = more predators/diversity in cali.
1998 - Orcas changes their dominat prey to otters, dramatic deacrease in otter population, urchins are released from top-down effect = decrease in kelp.
Removal of one taxonomy (top predator) in a system changes the communities dramatically.
Difference between Northern and Southern hemisphere kelp chemical - otters prefer northern less chemicals in kelp, they need less adaptions.
His research emphasied how important predators ar ein controlling the functioning of communities.
How common are trophic cascades?
Apex predators trophic cascades top down
affects:
- biodiversity
- fire regimes
- disease
- atmosphere
- soil
- water
- invasive species
Top-down vs bottom-up control?
consumer to resource
resource to consumer- still the most widely accepted way of control in food systems. Reflects the ineffcitiveness of biomass through the trophic levels.
Which is more important in the bigger picture? top-down/botton-up?
look at lecture lisde more info there
Why is the world green paper (Hairston et al 1960)
- why is not more of plant biomass removed, if there are herbivores that do eat them?
= green plant biomass accumulates because herbivores are kept in check by predators - top-down control.
=everything else is controlled by bottom-up
there is a consumer control dynamic going on between herbivores and plants.
but in more dry areas they can remove all plant biomass.
Plants can defend (thorns and chemically) themselves and herbivores compete for limited resources.
Bottom-up
- plant chemical defense.
- nutritional limitations.
Top-down
- predator control of herbivore numbers.
why are oceans blue?
1)Most water is too deep for photosynthesis
2)Large plants can only survive in restricted
condition
3)Seawater is too nutrient poor to maintain dense
phytoplankton populations
= bottom up control.
