21.2 - Food Webs and Trophic Cascades

food web

network of feeding relationships between species in an ecosystem - includes multiple food chains

role of primary producers in energy transfer

converts nutrients, light and water into biomass

role of primary consumers in energy transfer

species that directly consume primary producers and are essential for transferring energy to higher trophic levels.

food chain

sequence of organisms eating one another, from producers to consumers in which energy and nutrients are passed 

bottom-up limitation

ecosystems are primarily regulated by the availability of nutrients and energy through abundance of primary producers

• influenced by rain, temp, light and nutrient availability

• more producers = more organisms in higher levels = increased availability of nutrients and energy

• resources determine NPP

top-down limitation

ecosystems are primarily regulated by consumption pressure from higher trophic levels

• limits the population sizes of lower trophic levels, often leading to increased abundance of primary producers.

• high level consumption determines NPP

2 main ways energy flow is controlled 

bottom-up and top-down

why does 3-level food webs have higher NPP than 2-level webs

in 3-level there is pressure placed on primary consumers by secondary consumers so less pressure on producers. In 2-levels no pressure placed on primary consumers = more pressure on producers = lower NPP

why is NPP lower in food web with omnivore than one without

with = both secondary and primary consumers are placing pressures on producers = 2 sources of consumption pressure compared to webs without

trophic cascade

rate of consumption at one trophic level results in a change in species abundance or consumption at lower trophic levels

e.g. of trophic cascade

if dingo pop decreases, reduces rate of consumption of kangaroos = their pop increases = increased consumption of vegetation = decreased abundance

what happened when there were no wolves in Yellowstone

the elk population grew excessively, leading to overgrazing of vegetation and significant ecological imbalance.

what happened to Yellowstone when wolves were reintroduced

declined elk pop and changed their behaviour to avoid certain parts esp valleys and gorges = regenerate veg = forests began to regrow = increased biodiversity including birds and beavers

• beavers increased = created niches for other species including dams becoming habitats for otters, ducks and fish

• wolves killed coyotes = rabbits and mice increased = increased hawks, foxes, weasels

• bears increased due to increased food sources including berries and dead carrion

• less meandering and erosion of rivers = channels narrowed = more pools formed and more riffle sections = increased habitats