week 4- marine food webs

‘In general terms it is a pathway of consumption and energy flow from one trophic level to another” * Simple * Linear * Efficient transfer of energy

• Tertiary consumer (top predator) • Secondary consumer (heterotroph) • Primary consumer (heterotroph) • Primary producer (autotroph))

• Eating is inefficient • 70-99% of energy is lost as heat (respiration) • Energy cannot be created so diminishes through each level • Primary production in marine systems is very large but doesn’t scaffold more levels • Long food chains are unstable • Predator design is limited (apex predators can only be so effective) • Omnivory (eating both plant matter and animals) is common

• Energetic demand of consumers increases with body size • Marine primary producers are very small and numerous • But some apex predators begin life as eggs larvae and increase in size by 5 orders of magnitude

• Low levels of specialism • Openness of marine systems • Large size changes along a life history • Long lifespans – ontogenetic shifts

Bottom up control: resource availability drives food web stability E.g. More phytoplankton so more copepods, sand eels and seabird breeding success

driven by predator prey interactions sometimes grazing, important for ecosystem structure and function

Overfishing led to depletion of cod stocks in N. Atlantic * Subsequent increases in the abundance of shrimp, crabs, and lobster * Evidence of top-down control by cod on benthic macro-invertebrates

• An ecological phenomenon triggered by the addition or removal of top predators and involving reciprocal changes in the relative populations of predator and prey through a food chain, which often results in dramatic changes in ecosystem structure and nutrient cycling

• A phenomenon in which populations of medium-sized predators rapidly increase after the removal of predators